Publications

Crystal engineering offers a route to functional molecular solids through controlled noncovalent interactions. In centrosymmetric crystals, piezoelectricity can be induced by stereoselective doping, where chiral additives are incorporated in a biased orientation that lowers inversion symmetry and generates net polarization. However, the microscopic origin of this polarization, specifically whether it arises primarily from dipole mismatch between the dopant and the host molecule it replaces, or from dopant-induced lattice distortion, remains generally unclear. Here, we disentangle these two contributions by incorporating four chiral N-acetyl-L-amino acids into the centrosymmetric crystal N-acetyl-DL-valine and examining the resulting materials using piezoelectric measurements, density functional theory (DFT), and low-frequency Raman spectroscopy that reveals concentration-dependent symmetry-breaking phenomena not resolved by conventional X-ray diffraction. We show that in the systems studied, macroscopic piezoelectricity correlates directly with the magnitude and orientation of local lattice distortions, whereas dipole mismatch along the polar axis plays a secondary role. These results establish a direct structurefunction relation in doped molecular crystals and provide predictive design principles for engineering electromechanical response in molecular crystals through biased local distortions.

We introduce an automated workflow for generating non-empirical Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functionals. WOT-SRSH functionals have been shown to yield highly accurate fundamental band gaps, band structures, and optical spectra for bulk and 2D semiconductors and insulators. Our workflow automatically and efficiently determines the WOT-SRSH functional parameters for a given crystal structure and composition, approximately enforcing the correct screened long-range Coulomb interaction and an ionization potential ansatz. In contrast to previous manual tuning approaches, our tuning procedure relies on a new search algorithm that only requires a few hybrid functional calculations with minimal user input. We demonstrate our workflow on 23 previously studied semiconductors and insulators, reporting the same high level of accuracy. By automating the tuning process and improving its computational efficiency, the approach outlined here enables applications of the WOT-SRSH functional to compute spectroscopic and optoelectronic properties for a wide range of materials.

Density functional theory (DFT) has transformed our ability to investigate and understand electronic ground states. In its original formulation, however, DFT is not suited to addressing (e.g.) degenerate ground states, mixed states with different particle numbers, or excited states. All these issues can be handled, in principle exactly, via ensemble DFT (EDFT). This Perspective provides a detailed introduction to and analysis of EDFT, in an in-principle exact framework that is constructed to avoid uncontrolled errors and inconsistencies that may be associated with ad hoc extensions of conventional DFT. In particular, it focuses on the \u201censemblization\u201d of both exact and approximate density functionals, a term that we coined to describe a rigorous approach that lends itself to the construction of novel approximations consistent with the general ensemble framework, yet applicable to practical problems where traditional DFT tends to fail or does not apply at all. In particular, symmetry considerations and ensemble properties are shown to enable each other in shaping a practical DFT-based methodology that extends beyond the ground state and, in doing so, highlights the need to look outside the standard ground state KohnSham treatment.

Optimal tuning of functional parameters in density functional theory approximations, based on enforcing the ionization potential theorem, is a method of choice for the nonempirical prediction of the electronic structure of finite systems. This method has recently been extended to the bulk limit, based on an ansatz that generalizes the ionization potential theorem to the removal of an electron from a localized Wannier orbital. This Wannier-localization-based optimal tuning method has been shown to be highly successful for a wide range of periodic systems, accurately predicting electronic and optical properties. However, a rigorous theoretical justification for its foundational ansatz has been lacking. Here, we establish an ionization potential condition for the removal of an electron from a Wannier-localized orbital, by extending the piecewise linearity and Janaks theorems in density functional theory. We also provide numerical evidence supporting our theory.

Many animals produce vivid colors by reflecting and amplifying light with stacked guanine crystals within membrane-bound organelles called iridosomes. While the presence of guanine crystals in iridosomes is well documented, the mechanisms facilitating the accumulation of water-insoluble guanine and driving its crystallization remain unclear. Here we used cryo-electron microscopy, live-cell pH imaging, pharmacological perturbations and spectroscopy to study iridosome maturation in zebrafish. Cryo-electron and synchrotron-based soft X-ray microscopies revealed that amorphous guanine initially accumulates in early-stage iridosomes in its protonated state. Live-cell imaging with a pH sensor demonstrated that early iridosomes are acidic, with pH gradually neutralizing during development. Inhibiting V-ATPase disrupted this acidification and significantly reduced crystal formation, indicating its role in pH regulation. Our findings reveal insights into the molecular mechanisms facilitating guanine formation within iridosomes, emphasizing the pivotal role of pH alternations in the precise formation of biogenic crystals.

The pronounced electrochemical reactivity between halide perovskites and metal electrodes can introduce mobile extrinsic metal ions which can cause device instability or enable novel functionalities. Here we systematically investigate the kinetics of gold cation (Au⁺) migration in indium tin oxide (ITO)/methylammonium lead triiodide (MAPbI₃)/Au model devices under long-term potentiostatic biasing. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) analyses reveal that Au⁺ions, electrochemically generated at the Au anode, traverse the perovskite layer with diffusion coefficients on the order of 10⁻¹¹to 10⁻¹⁰cm²s⁻¹and are subsequently reduced at the cathode as Au⁰clusters, resembling metal plating behavior in electrolytic cells and solid-state batteries during charging. Furthermore, reversing the applied bias strips the plated Au⁰, revealing reversibility suitable for bipolar resistive switching devices and providing direct evidence of the electrochemical and ionic nature of Au transport within the perovskite matrix. Quantitatively determining diffusion coefficients and ion concentrations provides foundational inputs for future drift-diffusion modelling opportunities and allows us to relate our findings to implications on long term operation of devices like photovoltaic modules. These results clearly demonstrate the solid-state electrochemical nature of perovskite devices, highlight methods to be more quantitative about ion transport properties, provide and emphasize the importance of disentangling electro-, photo-, photoelectrochemical processes for understanding device performance and unlocking new functionalities.

Hydrogen bonds in molecular crystals are often modeled as double-well potentials, yet direct evidence linking this potential form to vibrational spectroscopic features remains elusive. In this study, we investigate alpha-glycine, a hydrogen-bonded crystal that exhibits pronounced Raman anomalies without undergoing a structural phase transition. Through temperature- and polarization-dependent Raman spectroscopy, supported by isotope substitution and first-principles calculations, we identify two peaks whose behavior violates conventional Raman selection rules. These peaks merge and narrow anomalously with temperature, an effect that cannot be explained by harmonic models or thermal broadening. Simulated spectra based on a weakly evolving asymmetric double-well potential reproduce this merging, indicating that both peaks originate from one double-well potential. Our results establish alpha-glycine as a model system directly linking microscopic hydrogen-bond potentials to vibrational spectroscopic features.

Thin xanthine crystal plates operate as mirrors in the ocelli (small eyes) of the insect L. rozsypali (jumping bristletail), where they form superstructures that are used to reflect missed incident light back to the light detectors. Here, we present the structure of the biogenic xanthine crystals. Structure determination is made possible by combining electron diffraction and 4D scanning transmission electron microscopy with fundamental symmetry considerations and first-principles calculations based on density functional theory. We find three possible structures, all layered, with individual planes comprised of the same planar hydrogen-bonded network. However, they are different polytypes (i.e., exhibit different stacking of the individual planes). One of these polytypes corresponds to the structure of the biogenic crystals, whereas a second polytype corresponds to the structure recently determined for synthetic xanthine crystals.

The anisotropic nature of layered materials is key to many of their unique physical properties. The design and control of novel layered architectures requires a microscopic understanding of their intra- and inter-layer interactions. Ab initio simulations, based on, e.g., density functional theory, often provide valuable insights regarding their structural, mechanical, dynamical, and electronic properties. However, such calculations are often computationally demanding, thus limiting the treatment to relatively small length and time scales. Classical molecular dynamic simulations, based on physically motivated force-fields, may offer a viable computationally efficient alternative, when parameterized appropriately against ab initio reference data for small model systems. The general strategy usually relies on a separate treatment of intra- and inter-layer interactions. When considering the latter, popular isotropic potentials, such as those presented by Lennard-Jones and Morse, often fail to simultaneously capture binding and sliding physics. Therefore, anisotropic interlayer force fields, based on the Kolmogorov-Crespi scheme, have become the tool-of-choice. In this review, we summarize progress in the field of anisotropic interlayer force field, including the fundamental theoretical framework, parameterization, and representative applications to selected physical properties. We also discuss potential directions for further advancement, based on state-of-the-art developments in simulation technologies.

Photoelectron satellites-the structures appearing on the low kinetic or high binding-energy side of the "main" or "elastic" photopeak-betray the complex many-body interactions set in motion by the sudden creation of the core hole. In this work, we demonstrate, using the technologically important ferromagnetic half-metal CrO2, how such satellites can manifest themselves in other core-level spectroscopies of the material and how they can reveal important details pertinent to its electronic structure. Specifically, we identify a fluorescence satellite in the Cr L3 resonant x-ray-emission spectra that radiates at a constant emission energy across the Cr L3 x-ray edge with energy approximate to 1.3 eV above the ordinary valence fluorescence. We provide evidence that this feature arises from the valence recombination of the Cr 2p core hole "dressed" by the same shakeup charge-transfer process present in both the Cr x-ray photoelectron and the Cr x-ray absorption spectra with its energy uniquely measuring the exchange splitting of the Cr 3d level. Further analysis of the x-ray emission data reveals three additional features that radiate at constant loss energy that are attributed to combinations of Cr 3d(t2g)-* Cr 3d(t2g), charge-transfer O 2p-* Cr 3d, and crystal-field Cr3d(t2g)-* Cr3d(eg) excitations. These assignments and their energies are supported by density-functional theory calculations, the accuracy of which we demonstrate by hard x-ray valence-photoemission measurements. Atomic multiplet calculations, which include crystal-field effects, help interpret x-ray photoelectron and x-ray absorption spectra of the covalently mixed Cr ion. Resonant Cr K-L2,3L2,3 Auger-electron emission spectra support a ligand-to-metal nature of the charge-transfer process while highlighting the charge sensitivity differences between photon-in/electron-out and photon-in/photon-out spectroscopies.

Previous studies indicated that defects in halide perovskites can generate shallow electronic states, which are crucial for their performance in devices. However, how shallow states persist amid pronounced atomic dynamics on halide perovskite surfaces remains unknown. We reveal that electronic states at surfaces of prototypical CsPbBr₃ are energetically distributed at room temperature, akin to well-passivated inorganic semiconductors, despite the presence of undercoordinated atoms and cleaved bonds. Notably, approximately 70% of surface-state energies appear within 0.2 eV of the valence-band edge. Although deep states can still form, they are rarely energetically isolated and are less likely to act as traps. Accelerating first-principles calculations via machine learning, we show that the unique atomic dynamics in halide perovskites render the formation of deep electronic states at their surfaces unlikely. These findings reveal the microscopic mechanism behind the low density of deep states at dynamic halide perovskite surfaces, which is key to their device performance.

Crystallographic anisotropy, be it inherent or externally induced, profoundly impacts the materials physical properties, contributing to their ground-state morphology and magnetic arrangement and fostering distinctive optical behavior. Two-dimensional (2D) materials provide a relatively non-complex platform to study these anisotropy-driven properties. This review explores the intricate relationship between structural anisotropy and the resulting physical phenomena in 2D materials, primarily focusing on 2D hybrid perovskites (2D HPs) and transition metal phosphorous trichalcogenides. Case studies of 2D PEA₂PbI₄ HPs and FePS₃ are provided, explaining how intrinsic structural anisotropy originates and manifests as ground state polymorphism in 2D HPs and zigzag antiferromagnetic arrangement in FePS₃. The case of alloyed MnPS₃ is examined, where extrinsically induced anisotropy induces magnetic disorder, impacting its magnetic phase stability and overall optical behavior. This account, thus, underscores the origin and significance of intrinsic and extrinsic anisotropy in manipulating materials properties.

We present a first principles strategy for developing approximations for excited states through ensemble density functionals. Central to our result is the recognition that density-driven correlations (ddc's) can be vitally important to address excited states individually through ensembles, yet standard density-functional approximations based on ground state physics miss ddc's altogether. To model the ddc, we exploit the recently understood low-density limit of electrons in excited states. The theory developments are then combined to produce a proof-of-concept excited state approximation that resolves urgent paradigmatic failures (double excitations, charge transfer excitations, piecewise linearity) of existing state-of-art density-functional approaches, directly from differences in self-consistent field calculations; i.e., ΔSCF. In light of its observed impressive performance, we conclude that the approach represents a major step toward unified and accurate modeling of neutral and charged excitations.

Understanding how crystals nucleate is a key goal in materials, biomineralization, and chemistry. Many inorganic materials are known to crystallize \u201cnonclassically\u201d by particle attachment. However, a molecular-level understanding of small molecule crystallization is hampered by the complexity and time scales of nucleation events, which are often too large to simulate and too small to observe. Here, by combining unbiased molecular dynamics simulations and in situ experiments, we uncover this nucleation \u201cblind spot\u201d to elucidate the nonclassical crystallization mechanism of the nucleobase, guanine. The multi-step nucleation process begins with stacked guanine clusters, whose H-bonding and π-stacking arrangement progressively orders as they attach into nanoscopic fibers (observed by simulation and electron microscopy), partially ordered bundles, and finally, 3D periodic crystals. This work provides a foundation for understanding how organisms exquisitely control the formation of guanine and other molecular crystals, which are used ubiquitously in biology as optical and nitrogen-storage materials.

Hybrid functionals have been considered insufficiently reliable for the prediction of band gaps in solids and surfaces. We revisit this issue with a new generation of optimally tuned range-separated hybrid functionals, focusing on the reconstructed Si(111)-(2×1) and Ge(111)-(2×1) surfaces. We show that certain hybrid functionals can accurately predict the surface-state and bulk fundamental and optical gaps, as well as projected band structures of these surfaces, by combining ground-state and time-dependent density functional theory.

Halide perovskites (HaPs) have emerged as promising new materials for a wide range of optoelectronic applications, notably solar energy conversion. These materials are well known to exhibit significant dynamical effects even at room temperature, which affect both their electronic properties and their long-term stability. Molecular dynamics (MD) simulations can provide significant insights into such effects. However, long time scale simulations require both accuracy and scalability. The latter is an issue for first principles methods and the former is challenging for classical force fields. Machine-learned force fields (MLFF) are a promising avenue for bridging across this seeming contradiction. Here, we apply the gradient-domain machine learning approach, using CsPbBr₃ as an example. We find that training based on room temperature density functional theory (DFT) data fails to generate an MLFF that provides long-term stable MD, owing to an insufficient sampling of rare events in the training set. We show that this problem is resolved by using a temperature ensemble (TE) method, which can be generated in parallel and yields a combined data set based on MD trajectories from a variety of temperatures. The MLFF model based on the TE method yields high accuracy for long-term simulations, showing remaining errors of the same magnitude of inherent errors in the DFT calculation.

We use density functional theory (DFT) with non-empirically tuned screened range-separated hybrid (SRSH) functionals to calculate the electronic properties of native zinc and oxygen vacancy point defects in ZnO, and we predict their defect levels for thermal and optical transitions in excellent agreement with available experiments and prior calculations that use empirical hybrid functionals. The ability of this non-empirical first-principles framework to accurately predict quantities of relevance to both bulk- and defect-level spectroscopy enables high-accuracy DFT calculations with non-empirical hybrid functionals for defect physics, at a reduced computational cost.

We investigate the effect of peripheral fluorination and chlorination on the rate of axial phenoxylation of boron subphthalocyanines (BsubPcs), with Br and Cl as axial ligands, specifically Br-BsubPc, Br-F₁₂BsubPc, Br-Cl₁₂BsubPc, Cl-BsubPc, and Cl-F₁₂BsubPc. For this study, we use various solvents at their reflux temperature to acquire conversion of axial phenoxylation and obtain kinetic data. We found the experimentally observed reactivity of the BsubPcs used in this study followed Br-BsubPc > Cl-BsubPc > Br-F₁₂BsubPc ≈ Br-Cl₁₂SubPc >> Cl-F₁₂BsubPc. This shows that peripheral fluorination or chlorination inhibits the rate of axial phenoxylation and confirms the axial BBr bonds to be more reactive than BCl bonds. Density functional theory (DFT) calculations confirm the observed kinetic data and suggest that phenoxylation proceeds primarily via the Torres mechanism.

Palladium diselenide (PdSe₂)a layered van der Waals materialis attracting significant attention for optoelectronics due to the wide tunability of its band gap from the infrared through the visible range as a function of the number of layers. However, there continues to be disagreement over the precise nature and value of the optical band gap of bulk PdSe₂, owing to the rather small value of this gap that complicates experimental measurements and their interpretation. Here, we design and employ a Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional to investigate the electronic band structures and optical absorption spectra of bulk and monolayer PdSe₂. In particular, we account carefully for the finite exciton center-of-mass momentum within a time-dependent WOT-SRSH framework to calculate the indirect optical gap and absorption onset accurately. Our results agree well with the best available photoconductivity measurements, as well as with state-of-the-art many-body perturbation theory calculations, confirming that bulk PdSe₂ has an optical gap in the mid-infrared (upper bound of 0.44 eV). More generally, this work further bolsters the utility of the WOT-SRSH approach for predictive modeling of layered semiconductors.

Charge transfer (CT) excitation energies are known to be challenging for standard time-dependent (TD) density functional theory (DFT) calculations. Perturbative ensemble DFT (pEDFT) was suggested as an easy-to-implelemt, low-cost alternative to TDDFT, because it is an in principle exact theory for calculating excitation energies that produces useful valence excitation energies. Here, we examine analytically and numerically (based on the benzene-tetracyanoethylene complex) how well pEDFT performs in the CT limit. We find that pEDFT is qualitatively correct in that it follows the Mulliken limit while being only weakly dependent on the underlying density functional approximation. We observe, however, that quantitatively pEDFT is not as accurate as TDDFT. We attribute this to the emergence of a new type of self-interaction-like term that adversely affects the computation.

Refractive materials found in the natural world often exhibit unique structures that result in intriguing physical properties and offer a valuable resource for designing tailored bio-inspired materials. Here, we investigate from first principles the factors that govern the refractive index of metalamino-acid crystals. We specifically focus on the influence of crystal structure, metal ion inclusion, and spin configuration in phenylalanine- and cysteine-based materials. We find that the inclusion of copper and zinc metal ions in the crystal lattice has an important structural role that directly influences the refractive properties. In addition, the metal ions may contribute significantly to the dielectric response and therefore to the refractive index even within a given structure. Furthermore, in the synthetically available case of phenylalaninecopper we verify the results experimentally. Our results demonstrate the role of the inclusion of metal atoms in biogenic assemblies, emphasizing the potential use of this concept in bio-inspired molecular crystals that offer a flexible platform for the design of novel materials with desired optical features.

α-Moganite is a polymorph of silicon dioxide that, together with α-quartz, comprises the inorganic component of flint/chert─a material of overwhelming importance in archeology, anthropology, and paleontology. Whereas α-quartz has been studied extensively, both theoretically and experimentally, α-moganite has received considerably less attention, possibly owing to an absence of pure-phase material. In this study, we address this gap by providing a detailed first-principles analysis of the infrared (IR) absorption spectrum of α-moganite and its relation to the underlying structure. We find that the 4-membered rings of silicon and oxygen atoms, that do not appear in α-quartz, constitute a fundamental structural motif of this mineral. This distinctive arrangement results in moganite-specific fingerprints in the IR spectrum of flint/chert. This opens the door to evaluation and quantification of α-moganite content in archeological samples, providing insights into flint/chert raw material preferences and stone tool production by hominins.

An anisotropic interlayer potential (ILP), designed to describe the interlayer interaction in graphene/MX₂ and h-BN/MX₂ (M = Mo, W; X = S, Se) heterostructures, is presented. The ILP is parametrized against density functional theory (DFT) calculations within the Perdew-Burke-Ernzerhof (PBE) generalized-gradient approximation, augmented by nonlocal many-body dispersive (MBD-NL) interactions. The parametrized force field demonstrates excellent agreement with the DFT reference data of binding energy curves and sliding energy surfaces across all heterostructures considered. The transferability of the developed ILP is demonstrated for the phenalenyl (C₁₃H₉)/MoS₂ and B₇N₆H₉/MoS₂ interfaces, which are outside the training set. The force field is then used to study equilibrium interlayer distances, bulk moduli, and phonon spectra by means of molecular dynamics simulations.

Accurate prediction of electronic and optical excitations in van der Waals (vdW) materials is a long-standing challenge for density functional theory. The recent Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functional has proven successful in non-empirical determination of electronic band gaps and optical absorption spectra for covalent and ionic crystals. However, for vdW materials the tuning of the material- and structure-dependent functional parameters has only been attained semi-empirically. Here, we present a non-empirical WOT-SRSH approach applicable to vdW materials, with the optimal functional parameters transferable between monolayer and bulk. We apply this methodology to prototypical vdW materials: black phosphorus, molybdenum disulfide, and hexagonal boron nitride (in the latter case including zero-point renormalization). We show that the WOT-SRSH approach consistently achieves accuracy levels comparable to experiments and many-body perturbation theory (MBPT) calculations for band structures and optical absorption spectra, both on its own and as an optimal starting point for MBPT calculations.

Halide perovskites (HPs) are crystalline solids that feature a unique softness, absent in conventional semiconducting materials. In recent years, this softness has been pivotal to many properties in these materials, in both static and dynamic regimes. Here, we focus on the two-dimensional (2D) (PEA)₂PbI₄ crystal. We employ extensive density functional theory calculations and structural analysis to uncover a rich mosaic of ground-state configurations, identifying several stable configurations with distinct electronic properties. Our study uncovers an intrinsic Rashba effect within a structure traditionally considered as globally centrosymmetric, presenting a challenge to conventional understanding in the field. The observed effect emerges from a local symmetry-breaking induced by specific spatial orientations of the organic PEA molecules. This intrinsic Rashba effect, observed in select configurations, underscores the nuanced symmetrical complexities of 2D HPs and highlights their potential for spin-related applications. Additionally, our investigation demonstrates the exceptional flexibility of 2D HPs, as evidenced by an observed significant tolerance toward single-molecule rotations. This flexibility suggests potential pathways for smoother transitions between different molecular domains within these materials. Overall, our findings emphasize the intricate interplay between the organic/inorganic counterparts and the electronic properties in 2D HPs, paving the way for further exploration and exploitation of their unique characteristics in various optoelectronic and spintronic applications.

Halide double perovskites are a chemically diverse and growing class of compound semiconductors that are promising for optoelectronic applications. However, the prediction of their fundamental gaps and optical properties with density functional theory (DFT) and ab initio many-body perturbation theory has been a significant challenge. Recently, a nonempirical Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional has been shown to accurately produce the fundamental band gaps of a wide set of semiconductors and insulators, including lead halide perovskites. Here, we apply the WOT-SRSH functional to five halide double perovskites and compare the results with those obtained from other known functionals and previous GW calculations. We also use the approach as a starting point for GW calculations and we compute the band structures and optical absorption spectrum for Cs2AgBiBr6, using both time-dependent DFT and the GW-Bethe-Salpeter equation approach. We show that the WOT-SRSH functional leads to accurate fundamental and optical band gaps, as well as optical absorption spectra, consistent with spectroscopic measurements, thereby establishing WOT-SRSH as a viable method for the accurate prediction of optoelectronic properties of halide double perovskites.

Ion migration is a broad term used to account for the degradation of halide perovskite materials and devices. However, ion mobility is only one piece of the full picture─mobile ions/defects are first created, then transported, and eventually annihilated or immobilized. In this Perspective, we summarize emerging work that shows how tractable photochemical and Faradaic reactions provide a continuous source of ions to migrate. Furthermore, we discuss strategies to fundamentally manipulate ion migration by targeting specific electrochemical and reduction/oxidation mechanisms. This highlights the important role of defect photoelectrochemistry, as well as the soft nature of the perovskite lattice, in ion migration and self-healing. We conclude that distinguishing more detailed processes involved in \u201cion migration\u201d, with an emerging focus on the reactions that form mobile ionic defects, is necessary to greatly improve the stability of devices and open up more technological applications.

Organic crystals are widely used by animals to manipulate light for producing structural colors and for improving vision. To date only seven crystal types are known to be used, and among them β-guanine crystals are by far the most widespread. The fact that almost all these crystals have unusually high refractive indices (RIs) is consistent with their light manipulation function. Here, the physical, structural, and optical principles of how light interacts with the polarizable free-electron-rich environment of these quasiaromatic molecules are addressed. How the organization of these molecules into crystalline arrays introduces optical anisotropy and finally how organisms control crystal morphology and superstructural organization to optimize functions in light reflection and scattering are also discussed. Many open questions remain in this fascinating field, some of which arise out of this in-depth analysis of the interaction of light with crystal arrays. More types of organic crystals will probably be discovered, as well as other organisms that use these crystals to manipulate light. The insights gained from biological systems can also be harnessed for improving synthetic light-manipulating materials.

Crystalline riboflavin (vitamin B₂) performs an important biological role as an optically functional material in the tapetum lucidum of certain animals, notably lemurs and cats. The tapetum lucidum is a reflecting layer behind the retina, which serves to enhance photon capture and vision in low-light settings. Motivated by the aim of rationalizing its biological role, and given that the structure of biogenic solid-state riboflavin remains unknown, we have used a range of experimental and computational techniques to determine the solid-state structure of synthetic riboflavin. Our multitechnique approach included microcrystal XRD, powder XRD, three-dimensional electron diffraction (3D-ED), high-resolution solid-state ¹³C NMR spectroscopy, and dispersion-augmented density functional theory (DFT-D) calculations. Although an independent report of the crystal structure of riboflavin was published recently, our structural investigations reported herein provide a different interpretation of the intermolecular hydrogen-bonding arrangement in this material, supported by all the experimental and computational approaches utilized in our study. We also discuss, more generally, potential pitfalls that may arise in applying DFT-D geometry optimization as a bridging step between structure solution and Rietveld refinement in the structure determination of hydrogen-bonded materials from powder XRD data. Finally, we report experimental and computational values for the refractive index of riboflavin, with implications for its optical function.

The ionization potential of molecular chains is well-known to be a tunable nanoscale property that exhibits clear quantum confinement effects. State-of-the-art methods can accurately predict the ionization potential in the small molecule limit and in the solid-state limit, but for intermediate, nanosized systems prediction of the evolution of the electronic structure between the two limits is more difficult. Recently, optimal tuning of range-separated hybrid functionals has emerged as a highly accurate method for predicting ionization potentials. This was first achieved for molecules using the ionization potential theorem (IPT) and more recently extended to solid-state systems, based on an ansatz that generalizes the IPT to the removal of charge from a localized Wannier function. Here, we study one-dimensional molecular chains of increasing size, from the monomer limit to the infinite polymer limit using this approach. By comparing our results with other localization-based methods and where available with experiment, we demonstrate that Wannier-localization-based optimal tuning is highly accurate in predicting ionization potentials for any chain length, including the nanoscale regime.

The high-order finite difference real-space pseudopotential density functional theory (DFT) approach is a valuable method for large-scale, massively parallel DFT calculations. A significant challenge in the approach is the oscillating \u201cegg-box\u201d error introduced by aliasing associated with a coarse grid spacing. To address this issue while minimizing computational cost, we developed a finite difference interpolation (FDI) scheme [Roller et al., J. Chem. Theory Comput. 19, 3889 (2023)] as a means of exploiting the high resolution of the pseudopotential to reduce egg-box effects systematically. Here, we show an implementation of this method in the PARSEC code and examine the practical utility of the combination of FDI with additional methods for improving force precision and/or reducing its computational cost, including orbital-based forces, compensating charges (namely, adding and subtracting a judiciously chosen charge density such that the total density is unaltered), and a modified spatial domain in which the real-space grid is defined. Using selected small molecules, as well as metallic Li, as test cases, we show that a combination of all four aspects leads to a significant reduction in computational cost while retaining a high level of precision that supports accurate structures and vibrational spectra, as well as stable and accurate molecular dynamics runs.

Polymorphism is a well-known property of molecular crystals, which allows the same molecule to form solids with several crystalline structures that can differ significantly in physical properties. Polymorphs that possess different optical absorption properties in the visible range may exhibit different perceived colors, a phenomenon known as color polymorphism. One striking example of color polymorphism is given by 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, known as ROY for its red-orange-yellow colors. First-principles prediction of color polymorphism may help in polymorph assignment and design but has proven to be challenging. Here, we predict the absorption spectra and simulate the colors of 12 ROY polymorphs using the general, nonempirical method of time-dependent (TD) optimally tuned screened range-separated hybrid (OT-SRSH) functional. For 5 ROY polymorphs with known experimental absorption spectra, we show that the TD-OT-SRSH approach predicts absorption spectra in quantitative agreement with experiment. For all polymorphs, we show that an accurate simulation of the colors is obtained, paving the way to a fully predictive, low-cost calculation of color polymorphism.

Van der Waals polytypes of broken inversion and mirror symmetries have been recently shown to exhibit switchable electric polarization even at the ultimate two-layer thin limit. Their out-of-plane polarization has been found to accumulate in a ladder-like fashion with each successive layer, offering 2D building blocks for the bottom-up construction of 3D ferroelectrics. Here, it is demonstrated experimentally that beyond a critical stack thickness, the accumulated polarization in rhombohedral polytypes of molybdenum disulfide saturates. The underlying saturation mechanism, deciphered via density functional theory and self-consistent PoissonSchrödinger calculations, point to a purely electronic redistribution involving: 1. Polarization-induced bandgap closure that allows for cross-stack charge transfer and the emergence of free surface charge; 2. Reduction of the polarization saturation value, as well as the critical thickness at which it is obtained, by the presence of free carriers. The resilience of polar layered structures to atomic surface reconstruction, which is essentially unavoidable in polar 3D crystals, potentially allows for the design of new devices with mobile surface charges. The findings, which are of general nature, should be accounted for when designing switching and/or conductive devices based on ferroelectric layered materials.

Graphullerene is a novel two-dimensional carbon allotrope with unique optoelectronic properties. Despite significant experimental characterization and prior density functional theory calculations, unanswered questions remain as to the nature, energy, and intensity of the electronic and optical excitations. Here, we present first-principles calculations of the quasiparticle band structure, neutral excitations, and absorption spectra of monolayer graphullerene and bulk graphullerite, employing the GW-Bethe-Salpeter equation (GW-BSE) approach. We show that strongly bound excitons dominate the absorption spectrum of monolayer graphullerene with binding energies up to 0.8 eV, while graphullerite exhibits less pronounced excitonic effects. Our calculations also reveal a strong linear polarization anisotropy, reflecting the in-plane structural anisotropy from intermolecular coupling between neighboring C₆₀ units. We further show that the presence of Mg atoms, crucial to the synthesis process, induces structural modifications and polarizability effects, resulting in a ∼1 eV quasiparticle gap renormalization and a reduction in the exciton binding energy to ∼0.6 eV.

The ultrahigh surface area of two-dimensional materials can drive multimodal coupling between optical, electrical, and mechanical properties that leads to emergent dynamical responses not possible in three-dimensional systems. We observed that optical excitation of the WS₂ monolayer above the exciton energy creates symmetrically patterned mechanical protrusions which can be controlled by laser intensity and wavelength. This observed photostrictive behavior is attributed to lattice expansion due to the formation of polarons, which are charge carriers dressed by lattice vibrations. Scanning Kelvin probe force microscopy measurements and density functional theory calculations reveal unconventional charge transport properties such as the spatially and optical intensity-dependent conversion in the WS₂ monolayer from apparent n- to p-type and the subsequent formation of effective p-n junctions at the boundaries between regions with different defect densities. The strong opto-electrical-mechanical coupling in the WS₂ monolayer reveals previously unexplored properties, which can lead to new applications in optically driven ultrathin microactuators.

Spontaneous electric polarization is recently observed in multilayered van der Waals stacked materials, arising from a symmetry breaking in a unit cell with two or more constituent species, or non-centrosymmetric intra-layer atom displacement in single-atom-species materials. Here, it is shown that even elemental crystals, consisting of one type of atom and composed of non-polar and centrosymmetric layers, exhibit electric polarization if arranged in an appropriate three-dimensional architecture. This concept is demonstrated here for mixed-stacking tetra-layer polytypes of non-polar graphene sheets. Surprisingly, it is find that the room temperature out-of-plane electric polarization increases with external electrostatic hole doping, rather than decreases with it owing to screening. Using first-principles calculations, as well as a self-consistent tight-binding model, the emergence of polarization is explain in terms of inter-layer charge rearrangement and the doping dependence in terms of gating-induced inter-layer charge transfer. This newly discovered intrinsic polarization may therefore offer new venues for designing the electronic response of graphene-based polytypes to external fields.Internal electric polarization in a periodic crystal requires a non-centrosymmetric unit cell. Usually, the different atomic species in each cell break the symmetry and polarize the inter-atomic bonds, thus hiding the purely geometrical contribution of the atomic positions. The current report reveals a room-temperature polarization in elemental layered crystals of carbon atoms only. It explores fascinating electronic distributions in such metastable van der Waals polytypes of various structural configurations and symmetries. image

Numerous bio-organisms employ template-assisted crystallization of molecular solids to yield crystal morphologies with unique optical properties that are difficult to reproduce synthetically. Here, a facile procedure is presented to deposit bio-inspired birefringent crystals of xanthine derivatives on a template of single-crystal quartz. Crystalline sheets that are several millimeters in length, several hundred micrometers in width, and 300600 nm thick, are obtained. The crystal sheets are characterized with a well-defined orientation both in and out of the substrate plane, giving rise to high optical anisotropy in the plane parallel to the quartz surface, with a refractive index difference Δn ≈ 0.25 and a refractive index along the slow axis of n ≈ 1.7. It is further shown that patterning of the crystalline stripes with a tailored periodic grating leads to a thin organic polarization-dependent diffractive meta-surface, opening the door to the fabrication of various optical devices from a platform of small-molecule based organic dielectric crystals.

Using both time-dependent density functional theory (TDDFT) and the "single-shot"GW plus Bethe-Salpeter equation (GW-BSE) approach, we compute optical band gaps and optical absorption spectra from first principles for eight common binary and ternary closed-shell metal oxides (MgO, Al2O3, CaO, TiO2, Cu2O, ZnO, BaSnO3, and BiVO4), based on the nonempirical Wannier-localization-based, optimally tuned, screened range-separated hybrid functional. Overall, we find excellent agreement between our TDDFT and GW-BSE results and experiment, with a mean absolute error smaller than 0.4 eV, including for Cu2O and ZnO that are traditionally considered to be challenging for both methods.

In-gap states and their effect on recombination rates in quasi-2D lead-iodide-based perovskites, intercalated with various spacer molecules, are studied using a combination of scanning tunneling spectroscopy and temperature-dependent photoconductivity measurements. The results are further analyzed by a Shockley-Read-Hall model. Indications for shallow in-gap states, positioned at about 0.15-0.2 eV below the bottom of the conduction band, are found. These states are identified as dominating the recombination route of photogenerated carriers in these systems, with a relatively large capture coefficient of about 10-5-10-6 cm3 s-1 at room temperature. First-principles calculations based on density functional theory imply that these states are not an intrinsic effect of the inclusion of the spacer molecules, but rather one that arises from chemical defect formation or structural deformation of the perovskite layers. The results suggest that further improvement of the performance of solar cells that are based on quasi-2D perovskites requires, along with enhancing carrier mobility, efforts to suppress the concentration of these detrimental defect states.The effect of in-gap states on recombination rates in quasi-2D lead-iodide-based perovskites, intercalated with various spacer molecules, is studied using a combination of scanning tunneling spectroscopy, temperature-dependent photoconductivity measurements, and theoretical calculations. Tunneling spectra reveal shallow in-gap states that appear to dominate the recombination kinetics of photogenerated carriers in these systems.image (c) 2023 WILEY-VCH GmbH

The halide perovskite CsPbBr3 belongs to the Cs-Pb-Br material system, which features two additional thermodynamically stable ternary phases, Cs4PbBr6 and CsPb2Br5. The coexistence of these phases and their reportedly similar photoluminescence (PL) have resulted in a debate on the nature of the emission in these systems. Herein, optical and microscopic characterizations are combined with an effective mass, correlated electronhole model of excitons in confined systems, to investigate the emission properties of the ternary phases in the Cs-Pb-Br system. It is found that all Cs-Pb-Br phases exhibit green emission and the non-perovskite phases exhibit PL quantum yields orders of magnitude larger than CsPbBr3. In particular, blue- and red-shifted emission for the Cs- and Pb-rich phases, respectively, are measured, stemming from embedded CsPbBr3 nanocrystals (NCs). This model reveals that the difference in emission shift is caused by the combined effects of NC size and different band mismatch. Furthermore, the importance of including the dielectric mismatch in the calculation of the emission energy for Cs-Pb-Br composites is demonstrated. The results explain the reportedly limited blue shift in CsPbBr3@Cs4PbBr6 composites and rationalize some of its differences with CsPb2Br5.

An anisotropic interlayer force field that describes the interlayer interactions in homogeneous and heterogeneous interfaces of group-VI transition metal dichalcogenides (MX₂, where M = Mo, W, and X = S, Se) is presented. The force field is benchmarked against density functional theory calculations for bilayer systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, augmented by a nonlocal many-body dispersion treatment of long-range correlation. The parametrization yields good agreement with the reference calculations of binding energy curves and sliding potential energy surfaces. It is found to be transferable to transition metal dichalcogenide (TMD) junctions outside of the training set that contain the same atom types. Calculated bulk moduli agree with most previous dispersion-corrected density functional theory predictions, which underestimate the available experimental values. Calculated phonon spectra of the various junctions under consideration demonstrate the importance of appropriately treating the anisotropic nature of the layered interfaces. Considering our previous parametrization for MoS₂, the anisotropic interlayer potential enables accurate and efficient large-scale simulations of the dynamical, tribological, and thermal transport properties of a large set of homogeneous and heterogeneous TMD interfaces.

Reliable prediction of the ground-state spin and magnetic coupling constants in transition-metal complexes is a well-known challenge for density functional theory (DFT). One popular strategy for addressing this long-standing issue involves the modification of the fraction of Fock exchange in a hybrid functional. Here we explore the viability of this approach using three polynuclear metal-organic complexes based on a Ni4O4 cubane motif, having different ground state spin values (S = 0, 2, 4) owing to the use of different ligands. We systematically search for an optimum fraction of Fock exchange, across various global, range-separated, and double hybrid functionals. We find that for all functionals tested, at best there only exists a very narrow range of Fock exchange fractions which results in a correct prediction of the ground-state spin for all three complexes. The useful range is functional dependent, but general trends can be identified. Typically, at least two similar systems must be used in order to determine both an upper and lower limit of the optimal range. This is likely owing to conflicting demands of minimizing delocalization errors, which typically requires a higher percentage of Fock exchange, and addressing static correlation, which typically requires a lower one. Furthermore, we find that within the optimal range of Fock exchange, the sign and relative magnitude of Ni-Ni magnetic coupling constants are reasonably well reproduced, but there is still room for quantitative improvement in the prediction. Thus, the prediction of spin state and magnetic coupling in polynuclear complexes remains an ongoing challenge for DFT.

Consistency between the exchange-correlation (XC) functional used during pseudopotential construction and planewave-based electronic structure calculations is important for an accurate and reliable description of the structure and properties of condensed-phase systems. In this work, we present a general scheme for constructing pseudopotentials with range-separated hybrid (RSH) XC functionals based on the solution of the all-electron radial integro-differential equation for a spherically symmetrized reference atomic configuration. As a proof of principle, we demonstrate pseudopotential construction with the Perdew-Burke-Ernzerhof (PBE), hybrid PBE (PBE0), Heyd-Scuseria-Ernzerhof RSH (HSE06), and screened RSH (SRSH, based on the long-range corrected LC-ωPBE0 RSH) XC functionals for a select set of atoms and then investigate the importance of pseudopotential consistency when computing band gaps, equilibrium lattice parameters, bulk moduli, and atomization energies of several solid-state systems. In doing so, we find that pseudopotential consistency errors tend to be systematic and can be as large as 0.1 eV (or 1.4%) when computing band gaps.

The accurate description of electronic properties and optical absorption spectra is a long-standing challenge for density functional theory. Recently, the introduction of screened range-separated hybrid (SRSH) functionals for solid-state materials has allowed for the calculation of fundamental band gaps and optical absorption spectra that are in very good agreement with many-body perturbation theory. However, since solid-state SRSH functionals are typically tuned to reproduce the properties of bulk phases, their transferability to low-dimensional structures, which experience substantially different screening than in the bulk, remains an open question. In this work, we explore the transferability of SRSH functionals to several prototypical van der Waals materials, including transition-metal sulfides and selenides, indium selenide, black phosphorus, and hexagonal boron nitride. Considering the bulk and a monolayer of these materials as limiting cases, we show that the parameters of the SRSH functional can be determined systematically, using only the band-edge quasiparticle energies of these extremal structural phases as fitting targets. The resulting SRSH functionals can describe both electronic band structures and optical absorption spectra with accuracy comparable to more demanding ab initio many-body perturbation theory (GW and Bethe-Salpeter equation) approaches. Selected examples also demonstrate that the SRSH parameters, obtained from the bulk and monolayer reference structures, display good accuracy for band structures and optical spectra of bilayers, indicating a degree of transferability that is independent of the fitting procedure.

The driven Liouville von Neumann approach is a method to computationally explore electron dynamics and transport in nanoscale systems. It does so by imposing open boundary conditions on finite atomistic model systems, which drive them out of equilibrium. The approach is compatible with any underlying electronic structure treatment that can be phrased in terms of a single-particle framework, ranging from simple tight-binding descriptions to state-of-the-art density functional theory treatments of the interacting system. In this perspective, we motivate the approach, discuss its theoretical foundations, explain its essential elements, overview recent extensions and applications, and present remaining challenges and opportunities.

Metal halide perovskites are an attractive class of semiconductors, but it has proven difficult to control their electronic doping by conventional strategies due to screening and compensation by mobile ions or ionic defects. Noble-metal interstitials represent an under-studied class of extrinsic defects that plausibly influence many perovskite-based devices. In this work, doping of metal halide perovskites is studied by electrochemically formed Au+ interstitial ions, combining experimental data on devices with a computational analysis of Au+ interstitial defects based on density functional theory (DFT). Analysis suggests that Au+ cations can be easily formed and migrate through the perovskite bulk via the same sites as iodine interstitials (I-i(+)). However, whereas I-i(+) compensates n-type doping by electron capture, the noble-metal interstitials act as quasi-stable n-dopants. Experimentally, voltage-dependent, dynamic doping by current density-time (J-t), electrochemical impedance, and photoluminescence measurements are characterized. These results provide deeper insight into the potential beneficial and detrimental impacts of metal electrode reactions on long-term performance of perovskite photovoltaic and light-emitting diodes, as well as offer an alternative doping explanation for the valence switching mechanism of halide-perovskite-based neuromorphic and memristive devices.

The real-space pseudopotential approach is a well-knownmethodfor large-scale density functional theory (DFT) calculations. Oneof its main limitations, however, is the introduction of errors associatedwith the positioning of the underlying real-space grid, a phenomenonusually known as the "egg-box" effect. The effect canbe controlled by using a finer grid, but this raises the cost of thecalculations or even undermines their feasibility altogether. Therefore,there is ongoing interest in the reduction of the effect per a givenreal-space grid. Here, we present a finite difference interpolationof electron orbitals as a means of exploiting the high resolutionof the pseudopotential to reduce egg-box effects systematically. Weimplement the method in PARSEC, a finite difference real-space pseudopotentialDFT code, and demonstrate error mitigation and improved convergenceat a low additional computational cost.

We present first principles calculations of the interface between GaN and strained AlN, using a slab model in which polarization is compensated via surface fractional-charge pseudo-hydrogen atoms. We show that an interface two-dimensional carrier electron or hole gas emerges naturally in response to different compensating surface charges, but that this need not involve in-gap surface states.

Two-dimensional (2D) halide perovskitesoffer a uniqueplatformfor investigating the ground state of materials possessing significantanharmonicity. In contrast to three-dimensional perovskites, their2D counterparts offer substantially fewer degrees of freedom, resultingin multiple well-defined crystal structures. In this work, we thoroughlyinvestigate the anharmonic ground state of the benchmark (PEA)(2)PbI4 compound, using complementary informationfrom low-temperature X-ray diffraction (XRD) and photoluminescencespectroscopy, supported by density functional theory calculations.We extrapolate four crystallographic configurations from low-temperatureXRD. These configurations imply that the ground state has an intrinsicdisorder stemming from two coexisting chiral sublattices, each witha bioriented organic spacer molecule. We further show evidence thatthese chiral structures form unevenly populated ground states, portrayinguneven anharmonicity, where the state population may be tuned by surfaceeffects. Our results uncover a disordered ground state that may induceintrinsic grain boundaries, which cannot be ignored in practical applications.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a β,β-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H₃tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [Fe^IIItpfc(py)]₂COT and 6-coordinate [Fe^IIItpfc(py)₂]₂COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [Fe^IIItpfc(py)]₂COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022.

Ferroelectricity in atomically thin bilayer structures has been recently predicted¹ and measured²⁴ in two-dimensional materials with hexagonal non-centrosymmetric unit-cells. The crystal symmetry translates lateral shifts between parallel two-dimensional layers to sign changes in their out-of-plane electric polarization, a mechanism termed slide-tronics⁴. These observations have been restricted to switching between only two polarization states under low charge carrier densities⁵¹², limiting the practical application of the revealed phenomena¹³. To overcome these issues, one should explore the nature of polarization in multi-layered van der Waals stacks, how it is governed by intra- and interlayer charge redistribution and to what extent it survives the addition of mobile charge carriers¹⁴. To explore these questions, we conduct surface potential measurements of parallel WSe₂ and MoS₂ multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields that provide a means to design multi-state ladder-ferroelectrics. Furthermore, we find that the internal polarization remains notable on electrostatic doping of mobile charge carrier densities as high as 10¹³ cm⁻², with substantial in-plane conductivity. Using density functional theory calculations, we trace the extra charge redistribution in real and momentum spaces and identify an eventual doping-induced depolarization mechanism.

The accurate prediction of the band gaps of halide perovskites within density functional theory is known to be challenging. The recently developed Wannier-localized optimally tuned screened range-separated hybrid functional was shown to be highly accurate for fundamental band gaps of standard semiconductors and insulators. This was achieved by selecting the parameters of the functional to satisfy an ansatz that generalizes the ionization potential theorem to the removal of charge from a state that corresponds to a Wannier function. Here, we present applications of the method to the band gaps of typical halide perovskites. We find a satisfyingly small formal mean absolute error of ∼0.1 eV with respect to experimental band gaps and very good agreement with previous many-body perturbation theory calculations.

The nonadditive kinetic potential vNAD is a key quantity in density-functional theory (DFT) embedding methods, such as frozen density embedding theory and partition DFT. vNAD is a bifunctional of electron densities ρB and ρtot=ρA+ρB. It can be evaluated using approximate kinetic-energy functionals, but accurate approximations are challenging. The behavior of vNAD in the vicinity of the nuclei has long been questioned, and singularities were seen in some approximate calculations. In this article, the existence of singularities in vNAD is analyzed analytically for various choices of ρB and ρtot, using the nuclear cusp conditions for the density and Kohn-Sham potential. It is shown that no singularities arise from smoothly partitioned ground-state Kohn-Sham densities. We confirm this result by numerical calculations on diatomic test systems HeHe, HeLi+, and H2, using analytical inversion to obtain a numerically exact vNAD for the local density approximation. We examine features of vNAD which can be used for development and testing of approximations to vNAD[ρB,ρtot] and kinetic-energy functionals.

Structural analysis of beam-sensitive materials by transmission electron microscopy (TEM) represents a significant challenge, as high resolution TEM (HRTEM) requires high electron doses that limits its applicability to stable inorganic materials. Beam sensitive materials, such as organic crystals (of key importance in pharmaceuticals, organic electronics, and biology) must be imaged under low dose conditions, leading to problematic contrast interpretation and the loss of fine structural details. Here, we describe HRTEM imaging of organic crystalline materials with near-atomic resolution of up to 1.6 Å that enabled the real-space study of crystal structures, as well as observation of co-existing polymorphs, crystal defects, and atoms. This is made possible by a low-dose focal series reconstruction (LD-FSR) methodology developed by us, which provides HRTEM images where contrast reflects true object structure and can be performed on contemporary cryo-EM instruments available to many research institutions. We imaged copper phthalocyanine (CuPc), perchlorinated analogue of CuPc, and indigo crystalline films. In the case of indigo crystals, we were able to observe co-existing polymorphs and individual atoms (carbonyl oxygen). In the case of CuPc, we observed several polymorphs, including a new one, for which we elucidated the crystal structure based on direct in-focus imaging, accomplishing real-space crystal structure elucidation. Direct structural analysis of beam sensitive materials with high resolution that enables the real-space study of crystals can be transformative for structural science of organic materials.

The dependence of ab initio many-body perturbation theory within the GW approximation on the eigensystem used in calculating quasiparticle corrections limits this methods predictive power. Here, we investigate the accuracy of the recently developed Wannier-localized optimally tuned screened range-separated hybrid (WOTSRSH) functional as a generalized Kohn-Sham starting point for single-shot GW (G0W0) calculations for a range of semiconductors and insulators. Comparison to calculations based on well-established functionals, namely, PBE, PBE0, and HSE, as well as to self-consistent GW schemes and to experiment, shows that band gaps computed via G0W0@WOT-SRSH have a level of precision and accuracy that is comparable to that of more advanced methods such as quasiparticle self-consistent GW and eigenvalue self-consistent GW . We also find that G0W0@WOT-SRSH improves the description of states deeper in the valence band manifold. Finally, we show that G0W0@WOT-SRSH significantly reduces the sensitivity of computed band gaps to ambiguities in the underlying WOT-SRSH tuning procedure.

We study the optimal tuning of the free parameters in range-separated double hybrid functionals, based on enforcing the exact conditions of piecewise linearity and spin constancy. We find that introducing the range separation in both the exchange and the correlation terms allows for the minimization of both fractional charge and fractional spin errors for singlet atoms. The optimal set of parameters is system specific, underlining the importance of the tuning procedure. We test the performance of the resulting optimally tuned functionals for the dissociation curves of diatomic molecules. We find that they recover the correct dissociation curve for the one-electron system, H2+, and improve the dissociation curves of many-electron molecules such as H2 and Li2, but they also yield a nonphysical maximum and only converge to the correct dissociation limit at very large distances.

A critical overview of the theory of the chirality-induced spin selectivity (CISS) effect, that is, phenomena in which the chirality of molecular species imparts significant spin selectivity to various electron processes, is provided. Based on discussions in a recently held workshop, and further work published since, the status of CISS effects-in electron transmission, electron transport, and chemical reactions-is reviewed. For each, a detailed discussion of the state-of-the-art in theoretical understanding is provided and remaining challenges and research opportunities are identified.

Halides and pseudohalides of boron subphthalocyanine (BsubPc) are promising candidates for efficient yet stable organic photovoltaics. Here, the electronic structure of such molecules, obtained using density functional theory, is considered. Based on the calculations, it is found that the tetrameric boron bond is stabilized by an inductive effect at the axial substituent and by conjugative effects across the ring system. It is further found that stability is dictated mostly by the axial moiety, such that Br-BsubPc is the most fitting precursor structure for further synthesis steps, whereas F-BsubPc is the most suitable candidate for long-term device performance. H-BsubPc is examined as a new BsubPc derivative, and found to be too volatile for long term device performance. Finally, it is shown that peripheral substitution dictates the position of frontier orbitals, thereby allowing for essentially separate optimization of material properties and material stability.

A generalized adiabatic connection that applies to any type of range-separated hybrid (RSH) functional employed within generalized KohnSham (KS) theory is presented. This generalized relation is then used to derive a definition for a rigorous distinction between multiplicative exchange and correlation components. The developed adiabatic connection is defined in terms of both generalized and conventional KS orbitals. It is shown, however, that using only the KS orbitals produces an error that is (Formula presented.), where (Formula presented.) is fully defined in terms of parameters in the RSH functional, and is found to be small in practical calculations. It is expected that this new adiabatic connection can assist in the development of new RSH functionals and the assessment of existing ones.

We revisit the important issues of polymorphism, structure, and nucleation of cholesterol·H₂O using first-principles calculations based on dispersion-augmented density functional theory. For the lesser known monoclinic polymorph, we obtain a fully extended H-bonded network in a structure akin to that of hexagonal ice. We show that the energy of the monoclinic and triclinic polymorphs is similar, strongly suggesting that kinetic and environmental effects play a significant role in determining polymorph nucleation. Furthermore, we find evidence in support of various O-H···O bonding motifs in both polymorphs that may result in hydroxyl disorder. We have been able to explain, via computation, why a single cholesterol bilayer in hydrated membranes always crystallizes in the monoclinic polymorph. We rationalize what we believe is a single-crystal to single-crystal transformation of the monoclinic form on increased interlayer growth beyond that of a single cholesterol bilayer, interleaved by a water bilayer. We show that the ice-like structure is also relevant to the related cholestanol·2H₂O and stigmasterol·H₂O crystals. The structure of stigmasterol hydrate both as a trilayer film at the air-water interface and as a macroscopic crystal further assists us in understanding the polymorphic and thermal behavior of cholesterol·H₂O. Finally, we posit a possible role for one of the sterol esters in the crystallization of cholesterol·H₂O in pathological environments, based on a composite of a crystalline bilayer of cholesteryl palmitate bound epitaxially as a nucleating agent to the monoclinic cholesterol·H₂O form.

In calculations based on density functional theory, the "HOMO-LUMO gap"(difference between the highest occupied and lowest unoccupied molecular orbital energies) is often used as a low-cost, ad hoc approximation for the lowest excitation energy. Here we show that a simple correction based on rigorous ensemble density functional theory makes the HOMO-LUMO gap exact in principle and significantly more accurate in practice. The introduced perturbative ensemble density functional theory approach predicts different and useful values for singlet-singlet and singlet-triplet excitations, using semilocal and hybrid approximations. Excitation energies are similar in quality to time-dependent density functional theory, especially at high fractions of exact exchange. The approach therefore offers an easy-to-implement and low-cost route to robust prediction of molecular excitation energies.

An anisotropic interlayer force field that describes the interlayer interactions in molybdenum disulfide (MoS2) is presented. The force field is benchmarked against density functional theory calculations for both bilayer and bulk systems within the Heyd- Scuseria-Ernzerhof hybrid density functional approximation, augmented by a nonlocal many-body dispersion treatment of long-range correlation. The parametrization yields good agreement with the reference calculations of binding energy curves and sliding potential energy surfaces for both bilayer and bulk configurations. Benchmark calculations for the phonon spectra of bulk MoS2 provide good agreement with experimental data, and the calculated bulk modulus falls in the lower part of experimentally measured values. This indicates the accuracy of the interlayer force field near equilibrium. Under external pressures up to 20 GPa, the developed force field provides a good description of compression curves. At higher pressures, deviations from experimental data grow, signifying the validity range of the developed force field.

Among many riddles posed by halide perovskites, the surprising apparent near-absence of harmful defects stands out. This is commonly explained by invoking defect tolerance (DT), but the term is used loosely, sometimes interchangeably with self-healing (SH). Also, the relation between underlying physical and chemical mechanisms and device behavior is often murky. Here, we offer our views as to what DT and SH constitute, the evidence for and against them, and what research challenges remain.

Accurate prediction of fundamental band gaps of crystalline solid-state systems entirely within density functional theory is a long-standing challenge. Here, we present a simple and inexpensive method that achieves this by means of nonempirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron from an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band-gap semiconductors to large band-gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 electronvolts (eV), and is found to yield quantitative accuracy across the board, with a mean absolute error of ∼0.1 eV and a maximal error of ∼0.2 eV.

Double excitations, which are dominated by a Slater determinant with both electrons in the highest occupied molecular orbital promoted to the lowest unoccupied orbital(s), pose significant challenges for low-cost electronic structure calculations based on density-functional theory (DFT). Here, we demonstrate that recent advances in ensemble DFT [Gould et al., Phys. Rev. Lett. 125, 233001 (2020)], which extend concepts of groundstate DFT to excited states via a rigorous physical framework based on the ensemble fluctuation-dissipation theorem, can be used to shed light on the double-excitation problem. We find that the exchange physics of double excitations is reproducible by standard DFT approximations using a linear combination formula, but correlations are more complex. In passing, to analyze correlation, we extend the random-phase approximation to ensembles. We then show, using selected test systems, that standard DFT approximations may be adapted to tackle double excitations based on theoretically motivated simple formulas that employ ensemble extensions of expressions that use the on-top pair density.

Hamiltonian matrices for Kohn-Sham calculations implemented in real space are often large (millions by millions) but very sparse. This poses challenges and opportunities for iterative eigensolvers, which often require a large number of matrix-vector multiplications. As a consequence, an efficient parallel sparse matrix-vector multiplication algorithm is desired. Here, we investigate the benefits of using Hilbert space-filling curves (SFCs) in domain partitioning. We show that the use of Hilbert SFCs in grid-point partitioning brings better locality of the grid points, improves balance of communication, and reduces communication overhead. We also demonstrate an extension of Hilbert SFCs coupled with blockwise operations. The use of blockwise operations helps exploit the vector-processing units in contemporary computational platforms. We illustrate speedup and scalability improvements for an iterative eigensolver based on the Chebyshev-filtered subspace iteration method. Using blockwise Hilbert SFCs, we solve the Kohn-Sham problem for silicon nanocrystals up to 10 nm in diameter, which contain over 26,000 atoms. We illustrate how the density of states of silicon nanocrystals evolves to the bulk limit, where Van Hove singularities are clearly apparent.

First-principles calculations on a prototypical hybrid organicinorganic perovskite reveal an unexpected role for hydrogen defects in the optoelectronic properties of this material.

Understanding and controlling the energy level alignment at interfaces with metal halide perovskites (MHPs) is essential for realizing the full potential of these materials for use in optoelectronic devices. To date, however, the basic electronic properties of MHPs are still under debate. Particularly, reported Fermi level positions in the energy gap vary from indicating strong n- to strong p-type character for nominally identical materials, raising serious questions about intrinsic and extrinsic defects as dopants. \u200bIn this work, photoemission experiments demonstrate that thin films of the prototypical methylammonium lead triiodide (MAPbI3) behave like an intrinsic semiconductor in the absence of oxygen. Oxygen is then shown to be able to reversibly diffuse into and out of the MAPbI3 bulk, requiring rather long saturation timescales of ≈1 h (in: ambient air) and over 10 h (out: ultrahigh vacuum), for few 100 nm thick films. Oxygen in the bulk leads to pronounced p-doping, positioning the Fermi level universally ≈0.55 eV above the valence band maximum. The key doping mechanism is suggested to be molecular oxygen substitution of iodine vacancies, supported by density functional theory calculations. This insight rationalizes previous and future electronic property studies of MHPs and calls for meticulous oxygen exposure protocols.

A new family of 3d4f coordination polymers with the molecular formula [NiIILnIII(L)(dca)2(NO3)]n [Ln = Eu (1), Gd (2), Tb (3), Dy (4), and Ho (5); H2L = N,N-bis(2-hydroxy-3-methoxy-5-methylbenzyl)homopiperazine; dca = dicyanamide] has been synthesized in search for new single-molecule magnet (SMM) materials containing highly anisotropic lanthanides. The magnetic properties of these materials have been established by DC and AC magnetometry and explained quantitatively by comprehensive phenomenological modeling based on a generalized Heisenberg-type model, hypothesized previously based on first-principles calculations. Single-crystal X-ray diffraction has shown that the compounds are isostructural, with lanthanide atoms occupying a nine-coordination site with muffin-like geometry and individual NiIILnIII units linked by dca anions. Other than the paramagnetic compound 1, 25 exhibit intra-unit ferromagnetic 3d4f interactions, favorable for a large spin ground state. A slow field-induced relaxation of magnetization has been observed in compound 4 (only), displaying a substantial energy barrier of Ueff/kB = 26.2(5) K, below 6 K. This is attributed to an easy-plane anisotropy and is consistent with the relaxation in systems with a Kramers ground state doublet and hyperfine interactions.

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

Two important extensions of Kohn-Sham (KS) theory are generalized: KS theory and ensemble KS theory. The former allows for non-multiplicative potential operators and greatly facilitates practical calculations with advanced, orbital-dependent functionals. The latter allows for quantum ensembles and enables the treatment of open systems and excited states. Here, we combine the two extensions, both formally and practically, first via an exact yet complicated formalism and then via a computationally tractable variant that involves a controlled approximation of ensemble "ghost interactions"by means of an iterative algorithm. The resulting formalism is illustrated using selected examples. This opens the door to the application of generalized KS theory in more challenging quantum scenarios and to the improvement of ensemble theories for the purpose of practical and accurate calculations.

Schottky barrier heights (SBHs) measured at polycrystalline metal-semiconductor (MS) interfaces have displayed an insensitivity to the work function (WF) of the metal, known as the "Fermi level pinning" (FLP) phenomenon. The obstacle presented by FLP in thwarting technological efforts to tune the SBH has been difficult to overcome because of a lack of understanding of the origin of the FLP effect. Presently, SBH explanation still largely relies on empirical models, and FLP remains a mystery. Here, the phenomenon of FLP for zinc-blende/diamond (ZBD) semiconductors is explicitly demonstrated to originate from interface metal-cation bonds, which are metallic in nature. Based on the analysis of two representative and electrically distinct types of interface, it is shown that screening by metallic bonds weakens the dependence of SBH on the metal and results in a SBH close to that found between the semiconductor and its own cation in elemental metal form. The latter SBH is shown to agree well with experimentally measured SBH from polycrystalline interface, i.e., the apparent pinning levels. A fundamental, self-consistent explanation of the FLP phenomenon thus emerges, and with it, strategies to avoid FLP for technological applications may also be suggested.

Eight decades ago, Schottky proposed that the energy barrier at the metal-semiconductor interface, which now bears his name, should be compared with the difference of two surface quantities, the work function (WF) of the metal and the ionization potential of the semiconductor. This tradition of plotting and modeling the Schottky barrier height (SBH) against the metal WF has been followed ever since. However, success in general quantitative understanding of the SBH from physical principles has been limited, and empirical models are still relied upon. Here, we show that the stumbling block that has prevented a broadly applicable physical explanation of the SBH is the presence of surface dipole terms, inherently included in the traditional, Schottky-Mott styled analyses. By removing these surface contributions with the help of the recently developed neutral polyhedra theory, we show that the SBHs calculated for a very large number of epitaxial interfaces between metals and zinc-blende semiconductors are quantitatively explained from general chemical principles. Amazingly, SBHs calculated for 17 different semiconductors fit onto the same universal plot. Previously, SBHs needed to be grouped according to the semiconductor before analyses could be conducted separately for, and with empirical parameters specific to, each semiconductor. This work shows that the mechanism for SBH formation at metal-semiconductor interface is none other than the same chemistry responsible for charge distribution in molecules. There is no need for empirical modeling once the traditional beginning-point of SBH analysis is abandoned and the proposed new one is used.

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metalorganic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above ∼50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Predicting the band structure and optical absorption spectra of narrow band gap semiconductors is challenging for electronic structure methods. Here, it is shown shown that density functional theory can yield accurate band structures and time-dependent density functional theory (TDDFT) can yield accurate optical absorption spectra for these systems. This is achieved by using a screened range-separated hybrid (SRSH) functional with a single empirical parameter, fit to reproduce the experimental band gap. By comparing TDDFT results based on the SRSH approach with those obtained based on the HeydScuseriaErnzerhof functional it is shown that screened long-range exact exchange improves the accuracy of the TDDFT spectra for these systems.

The exact energy functional of density functional theory (DFT) is well known to obey various constraints. Three conditions that must be obeyed by the exact energy functional, but may or may not be obeyed by approximate ones, are often pointed out as important in general and for accurate computation of spectroscopic observables in particular. These are: (1) piecewise linearity as a function of the fractional particle number, (2) freedom from one-electron self-interaction, and (3) for a finite system, the functional derivative with respect to the density results in an asymptotic -1/rpotential (in Hartree atomic units), whereris the distance from the system center. In this overview, we explain what these conditions are, what they address, and why each one is of importance for spectroscopy. We then show, using specific examples from the literature, that these three properties are related, but are not equivalent and need to be assessed individually.

Predicting the degree of localization and calculating the trapping energies of polarons in insulators by density functional theory (DFT) is challenging. Hybrid functionals are often reparametrized to obtain accurate results and the a priori selection of these parameters is still an open question. Here we test the accuracy of several range-separated hybrid functionals, all reparametrized to produce an accurate band gap, by calculating the charge transition levels (CTLs) of experimentally well-studied hole polaron defect centers in MgO. We show that the functional with screened long-range exact exchange is moderately but consistently more accurate than functionals which do not include long-range exact exchange. We provide evidence that the source of the improved accuracy is the eigenvalue associated with the valence band maximum of the bulk material. We discuss the extent to which this accuracy relates to Koopmans & rsquo; compliance of the defect energy level.

The performance of optimally tuned range-separated hybrid (OT-RSH) functional calculations in predicting accurate isotropic nuclear magnetic shielding (sigma) and chemical shift values is examined. To that end, the results of OT-RSH and other approximate density functional theory calculations are assessed against recently published benchmark CCSD(T) calculations for a test set consisting of several molecules and bond types. It is found that for atoms in single bonds with a large paramagnetic contribution to sigma, OT-RSH offers a significant improvement in prediction of shielding constants over popular semi-local and hybrid density functionals, yielding non-empirical results that are as accurate as those of semi-empirical density functionals often used for prediction of shielding constants. This success is attributed to the improved fundamental gap prediction of the OT-RSH approach. For atoms in multiple bonds, however, larger errors often persist. By comparing OT-RSH and recently reported double-hybrid functional results, the remaining difficulties are traced to significant non-local correlation.

We provide a new scheme for representing pseudopotentials on a finite real-space grid, designed to significantly reduce the "egg box'' effect, i.e., unphysical fluctuations of computed quantities upon real-space translation. Instead of interpolating the electron-ion potential onto the grid, our scheme starts at a reference position and then uses a weighted sum of translation operators to account for the positions of atoms in real space. This results in a nonlocal but banded representation (even for local potentials) which is fully compatible with nonlocal pseudopotential operators. As a demonstration, this scheme is tested in one dimension for three types of potentials: a local pseudopotential, a nonlocal pseudopotential, and a local pseudopotential with self-consistent Hartree and exchange-correlation potentials. This scheme is found to reduce fluctuations of examined quantities by at least three orders of magnitude. The approach requires neither grid adaptation nor pseudopotential modification and can be readily extended to the three-dimensional case.

Hybrid functionals have proven to be of immense practical value in density-functional-theory calculations. While they are often thought to be a heuristic construct, it has been established that this is in fact not the case. Here, we present a rigorous and formally exact analysis of generalized Kohn-Sham (GKS) density-functional theory of hybrid functionals, in which exact remainder exchange-correlation potentials combine with a fraction of Fock exchange to produce the correct ground-state density. First, we extend formal GKS theory by proving a generalized adiabatic connection theorem. We then use this extension to derive two different definitions for a rigorous distinction between multiplicative exchange and correlation components-one new and one previously postulated. We examine their density-scaling behavior and discuss their similarities and differences. We then present a new algorithm for obtaining exact GKS potentials by inversion of accurate reference electron densities and employ this algorithm to obtain exact potentials for simple atoms and ions. We establish that an equivalent description of the many-electron problem is indeed obtained with any arbitrary global fraction of Fock exchange, and we rationalize the Fock-fraction dependence of the computed remainder exchange-correlation potentials in terms of the new formal theory. Finally, we use the exact theoretical framework and numerical results to shed light on the exchange-correlation potential used in approximate hybrid functional calculations and to assess the consequences of different choices of fractional exchange.

We present a comparative density functional theory (DFT) investigation of the interaction of the iron porphyrin (FeP) molecule with the metallic Co(001) and Cu(001) surfaces, with the aim of elucidating the effect of different choices for the treatment of dispersion. We compare a GGA+U approach, several flavors of dispersion-augmented terms, and two variants of the vdW-DF approach, which treats long-range correlation explicitly. For the Co surface, we find that all approaches predict chemisorption and a high-spin state, although vdW-DF functionals generally predict weaker bonds and weaker chemisorption. For the Cu surface, we find that the functionals augmented by pair-wise dispersion once again predict chemisorption and a preferred HS state, but the vdW-DF functionals predict physisorption and a LS state. These results demonstrate the importance of careful assessment of the level of theory at which dispersion is treated, as this may have significant quantitative and even qualitative effects on the predictions made. The results also call for additional experimental data for these systems.

Calcium oxalate minerals are broadly present in nature. They form through biogenic, geogenic, and pathogenic processes that lead to different pseudopolymorphs. Being the most common solid phase in human nephrolithiasis, calcium oxalate monohydrate (COM) in particular has been the focus of much investigation. It exists in several crystalline forms, two of which appear to be of biological and medical relevance: the low- and high-temperature forms (COM-LT and COM-HT, respectively). While there is broad consensus on the ordered structure of COM-LT, which possesses the P2(1)/n space group symmetry, for COM-HT controversy remains. Experimental results suggest that there is a certain degree of structural disorder in the high-temperature form. However, the exact character of disorder in COM-HT is yet an open question. Here, we examine the effect of the disorder of water molecules on the structure of COM using first-principles calculations based on dispersion-augmented density functional theory. Such calculations allow for controlled examination of specific disorder features and their effect on crystal structure and stability. On the basis of our first-principles analysis, we suggest that in COM-HT each water dimer site is randomly occupied by any of four water dimer arrangements present in COM-LT, leading to statistical 2/m point symmetry at each site and a statistical I2/m space group symmetry.

The importance of many-body dispersion effects in layered materials subjected to high external loads is evaluated. State-of-the-art many-body dispersion density functional theory calculations performed for graphite, hexagonal boron nitride, and their heterostructures were used to fit the parameters of a classical registry-dependent interlayer potential. Using the latter, we performed extensive equilibrium molecular dynamics simulations and studied the mechanical response of homogeneous and heterogeneous bulk models under hydrostatic pressures up to 30 GPa. Comparison with experimental data demonstrates that the reliability of the many-body dispersion model extends deep into the subequilibrium regime. Friction simulations demonstrate the importance of many-body dispersion effects for the accurate description of the tribological properties of layered material interfaces under high pressure.

Key spin transport phenomena, including magnetoresistance and spin transfer torque, cannot be activated without spin-polarized currents, in which one electron spin is dominant. At the nanoscale, the relevant length-scale for modern spintronics, spin current generation is rather limited due to unwanted contributions from poorly spin-polarized frontier states in ferromagnetic electrodes, or too short length-scales for efficient spin splitting by spin-orbit interaction and magnetic fields. Here, we show that spin-polarized currents can be generated in silver-vanadocene-silver single molecule junctions without magnetic components or magnetic fields. In some cases, the measured spin currents approach the limit of ideal ballistic spin transport. Comparison between conductance and shot-noise measurements to detailed calculations reveals a mechanism based on spin-dependent quantum interference that yields very efficient spin filtering. Our findings pave the way for nanoscale spintronics based on quantum interference, with the advantages of low sensitivity to decoherence effects and the freedom to use non-magnetic materials.

The eyes of many fish contain a reflecting layer of organic crystals partially surrounding the photoreceptors of the retina, which are commonly believed to be composed of guanine. Here we study an unusual fish eye from Stizostedion lucioperca that contains two layers of organic crystals. The crystals in the outer layer are thin plates, whereas the crystals in the inner tapetum layer are block-shaped. We show that the outer layer indeed contains guanine crystals. Analyses of solutions of crystals from the inner layer indicated that the block-shaped crystals are composed of xanthopterin. A model of the structure of the block-shaped crystals was produced using symmetry arguments based on electron diffraction data followed by dispersion-augmented DFT calculations. The resulting crystal structure of xanthopterin included, however, a problematic repulsive interaction between C=O and N of two adjacent molecules. Knowing that dissolved 7,8-dihydroxanthopterin can oxidize to xanthopterin, we replaced xanthopterin with 7,8-dihydroxanthopterin in the model. An excellent fit was obtained with the powder X-ray diffraction pattern of the biogenic crystals. We then analyzed the biogenic block-shaped crystals in their solid state, using MALDI-TOF and Raman spectroscopy. All three methods unequivocally prove that the block-shaped crystals in the eye of S. lucioperca are crystals of 7,8-dihydroxanthopterin. On the basis of the eye anatomy, we deduce that the guanine crystals form a reflective layer producing the silvery color present on part of the eye surface, whereas the block-shaped crystals backscatter light into the retina in order to increase the light sensitivity of the eye.

The variety and complexity of DNA-based structures make them attractive candidates for nanotechnology, yet insufficient stability and mechanical rigidity, compared to polyamide-based molecules, limit their application. Here, we combine the advantages of polyamide materials and the structural patterns inspired by nucleic-acids to generate a mechanically rigid fluorenylmethyloxycarbonyl (Fmoc)-guanine peptide nucleic acid (PNA) conjugate with diverse morphology and photoluminescent properties. The assembly possesses a unique atomic structure, with each guanine head of one molecule hydrogen bonded to the Fmoc carbonyl tail of another molecule, generating a non-planar cyclic quartet arrangement. This structure exhibits an average stiffness of 69.6 +/- 6.8 Nm(-1) and Young's modulus of 17.8 +/- 2.5 GPa, higher than any previously reported nucleic acid derived structure. This data suggests that the unique cation-free "basket" formed by the Fmoc-G-PNA conjugate can serve as an attractive component for the design of new materials based on PNA self-assembly for nanotechnology applications.

Controlled modification of the semiconductor surface work function is of fundamental importance for improvements in the efficiency of (opto-)electronic devices. Binding amino acids to a semiconductor surface through their common carboxylic group offers a versatile tool for modulation of surface properties by the choice of their side chain. This approach is demonstrated here by tailoring the surface work function of indium tin oxide, one of the most abundant transparent electrodes in organic optoelectronic devices. We find that the work function can be systematically tuned by the side chain of the amino acid, resulting in either an increase or a decrease of the work function, over a large range of similar to 250 meV. This side chain effect is mostly due to alteration of the dipole component perpendicular to the surface, with a generally smaller contribution for changes in surface band bending. These findings also shed light on electronic interactions at the interface between proteins and semiconductors, which are of importance for future bio-electronic devices.

We report on previously unnoticed features of the exact Hartree-exchange and correlation potentials for atoms and ions treated via ensemble density functional theory, demonstrated on fractional ions of Li, C, and F. We show that these potentials, when treated separately, can reach non-vanishing asymptotic constant values in the outer region of spherical, spin unpolarized atoms. In the next leading order, the potentials resemble Coulomb potentials created by effective charges which have the peculiarity of not behaving as piecewise constants as a function of the electron number. We provide analytical derivations and complement them with numerical results using the inversion of the Kohn-Sham equations for interacting densities obtained by accurate quantum Monte Carlo calculations. The present results expand on the knowledge of crucial exact properties of Kohn-Sham systems, which can guide development of advanced exchange-correlation approximations.

We present a comprehensive analysis of magnetic coupling in a group of three popular chromium-based molecular rings, the homometallic Cr-8 ring and the heterometallic Cr7Ni and Cr7Zn molecules. We show conclusively that the broken symmetry approach within density functional theory (DFT), based on suitable conventional or range-separated hybrid functionals, provides a quantitatively reliable tool to extract magnetic exchange coupling parameters in all rings considered, which opens a window for additional applications in molecular magnetism. We further show that a nonempirical model spin Hamiltonian, based on the parameters extracted from DFT, leads to excellent agreement with experimental susceptibility data and energy spectra. Moreover, based on an optimally tuned range-separated hybrid functional approach, we find that gas-phase gaps of the studied molecular rings are much larger than previously calculated and discuss the implications of the revised electronic structure to potential applications in molecular spintronics.

Screened range-separated hybrid (SRSH) functionals are of potential interest as a computationally inexpensive yet accurate alternative approach for studying (opto)electronic properties in the solid state. At present, SRSH functionals are typically tuned to reproduce with high accuracy the properties of either bulk or low-dimensional structures, rendering such functionals not only material specific, but also structure specific. The transferability of tuned SRSH functionals between bulk and low-dimensional phases has not been examined systematically and is desirable for straightforward and consistent modeling of size effects in nanostructures. We present a simple yet effective approach for simultaneous tuning of the fraction of short-range exact exchange and the range-separation parameter, which delivers accurate and transferable SRSH functionals for two-dimensional and bulk phases of two prototypical layered materials, molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN). The ground-state SRSH band structures, resulting from minimal fitting of the SRSH to a single quasiparticle energy, are found to be in excellent agreement with GW calculations over the entire Brillouin zone. Excited-state properties are predicted using time-dependent density functional theory calculations, based on the SRSH (TD-SRSH) functional. Calculated absorption spectra are found to be in excellent agreement with GW and Bethe-Salpeter equation (BSE) calculations for MoS2, but less so for h-BN; the failure of the TD-SRSH for the latter material is examined and a BSE approach based on the SRSH ground state is shown to restore accuracy.

For nanostructured interfaces between open-shell molecules and metal surfaces that involve charge transfer upon adsorption, the investigation of molecular magnetic properties is an interesting yet difficult task, because in principle different magnetic configurations with distinct properties can be found. Here, we study the magnetic properties of CuPc-Ag and CoPc-Ag interfaces, which constitute interesting test cases because charge is transferred to the initially open-shell Pc molecules upon adsorption. Using hybrid density functional theory, we examine the stability of the various magnetic configurations occurring at these nanoscale interfaces, as well as for the corresponding gas-phase anions, and compare our findings to those of previous experimental studies. For CuPc-Ag, we identify a high-spin triplet configuration as the most likely configuration at the interface, whereas for CoPc-Ag a quenching of the total magnetic moment is found. Interestingly, such quenching is consistent with two distinctly different interfacial electronic configurations. These important differences in the magnetic properties of CuPc and CoPc on Ag are rationalized by variations in the interaction of their central metal atoms with the substrate. Our work facilitates a deeper understanding of the magnetic configuration and interlinked electronic-structure properties of molecule-metal interfaces. Furthermore, it highlights the necessity of an appropriate choice of methodology in tandem with a detailed evaluation of the different emerging magnetic properties.

We consider the Br vacancy in CsPbBr3 as a prototype for the impact of structural dynamics on defect energetics in halide perovskites (HaPs). Using first-principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy level associated with a Br vacancy is found to be intrinsically dynamic, oscillating by as much as 1 eV on the picosecond time scale at room temperature. These significant energy fluctuations are correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at these Pb atomic sites. We expect this unusually strong coupling of structural dynamics and defect energetics to bear important implications for both experimental and theoretical analyses of defect characteristics in HaPs. It may also hold significant ramifications for carrier transport and defect tolerance in this class of photovoltaic materials.

In recent years, inorganic cesium-lead-halide perovskites, CsPbX3 (X = I, Br, Cl), have attracted interest for optoelectronic applications such as highly efficient thin-film light-emitting diodes or wide-gap absorber materials for photovoltaics. However, phase segregation and secondary phases in as-deposited thin films are still considered to be limiting factors for devices based on CsPbX3. Here, we report a correlative electron microscopy and spectroscopy approach for the identification of secondary phases and their distributions in Cs-Pb-Br thin films, deposited by solution based and coevaporation methods on various substrates. We identified phases by their compositional, structural, and optoelectronic properties, using X-ray diffraction, spectroscopy, and a variety of microscopy techniques. We found that the Cs-Pb-Br films contain ternary Cs4PbBr6 and CsPb2Br5 phases in addition to CsPbBr3, a finding consistent with calculations of formation enthalpies by means of the density functional theory showing that these values are very similar for the three ternary phases. We find that these phases can exhibit different spatial distributions inside the film and discuss the influence of the deposition method and synthesis parameters on the resulting phase composition of the Cs-Pb-Br layers.

We present band structure and optical absorption spectra obtained from density functional theory (DFT) and linear response time-dependent DFT (TDDFT) calculations using a screened range-separated hybrid (SRSH) functional, including spin-orbit coupling, for seven prototypical semiconductors. The results are compared to those obtained from highly converged many-body perturbation theory calculations using the GW approximation and the GW plus Bethe-Salpeter equation (GW-BSE) approaches. We use a single empirical parameter for our SRSH calculations, fit such that the SRSH band gap reproduces the GW band gap at the Γ point. We then find that ground-state generalized Kohn-Sham SRSH eigenvalues accurately reproduce the band structure obtained from GW calculations, typically to within 0.1-0.2 eV, and optical absorption spectra obtained using TDDFT with the SRSH functional agree well with those of GW-BSE, with a mean deviation of 0.03 and 0.11 eV for the location of the first and second absorption peaks, respectively, at a fraction of the computational cost.

Until recently it was thought that the only optical function of pteridines in biology was to act as light-absorbing pigments, but a recent report by some of us revealed that crystalline isoxanthopterin is a reflector in the eyes of decapod crustaceans. Here, we report the formation of crystalline isoxanthopterin synthetically from the polar dimethyl sulfoxide solvent, with X-ray diffraction analysis revealing a crystal structure different from that of biogenic isoxanthopterin. The structure of the new polymorph was determined in two independent ways. In one approach, it was generated and optimized using first-principles calculations, followed by comparison of simulation and experiment for high-resolution powder X-ray diffraction (PXRD) and electron diffraction. In the other approach, the structure was obtained definitively from PXRD data using a direct-space genetic algorithm for structure solution followed by Rietveld refinement. The synthetic structure is different from its biogenic counterpart, especially in having a nonplanar criss-cross H-bonded arrangement. We also rationalized the morphology of the crystals and the effect of the DMSO thereon, via a comparison between observed and theoretical growth morphologies. In addition, we calculated the optical properties of the synthetic structure and found its two dominant refractive indices to be somewhat lower than those of its biogenic counterpart, but still as high as those of reflecting guanine crystals. Synthetic isoxanthopterin therefore emerges as a promising candidate for incorporation in artificial optical systems.

The formation of band offset (BO) at isovalent semiconductor heterojunctions has been branded "bulklike" because of an insensitivity of the BO to the interface orientation and atomic structure and a transitivity in BOs. Even though tunability and nontransitivity of BO are frequently found for heterovalent interfaces, empirical theories with built-in bulklike characteristics have thus far dominated the explanation of experimental BOs. Presently, the distribution of charge density and the formation of BO at a large number of interfaces between lattice-matched perovskite oxides are studied in detail using density functional theory. Ionic screening is found to dominate the formation of the BO, as a sharp dependence of the (apparently tunable) BO on atomic structure for unrelaxed interfaces is essentially washed out upon lattice relaxation. Numerical experimentation with deliberate embedding of dipolar layers in perovskite oxides and their interfaces corroborates the effectiveness of ionic screening. The relaxed, converged (bulklike) BOs are found to be in good agreement with the prediction of the neutral polyhedra theory (NPT). The success of the NPT, presently for ionic interfaces and previously for covalent zinc-blende interfaces, unmasks a possible connection between the partition into neutral symmetric cells and the energy-minimization requirement on the interface charge distribution. The independence of the BO on interface specifics, i.e., bulklike behavior, is shown to stem directly from such a property of charge distribution with minimized electrostatic energy. As energy minimization governs the formation of charge distribution in general, NPT is expected to describe the band offset of a wide variety of material interfaces.

Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3), we reveal the band dispersion in two high-symmetry directions and identify the global valence band maxima. With these benchmark data, we construct "standard" photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature for determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.

Range-separated hybrid (RSH) functionals have been shown to overcome the tendency of traditional density functional theory to underestimate the fundamental orbital gap. More recently, the screened RSH (SRSH) approach has been developed as a means to extend these functionals to address the effect of the electrostatic environment on the fundamental gap. Here, we report a scheme that combines the SRSH formulation with the polarized continuum model (PCM) within a consistent framework for addressing long-range screened electrostatic interactions, which is further improved by optimal tuning (OT). The quantitative predictive power of the new OT-SRSH-PCM scheme by addressing fundamental gaps in thin films of organic semiconducting materials. This is especially impressive as the approach is based on single molecule calculations. We also discuss the advantages of this approach over alternative schemes combining PCM with RSH. In particular, we show that it avoids the well-documented tendency of standard OT to collapse the range separation parameter when performed within a dielectric continuum.

Isotope enrichment is widely used to affect atomic masses, facilitating data acquisition and peak assignments in experiments such as nuclear magnetic resonance and infrared spectroscopy. It is also used for elucidating the origin of weak features in systems where natural isotopic abundances are low. However, it is not possible to always know a priori precisely how vibrational modes change for arbitrary levels of isotopic substitution. Here, we examine this issue by presenting a joint experimental and theoretical study for the important case of C-13 isotope substitution effects on the infrared spectra of calcite. By systematically varying the C-13 : C-12 ratio, we find that the relative positions and intensities of infrared-active vibrational modes can vary, in a non-linear and mode-dependent fashion, with minority isotope content and proximity. This allows us to determine the origin of weak spectral features due to the natural abundance of isotopes and to show that even relatively low levels of substitution are not necessarily within the "dilute limit," below which isotopic substitutions do not interact.

Electronic coupling matrix elements are important to the theoretical description of electron transfer processes. However, they are notoriously difficult to obtain accurately from time-dependent density functional theory (TDDFT). Here, we use the HAB11 benchmark dataset of coupling matrix elements to assess whether TDDFT using optimally tuned range-separated hybrid functionals, already known to be successful for the description of charge transfer excitation energies, also allows for an improved accuracy in the prediction of coupling matrix elements. We find that this approach outperforms all previous TDDFT calculations, based on semi-local, hybrid or non-tuned range-separated hybrid functionals, with a remaining average deviation as low as ∼12%. We discuss potential sources for the remaining error.

A short overview of recent attempts at merging two independently developed methods is presented. These are the optimal tuning of a range-separated hybrid (OT-RSH) functional, developed to provide an accurate first-principles description of the electronic structure and optical properties of gas-phase molecules, and the polarizable continuum model (PCM), developed to provide an approximate but computationally tractable description of a solvent in terms of an effective dielectric medium. After a brief overview of the OT-RSH approach, its combination with the PCM as a potentially accurate yet low-cost approach to the study of molecular assemblies and solids, particularly in the context of photocatalysis and photovoltaics, is discussed. First, solvated molecules are considered, with an emphasis on the challenge of balancing eigenvalue and total energy trends. Then, it is shown that the same merging of methods can also be used to study the electronic and optical properties of molecular solids, with a similar discussion of the pros and cons. Tuning of the effective scalar dielectric constant as one recent approach that mitigates some of the difficulties in merging the two approaches is considered.

We present results and analyses for the photoelectron spectra of small copper oxide cluster anions (CuO-, CuO2-, CuO3-, and Cu2O-). The spectra are computed using various techniques, including density functional theory (DFT) with semi-local, global hybrid, and optimally tuned range-separated hybrid functionals, as well as many-body perturbation theory within the GW approximation based on various DFT starting points. The results are compared with each other and with the available experimental data. We conclude that as in many metal-organic systems, self-interaction errors are a major issue that is mitigated by hybrid functionals. However, these need to be balanced against a strong role of non-dynamical correlation-especially in smaller, more symmetric systems-where errors are alleviated by semi-local functionals. The relative importance of the two phenomena, including practical ways of balancing the two constraints, is discussed in detail. Published by AIP Publishing.

Generalized Kohn-Sham (GKS) theory extends the realm of density functional theory (DFT) by providing a rigorous basis for non-multiplicative potentials, the use of which is outside original Kohn{Sham theory. GKS theory is of increasing importance as it underlies commonly used approximations, notably (conventional or range-separated) hybrid functionals and meta-generalized-gradient-approximation (meta-GGA) functionals. While this approach is often extended in practice to time-dependent DFT (TDDFT), the theoretical foundation for this extension has been lacking, because the Runge-Gross theorem and the van Leeuwen theorem that serve as the basis of TDDFT have not been generalized to non-multiplicative potentials. Here, we provide the necessary generalization. Specifically, we show that with one simple but non-trivial additional caveat - upholding the continuity equation in the GKS electron gas - the Runge-Gross and van Leeuwen theorems apply to time-dependent GKS theory. We also discuss how this is manifested in common GKS-based approximations.

It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

We show that fundamental gaps and optical spectra of molecular solids can be predicted quantitatively and nonempirically within the framework of time-dependent density functional theory (TDDFT) using the recently developed optimally tuned screened range-separated hybrid (OT-SRSH) functional approach. In this scheme, the electronic structure of the gas-phase molecule is determined by optimal tuning of the range-separation parameter in a range-separated hybrid functional. Screening and polarization in the solid state are taken into account by adding long-range dielectric screening to the functional form, with the modified functional used to perform self-consistent periodic-boundary calculations for the crystalline solid. We provide a comprehensive benchmark for the accuracy of our approach by considering the X23 set of molecular solids and comparing results obtained from TDDFT with those obtained from many-body perturbation theory in the GW-BSE approximation. We additionally compare results obtained from dielectric screening computed within the random-phase approximation to those obtained from the computationally more efficient many-body dispersion approach and find that this influences the fundamental gap but has little effect on the optical spectra. Our approach is therefore robust and can be used for studies of molecular solids that are typically beyond the reach of computationally more intensive methods.

By studying the lowest excitations of an exactly solvable one-dimensional soft-Coulomb molecular model, we show that components of Kohn-Sham ensembles can be used to describe charge transfer processes. Furthermore, we compute the approximate excitation energies obtained by using the exact ensemble densities in the recently formulated ensemble Hartree-exchange theory [T. Gould and S. Pittalis, Phys. Rev. Lett. 119, 243001 (2017)]. Remarkably, our results show that triplet excitations are accurately reproduced across a dissociation curve in all cases tested, even in systems where ground state energies are poor due to strong static correlations. Singlet excitations exhibit larger deviations from exact results but are still reproduced semi-quantitatively. Published by AIP Publishing.

The notion that halide perovskite crystals (ABX(3), where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.

We assess the performance of the optimally tuned range-separated hybrid (OT-RSH) functional approach in predicting the ground-state electronic configuration and spin-state energetics of complexes that can potentially exhibit multiple spin configurations. To that end, we investigate eight iron complexes: four spin-crossover complexes, for which reference data from other approximate density functionals are available, and four smaller complexes, for which reference ab initio data are available. We show that the spin-state energetics are mostly governed by the percentage of short-range exact exchange and are only weakly influenced by the choice of the range-separation parameter. However, the electronic structure, especially the fundamental gap, is much more sensitive to the range-separation parameter. We further find that correct prediction of the ground state in spin-crossover compounds requires a reduction in the amount of short-range exact exchange, likely owing to a larger role of static correlation.

Hybrid organic-inorganic perovskites (HOIPs) have recently emerged as highly promising solution-processable materials for photovoltaic (PV) and other optoelectronic devices. HOIPs represent a broad family of materials with properties highly tuneable by the ions that make up the perovskite structure as well as their multiple combinations. Interestingly, recent high-efficiency PV devices using HOIPs with substantially improved long-term stability have used combinations of different ionic compositions. The structural dynamics of these systems are unique for semiconducting materials and are currently argued to be central to HOIPs stability and charge-transport properties. Here, we studied the impact of ionic composition on phonon speeds of HOIPs from Brillouin spectroscopy experiments and density functional theory calculations for FAPbBr(3), MAPbBr(3), MAPbCl(3), and the mixed halide MAPbBr(1.25)Cl(1.75). Our results show that the acoustic phonon speeds can be strongly modified by ionic composition, which we explain by analysing the lead-halide sublattice in detail. The vibrational properties of HOIPs are therefore tuneable by using targeted ionic compositions in the perovskite structure. This tuning can be rationalized by non-trivial effects, for example, considering the influence of the shape and dipole moment of organic cations. This has an important implications for further improvements in the stability and charge-transport properties of these systems.

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.

We compare electronic structure characteristics of three different kinds of benzene-Adsorbed (111) surfaces: That of Bi₂Te₃, a prototypical topological insulator, that of Au, a prototypical inert metal, and that of Pt, a prototypical catalytic metal. Using first-principles calculations based on dispersion-corrected density functional theory, we show that benzene is chemisorbed on Pt, but physisorbed on Au and Bi₂Te₃. The adsorption on Bi₂Te₃ is particularly weak, consistent with a minimal perturbation of the electronic structure at the surface of the topological insulator, revealed by a detailed analysis of the interaction of the molecular orbitals with the topological surface states.

Peptide-based molecular electronic devices are promising due to the large diversity and unique electronic properties of biomolecules. These electronic properties can change considerably with peptide structure, allowing diverse design possibilities. In this work, we explore the effect of the side-chain of the peptide on its electronic properties, by using both experimental and computational tools to detect the electronic energy levels of two model peptides. The peptides include 2Ala and 2Trp as well as their 3-mercaptopropionic acid linker which is used to form monolayers on an Au surface. Specifically, we compare experimental ultraviolet photoemission spectroscopy measurements with density functional theory based computational results. By analyzing differences in frontier energy levels and molecular orbitals between peptides in gas-phase and in a monolayer on gold, we find that the electronic properties of the peptide side-chain are maintained during binding of the peptide to the gold substrate. This indicates that the energy barrier for the peptide electron transport can be tuned by the amino acid compositions, which suggests a route for structural design of peptide-based electronic devices.

We present a computational analysis of the terahertz spectra of the monoclinic and the orthorhombic polymorphs of 2,4,6-trinitrotoluene. Very good agreement with experimental data is found when using density functional theory that includes Tkatchenko-Scheffler pair-wise dispersion interactions. Furthermore, we show that for these polymorphs the theoretical results are only weakly affected by many-body dispersion contributions. The absence of dispersion interactions, however, causes sizable shifts in vibrational frequencies and directly affects the spatial character of the vibrational modes. Mode assignment allows for a distinction between the contributions of the monoclinic and orthorhombic polymorphs and shows that modes in the range from 0 to ca. 3.3 THz comprise both inter- and intramolecular vibrations, with the former dominating below ca. 1.5 THz. We also find that intramolecular contributions primarily involve the nitro and methyl groups. Finally, we present a prediction for the terahertz spectrum of 1,3,5-trinitrobenzene, showing that a modest chemical change leads to a markedly different terahertz spectrum.

Molecular monolayers at metal/semiconductor heterointerfaces affect electronic energy level alignment at the interface by modifying the interface's electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (PhO), aryl vinyl sulfide (PhS), and the corresponding molecule with a CH2 group allyl benzene (PhC). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high-quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current voltage and capacitance voltage analysis of Hg/PhX (X = C, 0, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor's effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact.

One major challenge of functional material fabrication is combining flexibility, strength, and toughness. In several biological and artificial systems, these desired mechanical properties are achieved by hierarchical architectures and various forms of anisotropy, as found in bones and nacre. Here, it is reported that crystals of N-capped diphenylalanine, one of the most studied self-assembling systems in nanotechnology, exhibit well-ordered packing and diffraction of sub-angstrom resolution, yet display an exceptionally flexible nature. To explore this flexibility, the mechanical properties of individual crystals are evaluated, assisted by density functional theory calculations. High-resolution scanning electron microscopy reveals that the crystals are composed of layered self-assembled structures. The observed combination of strength, toughness, and flexibility can therefore be explained in terms of weak interactions between rigid layers. These crystals represent a novel class of self-assembled layered materials, which can be utilized for various technological applications, where a combination of usually contradictory mechanical properties is desired.

Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with first-principles calculations for the absorption based on the GW plus Bethe-Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying excitations in perylene emerges, including evidence of an exciton-polariton stopband, as well as an assessment of the commonly used Tamm-Dancoff approximation to the GW-BSE approach. Our findings on this well-controlled system can guide understanding and development of advanced molecular solids and functionalization for applications.

The well-known insensitivity of the band offset (BO) of isovalent heterojunctions with the zincblende structure to the orientation, abruptness, and atomic structure of the interface was recently shown to be attributable to a localness in the dependence of charge density on the atomic structure. In contrast, a sharp dependence of the BO on interface specifics has been observed at heterovalent heterojunctions. Here, detailed analyses of the relationship between the BO, interface structure, and charge distribution have been carried out for many lattice-matched heterovalent interfaces between zincblende and diamond structure semiconductors. From thermodynamic considerations, three types of neutral interfaces were investigated, each with equal densities of donor- and acceptor-like heterovalent bonds, constructible in all orientations. Distinctively different, yet approximately orientation-independent, valence BOs were found. The equilibrium charge density of the heterovalent interface could be recreated with the charge densities of bulk semiconductors and oligo-cells. Because charge transfer between heterovalent bonds is identifiable with that for dopants in semiconductor and its effect accountable by linear response, a combination of neutral polyhedra theory, previously developed for isovalent heterojunctions, and dielectric screening theory was found to explain BO trends throughout, allowing a strategy that facilitates adjustment in the BO of all isovalent heterojunctions.

A new parametrization of the anisotropic interlayer potential for hexagonal boron nitride (h-BN ILP) is presented. The force-field is benchmarked against density functional theory calculations of several dimer systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, corrected for many-body dispersion effects. The latter, more advanced method for treating dispersion, is known to produce binding energies nearly twice as small as those obtained with pairwise correction schemes, used for an earlier ILP parametrization. The new parametrization yields good agreement with the reference calculations to within ∼1 and ∼0.5 meV/atom for binding and sliding energies, respectively. For completeness, we present a complementary parameter set for homogeneous graphitic systems. Together with our previously suggested ILP parametrization for the heterogeneous graphene/h-BN junction, this provides a powerful tool for consistent simulation of the structural, mechanical, tribological, and heat transport properties of both homogeneous and heterogeneous layered structures based on graphene and h-BN.

Guanine crystals are widely used in nature as components of multilayer reflectors. Guanine-based reflective systems found in the copepod cuticle and in the mirror of the scallop eye are unique in that the multilayered reflectors are tiled to form a contiguous packed array. In the copepod cuticle, hexagonal crystals are closely packed to produce brilliant colors. In the scallop eye, square crystals are tiled to obtain an image-forming reflecting mirror. The tiles are about 1 mm in size and 70 nm thick. According to analysis of their electron diffraction patterns, the hexagon and square tiles are not single crystals. Rather, each tile type is a composite of what appears to be three crystalline domains differently oriented and stacked onto one another, achieved through a twice-repeated twinning about their and crystal axes, respectively. By these means, the monoclinic guanine crystal mimics higher symmetry hexagonal and tetragonal structures to achieve unique morphologies.

Femtosecond resolution electron scattering techniques are applied to resolve the first atomic-scale steps following absorption of a photon in the prototypical hybrid perovskite methylammonium lead iodide. Following above-gap photoexcitation, we directly resolve the transfer of energy from hot carriers to the lattice by recording changes in the mean square atomic displacements on 10-ps time scales. Measurements of the time-dependent pair distribution function show an unexpected broadening of the iodine-iodine correlation function while preserving the PbI distance. This indicates the formation of a rotationally disordered halide octahedral structure developing on picosecond time scales. This work shows the important role of light-induced structural deformations within the inorganic sublattice in elucidating the unique optoelectronic functionality exhibited by hybrid perovskites and provides new understanding of hot carrierlattice interactions, which fundamentally determine solar cell efficiencies.

We analyse the valence electronic structure of cobalt phthalocyanine (CoPc) by means of optimally tuning a range-separated hybrid functional. The tuning is performed by modifying both the amount of short-range exact exchange (α) included in the hybrid functional and the range-separation parameter (γ), with two strategies employed for finding the optimal γ for each α. The influence of these two parameters on the structural, electronic, and magnetic properties of CoPc is thoroughly investigated. The electronic structure is found to be very sensitive to the amount and range in which the exact exchange is included. The electronic structure obtained using the optimal parameters is compared to gas-phase photo-electron data and GW calculations, with the unoccupied states additionally compared with inverse photo-electron spectroscopy measurements. The calculated spectrum with tuned γ, determined for the optimal value of α = 0.1, yields a very good agreement with both experimental results and with GW calculations that well-reproduce the experimental data.

We develop a robust approach for the description of the energetics of charge transfer (CT) excitations and transport levels at organic interfaces based on a screened range-separated hybrid (SRSH) functional. We find that SRSH functionals correctly capture the effect of solid-state electronic polarization on transport gap renormalization and on screening of the electrostatic electron hole interaction. With respect to calculations based on nonscreened optimally tuned RSH (long-range corrected) functionals, the SRSH-based calculations can be performed for both isolated molecular complexes and systems embedded in a dielectric medium with the same range-separation parameter, which allows a clear physical interpretation of the results in terms of solid-state polarization without any perturbation of the molecular electronic structure. By considering weakly interacting donor/acceptor complexes of pentacene with C-60 and poly-3-hexylthiophene (P3HT) with PCBM, we show that this new approach provides CT-state energies that compare very well with experimental data.

The alignment of the frontier orbital energies of an adsorbed molecule with the substrate Fermi level at metal-organic interfaces is a fundamental observable of significant practical importance in nanoscience and beyond. Typical density functional theory calculations, especially those using local and semi-local functionals, often underestimate level alignment leading to inaccurate electronic structure and charge transport properties. In this work, we develop a new fully self-consistent predictive scheme to accurately compute level alignment at certain classes of complex heterogeneous molecule-metal interfaces based on optimally tuned range-separated hybrid functionals. Starting from a highly accurate description of the gas-phase electronic structure, our method by construction captures important nonlocal surface polarization effects via tuning of the long-range screened exchange in a range-separated hybrid in a non-empirical and system-specific manner. We implement this functional in a plane-wave code and apply it to several physisorbed and chemisorbed molecule-metal interface systems. Our results are in quantitative agreement with experiments, for both the level alignment and work function changes. Our approach constitutes a new practical scheme for accurate and efficient calculations of the electronic structure of molecule-metal interfaces. (C) 2017 Author(s).

A parameter-free version of the recently developed driven Liouville-von Neumann equation [T. Zelovich et al., J. Chem. Theory Comput. 10(8), 2927-2941 (2014)] for electronic transport calculations in molecular junctions is presented. The single driving rate, appearing as a fitting parameter in the original methodology, is replaced by a set of state-dependent broadening factors applied to the different single-particle lead levels. These broadening factors are extracted explicitly from the self-energy of the corresponding electronic reservoir and are fully transferable to any junction incorporating the same lead model. The performance of the method is demonstrated via tight-binding and extended Hückel calculations of simple junction models. Our analytic considerations and numerical results indicate that the developed methodology constitutes a rigorous framework for the design of "black-box" algorithms to simulate electron dynamics in open quantum systems out of equilibrium.

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH3NH3PbBr3) and all-inorganic (CsPbBr3) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr3.

The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the C13 position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.

Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (

The vibrational Stark shift is an important effect in determining the electrostatic environment for molecular or condensed matter systems. However, accurate ab initio calculations of the vibrational Stark effect are a technically demanding challenge. We make use of density functional theory constructed on a real-space grid to expedite the computation of this effect. Our format is especially advantageous for the investigation of small molecules in finite fields as cluster boundary conditions eliminate spurious supercell interactions and allow for charged systems, while convergence is controlled by a single parameter, the grid spacing. The Stark tuning rate is highly sensitive to the interaction between anharmonicity in a vibrational mode and the applied field. To ensure this subtle interaction is fully captured, we apply three parallel approaches: a direct finite field, a perturbative method, and a molecular dynamics method. We illustrate this method by applying it to several small molecules containing C-O and C-N bonds and show that a consistent result can be obtained.

Lead-halide perovskites are promising materials for opto-electronic applications. Recent reports indicated that their mechanical and electronic properties are strongly affected by the lattice vibrations. Herein we report far-infrared spectroscopy measurements of CH₃NH₃Pb(I/Br/Cl)₃ thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine the frequencies of the transversal and longitudinal optical phonons, and use them to calculate, via appropriate models, the static dielectric constants, polaron masses, electron-phonon coupling constants, and upper limits for the phonon-scattering limited charge carrier mobilities. Within the limitations of the model used, we can place an upper limit of 200 cm² V s^-1 for the room temperature charge carrier mobility in MAPbI₃ single crystals. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

Charge migration for electron transfer via the polypeptide matrix of proteins is a key process in biological energy conversion and signaling systems. It is sensitive to the sequence of amino acids composing the protein and, therefore, offers a tool for chemical control of charge transport across biomaterial-based devices. We designed a series of linear oligoalanine peptides with a single tryptophan substitution that acts as a "dopant," introducing an energy level closer to the electrodes' Fermi level than that of the alanine homopeptide. We investigated the solid-state electron transport (ETp) across a selfassembled monolayer of these peptides between gold contacts. The single tryptophan "doping" markedly increased the conductance of the peptide chain, especially when its location in the sequence is close to the electrodes. Combining inelastic tunneling spectroscopy, UV photoelectron spectroscopy, electronic structure calculations by advanced density-functional theory, and dc current-voltage analysis, the role of tryptophan in ETp is rationalized by charge tunneling across a heterogeneous energy barrier, via electronic states of alanine and tryptophan, and by relatively efficient direct coupling of tryptophan to a Au electrode. These results reveal a controlled way of modulating the electrical properties of molecular junctions by tailormade "building block" peptides.

It is well known that the magnitude of band offset (BO) at any semiconductor heterojunction is directly derivable from the distribution of charge at that interface and that the latter is decided by a minimization of total energy. However, the fact that BO formation is governed by energy minimization has not been explicitly used in theoretical BO models, likely because the equilibrium charge densities at heterojunction interfaces appear difficult to predict, except via explicit calculation. In this paper, electron densities at a large number of (100), (110), and (111) oriented heterojunctions between lattice-matched, isovalent semiconductors with the zinc blende (ZB) structure have been calculated by first-principles methods and analyzed in detail for possible common characteristics among energy-minimized densities. Remarkably, the heterojunction electron density was found to largely depend only on the immediate, local atomic arrangement. In fact, it is so much so that a juxtaposition of local electron-densities generated in oligo-cells (LEGOs) accurately reproduced the charge densities that minimize the energy for the heterojunctions. Furthermore, the charge distribution for each bulk semiconductor was found to display a striking separability of its electrostatic effect into two neutral parts, associated with the cation and the anion, which are approximately transferrable among semiconductors. These discoveries form the basis of a neutral polyhedra theory (NPT) that approximately predicts the equilibrium charge density and BO of relaxed heterojunctions from the energy minimization requirement. Well-known experimentally observed characteristics of heterojunctions, such as the insensitivity of BO to heterojunction orientation and the identity of interface bonds, the transitivity rule, etc., are all in good agreement with the NPT. Therefore, energy minimization, which essentially decides the electronic properties of all other solid and molecular systems, also governs the formation of the charge density at these heterojunction interfaces. In particular, the approach presented here eliminates the need to invoke mechanisms that are specific to semiconductor interfaces.

A nonorthogonal localized basis-set implementation of the driven Liouville von Neumann (DLvN) approach is presented. The method is based on block-orthogonalization of the Hamiltonian and overlap matrix representations, yielding nonoverlapping blocks that correspond to the various system sections. An extended Hückel description of gold/benzene-dithiol/gold and gold/pyridine-dithiol/gold junctions is used to demonstrate the performance of the method. The presented generalization is an important milestone toward using the DLvN approach for performing accurate dynamic electronic transport calculations in realistic model systems, based on density functional theory packages that rely on atom-centered basis-set representations.

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI₃, MAPbBr₃, CsPbBr₃), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI₃ VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

Cesium lead iodide possesses an excellent combination of band gap and absorption coefficient for photovoltaic applications in its perovskite phase. However, this is not its equilibrium structure under ambient conditions. In air, at ambient temperature it rapidly transforms to a nonfunctional, so-called yellow phase. Here we show that chloride doping, particularly at levels near the solubility limit for chloride in a cesium lead iodide host, provides a new approach to stabilizing the functional perovskite phase. In order to achieve high doping levels, we first co-deposit colloidal nanocrystals of pure cesium lead chloride and cesium lead iodide, thereby ensuring nanometer-scale mixing even at compositions that potentially exceed the bulk miscibility of the two phases. The resulting nanocrystal solid is subsequently fused into a polycrystalline thin film by chemically induced, room-temperature sintering. Spectroscopy and X-ray diffraction indicate that the chloride is further dispersed during sintering and a polycrystalline mixed phase is formed. Using density functional theory (DFT) methods in conjunction with nudged elastic band techniques, low-energy pathways for interstitial chlorine diffusion into a majority-iodide lattice were identified, consistent with the facile diffusion and fast halide exchange reactions observed. By comparison to DFT-calculated values (with the PBE exchange-correlation functional), the relative change in band gap and the lattice contraction are shown to be consistent with a Cl/I ratio of a few percent in the mixed phase. At these incorporation levels, the half-life of the functional perovskite phase in a humid atmosphere increases by more than an order of magnitude.

We present a new force-field potential that describes the interlayer interactions in heterojunctions based on graphene and hexagonal boron nitride (h-BN). The potential consists of a long-range attractive term and a short-range anisotropic repulsive term. Its parameters are calibrated against reference binding and sliding energy profiles for a set of finite dimer systems and the periodic graphene/h-BN bilayer, obtained from density functional theory using a screened-exchange hybrid functional augmented by a many-body dispersion treatment of long-range correlation. Transferability of the parametrization is demonstrated by considering the binding energy of bulk graphene/h-BN alternating stacks. Benchmark calculations for the superlattice formed when relaxing the supported periodic heterogeneous bilayer provide good agreement with both experimental results and previous computational studies. For a free-standing bilayer we predict a highly corrugated relaxed structure. This, in turn, is expected to strongly alter the physical properties of the underlying monolayers. Our results demonstrate the potential of the developed force-field to model the structural, mechanical, tribological, and dynamic properties of layered heterostructures based on graphene and h-BN.

Molecular solids have attracted attention recently in the context of organic (opto)electronics. These materials exhibit unique charge carrier generation and transport phenomena that are distinct from those of conventional semiconductors. Understanding these phenomena is fundamental to optoelectronics and requires a detailed description of the excited-state properties of molecular solids. Recent advances in many-body perturbation theory (MBPT) and density functional theory (DFT) have made such description possible and have revealed many surprising electronic and optical properties of molecular crystals. Here, we review this progress. We summarize the salient aspects of MBPT and DFT as well as various properties that can be described by these methods. These properties include the fundamental gap and its renormalization, hybridization and band dispersion, singlet and triplet excitations, optical spectra, and excitonic properties. For each, we present concrete examples, a comparison to experiments, and a critical discussion.

We provide a quantitative examination of the ionisation potential, protonation, and de-protonation energies of a set of 22 different amino acids. Specifically, we compare results obtained using the conventional hybrid functionals B3LYP and BHLYP (which use 20% and 50% of Fock exchange, respectively) with those obtained from the recently developed optimally tuned range-separated hybrid (OT-RSH) functional approach, as well as to literature coupled-cluster calculations with single and double excitations (CCSD) data. We find the OT-RSH results to be quantitatively close to those of the BHLYP functional and a significant improvement over those of B3LYP, with respect to the CCSD data. We conclude that the results of the OT-RSH for these quantities are as reliable and inexpensive as those of BHLYP, but possess one distinct and important advantage: they overcome the empiricism and limited predictive power associated with the arbitrary choice of the amount of Fock exchange in the BHLYP. We further discuss quantitative and qualitative trends in the optimal-tuning procedure of the amino acids studied.

Molecular junctions based on ferromagnetic electrodes allow the study of electronic spin transport near the limit of spintronics miniaturization. However, these junctions reveal moderate magnetoresistance that is sensitive to the orbital structure at their ferromagnet-molecule interfaces. The key structural parameters that should be controlled in order to gain high magnetoresistance have not been established, despite their importance for efficient manipulation of spin transport at the nanoscale. Here, we show that single-molecule junctions based on nickel electrodes and benzene molecules can yield a significant anisotropic magnetoresistance of up to ∼200% near the conductance quantum G₀. The measured magnetoresistance is mechanically tuned by changing the distance between the electrodes, revealing a nonmonotonic response to junction elongation. These findings are ascribed with the aid of first-principles calculations to variations in the metal-molecule orientation that can be adjusted to obtain highly spin-selective orbital hybridization. Our results demonstrate the important role of geometrical considerations in determining the spin transport properties of metal-molecule interfaces.

Hybrid organic-inorganic perovskites (HOIPs) are crystals with the structural formula ABX₃, where A, B, and X are organic and inorganic ions, respectively. While known for several decades, HOIPs have only in recent years emerged as extremely promising semiconducting materials for solar energy applications. In particular, power-conversion efficiencies of HOIP-based solar cells have improved at a record speed and, after only little more than 6 years of photovoltaics research, surpassed the 20% threshold, which is an outstanding result for a solution-processable material. It is thus of fundamental importance to reveal physical and chemical phenomena that contribute to, or limit, these impressive photovoltaic efficiencies.To understand charge-transport and light-absorption properties of semiconducting materials, one often invokes a lattice of ions displaced from their static positions only by harmonic vibrations. However, a preponderance of recent studies suggests that this picture is not sufficient for HOIPs, where a variety of structurally dynamic effects, beyond small harmonic vibrations, arises already at room temperature.In this Account, we focus on these effects. First, we review structure and bonding in HOIPs and relate them to the promising charge-transport and absorption properties of these materials, in terms of favorable electronic properties. We point out that HOIPs are much "softer" mechanically, compared to other efficient solar-cell materials, and that this can result in large ionic displacements at room temperature. We therefore focus next on dynamic structural effects in HOIPs, going beyond a static band-structure picture. Specifically, we discuss pertinent experimental and theoretical findings as to phase-transition behavior and molecular/octahedral rearrangements. We then discuss atomic diffusion phenomena in HOIPs, with an emphasis on the migration of intrinsic and extrinsic ionic species. From this combined perspective, HOIPs appear as highly dynamic materials, in which structural fluctuations and long-range ionic motion have an unusually strong impact on charge-transport and optical properties. We highlight the potential implications of these effects for several intriguing phenomenological observations, ranging from scattering mechanisms and lifetimes of charge carriers to light-induced structural effects and ionic conduction. (Figure Presented).

Molecular crystals are a prototypical class of van der Waals (vdW) bound organic materials with excited-state properties relevant for optoelectronics applications. Predicting the structure and excited-state properties of molecular crystals presents a challenge for electronic structure theory, as standard approximations to density functional theory (DFT) do not capture long-range vdW dispersion interactions and do not yield excited-state properties. In this work, we use a combination of DFT including vdW forces, using both nonlocal correlation functionals and pairwise correction methods, together with many-body perturbation theory (MBPT) to study the geometry and excited states, respectively, of the entire series of oligoacene crystals, from benzene to hexacene. We find that vdW methods can predict lattice constants within 1% of the experimental measurements, on par with the previously reported accuracy of pairwise approximations for the same systems. We further find that excitation energies are sensitive to geometry, but if optimized geometries are used MBPT can yield excited-state properties within a few tenths of an eV from experiment. We elucidate trends in MBPT-computed charged and neutral excitation energies across the acene series and discuss the role of common approximations used in MBPT.

Chirality-induced spin selectivity is a recently-discovered effect, which results in spin selectivity for electrons transmitted through chiral peptide monolayers. Here, we use this spin selectivity to probe the organization of self-assembled α-helix peptide monolayers and examine the relation between structural and spin transfer phenomena. We show that the α-helix structure of oligopeptides based on alanine and aminoisobutyric acid is transformed to a more linear one upon cooling. This process is similar to the known cold denaturation in peptides, but here the self-assembled monolayer plays the role of the solvent. The structural change results in a flip in the direction of the electrical dipole moment of the adsorbed molecules. The dipole flip is accompanied by a concomitant change in the spin that is preferred in electron transfer through the molecules, observed via a new solid-state hybrid organic-inorganic device that is based on the Hall effect, but operates with no external magnetic field or magnetic material.

We observe large, reversible, bias driven changes in the vibrational energies of PCBM based on simultaneous transport and surface-enhanced Raman spectroscopy (SERS) measurements on PCBM-gold junctions. A combination of linear and quadratic shifts in vibrational energies with voltage is analyzed and compared with similar measurements involving C₆₀-gold junctions. A theoretical model based on density functional theory (DFT) calculations suggests that both a vibrational Stark effect and bias-induced charging of the junction contribute to the shifts in vibrational energies. In the PCBM case, a linear vibrational Stark effect is observed due to the permanent electric dipole moment of PCBM. The vibrational Stark shifts shown here for PCBM junctions are comparable to or larger than the charging effects that dominate in C₆₀ junctions.

Solution-processed hybrid organic-inorganic perovskites (HOIPs) exhibit long electronic carrier diffusion lengths, high optical absorption coefficients and impressive photovoltaic device performance. Recent results allow us to compare and contrast HOIP charge-transport characteristics to those of III-V semiconductors - benchmarks of photovoltaic (and light-emitting and laser diode) performance. In this Review, we summarize what is known and unknown about charge transport in HOIPs, with particular emphasis on their advantages as photovoltaic materials. Experimental and theoretical findings are integrated into one narrative, in which we highlight the fundamental questions that need to be addressed regarding the charge-transport properties of these materials and suggest future research directions.Errata: In the originally published version of this article, the conduction bands in Figure 3c were incorrectly aligned. This has been corrected in both the HTML and PDF versions. We apologize to the readers for this error.

Living organisms display a spectrum of wondrous colors, which can be produced by pigmentation, structural coloration, or a combination of the two. A relatively well-studied system, which produces colors via an array of alternating anhydrous guanine crystals and cytoplasm, is responsible for the metallic luster of many fish. The structure of biogenic anhydrous guanine was so far believed to be the same as that of the synthetic one, a monoclinic polymorph (denoted as α). Here we re-examine the structure of biogenic guanine, using detailed experimental X-ray and electron diffraction data, exposing troublesome inconsistencies, namely, a "guanigma". To address this, we sought alternative candidate polymorphs using symmetry and packing considerations and then utilized first-principles calculations to determine whether the selected candidates could be energetically stable. We identified theoretically a different monoclinic polymorph (denoted as β), were able to synthesize it, and confirmed using X-ray diffraction that it is this polymorph that occurs in biogenic samples. However, the electron diffraction data were still not consistent with this polymorph but rather with a theoretically generated orthorhombic polymorph (denoted as γ). This apparent inconsistency was resolved by showing how the electron diffraction pattern could be affected by crystal structural faults composed of offset molecular layers.

Young's moduli of selected amino acid molecular crystals were studied both experimentally and computationally using nanoindentation and dispersion-corrected density functional theory. The Young modulus is found to be strongly facet-dependent, with some facets exhibiting exceptionally high values (as large as 44 GPa). The magnitude of Young's modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding network and the measured facet. Furthermore, we show computationally that the Young modulus can be as large as 70-90 GPa if facets perpendicular to the primary direction of the hydrogen-bonding network can be stabilized. This value is remarkably high for a molecular solid and suggests the design of hydrogen-bond networks as a route for rational design of ultra-stiff molecular solids.

We present insights into the lead-molecule coupling scheme in molecular electronics junctions. Using a "site-to-state" transformation that provides direct access to the coupling matrix elements between the molecular states and the eigenstate manifold of each lead, we find coupling bands whose character depends on the geometry and dimensionality of the lead. We use a standard tight-binding model to elucidate the origin of the coupling bands and explain their nature via simple "particle-in-a-box" type considerations. We further show that these coupling bands can shed light on the charge transport behavior of the junction. The picture presented in this study is not limited to the case of molecular electronics junctions and is relevant to any scenario where a finite molecular entity is coupled to a (semi)infinite system.

Solar cells based on organic-inorganic halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current-voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calculations based on density functional theory, we study the properties of an important defect in hybrid perovskites - interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin-Corbett mechanism. Our analysis highlights the structural flexibility of organic-inorganic perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells. On the move: An examination of the pathways for hydrogen migration in organic-inorganic halide perovskites has shown that hydrogen defects are likely mobile in these materials. These findings may bear important consequences for the hysteresis, stability, and ionic conductivity of hybrid perovskites and related solar cells.

We report a combined ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) study of the electronic structure of aromatic self-assembled monolayers covalently bound to Si, using several different aromatic groups (phenyl, biphenyl, and fluorene) and binding groups (O, NH, and CH₂). We obtain excellent agreement between theory and experiment, which allows for a detailed interpretation of the experimental results. Our analysis reveals a significant effect of the binding group on state hybridization at the organic/inorganic interface. Specifically, it highlights that lone-pair electrons in the binding atom facilitate hybridization between the aromatic system and the Si substrate, resulting in a significant induced density of interface states (IDIS). These interface states are manifested as a broadened HOMO peak in the experimental UPS data and are clearly observed in a theoretical spatially-resolved density of states map. This provides means to control the degree of coupling between substrate and molecule, which may prove useful in the design of transport across organic/inorganic interfaces.

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

There are several approximations to the exchange-correlation functional in density-functional theory, which accurately predict total energy-related properties of many-electron systems, such as binding energies, bond lengths, and crystal structures. Other approximations are designed to describe potential-related processes, such as charge transfer and photoemission. However, the development of a functional which can serve the two purposes simultaneously is a long-standing challenge. Trying to address it, we employ in the current work the ensemble generalization procedure proposed by Kraisler and Kronik [Phys. Rev. Lett. 110, 126403 (2013)]. Focusing on the prediction of the ionization potential via the highest occupied Kohn-Sham eigenvalue, we examine a variety of exchange-correlation approximations: the local spin-density approximation, semi-local generalized gradient approximations, and global and local hybrid functionals. Results for a test set of 26 diatomic molecules and single atoms are presented. We find that the aforementioned ensemble generalization systematically improves the prediction of the ionization potential, for various systems and exchange-correlation functionals, without compromising the accuracy of total energy-related properties. We specifically examine hybrid functionals. These depend on a parameter controlling the ratio of semi-local to non-local functional components. The ionization potential obtained with ensemble-generalized functionals is found to depend only weakly on the parameter value, contrary to common experience with non-generalized hybrids, thus eliminating one aspect of the so-called "parameter dilemma" of hybrid functionals.

Many novel applications in bioelectronics rely on the interaction between biomolecules and electronically conducting substrates. However, crucial knowledge about the relation between electronic transport via peptides and their amino-acid composition is still absent. Here, we report results of electronic transport measurements via several homopeptides as a function of their structural properties and temperature. We demonstrate that the conduction through the peptide depends on its length and secondary structure as well as on the nature of the constituent amino acid and charge of its residue. We support our experimental observations with high-level electronic structure calculations and suggest off-resonance tunneling as the dominant conduction mechanism via extended peptides. Our findings indicate that both peptide composition and structure can affect the efficiency of electronic transport across peptides.

We present a framework for obtaining reliable solid-state charge and optical excitations and spectra from optimally tuned range-separated hybrid density functional theory. The approach, which is fully couched within the formal framework of generalized Kohn-Sham theory, allows for the accurate prediction of exciton binding energies. We demonstrate our approach through first principles calculations of one- and two-particle excitations in pentacene, a molecular semiconducting crystal, where our work is in excellent agreement with experiments and prior computations. We further show that with one adjustable parameter, set to produce the known band gap, this method accurately predicts band structures and optical spectra of silicon and lithium fluoride, prototypical covalent and ionic solids. Our findings indicate that for a broad range of extended bulk systems, this method may provide a computationally inexpensive alternative to many-body perturbation theory, opening the door to studies of materials of increasing size and complexity.

A key quantity for molecule-metal interfaces is the energy level alignment of molecular electronic states with the metallic Fermi level. We develop and apply an efficient theoretical method, based on density functional theory (DFT) that can yield quantitatively accurate energy level alignment information for physisorbed metal-molecule interfaces. The method builds on the "DFT+Σ" approach, grounded in many-body perturbation theory, which introduces an approximate electron self-energy that corrects the level alignment obtained from conventional DFT for missing exchange and correlation effects associated with the gas-phase molecule and substrate polarization. Here, we extend the DFT+Σ approach in two important ways: first, we employ optimally tuned range-separated hybrid functionals to compute the gas-phase term, rather than rely on GW or total energy differences as in prior work; second, we use a nonclassical DFT-determined image-charge plane of the metallic surface to compute the substrate polarization term, rather than the classical DFT-derived image plane used previously. We validate this new approach by a detailed comparison with experimental and theoretical reference data for several prototypical molecule-metal interfaces, where excellent agreement with experiment is achieved: benzene on graphite (0001), and 1,4-benzenediamine, Cu-phthalocyanine, and 3,4,9,10-perylene-tetracarboxylic-dianhydride on Au(111). In particular, we show that the method correctly captures level alignment trends across chemical systems and that it retains its accuracy even for molecules for which conventional DFT suffers from severe self-interaction errors. (Figure Presented).

Theory and experiment are combined to investigate the nature of low-energy excitons within ordered domains of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) polycrystalline thin films. First-principles density functional theory and many-body perturbation theory calculations, along with polarization-dependent optical absorption spectro-microscopy on ordered domains, show multiple low-energy absorption peaks that are composed of excitonic states delocalized over several molecules. While the first absorption peak is composed of a single excitonic transition and retains the polarization-dependent behavior of the molecule, higher energy peaks are composed of multiple transitions with optical properties that can not be described by those of the molecule. The predicted structure-dependence of polarization-dependent absorption reveals the exact inter-grain orientation within the TIPS-PEN film. Additionally, the degree of exciton delocalization can be significantly tuned by modest changes in the solid-state structure and the spatial extent of the excitations along a given direction is correlated with the degree of electronic dispersion along the same direction. These findings pave the way for tailoring the singlet fission efficiency of organic crystals by solid-state structure.

Many approximations within density-functional theory spuriously predict that a many-electron system can dissociate into fractionally charged fragments. Here, we revisit the case of dissociated diatomic molecules, known to exhibit this problem when studied within standard approaches, including the local spin-density approximation (LSDA). By employing our recently proposed [E. Kraisler and L. Kronik, Phys. Rev. Lett. 110, 126403 (2013)PRLTAO0031-900710.1103/PhysRevLett.110.126403] ensemble generalization we find that asymptotic fractional dissociation is eliminated in all systems examined, even if the underlying exchange correlation (xc) is still the LSDA. Furthermore, as a result of the ensemble-generalization procedure, the Kohn-Sham potential develops a spatial step between the dissociated atoms, reflecting the emergence of the derivative discontinuity in the xc energy functional. This step, predicted in the past for the exact Kohn-Sham potential and observed in some of its more advanced approximate forms, is a desired feature that prevents any fractional charge transfer between the system's fragments. It is usually believed that simple xc approximations such as the LSDA cannot develop this step. Our findings show, however, that ensemble generalization to fractional electron densities automatically introduces the desired step even to the most simple approximate xc functionals and correctly predicts asymptotic integer dissociation.

The inclusion of the global effects of semiconductor doping poses a unique challenge for first-principles simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the space-charge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. Here, we present a multiscale technique that fully addresses these difficulties. It is based on the introduction of a charged sheet, mimicking the SCR-related field, along with free charge which mimics the bulk charge reservoir, such that the system is neutral overall. These augment a slab comprising "pseudoatoms" possessing a fractional nuclear charge matching the bulk doping concentration. Self-consistency is reached by imposing charge conservation and Fermi level equilibration between the bulk, treated semiclassically, and the electronic states of the slab, which are treated quantum-mechanically. The method, called CREST - the charge-reservoir electrostatic sheet technique - can be used with standard electronic structure codes. We validate CREST using a simple tight-binding model, which allows for comparison of its results with calculations encompassing the full SCR explicitly. Specifically, we show that CREST successfully predicts scenarios spanning the range from no to full Fermi level pinning. We then employ it with density functional theory, obtaining insight into the doping dependence of the electronic structures of the metallic "clean-cleaved" Si(111) surface and its semiconducting (2×1) reconstructions.

In exact density functional theory, the total ground-state energy is a series of linear segments between integer electron points, a condition known as "piecewise linearity." Deviation from this condition is indicative of poor predictive capabilities for electronic structure, in particular of ionization energies, fundamental gaps, and charge transfer. In this article, we take a new look at the deviation from linearity (i.e., curvature) in the solid-state limit by considering two different ways of approaching it: a large finite system of increasing size and a crystal represented by an increasingly large reference cell with periodic boundary conditions. We show that the curvature approaches vanishing values in both limits, even for functionals which yield poor predictions of electronic structure, and therefore cannot be used as a diagnostic or constructive tool in solids. We find that the approach towards zero curvature is different in each of the two limits, owing to the presence of a compensating background charge in the periodic case. Based on these findings, we present a new criterion for functional construction and evaluation, derived from the size-dependence of the curvature, along with a practical method for evaluating this criterion. For large finite systems, we further show that the curvature is dominated by the self-interaction of the highest occupied eigenstate. These findings are illustrated by computational studies of various solids, semiconductor nanocrystals, and long alkane chains.

Molecular crystals are ubiquitous in many areas of science and engineering, including biology and medicine. Until recently, our ability to understand and predict their structure and properties using density functional theory was severely limited by the lack of approximate exchange-correlation functionals able to achieve sufficient accuracy. Here we show that there are many cases where the simple, minimally empirical pairwise correction scheme of Tkatchenko and Scheffler provides a useful prediction of the structure and properties of molecular crystals.After a brief introduction of the approach, we demonstrate its strength through some examples taken from our recent work. First, we show the accuracy of the approach using benchmark data sets of molecular complexes. Then we show its efficacy for structural determination using the hemozoin crystal, a challenging system possessing a wide range of strong and weak binding scenarios. Next, we show that it is equally useful for response properties by considering the elastic constants exhibited by the supramolecular diphenylalanine peptide solid and the infrared signature of water libration movements in brushite. Throughout, we emphasize lessons learned not only for the methodology but also for the chemistry and physics of the crystals in question.We further show that in many other scenarios where the simple pairwise correction scheme is not sufficiently accurate, one can go beyond it by employing a computationally inexpensive many-body dispersive approach that results in useful, quantitative accuracy, even in the presence of significant screening and/or multibody contributions to the dispersive energy. We explain the principles of the many-body approach and demonstrate its accuracy for benchmark data sets of small and large molecular complexes and molecular solids.

A microscopic picture of structure and bonding in organic-inorganic perovskites is imperative to understanding their remarkable semiconducting and photovoltaic properties. On the basis of a density functional theory treatment that includes both spin-orbit coupling and dispersive interactions, we provide detailed insight into the crystal binding of lead-halide perovskites and quantify the effect of different types of interactions on the structural properties. Our analysis reveals that cohesion in these materials is characterized by a variety of interactions that includes important contributions from both van der Waals interactions among the halide atoms and hydrogen bonding. We also assess the role of spin-orbit coupling and show that it causes slight changes in lead-halide bonding that do not significantly affect the lattice parameters. Our results establish that consideration of dispersive effects is essential for understanding the structure and bonding in organic-inorganic perovskites in general and for providing reliable theoretical predictions of structural parameters in particular.

A self-consistent optimally tuned range-separated hybrid density functional (scOT-RSH) approach is developed. It can simultaneously predict accurate geometries, vibrational modes, and frontier orbital energies. This is achieved by optimizing the range-separation parameter, γ to both satisfy the ionization energy theorem and minimize interatomic forces. We benchmark our approach against an established hybrid functional, B3LYP, using the G2 test set. scOT-RSH greatly improves the accuracy of occupied frontier orbital energies, with a mean absolute error (MAE) of only 0.2 eV relative to experimental ionization energies compared to 2.96 eV with B3LYP. Geometries do not change significantly compared to those obtained from B3LYP, with a bond length MAE of 0.012 Å compared to 0.008 Å for B3LYP, and a 6.5% MAE for zero-point energies, slightly larger than that of B3LYP (3.1%). scOT-RSH represents a new paradigm in which accurate geometries and ionization energies can be predicted simultaneously from a single functional approach.

The energy positions of frontier orbitals in organic electronic materials are often studied experimentally by (inverse) photoemission spectroscopy and theoretically within density functional theory. However, standard exchange-correlation functionals often result in too small fundamental gaps, may lead to wrong orbital energy ordering, and do not capture polarization-induced gap renormalization. Here we examine these issues and a strategy for overcoming them by studying the gas phase and bulk electronic structure of the organic molecule quinacridone (5Q), a promising material with many interesting properties for organic devices. Experimentally we perform angle-resolved photoemission spectroscopy (ARUPS) on thin films of the crystalline β phase of 5Q. Theoretically we employ an optimally tuned range-separated hybrid functional (OT-RSH) within density functional theory. For the gas phase molecule, our OT-RSH result for the ionization potential (IP) represents a substantial improvement over the semilocal PBE and the PBE0 hybrid functional results, producing an IP in quantitative agreement with experiment. For the bulk crystal we take into account the correct screening in the bulk, using the recently developed optimally tuned screened range-separated hybrid (OT-SRSH) approach, while retaining the optimally tuned parameters for the range separation and the short-range Fock exchange. This leads to a band gap narrowing due to polarization effects and results in a valence band spectrum in excellent agreement with experimental ARUPS data, with respect to both peak positions and heights. Finally, full-frequency G0W0 results based on a hybrid functional starting point are shown to agree with the OT-SRSH approach, improving substantially on the PBE-starting point.

We propose a new method for simulating electron dynamics in open quantum systems out of equilibrium, using a finite atomistic model. The proposed method is motivated by the intuitive and practical nature of the driven Liouville-von-Neumann equation approach of Sánchez et al. [J. Chem. Phys. 2006, 124, 214708] and Subotnik et al. [J. Chem. Phys. 2009, 130, 144105]. A key ingredient of our approach is a transformation of the Hamiltonian matrix from an atomistic to a state representation of the molecular junction. This allows us to uniquely define the bias voltage across the system while maintaining a proper thermal electronic distribution within the finite lead models. Furthermore, it allows us to investigate complex molecular junctions, including multilead configurations. A heuristic derivation of our working equation leads to explicit expressions for the damping and driving terms, which serve as appropriate electron sources and sinks that effectively "open" the finite model system. Although the method does not forbid it, in practice we find neither violation of Pauli's exclusion principles nor deviation from density matrix positivity throughout our numerical simulations of various tight-binding model systems. We believe that the new approach offers a practical and physically sound route for performing atomistic time-dependent transport calculations in realistic molecular junction models.

We present a real-space method for first-principles nanoscale electronic transport calculations. We use the nonequilibrium Green's function method with density functional theory and implement absorbing boundary conditions (ABCs, also known as complex absorbing potentials, or CAPs) to represent the effects of the semi-infinite leads. In real space, the Kohn-Sham Hamiltonian matrix is highly sparse. As a result, the transport problem parallelizes naturally and can scale favorably with system size, enabling the computation of conductance in relatively large molecular junction models. Our use of ABCs circumvents the demanding task of explicitly calculating the leads' self-energies from surface Green's functions, and is expected to be more accurate than the use of the jellium approximation. In addition, we take advantage of the sparsity in real space to solve efficiently for the Green's function over the entire energy range relevant to low-bias transport. We illustrate the advantages of our method with calculations on several challenging test systems and find good agreement with reference calculation results.

One-electron self-interaction and an incorrect asymptotic behavior of the Kohn-Sham exchange-correlation potential are among the most prominent limitations of many present-day density functionals. However, a one-electron self-interaction-free energy does not necessarily lead to the correct long-range potential. This is shown here explicitly for local hybrid functionals. Furthermore, carefully studying the ratio of the von Weizsäcker kinetic energy density to the (positive) Kohn-Sham kinetic energy density, τ_W/τ, reveals that this ratio, which frequently serves as an iso-orbital indicator and is used to eliminate one-electron self-interaction effects in meta-generalized-gradient approximations and local hybrid functionals, can fail to approach its expected value in the vicinity of orbital nodal planes. This perspective article suggests that the nature and consequences of one-electron self-interaction and some of the strategies for its correction need to be reconsidered.

We investigate periodical oscillations in the conductance of suspended Au and Pt atomic chains during elongation under mechanical stress. Analysis of conductance and shot noise measurements reveals that the oscillations are mainly related to variations in a specific conduction channel as the chain undergoes transitions between zigzag and linear atomic configurations. The calculated local electronic structure shows that the oscillations originate from varying degrees of hybridization between the atomic orbitals along the chain as a function of the zigzag angle. These variations are highly dependent on the directionally and symmetry of the relevant orbitals, in agreement with the order-of-magnitude difference between the Pt and Au oscillation amplitudes observed in experiment. Our results demonstrate that the sensitivity of conductance to structural variations can be controlled by designing atomic-scale conductors in view of the directional interactions between atomic orbitals.

The fundamental gap is a central quantity in the electronic structure of matter. Unfortunately, the fundamental gap is not generally equal to the Kohn-Sham gap of density functional theory (DFT), even in principle. The two gaps differ precisely by the derivative discontinuity, namely, an abrupt change in slope of the exchange-correlation energy as a function of electron number, expected across an integer-electron point. Popular approximate functionals are thought to be devoid of a derivative discontinuity, strongly compromising their performance for prediction of spectroscopic properties. Here we show that, in fact, all exchange-correlation functionals possess a derivative discontinuity, which arises naturally from the application of ensemble considerations within DFT, without any empiricism. This derivative discontinuity can be expressed in closed form using only quantities obtained in the course of a standard DFT calculation of the neutral system. For small, finite systems, addition of this derivative discontinuity indeed results in a greatly improved prediction for the fundamental gap, even when based on the most simple approximate exchange-correlation density functional - the local density approximation (LDA). For solids, the same scheme is exact in principle, but when applied to LDA it results in a vanishing derivative discontinuity correction. This failure is shown to be directly related to the failure of LDA in predicting fundamental gaps from total energy differences in extended systems.

We present and test a new approximation for the exchange-correlation (xc) energy of Kohn-Sham density functional theory. It combines exact exchange with a compatible non-local correlation functional. The functional is by construction free of one-electron self-interaction, respects constraints derived from uniform coordinate scaling, and has the correct asymptotic behavior of the xc energy density. It contains one parameter that is not determined ab initio. We investigate whether it is possible to construct a functional that yields accurate binding energies and affords other advantages, specifically Kohn-Sham eigenvalues that reliably reflect ionization potentials. Tests for a set of atoms and small molecules show that within our local-hybrid form accurate binding energies can be achieved by proper optimization of the free parameter in our functional, along with an improvement in dissociation energy curves and in Kohn-Sham eigenvalues. However, the correspondence of the latter to experimental ionization potentials is not yet satisfactory, and if we choose to optimize their prediction, a rather different value of the functional's parameter is obtained. We put this finding in a larger context by discussing similar observations for other functionals and possible directions for further functional development that our findings suggest.

Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. Phys. Rev. Lett. 2012, 109, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1-0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception found-a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

Brushite, CaHPO₄·2H₂O, is a crystalline hydrated acidic form of calcium phosphate that occurs in both physiological and pathological biomineralization processes. Additionally, it is biocompatible in humans. Several groups have investigated the experimental Fourier transform infrared vibrational spectrum of brushite. These investigations have led to a long-standing debate concerning the correct assignment for a few of the observed frequencies, particularly, the water stretching and libration modes. Here, we perform a comprehensive first principles theoretical investigation of the vibrational spectrum of brushite with calculations based on dispersion-corrected density functional theory. We obtain both the vibrational frequencies, using the frozen phonon approach, and their corresponding peak intensities, based on the Born effective charge tensor. This allows for an unambiguous assignment of all vibrations, including water vibration and libration modes.

A new interlayer force-field for layered hexagonal boron nitride (h-BN) based structures is presented. The force-field contains three terms representing the interlayer attraction due to dispersive interactions, repulsion due to anisotropic overlaps of electron clouds, and monopolar electrostatic interactions. With appropriate parameterization, the potential is able to simultaneously capture well the binding and lateral sliding energies of planar h-BN based dimer systems as well as the interlayer telescoping and rotation of double walled boron-nitride nanotubes of different crystallographic orientations. The new potential thus allows for the accurate and efficient modeling and simulation of large-scale h-BN based layered structures.

Vibrational modes of molecules are fundamental properties determined by intramolecular bonding, atomic masses, and molecular geometry, and often serve as important channels for dissipation in nanoscale processes. Although single-molecule junctions have been used to manipulate electronic structure and related functional properties of molecules, electrical control of vibrational mode energies has remained elusive. Here we use simultaneous transport and surface-enhanced Raman spectroscopy measurements to demonstrate large, reversible, voltage-driven shifts of vibrational mode energies of C60 molecules in gold junctions. C60 mode energies are found to vary approximately quadratically with bias, but in a manner inconsistent with a simple vibrational Stark effect. Our theoretical model instead suggests that the mode shifts are a signature of bias-driven addition of electronic charge to the molecule. These results imply that voltage-controlled tuning of vibrational modes is a general phenomenon at metal-molecule interfaces and is a means of achieving significant shifts in vibrational energies relative to a pure Stark effect.

The diphenylalanine peptide self-assembles to form nanotubular structures of remarkable mechanical, piezolelectrical, electrical, and optical properties. The tubes are unexpectedly stiff, with reported Young's moduli of 19-27 GPa that were extracted using two independent techniques. Yet the physical basis for the remarkable rigidity is not fully understood. Here, we calculate the Young's modulus for bulk diphenylalanine peptide from first principles, using density functional theory with dispersive corrections. The calculation demonstrates that at least half of the stiffness of the material is the result of dispersive interactions. We further quantify the nature of various inter- and intramolecular interactions. We reveal that despite the porous nature of the lattice, there is an array of rigid nanotube backbones with interpenetrating "zipper-like" aromatic interlocks that result in stiffness and robustness. This presents a general strategy for the analysis of bioinspired functional materials and may pave the way for rational design of bionanomaterials.

We present a tutorial overview of the simulation of gas-phase valenceelectron photoemission spectra using density functional theory (DFT), emphasizing both fundamental considerations and practical applications, and making appropriate links between the two. We explain how an elementary quantum mechanics view of photoemission couples naturally to a many-body perturbation theory view. We discuss a rigorous approach to photoemission within the framework of time-dependent DFT. Then we focus our attention on ground-state DFT. We clarify the extent to which it can be used to mimic many-body perturbation theory in principle, and then provide a detailed discussion of the accuracy one can and cannot expect in practice with various approximate DFT forms.

We present a fully numerical multigrid approach for solving the all-electron Kohn-Sham equation in molecules. The equation is represented on a hierarchy of Cartesian grids, from coarse ones that span the entire molecule to very fine ones that describe only a small volume around each atom. This approach is adaptable to any type of geometry. We demonstrate it for a variety of small molecules and obtain high accuracy agreement with results obtained previously for diatomic molecules using a prolate-spheroidal grid. We provide a detailed presentation of the numerical methodology and discuss possible extensions of this approach.

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S_N and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si-C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

Fundamental gap renormalization due to electronic polarization is a basic phenomenon in molecular crystals. Despite its ubiquity and importance, all conventional approaches within density-functional theory completely fail to capture it, even qualitatively. Here, we present a new screened range-separated hybrid functional, which, through judicious introduction of the scalar dielectric constant, quantitatively captures polarization-induced gap renormalization, as demonstrated on the prototypical organic molecular crystals of benzene, pentacene, and C⁶⁰. This functional is predictive, as it contains system-specific adjustable parameters that are determined from first principles, rather than from empirical considerations.

We propose a nonempirical, pair-wise or many-body dispersion-corrected, optimally tuned range-separated hybrid functional. This functional retains the advantages of the optimal-tuning approach in the prediction of the electronic structure. At the same time, it gains accuracy in the prediction of binding energies for dispersively bound systems, as demonstrated on the S22 and S66 benchmark sets of weakly bound dimers.

The nature of low energy optical excitations, or excitons, in organic solids is of central relevance to many optoelectronic applications, including solar energy conversion. Excitons in solid pentacene, a prototypical organic semiconductor, have been the subject of many experimental and theoretical studies, with differing conclusions as to the degree of their charge-transfer character. Using first-principles calculations based on density functional theory and many-body perturbation theory, we compute the average electron-hole distance and quantify the degree of charge-transfer character within optical excitations in solid-state pentacene. We show that several low-energy singlet excitations are characterized by a weak overlap between electron and hole and an average electron-hole distance greater than 6 Å. Additionally, we show that the character of the lowest-lying singlet and triplet excitons is well-described with a simple analytic envelope function of the electron-hole distance.

Semiconductor doping is a process of fundamental importance to semiconductor physics and solid-state electronics, but cannot be explicitly simulated from first principles due to the huge system size needed for most doping scenarios. We examine the efficacy of the simulation of doping in silicon by the inclusion of "pseudoatoms" with fractional nuclear charge, introduced via specially constructed pseudopotentials. These provide a net charge carrier concentration matching an arbitrarily chosen doping level, at no increase of the computational cost. By extending this approach to consider minute deviations from the integer charge, we demonstrate that the electron Fermi level can be set to any value within the forbidden gap, at minimal perturbation of the electronic structure. Beyond the bulk scenario, we successfully simulate the development of the space-charge region in a heavily doped p-n junction and the doping dependence of the work function of the hydrogen-passivated (semiconducting) Si(111) surface.

Using a break junction technique, we find a clear signature for the formation of conducting hybrid junctions composed of a single organic molecule (benzene, naphthalene, or anthracene) connected to chains of platinum atoms. The hybrid junctions exhibit metallic-like conductance (∼0.1-1G₀), which is rather insensitive to further elongation by additional atoms. At low bias voltage the hybrid junctions can be elongated significantly beyond the length of the bare atomic chains. Ab initio calculations reveal that benzene based hybrid junctions have a significant binding energy and high structural flexibility that may contribute to the survival of the hybrid junction during the elongation process. The fabrication of hybrid junctions opens the way for combining the different properties of atomic chains and organic molecules to realize a new class of atomic scale interfaces.

Optimally tuned range separated hybrid functionals are a new class of implicitly defined functionals. Their important new aspect is that the range separation parameter in these functionals is determined individually for each system by iteratively tuning it until a fundamental, non-empirical condition is fulfilled. Such functionals have been demonstrated to be extremely successful in predicting electronic excitations. In this paper, we explore the use of the tuning approach for predicting ground state properties. This sheds light on one of its downsides - the violation of size consistency. By analyzing diatomic molecules, we reveal size consistency errors up to several electron volts and find that binding energies cannot be predicted reliably. Further consequences of the consistent ground-state use of the tuning approach are potential energy surfaces that are qualitatively in error and an incorrect prediction of spin states. We discuss these failures, their origins, and possibilities for overcoming them.

In the exact Kohn-Sham density-functional theory, the total energy versus the number of electrons is a series of linear segments between integer points. However, commonly used approximate density functionals produce total energies that do not exhibit this piecewise-linear behavior. As a result, the ionization potential theorem, equating the highest occupied eigenvalue with the ionization potential, is grossly disobeyed. Here, we show that, contrary to conventional wisdom, most of the required piecewise linearity of an arbitrary approximate density functional can be restored by careful consideration of the ensemble generalization of density-functional theory. Furthermore, the resulting formulation introduces the desired derivative discontinuity to any approximate exchange-correlation functional, even one that is explicitly density dependent. This opens the door to calculations of the ionization potential and electron affinity, even without explicit electron removal or addition. All these advances are achieved while neither introducing empiricism nor changing the underlying functional form. The power of the approach is demonstrated on benchmark systems using the local density approximation as an illustrative example.

The electronic structure of cobalt-phthalocyanine (CoPc) molecules adsorbed on Ag(100) is investigated by photoemission spectroscopy. The results are compared to first-principles electronic structure calculations, based on many-body perturbation theory in the GW approximation. The photoemission data, obtained from both multilayer and monolayer films of CoPc, show that charge transfer occurs between the first molecular layer and the metal surface. Varying the photon energy, to tune the photoionization cross sections, reveals that the charge-transfer-related interface states mainly involve the Co 3d atomic orbitals of the Co central atom. GW calculations for the neutral CoPc molecule and its anion compare well with the experimental observations for a multilayer and a monolayer CoPc film, respectively. They confirm the major role played by the Co atom in the charge-transfer process and elucidate the complex energy rearrangement of the molecular electronic levels upon metal adsorption.

Perdew et al. discovered two different properties of exact Kohn-Sham density functional theory (DFT): (i) The exact total energy versus particle number is a series of linear segments between integer electron points. (ii) Across an integer number of electrons, the exchange-correlation potential "jumps" by a constant, known as the derivative discontinuity (DD). Here we show analytically that in both the original and the generalized Kohn-Sham formulation of DFT the two properties are two sides of the same coin. The absence of a DD dictates deviation from piecewise linearity, but the latter, appearing as curvature, can be used to correct for the former, thereby restoring the physical meaning of orbital energies. A simple correction scheme for any semilocal and hybrid functional, even Hartree-Fock theory, is shown to be effective on a set of small molecules, suggesting a practical correction for the infamous DFT gap problem. We show that optimally tuned range-separated hybrid functionals can inherently minimize both DD and curvature, thus requiring no correction, and that this can be used as a sound theoretical basis for novel tuning strategies.

In conjugated organic molecules, excitation gaps typically decrease reciprocally with increasing the number of repeat units, n. This usually holds for individual molecules as well as for the corresponding bulk materials. Here, we show using density-functional theory calculations that a qualitatively different evolution is found for layers built from molecules consisting of polar repeat units. Whereas a 1/n-dependence is still observed in the case of isolated polar molecules, the global gap decreases essentially linearly with n in the corresponding 2D-periodic systems and vanishes beyond a certain molecular length, with the frontier states being localized at opposite ends of the layer. The latter is accompanied by a saturation of the dipole moment per molecule, an effect not observed in the isolated polar molecules. Interestingly, in both cases the limit of the gap for long (but finite) molecules differs qualitatively from that of infinite length obtained in 1D-periodic and 3D-periodic calculations, the latter serving as models for polymers and the bulk. We rationalize these dimensionality effects as a consequence of the potential gradient within the finite-length layers. They arise from the collective action of intra-molecular dipoles in the 2D periodic layers and can be traced back to surface effects.

We present a method for obtaining outer-valence quasiparticle excitation energies from a density-functional-theory-based calculation, with an accuracy that is comparable to that of many-body perturbation theory within the GW approximation. The approach uses a range-separated hybrid density functional, with an asymptotically exact and short-range fractional Fock exchange. The functional contains two parameters, the range separation and the short-range Fock fraction. Both are determined nonempirically, per system, on the basis of the satisfaction of exact physical constraints for the ionization potential and frontier-orbital many-electron self-interaction, respectively. The accuracy of the method is demonstrated on four important benchmark organic molecules: perylene, pentacene, 3,4,9,10-perylene-tetracarboxylic-dianydride (PTCDA), and 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA). We envision that for the outer-valence excitation spectra of finite systems the approach could provide an inexpensive alternative to GW, opening the door to the study of presently out of reach large-scale systems.

Free-standing ZnO nanocrystals simultaneously doped with Fe and Cu with varying Fe/Cu compositions have been synthesized using colloidal methods with a mean size of ∼7.7 nm. Interestingly, while the Cu-doped ZnO nanocrystal remains diamagnetic and Fe-doped samples show antiferromagnetic interactions between Fe sites without any magnetic ordering down to the lowest temperature investigated, samples doped simultaneously with Fe and Cu show a qualitative departure in exhibiting ferromagnetic interactions, with suggestions of ferromagnetic order at low temperature. XAS measurements establish the presence of Fe ²⁺ and Fe ³⁺ ions, with the concentration of the trivalent species increasing in the presence of Cu doping, providing direct evidence of the Fe ²⁺ + Cu ²⁺ ⇌ Fe ³⁺ + Cu ⁺ redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagnetic nature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreement with experimental observations.

Since the first report of Si-C bound organic monolayers on oxide-free Si almost two decades ago, a substantial amount of research has focused on studying the fundamental mechanical and electronic properties of these Si/molecule surfaces and interfaces. This feature article covers three closely related topics, including recent advances in achieving high-density organic monolayers (i.e., atomic coverage >55%) on oxide-free Si(111) substrates, an overview of progress in the fundamental understanding of the energetics and electronic properties of hybrid Si/molecule systems, and a brief summary of recent examples of subsequent functionalization on these high-density monolayers, which can significantly expand the range of applicability. Taken together, these topics provide an overview of the present status of this active area of research.

Excitation gaps are of considerable significance in electronic structure theory. Two different gaps are of particular interest. The fundamental gap is defined by charged excitations, as the difference between the first ionization potential and the first electron affinity. The optical gap is defined by a neutral excitation, as the difference between the energies of the lowest dipole-allowed excited state and the ground state. Within many-body perturbation theory, the fundamental gap is the difference between the corresponding lowest quasi-hole and quasi-electron excitation energies, and the optical gap is addressed by including the interaction between a quasi-electron and a quasi-hole. A long-standing challenge has been the attainment of a similar description within density functional theory (DFT), with much debate on whether this is an achievable goal even in principle. Recently, we have constructed and applied a new approach to this problem. Anchored in the rigorous theoretical framework of the generalized Kohn-Sham equation, our method is based on a range-split hybrid functional that uses exact long-range exchange. Its main novel feature is that the range-splitting parameter is not a universal constant but rather is determined from first principles, per system, based on satisfaction of the ionization potential theorem. For finite-sized objects, this DFT approach mimics successfully, to the best of our knowledge for the first time, the quasi-particle picture of many-body theory. Specifically, it allows for the extraction of both the fundamental and the optical gap from one underlying functional, based on the HOMO-LUMO gap of a ground-state DFT calculation and the lowest excitation energy of a linear-response time-dependent DFT calculation, respectively. In particular, it produces the correct optical gap for the difficult case of charge-transfer and charge-transfer-like scenarios, where conventional functionals are known to fail. In this perspective, we overview the formal and practical challenges associated with gap calculations, explain our new approach and how it overcomes previous difficulties, and survey its application to a variety of systems.

The broad use of organic semiconductors for optoelectronic applications relies on quantitative understanding and control of their spectroscopic properties. Of paramount importance are the transport gap-the difference between ionization potential and electron affinity-and the exciton binding energy-inferred from the difference between the transport and optical absorption gaps. Transport gaps are commonly established via photoemission and inverse photoemission spectroscopy (PES/IPES). However, PES and IPES are surface-sensitive, average over a dynamic lattice, and are subject to extrinsic effects, leading to significant uncertainty in gaps. Here, we use density functional theory and many-body perturbation theory to calculate the spectroscopic properties of two prototypical organic semiconductors, pentacene, and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), quantitatively comparing with measured PES, IPES, and optical absorption spectra. For bulk pentacene and PTCDA, the computed transport gaps are 2.4 and 3.0 eV, and optical gaps are 1.7 and 2.1 eV, respectively. Computed bulk quasiparticle spectra are in excellent agreement with surface-sensitive photoemission measurements over several eV only if the measured gap is reduced by 0.6 eV for pentacene and 0.6-0.9 eV for PTCDA. We attribute this redshift to several physical effects, including incomplete charge screening at the surface, static and dynamical disorder, and experimental resolution. Optical gaps are in excellent agreement with experiment with solid-state exciton binding energies of ∼0.5 eV for both systems; for pentacene the exciton is delocalized over several molecules and exhibits significant charge transfer character. Our parameter-free calculations provide new interpretation of spectroscopic properties of organic semiconductors critical to optoelectronics.

We compare the charge transport characteristics of heavy-doped p ⁺⁺- and n ⁺⁺-Si-alkyl chain/Hg junctions. Based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction we suggest that for both p ⁺⁺- and n ⁺⁺-type junctions, the energy difference between the Fermi level and lowest unoccupied molecular orbital (LUMO), i.e., electron tunneling, controls charge transport. This conclusion is supported by results from photoelectron spectroscopy (ultraviolet photoemission spectroscopy, inverse photoelectron spectroscopy, and x-ray photoemission spectroscopy) for the molecule-Si band alignment at equilibrium, which clearly indicate that the energy difference between the Fermi level and the LUMO is much smaller than that between the Fermi level and the highest occupied molecular orbital (HOMO). Furthermore, the experimentally determined Fermi level - LUMO energy difference, agrees with the non-resonant tunneling barrier height, deduced from the exponential length attenuation of the current.

Self-assembled organic monolayers serve for modifying the work function of inorganic substrates. We examine the role of the molecular backbone in determining monolayer-adsorbed work function, by considering the adsorption of dithiols with either a partially conjugated or a saturated backbone on the GaAs(001) surface. Using a combination of chemically resolved electrical measurements based on X-ray photo-electron spectroscopy and contact potential difference, together with first principles electronic structure calculations, we are able to distinguish quantitatively between the contributions of the band bending and surface dipole components. We find that the substrates coated by partially conjugated layers possess a larger band-bending, relative to that of the substrates coated by saturated layers. This is associated with an increased density of surface states, likely related to the presence of oxygen. At the same time, the samples coated by partially conjugated layers also possess a larger bond-dipole, with the difference found to result primarily from an extended charge rearrangement on the molecular backbone. The two effects are, in this case, of opposite sign, but a significant net change in work function is still found. Thus, design of the molecular backbone emerges as an additional and important degree of freedom in the design of potential profiles and charge injection barriers in monolayer-based structures and devices.

We present a comparative assessment of the accuracy of two different approaches for evaluating dispersion interactions: interatomic pairwise corrections and semiempirical meta-generalized-gradient-approximation (meta-GGA)-based functionals. This is achieved by employing conventional (semi)local and (screened-)hybrid functionals, as well as semiempirical hybrid and nonhybrid meta-GGA functionals of the M06 family, with and without interatomic pairwise Tkatchenko-Scheffler corrections. All of those are tested against the benchmark S22 set of weakly bound systems, a representative larger molecular complex (dimer of NiPc molecules), and a representative dispersively bound solid (hexagonal boron nitride). For the S22 database, we also compare our results with those obtained from the pairwise correction of Grimme (DFT-D3) and nonlocal Langreth-Lundqvist functionals (vdW-DF1 and vdW-DF2). We find that the semiempirical kinetic-energy-density dependence introduced in the M06 functionals mimics some of the nonlocal correlation needed to describe dispersion. However, long-range contributions are still missing. Pair-wise interatomic corrections, applied to conventional semilocal or hybrid functionals, or to M06 functionals, provide for a satisfactory level of accuracy irrespectively of the underlying functional. Specifically, screened-hybrid functionals such as the Heyd-Scuseria-Ernzerhof (HSE) approach reduce self-interaction errors in systems possessing both localized and delocalized orbitals and can be applied to both finite and extended systems. Therefore, they serve as a useful underlying functional for dispersion corrections.

We present all-electron G₀W₀ calculations for the electronic structure of the organic semiconductor copper phthalocyanine, based on semilocal and hybrid density-functional theory (DFT) starting points. We show that G₀W₀ calculations improve the quantitative agreement with high resolution photoemission and inverse photoemission experiments. However, the extent of the improvement provided by G₀W₀ depends significantly on the choice of the underlying DFT functional, with the hybrid functional serving as a much better starting point than the semilocal one. In particular, strong starting-point dependence is observed in the energy positions of highly localized molecular orbitals. This is attributed to self-interaction errors (SIE), due to which the orbitals obtained from semilocal DFT do not approximate the quasi-particle (QP) orbitals as well as those obtained from hybrid DFT. Our findings establish the viability of the G ₀W₀ approach for describing the electronic structure of metal-organic systems, given a judiciously chosen DFT-based starting point.

The electronic structure of terpyrimidinethiols is investigated by means of density-functional theory calculations for isolated molecules and monolayers. In the transition from molecule to selfassembled monolayer (SAM), we observe that the band gap is substantially reduced, frontier states increasingly localize on opposite sides of the SAM, and this polarization in several instances is in the direction opposite to the polarization of the overall charge density. This behavior can be analyzed by analogy to inorganic semiconductor quantum-wells, which, as the SAMs studied here, can be regarded as semiperiodic systems. There, similar observations are made under the influence of a, typically external, electric field and are known as the quantum-confined Stark effect. Without any external perturbation, in oligopyrimidine SAMs one encounters an energy gradient that is generated by the dipole moments of the pyrimidine repeat units. It is particularly strong, reaching values of about 1.6 eV/nm, which corresponds to a substantial electric field of 1.6 × 10 ⁷ V/cm. Close-lying σ-and π-states turn out to be a particular complication for a reliable description of the present systems, as their order is influenced not only by the docking groups and bonding to the metal, but also by the chosen computational approach. In the latter context we demonstrate that deliberately picking a hybrid functional allows avoiding pitfalls due to the infamous self-interaction error. Our results show that when aiming to build a monolayer with a specific electronic structure one can not only resort to the traditional technique of modifying the molecular structure of the constituents, but also try to exploit collective electronic effects.

Hybrid functionals often exhibit a marked improvement over semi-local functionals in the description of the electronic structure of organic materials. Because short-range hybrid functionals, notably the Heyd-Scuseria-Ernzerhof (HSE) functional, can also describe the electronic structure of metals reasonably well, it is interesting to examine to which extent they can correctly describe the electronic structure at metal-organic interfaces. Here, we address this question by comparing HSE calculations with many-body perturbation theory calculations in the GW approximation, or with experimental photoemission data, for two prototypical systems: benzene on graphite and benzene diamine on gold. For both cases, we find that while HSE yields results that are somewhat closer to experiment than those of semi-local functionals, the HSE prediction is still lacking quantitatively by ∼1 eV. We show that this quantitative failure arises because HSE does not correctly capture the fundamental gap of the organic or its renormalization by the metal. These discrepancies are traced back to missing long-range exchange and correlation components, an explanation which applies to any conventional or short-range hybrid functional.

We address the conundrum posed by the well-known failure of time-dependent DFT (TDDFT) with conventional functionals for "charge-transfer-like" excitations in oligoacenes. We show that this failure is due to a small spatial overlap in orbitals obtained from the underlying single-electron orbitals by means of a unitary transformation. We further show that, as in true charge-transfer excitations, this necessarily results in failure of linear-response TDDFT with standard functionals. Range-separated hybrid functionals have been previously shown to mitigate such errors but at the cost of an empirically adjusted range-separation parameter. Here, we explain why this approach should succeed where conventional functionals fail. Furthermore, we show that optimal tuning of a range-separated hybrid functional, so as to enforce the DFT version of Koopmans' theorem, restores the predictive power of TDDFT even for such difficult cases, without any external reference data and without any adjustable parameters. We demonstrate the success of this approach on the oligoacene series and on related hydrocarbons. This resolves a long-standing question in TDDFT and extends the scope of molecules and systems to which TDDFT can be applied in a predictive manner.

Malaria, an infectious disease once considered eradicated, has reemerged in recent years, primarily due to parasite resistance to commonly used synthetic antimalarial drugs. These drugs act by inhibiting crystallization of the malaria pigment, hemozoin (HZ). Thus, there is a vital need for understanding the process of HZ nucleation. In a companion paper, the pseudopolymorphic behavior of β-hematin, the synthetic form of HZ, has been characterized by X-ray diffraction (XRD) (Straasø, T.; Kapishnikov, S.; Kato, K.; Takata, M.; Als-Nielsen, J.; Leiserowitz, L.Cryst. Growth Des. 2011, 11, DOI: 10.1021/cg200410b). Here, we employ van der Waals (vdW)-corrected density functional theory (DFT) to study the two β-hematin crystal structures and their repeat unit, a heme dimer. We find that vdW interactions play a major role in the binding of the heme dimer and the β-hematin crystal. In addition, accounting for the periodic nature of the system is essential to obtaining the correct geometry of the heme dimer, which is affected by vdW interactions with adjacent dimers in the β-hematin crystal. The different stereoisomers of the heme dimer and their molecular crystals are close in energy, which is consistent with pseudopolymorphism in β-hematin, in agreement with recent XRD experiments. Finally, we use our results to comment on β-hematin crystallization mechanisms. This work demonstrates the viability of vdW-corrected DFT as a tool for gaining valuable insight into pertinent problems involving biological systems.

The fundamental and optical gaps of relevant molecular systems are of primary importance for organic-based photovoltaics. Unfortunately, whereas optical gaps are accessible with time-dependent density functional theory (DFT), the highest-occupied - lowest-unoccupied eigenvalue gaps resulting from DFT calculations with semi-local or hybrid functionals routinely and severely underestimate the fundamental gaps of gas-phase organic molecules. Here, we show that a range-separated hybrid functional, optimally tuned so as to obey Koopmans' theorem, provides fundamental gaps that are very close to benchmark results obtained from many-body perturbation theory in the GW approximation. We then show that using this functional does not compromise the possibility of obtaining reliable optical gaps from time-dependent DFT. We therefore suggest optimally tuned range-separated hybrid functionals as a practical and accurate tool for DFT-based predictions of photovoltaically relevant and other molecular systems.

We examine the role of the exact-exchange (EXX) Kohn-Sham potential in curing the problem of fractional molecular dissociation. This is achieved by performing EXX calculations for the illustrative case of the LiF molecule. We show that by choosing the lowest-energy electronic configuration for each interatomic distance, a qualitatively correct binding energy curve, reflecting integer dissociation, is obtained. Surprisingly, for LiF this comes at the cost of violating the Aufbau principle, a phenomenon we discuss at length. Furthermore, we numerically confirm that in the EXX potential of the diatomic molecule, one of the atomic potentials is shifted by a constant while the other one is not, depending on where the highest occupied molecular orbital is localized. This changes the relative positions of the energies of each atom and enforces the integer configuration by preventing spurious charge transfer. The size of the constant shift becomes increasingly unstable numerically the larger the interatomic separation is, reflecting the increasing absence of coupling between the atoms.

Infrared spectral peak broadening due to atomic disorder and narrowing due to particle-size-related optical absorption effects can be decoupled experimentally and theoretically. Applied to different sources of polycrystalline calcite, the method provides a powerful diagnostic tool for archaeology, geology, and materials/biomaterials science.

We present a broadly applicable, physically motivated, first-principles approach to determining the fundamental gap of finite systems from single-electron orbital energies. The approach is based on using a range-separated hybrid functional within the generalized Kohn-Sham approach to density functional theory. Its key element is the choice of a range-separation parameter such that Koopmans' theorem for both neutral and anion is obeyed as closely as possible. We demonstrate the validity, accuracy, and advantages of this approach on first, second and third row atoms, the oligoacene family of molecules, and a set of hydrogen-passivated silicon nanocrystals. This extends the quantitative usage of density functional theory to an area long believed to be outside its reach.

The size-dependence of the polarizability, susceptibility, and dielectric constant of nanometer-scale molecular layers is explored theoretically. First-principles calculations based on density functional theory are compared to phenomenological modeling based on polarizable dipolar arrays for a model system of organized monolayers composed of oligophenyl chains. Size trends for all three quantities are primarily governed by a competition between out-of-plane polarization enhancement and in-plane polarization suppression. Molecular packing density is the single most important factor controlling this competition and it strongly affects the bulk limit of the dielectric constant as well as the rate at which it is approached. Finally, the polarization does not reach its "bulk" limit, as determined from the Clausius-Mossotti model, but the susceptibility and dielectric constant do converge to the correct bulk limit. However, whereas the Clausius-Mossotti model describes the dielectric constant well at low lateral densities, finite size effects of the monomer units cause it to be increasingly inaccurate at high lateral densities.

The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of ∼0.6eV are obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.

Metal-organic molecule-semiconductor junctions are controlled not only by the molecular properties, as in metal-organic molecule-metal junctions, but also by effects of the molecular dipole, the dipolar molecule-semiconductor link, and molecule-semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current-voltage, capacitance-voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance.

Organic-based interfaces can possess a range of surpising electronic properties that are of intense interest from both the basic science and the applied research points of view. In this issue of MRS Bulletin, we provided state-of-the-art overviews of selected topics involving three complementary aspects of the electronic properties of organicbased interfaces: the nascent electronics technologies that would gain from improved understanding and control of such interfaces: the novel properties that organic-based interfaces may possess: and the experimental and theoretical challenges afforded by such studies.

Noncovalent interactions, of which London dispersion is an important special case, are essential to many fields of chemistry. However, treatment of London dispersion is inherently outside the reach of (semi)local approximations to the exchange-correlation functional as well as of conventional hybrid density functionals based on semilocal correlation. Here, we offer an approach that provides a treatment of both dispersive interactions and the electronic structure within a computationally tractable scheme. The approach is based on adding the leading interatomic London dispersion term via pairwise ion-ion interactions to a suitably chosen nonempirical hybrid functional, with the dispersion coefficients and van der Waals radii determined from first-principles using the recently proposed "TS-vdW" scheme (Tkatchenko, A.; Scheffler, M. Phys. Rev. Lett. 2009, 102, 073005). This is demonstrated via the important special case of weakly bound metal-phthalocyanine dimers. The performance of our approach is additionally compared to that of the semiempirical M06 functional. We find that both the PBE-hybrid+vdW functional and the M06 functional predict the electronic structure and the equilibrium geometry well, but with significant differences in the binding energy and in their asymptotic behavior.

Intermolecular interactions can affect the conduction properties of molecular junctions in several ways: Direct and through-substrate electronic interactions affect the spectral properties (density of states) of the conducting junction, intermolecular electrostatic interactions affect the positioning of molecular electronic energies and thereby the nature of interface polarization. Such interactions also influence the screening properties of the junction and consequently the electrostatic potential profile across the biased junction. Other consequences include effects on junction mechanical properties that can be manifested by a different temperature dependence of conduction for a single molecule and for a molecular layer, as well as effects on optical response that may be important, for example, for the junction response associated with light induced switching. This article discusses some of these effects and their implications for the performance of molecular junctions.

We examine the current response of molecularly controlled semiconductor devices to the presence of weakly interacting analytes. We evaluate the response of two types of devices, a silicon oxide coated silicon device and a GaAs/AlGaAs device, both coated with aliphatic chains and exposed to the same set of analytes. By comparing the device electrical response with contact potential difference and surface photovoltage measurements, we show that there are two mechanisms that may affect the underlying substrate, namely, formation of layers with a net dipolar moment and molecular interaction with surface states. We find that whereas the Si device response is mostly correlated to the analyte dipole, the GaAs device response is mostly correlated to interactions with surface states. Existence of a silicon oxide layer, whether native on the Si or deliberately grown on the GaAs, eliminates analyte interaction with the surface states.

Spin injection efficiency is shown to strongly depend on the interfacial structure between Fe contacts and Al_xGa_1-xAs in spin-based light emitting diodes. Both the magnitude and sign of the injected carriers are dependent on the atomic structure of the contacts and can be controlled through changes in temperature both during and following growth. We propose that the observed dependence is due to phase formation resulting from Fe/GaAs interfacial reactions. This proposed mechanism is consistent with electronic structure calculations, which show that thin layers of DO₃ Fe₃Ga at the Fe/GaAs interface can produce the observed sign reversals in the spin polarization of injected carriers.

We present exact-exchange calculations of the Kohn-Sham gap, as well as the fundamental gap resulting from it, using highly accurate grid-based all-electron and pseudopotential approaches for prototypical diatomic molecules. Results obtained with pseudopotentials that have been constructed in a manner consistent with the exact-exchange functional agree with the all electron results for the cases studied. This confirms the reliability of the pseudopotential approximation for orbital-dependent functionals such as exact exchange.

Our recent calculation of the effect of intermolecular interactions on molecular conduction (J. Comput. Theor. Nanosci. 2008, 5, 535) is generalized to molecules adsorbed on a model semiconductor surface and in a metal-molecule-semiconductor junction. The metal and semiconductor electrodes are represented by cubic lattices within generic tight binding models, where the semiconductor two-band structure is described by using a simple site-alteration property. A physically motivated choice of parameters for the molecule(s) and the electrodes completes the model definition. The model encompasses direct intermolecular interactions as well as through-metal interactions and can be solved exactly to yield spectral properties (surface density of states) and transport characteristics (transmission coefficients and current-voltage behavior) for single-molecule junctions and molecular layers. The model is applied to analyzing the effect of intermolecular interactions on the predicted negative differential resistance in metal-molecule-semiconductor junctions (recently observed in scanning tunneling microscopy studies of adsorbates on Si(100)).

We present an approach for fully numerical, all-electron solutions of the optimized effective potential equation within Kohn-Sham density functional theory for diatomic molecules. The approach is based on a real-space, prolate-spheroidal coordinate grid for solving the allelectron Kohn-Sham equations and an iterative scheme for solving the optimized effective potential equation. The accuracy of this method is demonstrated by comparison with previously reported calculations. New fully numerical benchmark results for selected diatomic molecules are provided.

The combination of photoelectron spectroscopy and density functional theory is an important technique for clarifying a material's electronic structure. So far, however, it has been difficult to predict when the spectrum of occupied Kohn-Sham eigenvalues obtained from commonly used (semi-)local functionals bears physical relevance and when not. We demonstrate that a simple criterion based on evaluating each orbital's self-interaction allows prediction of the physical reliability of the eigenvalue spectrum. We further show that a self-interaction correction significantly improves the interpretability of eigenvalues also in difficult cases such as organic semiconductors where (semi-)local functionals fail.

We present a two-part systematic density functional theory study of the electronic structure of selected transition metal phthalocyanines. We use a semi-local generalized gradient approximation (GGA) functional, as well as several hybrid exchange-correlation functionals, and compare the results to experimental photoemission data. Here, we study the low-spin systems NiPc and CoPc. We show that hybrid functionals provide computed photoemission spectra in excellent agreement with experimental data, whereas the GGA functional fails qualitatively. This failure is primarily because of under-binding of localized orbitals due to self-interaction errors.

We present a two-part systematic density functional theory (DFT) study of the electronic structure of selected transition metal phthalocyanines. We use a semi-local generalized gradient approximation (GGA) functional, as well as several hybrid exchange-correlation functionals, and compare the results to experimental photoemission data. Here, we study the intermediate spin systems MnPc and FePc. We show that DFT calculations of these systems are extremely sensitive to the choice of functional and basis set with respect to the obtained electronic configuration and to symmetry breaking. Interestingly, all simulated spectra are in good agreement with experiment despite the differences in the underlying electronic configurations.

(Figure Presented) We show how charge transfer excitations at molecular complexes can be calculated quantitatively using time-dependent density functional theory. Predictive power is obtained from range-separated hybrid functionals using nonempirical tuning of the range-splitting parameter. Excellent performance of this approach is obtained for a series of complexes composed of various aromatic donors and the tetracyanoethylene acceptor, paving the way to systematic nonempirical quantitative studies of charge-transfer excitations in real systems.

We present a real-space pseudopotential method for first principles calculations of noncollinear magnetic phenomena within density functional theory. We demonstrate the validity of the method using the test cases of the Cr₃ cluster and the Cr (sqrt(3) × sqrt(3)) R 3 0^{{ring operator}} monolayer. The approach retains all the typical benefits of the real-space approach, notably massive parallelization. It can be employed with arbitrary boundary conditions and can be combined with the computation of pseudopotential-based spin-orbit coupling effects.

We study the description of charge-transfer excitations in a series of coumarin-based donor-bridge-acceptor dyes. We show that excellent predictive power for the excitation energies and oscillator strengths in these systems is obtained by using a range-separated hybrid functional within the generalized Kohn-Sham approach to time-dependent density functional theory. Key to this success is a step for tuning the range separation parameter from first principles. We explore different methods for this tuning step, which are variants of a recently suggested approach for charge-transfer excitations [T. Stein, J. Am. Chem. Soc. 131, 2818 (2009)]. We assess the quality of prediction by comparing to excitation energies previously published for the same systems using the approximate coupled-cluster singles and doubles (CC2) method.

Traditional curriculum for teaching bonding often fosters the use of over-simplifications and over-generalizations. Therefore, there is a need for a presentation that is consistent with current scientific knowledge and that provides the student with the proper intellectual infrastructure for further studies. In this article, we present a general framework for bonding that can be presented at different levels of sophistication depending on the student's level and needs. This is achieved without sacrificing the benefits of traditional qualification of different bond types as this qualification is presented along a continuum scale of chemical bonding. The pedagogical strategy for teaching this model is a "bottom-up" one, starting with basic principles and ending with specific properties. It is our hope that its use could remove learning impediments and enhance students understanding of the nature of chemical bonds.

We present a real-space method for electronic-structure calculations of systems with general full or partial periodicity. The method is based on the self-consistent solution of the Kohn-Sham equations, using first principles pseudopotentials, on a uniform three-dimensional non-Cartesian grid. Its efficacy derives from the introduction of a new generalized high-order finite-difference method that avoids the numerical evaluation of mixed derivative terms and results in a simple yet accurate finite difference operator. Our method is further extended to systems where periodicity is enforced only along some directions (e.g., surfaces), by setting up the correct electrostatic boundary conditions and by properly accounting for the ion-electron and ion-ion interactions. Our method enjoys the main advantages of real-space grid techniques over traditional plane-wave representations for density functional calculations, namely, improved scaling and easier implementation on parallel computers, as well as inherent immunity to spurious interactions brought about by artificial periodicity. We demonstrate its capabilities on bulk GaAs and Na for the fully periodic case and on a monolayer of Si-adsorbed polar nitrobenzene molecules for the partially periodic case.

Hydrogen-terminated and alkyl-chain (C₁₈H₃₇)- terminated Si(100) surfaces with different doping levels have been characterized using Kelvin probe force microscopy. n- and p-doped Si(100) and lateral p ⁺⁺n and n⁺⁺p silicon junctions were hydrogenated in dilute HF solution, followed with a self-assembly deposition of organic molecules by thermally activated free-radical reaction between C=C and Si - H. The surface band bending following the two different chemical treatments was almost identical for both p-type silicon (∼0.7 eV, with a surface charge of 9.4 ± 0.5 × 10¹¹cm²) and n-type silicon (0.6 eV, with a surface charge of 8.7 ± 0.5 × 10¹¹/cm ²). These results indicate that the self-assembly of the C ₁₈.H₃₇ monolayer on a Si (100) surface results in electrical properties similar to those of a hydrogenated Si surface, with the advantage of longer stability in an ambient environment. The hydrogen-terminated and alkyl-chain-terminated surface do differ, however, in the surface dipole, which is lower by ∼0.6 eV for the latter, a value deduced from both the measurements and independent first principles electronic structure calculations. This dipole change is essentially due to the change in bond dipole associated with the replacement of Si - H bonds by Si - C bonds and the dipole associated with the methyl group.

Investigations of charge transport mechanisms in thin CdSe nanoparticles films deposited on silicon, using surface photovoltage and chemically resolved electrical measurements, reveal a strongly nonlinear optical response and a negative differential resistance. Here, both phenomena are rationalized within a phenomenological model consisting of two spatially separated types of traps, one related to the CdSe nanoparticles and the other to the nanoparticle/ substrate interface. The model successfully explains a broad range of both old and new experimental observations and is used as a numerical framework for showing how the interplay between hole and electron processes dominates the photoelectrical properties of nanoparticle films.

Current experimental and theoretical studies on the effect of intermolecular interactions on molecular conduction appear to be in conflict with each other. In particular, some experimental results, e.g., the observation of 2-dimensional free-particle character for interface bound electrons indicate strong intermolecular interactions while other observations indicate an additive character of conduction properties. In this paper we use a generic tight binding model with a physically motivated choice of parameters in order to examine this issue. The model encompasses direct intermolecular interactions as well as through-metal interactions and can be solved exactly to yield spectral properties (surface density of states) and transport characteristics (transmission coefficients and current-voltage behaviors) for single molecule junctions, molecular islands and molecular layers. We find linear scaling of conduction properties with the number of conducting molecules in junctions characterized by molecular layers when the probe (STM tip) addresses different numbers of molecules; however, the conduction per molecule can differ significantly from that of an isolated single molecule. When a junction involves finite molecular islands of varying sizes, linear scaling sets in only beyond a certain molecular island size, of the order of a few tens of molecules. Implications for current observation of linear scaling behaviors are discussed.

This review provides a perspective on the use of orbital-dependent functionals, which is currently considered one of the most promising avenues in modern density-functional theory. The focus here is on four major themes: the motivation for orbital-dependent functionals in terms of limitations of semilocal functionals; the optimized effective potential as a rigorous approach to incorporating orbital-dependent functionals within the Kohn-Sham framework; the rationale behind and advantages and limitations of four popular classes of orbital-dependent functionals; and the use of orbital-dependent functionals for predicting excited-state properties. For each of these issues, both formal and practical aspects are assessed.

We present and experimentally verify a concept for electronic devices based on nanoscale vacuum phototubes. Such devices are expected to be both reliable and amenable to large-scale integration. We further suggest several generalizations of the concept and discuss possible applications and advantages.

We present a systematic density functional theory study of the electronic structure of copper phthalocyanine (CuPc) using several different (semi)local and hybrid functionals and compare the results to experimental photoemission data. We show that semilocal functionals fail qualitatively for CuPc primarily because of underbinding of localized orbitals due to self-interaction errors. We discuss an appropriate choice of functional for studies of CuPc/metal interfaces and suggest the Heyd-Scuseria-Ernzerhof screened hybrid functional as a suitable compromise functional.

Molecules in (or as) electronic devices are attractive because the variety and flexibility inherent in organic chemistry can be harnessed towards a systematic design of electrical properties. Specifically, monolayers of polar molecules introduce a net dipole, which controls surface and interface barriers and enables chemical sensing via dipole modification. Due to the long range of electrostatic phenomena, polar monolayer properties are determined not only by the type of molecules and/or bonding configuration to the substrate, but also by size, (dis-)order, and adsorption patterns within the monolayer. Thus, a comprehensive understanding of polar monolayer characteristics and their influence on electronic devices requires an approach that transcends typical chemical designs, i.e., one that incorporates long-range effects, in addition to short-range effects due to local chemistry. We review and explain the main uses of polar organic monolayers in shaping electronic device properties, with an emphasis on long-range cooperative effects and on the differences between electrical properties of uniform and non-uniform monolayers.

We present structural and optical properties of silver clusters Ag _n (n=2, 4, 6, 8) at two model support sites of MgO, stoichiometric MgO(100) and F_S-center defect, based on density functional theory and embedded cluster model. Our results provide the mechanism responsible for the absorption and emission patterns due to the specific interaction between the excitations within the cluster and the support site which is strongly cluster size and structure dependent. We propose Ag₄ at stoichiometric site as well as Ag₂, Ag₄ and Ag₆ at F _S-center defects as good candidates for the emissive centers in the visible regime.

We introduce a frequency-domain formalism for computing excited-state forces using linear response theory within the framework of time-dependent density-functional theory. We present an implementation of the formalism within a real-space computational framework. We demonstrate the validity and usefulness of the approach by comparing its results to those obtained with a Green's function approach using the GW+Bethe-Salpeter-equation method, for the simple case of the CO molecule. We expect the method to be advantageous for computing excited-state dynamics in large systems.

We present a formalism and implementation for real-space calculations that incorporate relativistic effects, including spin-orbit coupling. We demonstrate the validity of the method using the test cases of AuH and the Au2 dimer. The proposed approach differs from nonrelativistic real-space calculations in the addition of nonlocal pseudopotential projectors, which are translated to small rank-1 matrix "stencils" operating on the discretized wave functions. This formalism retains all the usual benefits of the real-space approach, especially with respect to massive parallelization. We expect it to be readily applicable for computational studies of large systems exhibiting spin-orbit coupling effects.

Site-specific x-ray photoelectron spectroscopy together with density functional theory calculations based on the local density approximation have identified the chemical bonding, single-particle matrix element, and many-body effects in the x-ray photoelectron spectrum of corundum V2 O3. Significant covalent bonding in both the upper and lower lobes of the photoelectron spectrum is found, despite the localized nature of the V 3d electrons that are responsible for the Mott behavior. We show that the approximate treatment of correlation dominates the discrepancy between theory and experiment in the near-Fermi-edge region and that many-body effects of the photoemission process can be modeled by Doniach-Šunjić [J. Phys. C 3, 285 (1970)] asymmetric loss. Correlation effects govern the relative intensity and energy position of the higher level electron bands, and many-body effects dominate the "tail" region of both the upper and lower lobes of the photoemission spectrum.

A quantitative comparison of first principles calculations with extended X-ray absorption fine structure and transmission electron microscopy measurements provides strong evidence that Au₁₃ nanocrystals are stabilized in a slightly distorted icosahedral structure by on-top phosphine ligands and a combination of on-top and bridging thiol ligands. Importantly, the ligands change the icosahedral strain (i.e., the radial-tangential bond length ratio) significantly, with the tangential bonds within the Au core exhibiting much more disorder than the radial ones.

We examine the applicability of density functional theory (DFT) to the polarizability of C_n^- (n = 3-9) cluster anions. This was achieved by comparing DFT calculations using two different exchange-correlation functionals (the non-empirical local density approximation, LDA, and the semiempirical hybrid functional B97-1) to quantum chemical calculations using the coupled cluster method in the CCSD(T) "gold standard" approximation. We find that, unless the extra electron is not bound at all by DFT, both LDA and B97-1 agree with the CCSD(T) calculation to within 5-10%, allowing for a meaningful qualitative and semiquantitative analysis. Furthermore, the polarizability is found to increase monotonically with chain size, consistent with the trend inferred from electron detachment experiments.

We compare the electrostatic behavior of a single polar molecule adsorbed on a solid substrate with that of an adsorbed polar monolayer. This is accomplished by comparing first principles calculations obtained within a cluster model and a periodic slab model, using benzene derivatives on the Si(111) surface as a representative test case. We find that the two models offer diametrically opposite descriptions of the surface electrostatic phenomena. Slab electrostatics is dominated by dipole reduction due to intermolecular dipole-dipole interactions that partially depolarize the molecules, with charge migration to the substrate playing a negligible role due to electric field suppression outside the monolayer. Conversely, cluster electrostatics is dominated by dipole enhancement due to charge migration to/from the substrate, with only a small polarization of the molecule. This establishes the important role played by long-range interactions, in addition to local chemical properties, in tailoring surface chemistry via polar molecule adsorption.

Shining light on local order: The relations between local crystalline order and peak intensities in the infrared spectra of calcite are explained in terms of different sensitivities to Ca-O and O⋯O distances (see picture) by comparing ab initio phonon spectra for ideal and distorted calcite unit cells with experimental spectra of various biogenic and geological calcites. (Figure Presented).

Monolayers of alkyl chains, attached-through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO₂, are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation- or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) π* resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.

We discuss methodological aspects of first principles calculations of surface dipoles and potentials in general, and surface-adsorbed self-assembled monolayers in particular, using density functional theory with a slab/super-cell approach. We show that calculations involving asymmetric slabs may yield highly erroneous results for the surface dipole and demonstrated the efficacy of a simple dipole correction scheme. We explain the importance of the electrostatic dipole distribution, show how to compute it, and establish conditions for the equivalence of calculations for the dipole distribution and the electrostatic potential distribution.

We present ab initio density functional theory calculations for the electronic structure of chalcopyrite Mn-IV-V₂, with IV = (Si,Ge) and V = (N,P,As), in their ferromagnetic phase. We find that the P and As containing compounds (some of which been recently synthesized) are normal ferromagnetic metals and are therefore of limited use for spintronic applications. The N containing compounds, however, are predicted to be semiconducting, with a valence band that has a wide energy range with 100% spin polarization. As such, they emerge as interesting candidate materials for spin-polarized transport.

We describe the formalism, as well as numerical and implementation issues behind PARSEC - the pseudopotential algorithm for real-space electronic structure calculations. Its current capabilities are illustrated via application of PARSEC to numerous problems in nanoscience.

First principles calculations can provide important insight into the mechanisms of non-linear molecular response. Chain molecules are of particular interest due to their large and directional response, but calculating their non-linear response coefficients is a computational challenge. We have developed a real-space approach to evaluate the linear polarizability and first and second hyperpolarizability of molecular chains within Kohn-Sham density functional theory. Different approaches to minimize uncertainties in particular in the second hyperpolarizability γ are presented. We tested our scheme by calculating the response of model hydrogen chains using local and non-local density functionals. Differences between explicit density functionals and orbital functionals are discussed.

We present gas phase ultraviolet photoemission spectra of 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) and compare them to condensed phase spectra. Contrary to common expectations for organic solids, we observe that the two differ in many details, necessitating a reexamination of our theoretical understanding of the PTCDA electronic structure. Thus, we use density functional theory with several exchange-correlation functionals, as well as the GW method. We find that the local density approximation and the generalized gradient approximation fail to describe the experimental data because of underbound orbitals that are concentrated on the anhydride groups. We show that this is corrected by either a hybrid functional or the GW method and use the results to interpret the experimental data.

We elucidate the electronic structure of both filled and empty states of ordered alkyl chains bound to the Si(111) surface by combining direct and inverse photoemission spectroscopy with first principles calculations based on density functional theory. We identify both filled and empty interface-induced gap states, distinguish between those and states extending throughout the monolayer, and discuss the importance of these findings for interpreting transport experiments through such monolayers.

Dipole formation processes at self-assembled monolayers of benzene derivatives chemisorbed on the Si(111) surface are investigated from first principles. The surface dipole is shown to be sensitive to the molecular coverage and dominated by intramolecular charge rearrangement due to long-range, cooperative behavior. This cooperative behavior suppresses substrate contributions to dipole formation.

The electronic structure and magnetic properties of Mn-doped Ge, GaAs, and ZnSe nanocrystals are investigated using real space ab initio pseudopotentials constructed within the local spin-density approximation. The ferromagnetic and half-metallicity trends found in the bulk are preserved in the nanocrystals. However, the Mn-related impurity states become much deeper in energy with decreasing nanocrystalline size, causing the ferromagnetic stabilization to be dominated by double exchange via localized holes rather than by a Zener-like mechanism.

Ab initia absorption spectra and optical gaps for hydrogen-passivated Ge nanocrystals are calculated using time-dependent density functional theory within the adiabatic local density approximation. The results are compared to previous effective mass, tight-binding, empirical pseudopotential, and "Δ self-consistent field" calculations and shed light on the validity of the various approximations used. By comparing our results with calculations for hydrogen-passivated Si nanocrystals, we predict that the Ge optical gap is smaller than that of Si for any nanocrystal size.

Surface photovoltage spectroscopy (SPS) is a well-established contactless technique for semiconductor characterization, which relies on analyzing illumination-induced changes in the surface voltage. SPS traces its origins to the pioneering work of Brattain and Bardeen in the early 1950s, and was extended into a powerful spectroscopic tool by Gatos, Lagowski and Balestra in the early 1970s. It has been used as an extensive source of surface and bulk information on various semiconductors and semiconductor interfaces. In the following, we present the basic theory behind SPS, its experimental setup, and a range of its applications.

We examine the utility of photoelectron spectroscopy (PES) as a structural probe of Sin- in the n=20-26 size range by determining isomers and associated photoelectron spectra from first principles calculations. Across the entire size range, we consistently obtain a good agreement between the theory and experiment [Hoffmann, Eur. Phys. J. D 16, 9 (2001)]. We find that PES can almost invariably distinguish between structurally distinct isomers at a given cluster size, but that structurally similar isomers usually cannot be reliably distinguished by PES. For many, but not all, sizes the isomer giving the best match to experiment is the lowest-energy one found theoretically. Thus, combining theory with PES experiments emerges as a useful source of structural information even for intermediate size clusters.

The use of Kohn-Sham density-functional theory to yield the electrical response of molecular chains of hydrogen was analyzed. The Kohn-Sham density-functional theory was also used to calculate the linear and nonlinear response of hydrogen chains. It was found that the field-counteracting term which appear in the exact exchange potential, had the same origin as the derivative discontinuity. The results show that Kohn-Sham density functional theory (DFT) could be used to systems which were traditionally too difficult for DFT.

The ab initio pseudopotential-density-functional calculations for the electronic structure of the dilute magnetic semiconductor Mn _xGa _1-xP, with a realistic x=0.063, in its ordered ferromagnetic phase, were analyzed. It was observed that it possesses a spin-polarized valence band that could support ideal spin-polarized hole transport. The calculated Mn _xGa _1-xP density of states (DOS) was shown, with respect to the electronic structure. Spin-polarized features in the conduction band that could support ideal spin-polarized transport of minority electrons were found.

We present a real-space method for electronic-structure calculations of periodic systems. Our method is based on the self-consistent solution of the Kohn-Sham equations on a uniform three-dimensional grid. A higher-order finite-difference method is combined with ab initio pseudopotentials. The kinetic energy operator, the nonlocal term of the ionic pseudopotential, and the Hartree and exchange-correlation potentials are set up directly on the real-space grid. The local contribution to the ionic pseudopotential is initially obtained in reciprocal space and is then transferred to the real-space grid by Fourier transform. Our method enjoys the main advantages of real-space grid techniques over traditional plane-wave representations for density-functional calculations, i.e., improved scaling and easier implementation on parallel computers. We illustrate the method by application to liquid silicon.

We present a massively parallel implementation of time-dependent density functional theory in real space, aimed at computing optical absorption spectra of realistic systems with hundreds of atoms from first principles. We provide details of the formalism and discuss its implementation, optimization, and efficient parallelization, as well as remaining limitations, in detail. The capabilities of the code are illustrated by calculations of optical properties of hydrogenated silicon quantum dots.

This chapter describes the use of real Space pseudo potentials for the electronic structure problem. The pseudopotential model of condensed matter is one of the most promising developments within the of area computational materials science. It has led the way in providing a workable science framework for describing the properties of materials, while modern computers have provided the computational resources to implement the pseudopotential method. The pseudopotential concept treats matter as a sea of valence electrons moving in a background of ion cores. The cores are composed of nuclei and inert inner electrons. Within this model many of the complexities of an all-electron calculation are avoided. For example, a group IV element such as C with 6 electrons is treated in a similar fashion to Ge with 32 electrons as both elements have 4 valence electrons. The chapter illustrates that, as the pseudopotential binds only valence electron states, the resulting potential is weak and the Coulombic 1/r singularity at the nucleus is removed. Without the pseudopotential approximation, real space methods would be considerably more difficult to implement, if not impossible. Grids for the full potential must be spatially adapted to account for the rapid changes in the potential at the nuclear positions.

In this review, we will describe calculations using time-dependent density-functional theory (DFT) combined with pseudopotentials to determine excited state properties of matter. While a computational framework for ground state properties of condensed matter is well established, calculations for excited state properties are at a more formative stage. Time-dependent DFT represents an important advance by providing an explicit treatment of relevant correlation effects for electronic excitations. As such, it offers an ab initio formalism for excited states that avoids many of the drawbacks associated with empirical or semi-empirical methods. We will illustrate applications of time-dependent DFT to a variety of systems ranging from molecules and atomic clusters to quantum dots, which contain several hundred atoms.

Using the TDLDA technique, calculations were performed for the optical absorption spectra and optical gaps of Cd_nSe_n quantum dots (n=17,26,38). A bare dot was considered, where the surface atoms were fixed to replicate the bulk geometry. The calculated optical gaps for the dots show good agreement with experimental results.

We compare experimentally measured and ab initio computed photoelectron spectra of negatively charged deuterated silicon clusters (Si_mD _n^-, 4 ≤ m ≤ 10, 0 ≤ n ≤ 2) produced in a plasma environment. Based on this comparison, we discuss the kinetics and thermodynamics of the cluster formation and the effect of deuterium on the geometrical and electronic structure of the clusters.

The optical constants epsilon(E)=epsilon(1)(E)+iepsilon(2)(E) of unintentionally doped In0.53Ga0.47As lattice matched to InP have been measured at 300 K using spectral ellipsometry in the range of 0.4 to 5.1 eV. The epsilon(E) spectra displayed distinct structures associated with critical points at E-0 (direct gap), spin-orbit split E-0+Delta(0) component, spin-orbit split E-1, E-1+Delta(1), E-0(') feature, as well as E-2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Holden , Phys. Rev. B 56, 4037 (1997)], plus a Kramers-Kronig consistent correction, described in this work, that improves the fitting at low energies. This extended model is based on the electronic energy-band structure near these critical points plus excitonic and band-to-band Coulomb-enhancement effects at E-0, E-0+Delta(0), and the E-1, E-1+Delta(1), doublet. In addition to evaluating the energies of these various band-to-band critical points, information about the binding energy (R-1) of the two-dimensional exciton related to the E-1, E-1+Delta(1) critical points was obtained. The value of R-1 was in good agreement with effective mass/k.p theory. The ability to evaluate R-1 has important ramifications for first-principles band-structure calculations that include exciton effects at E-0, E-1, and E-2 [M. Rohlfing and S. G. Louie, Phys. Rev. Lett. 81, 2312 (1998); S. Albrecht , Phys. Rev. Lett. 80, 4510 (1998)]. (C) 2002 American Institute of Physics.

Small clusters have a range of unique physical and chemical phenomena that are strongly size dependent. However, analysis of these phenomena often assumes that thermodynamic equilibrium conditions prevail. We compare experimentally measured and ab initio computed photoelectron spectra of bare and deuterated silicon cluster anions produced in a plasma environment. We find that the isomers detected experimentally are usually not the ground-state isomers, but metastable ones, which indicates that cluster relaxation is strongly limited kinetically by a dwell time that is much shorter than the relaxation time. We show that, under these conditions, the highest electron affinity replaces the traditional lowest total energy as the appropriate criterion for predicting isomer structures. These findings demonstrate that a stringent examination of non-equilibrium effects can be crucial for a correct analysis of cluster properties.

We have determined the Ti and O components of the rutile TiO2 valence band using the method of site-specific x-ray photoelectron spectroscopy. Comparisons with calculations based on pseudopotentials within the local density approximation reveal the hybridization of the Ti 3d, 4s, and 4p states, and the O 2s and 2p states on each site. These chemical effects are observed due to the large differences between the angular-momentum dependent matrix elements of the photoelectron process.

We present ab initio pseudopotential-density-functional calculations for the electronic structure of the dilute magnetic semiconductor MnxGa1-xN, with a realistic x=0.063, in its ordered ferromagnetic phase. We find that the introduction of Mn results in the formation of a 100% spin polarized similar to1.5 eV-wide impurity band, primarily due to hybridization of Mn 3d and N 2p orbitals. This band renders the material half metallic and supports effective-mass transport within it. As such, MnxGa1-xN is a highly suitable material for spin injectors. Coupled with the previously reported high Curie temperature and inherent compatibility with GaN technology of this material, it emerges as a serious candidate for the next generation of spintronic devices.

Ab initio absorption spectra for CdnSen clusters (1less than or equal tonless than or equal to8) are calculated using an adiabatic time-dependent density functional formalism within the local density approximation (LDA). We find that the calculated spectra differ significantly from those computed using a simple LDA approach. In particular, they correct for the well-known underestimate of the absorption gap in time-independent density functional theory. The calculated spectra exhibit a variety of features that can be used for comparison against future experimental investigations. In addition, we find a correlation between the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, the optical gap, and the cluster binding energy.

We present ab initio density-functional calculations for the electronic structure of the dilute magnetic semiconductors MnxGa1-xAs and MnxIn1-xAs with a realistic x = 0.063. We find that the introduction of Mn perturbs the position of the nearest As atoms, but does not break the tetrahedral symmetry. Neither material is found to be strictly half metallic. However, in both materials the Mn content results in a majority-spin valence-band maximum that is similar to0.5 eV above the minority-spin valence-band maximum. This large valence-band split is primarily due to the hybridization of As 4p and Mn 3d orbitals. It results in a significant energy range where holes have a well-defined spin. The effective masses of holes in this range are found to be comparable to those of GaAs and InAs. Hence, in an ideal, disorder-free situation, spin-polarized transport may be explained by conventional transport in the context of a simple band picture. This leads to a theoretical limit of 100% spin injection from these materials. Attaining this limit in a sufficiently ordered material also requires a careful "engineering" of the Fermi-level position and a sufficiently low temperature.

The possibility of obtaining a detailed picture of the electronic structure makes surface photovoltage spectroscopy (SPS) eminently suitable for bridging the gap between the chemical, physical, optical and electrical properties of semiconductors. In SPS, changes in band bending (both at the free semiconductor surface and at buried interfaces) are monitored as a function of external illumination. Surface photovoltage spectroscopy can provide detailed, quantitative information on bulk properties (e.g. bandgap and type, carrier diffusion length and lifetime) and can be used for complete construction of surface and interface band diagrams, including the measurement of energy levels in quantum structures. A particular strength is that a comprehensive analysis of surface and bulk defect state distributions and properties is made possible. Measurements using SPS are contactless and non-destructive. In addition, they can be performed both in situ and ex situ, at any reasonable temperature, on any semiconducting material, at any ambient and at any lateral resolution down to the atomic scale. This review starts with an overview of SPS-related surface and interface theory, describes the SPS experimental set-up and presents applications for surface and interface characterization of a wide variety of materials and structures, cross-correlating them with other methodologies. Copyright (C) 2001 John Wiley & Sons, Ltd.

We present quantitative ab initio calculations for Na cluster structures and polarizabilities, for all cluster sizes up to 20 atoms. Our calculations are performed by combining an ab initio core-corrected pseudopotential and a gradient-corrected density functional within a real space approach. We find the cluster bonding to be very floppy and catalog a host of low-energy quasi-degenerate isomers for all second-decade clusters. The existence of these isomers results in a band of polarizability values for each cluster size even at zero temperature. This eliminates any finer structure in the polarizability curve. We further show that the experimental polarizability values are consistently underestimated by calculations at zero temperature. By computing the effects of structure expansion and distortion due to a finite temperature we arrive at a quantitative agreement between theory and experiment. (C) 2001 American Institute of Physics.

We have evaluated the Burstein-Moss (BM) shift at 300 K in seven samples of n-In_0.53Ga_0.47As (1.3 x 10¹⁶≤n≤3.9x 10¹⁹ cm^-3) lattice matched to InP using spectral ellipsometry in the range of 0.4-5.1 eV. The data have been fitted over the entire spectral range to a model reported by Holden et al. [in Thermphotovoltaic Generation of Electricity, edited by T. J. Coutts, J. P. Brenner, and C. S. Allman, AIP Conf. Proc. No. 460 (AIP, Woodbury, NY, 1999), p. 39], based on the electronic energy-band structure near critical points plus relevant discrete and continuum excitonic effects. A Fermi-level filling factor in the region of the fundamental gap has been used to account for the BM effect. While our data exhibit nonparabolic effects, with a blueshift of 415 meV for the most highly doped sample, we did not observe the Fermi-level saturation at 130 meV for n ≥ 10¹⁹ cm^-3 reported by Tsukernik et al. [Proceedings of the 24th International Conference on the Physics of Semiconductors, Jerusalem, 1998, edited by D. Gershoni (World Scientific, Singapore, 1999)]. Our BM displacements are in agreement with a modified full-potential linearized augmented- plane-wave calculation [G. W. Charache et al., J. Appl. Phys. 86, 452 (1999)] plus possible band- gap-reduction effects.

We introduce an experimental method by which sire-specific valence-electronic structure may be obtained. It utilizes the spatial dependence of the electric-field intensity that results from the superposition of the incident and reflected x-ray beams within the vicinity of a crystal x-ray Brag:: reflection. Resolution of the anion and cation contributions to the GaAs valence band is demonstrated and compared to an ab initio theoretical calculation of the Ga and As partial density of states.

We present a rigorous, nb initio theoretical calculation of the dependence of Na cluster polarizability on cluster size, up to 20 atoms, obtained by combining ab initio pseudopotentials with a gradient-corrected density functional. Using molecular dynamics, we find that for clusters as small as nine atoms, a multitude of degenerate isomers exists even at T=0. By calculating the polarizability of these isomers, we reproduce the generally decreasing nature of the measured polarizability curve, as well as its dips at "magic" numbers corresponding to closed electronic shells. Moreover, we find that the effect of a finite temperature on the cluster structure suffices to account for most of the quantitative discrepancy between theory and experiment.

A correlation between Fermi level pinning and yellow luminescence in Pt/n-GaN junctions has been studied using Schottky barrier measurements by internal photoemission spectroscopy and complementary deep level spectroscopies. The results show that illumination by photons with energies in the yellow luminescence range causes an unpinning of the interface Fermi level, accompanied by a significant increase of the Schottky barrier height from similar to 1 to similar to 1.9 eV. This strongly suggests the presence of acceptor states related to the yellow luminescence at the Pt/GaN interface. These states are charged in equilibrium and pin the interface Fermi level but can be optically discharged, resulting in a nearly unpinned interface. (C) 2000 American Institute of Physics. [S0003-6951(00)02033-7].

An exponential dependence of the photoconductivity on the surface photovoltage at GaN layers is predicted theoretically and confirmed experimentally. The prediction is based on the assumption that the material is mainly an ordered polycrystal, consisting of columnar grains. Accordingly, transport is expected to be limited by potential barriers at the grain boundaries, arising from the charge trapped at grain-boundary defects. The observed exponential dependence provides evidence that strongly supports the model by establishing a direct link between the bulk conductivity and the surface potential barrier. The same model is shown to successfully explain several other defect-related findings as well.

Stability aspects of the Mo/Cu(In,Ga)Se₂/CdS/ZnO solar cell are reviewed and assessed. These include (i) the chemical stability of the various interfaces present in the device, (ii) the long-term behavior of metastable defects found in the Cu(In,Ga)Se₂ (CIGS) compound, and (iii) the impact of Cu migration on device performance and lifetime. We find that (i) all interfaces within the structure are chemically stable, (ii) metastable defects have a beneficial effect on performance, and (iii) Cu migration effects are reversible and their possible detrimental effects are eclipsed by the beneficial effect of the metastable states. Moreover, Cu out-diffusion from the CIGS layer is absent in photovoltaic-quality CIGS. Finally, we propose a model that explains the exceptional radiation hardness and impurity tolerance of CIGS-based devices, based on the synergetic effect of copper migration and point defect reactions.

The chemical effects of oxygenation of Cu(In,Ga)Se₂ (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the `good' oxygenation effect from the `bad' ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of Cd_Cu interface donors and possibly a removal of O_Se interface acceptors. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells.

Surface passivation due to the interaction of a given molecule with n- and p-GaAs surfaces is explained well by a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interaction between the frontier orbitals of the molecules and the semiconductor surface states. The observed electronic changes depend on the nature of the molecules, on the one hand, and on that of the surface states, on the other. Considering semiconductor surface passivation as a frontier orbital interaction mechanism is expected to lead to its quantitative understanding, and use of such a model for designing molecular treatments of electronic materials provides a new tool for fine-tuning semiconductor device structures.

We present 'design rules' for the selection of molecules to achieve electronic control over semiconductor surfaces, using a simple molecular orbital model. The performance of most electronic devices depends critically on their surface electronic properties, i.e., surface band-bending and surface recombination velocity. For semiconductors, these properties depend on the density and energy distribution of surface states. The model is based on a surface state-molecule, HOMO-LUMO-like interaction between molecule and semiconductor. We test it by using a combination of contact potential difference, surface photovoltage spectroscopy, and time- and intensity- resolved photoluminescence measurements. With these, we characterize the interaction of two types of bifunctional dicarboxylic acids, the frontier orbital energy levels of which can be changed systematically, with air- exposed CdTe, CdSe, InP, and GaAs surfaces. The molecules are chemisorbed as monolayers onto the semiconductors. This model explains the widely varying electronic consequences of such interaction and shows them to be determined by the surface state energy position and the strength of the molecule-surface state coupling. The present findings can thus be used as guidelines for molecule-aided surface engineering of semiconductors.

Surface photovoltage spectroscopy (SPS) is introduced as a powerful tool for band gap determination of semiconductor powders. The main advantage of SPS is that scattering and reflection do not interfere with the spectra. Therefore, it does not suffer from the inherent limitations of transmission/reflection based spectroscopies, most notably diffuse reflectance spectroscopy (DRS). The principles of the approach are presented and its usefulness is demonstrated by comparing it with DRS for band gap determination of GaAs, InP, CdTe, CdSe, and CdS semiconductor powders.

We present a simple, compact, and robust arrangement for surface photovoltage measurements of free semiconductor surfaces immersed in liquids. It is based on the classical Kelvin probe arrangement, where the semiconductor sample is put in a liquid-containing, electrically insulating vessel, with an optically transparent window, situated between the sample and the Kelvin probe. At the price of permitting relative, rather than absolute, contact potential difference values, this modification enables easy, routine surface photovoltage measurements of semiconductors in any kind of liquid ambient. The validity and efficiency of this approach are demonstrated by surface photovoltage spectra obtained from the p-InP(100) surface in various liquid etchants.

The proven remarkable stability and radiation hardness of Cu(In,Ga)Se₂ (CIGS) solar cells stand in contradiction to the fact that CIGS shows both short-range and long-range instabilities. This work suggests that these instabilities may in fact be a prerequisite for CIGS's stability as they allow a degree of flexibility or smartness in accommodating externally imposed changes. Two self-healing cycles are proposed, in which copper species play an important role.

Photoelectron spectroscopy and admittance spectroscopy were used to analyze Cu(In,Ga)Se₂ (CIGS) based thin films and heterojunction solar cells based on chemical oxygenation and post-deposition air-annealing effects. The effect of CIGS surface chemistry on the electronic structure of the heterojunction solar cells and the influence on the CIGS layer of oxygenation-induced Cu redistribution were analyzed. Results showed that charge redistribution and compensation of the effective acceptor density can be achieved in the bulk of the absorber.

The deep level energy distribution associated with the well-known "yellow luminescence" in GaN is studied by means of two complementary deep level techniques: photoluminescence and surface photovoltage spectroscopy. The combined experimental results show that the yellow luminescence is due to capture of conduction band electrons, or electrons from shallow donors (with a maximum depth on the order of the thermal energy) by a deep acceptor level with a broad energy distribution, centered at similar to 2.2 eV below die conduction band edge. In addition, the results show that the density of yellow luminescence related states possesses a significant surface component. [S0163-1829(99)16215-5].

The theoretical concepts, experimental tools, and applications of surface photovoltage (SPV) techniques are reviewed in derail. The theoretical discussion is divided into two sections. The first reviews the electrical properties of semiconductor surfaces and the second discusses SPV phenomena. Next, the most common tools for SPV measurements and their relative advantages and disadvantages are reviewed. These include the Kelvin probe and the use of MIS structures, as well as other less used techniques. Recent novel high-spatial-resolution SPV measurement techniques are also presented. Applications include surface photovoltage spectroscopy (SPS) which is a very effective tool for gap state spectroscopy. An in-depth review of quantitative analyses, which permit the extraction of various important surface and bulk parameters, follows. These analyses include: carrier diffusion length; surface band bending, charge, and dipole; surface and bulk recombination rates; surface state distribution and properties; distinction between surface and bulk states: spectroscopy of thin films, heterostructures and quantum structures; and construction of band diagrams. Finally, concluding remarks are given. (C) 1999 Elsevier Science B.V. All rights reserved.

The photosaturation technique is a well-known method for measuring the band-bending at semiconductor surfaces. It is based on the assumption that the bands can be flattened upon suffciently intense illumination. The validity of this approach has been a subject of considerable dispute. A rigorous, quantitative examination of the method is presented. The physical mechanisms governing the photosaturation experiment are identified and analyzed using both an analytical and a numerical model. We show that while the technique is essentially valid, the illumination intensity required to obtain band flattening may be unrealistically high. Criteria for attaining photosaturation are formulated in terms of surface state parameters. Numerous pitfalls and sources of misinterpretation are pointed out. Specifically, a previously undiscussed pseudo-saturation due to surface states with significantly different thermal cross-sections, is described. A systematic approach to future experiments is suggested. (C) 1998 Elsevier Science B.V. All rights reserved.

Photovoltaic effects in CdSe quantum dot (QD) films have been studied using surface photovoltage spectroscopy and complementary methods. The results show that, contrary to previous studies, nonnegligible electric fields can exist in QD films. As a result, driftlike currents must be considered, in addition to the well-known diffusionlike currents. However, it is found that the specific case of photovoltage sign reversal, observed after etching highly quantized CdSe QD films, is governed by diffusionlike transport. The latter is highly influenced by preferential trapping of one type of charge carrier. The preferential trapping is shown to be surface localized and is strongly ambient dependent. It is shown that the photovoltaic properties of these CdSe QD films are dominated by their surface state distribution.

Surface photovoltage spectroscopy (SPS) has been used for quality control of ZnO/CdS/Cu(In,Ga)Se-2 (CIGS) thin-film solar cells. The results show that SPS makes it possible to detect "hard failures" following CIGS deposition, and both "hard" and "soft" failures following CdS deposition and following ZnO deposition. In addition, semi-quantitative screening of CdS/CIGS and ZnO/CdS/CIGS samples is possible. Hence, SPS is suggested as a useful tool for in-line monitoring of CIGS-based solar cell production lines. Moreover, SPS is shown to yield important new information regarding CIGS-based solar cells: (a) A deep gap state is found in samples of superior performance. (b) As opposed to the CdS/CIGS structure, a marked decrease in the open-circuit voltage upon Na contamination in ZnO/CIGS structures is found.

The promising new generation of solar cells based on CIGS (Cu(I,Ga)Se-2) exhibits behavior differing from that of earlier cells because of changes in the method of preparation, leading, among other things, to a difference in sodium content. A simple defect chemical model is presented for the effects of sodium on the surface chemistry and electronic properties of CIGS thin films. The model, based on the well-known catalytic effect that alkali metals have on surface oxidation of semiconductors, is shown to be consistent with the experimental data available in the literature.

A novel technique for the development of latent fingerprints is presented. It is based on placing a fingerprint-bearing object inside a corona discharge induced plasma. The development of various real and artificial fingerprints on metallic substrates under a wide range of conditions is studied. Using the results of the development experiments and the results of X-ray photoelectron spectroscopy, it is shown that the development is based on oxidation of the fingerprint background. This is achieved by strong oxidizers generated by the discharge process, while saturated fatty-acids found in sebaceous fingerprints protect the area beneath them, resulting in a visible fingerprint. The process is optimized by minimizing the electrode gap distance and maximizing the peak discharge voltage and the pulse repetition frequency.

A direct technique for determining band offsets at semiconductor heterojunctions is presented, which is applicable to at least any type I heterojunction, where the top layer doping is sufficiently low. The technique is based on surface photovoltage spectroscopy measurements as a function of overlayer thickness, A numerically simulated example shows that the band offset is a very strong function of the critical overlayer thickness, at which the overlayer contribution to the surface photovoltage spectrum appears, The method is applied to the technologically important InP/InGaAs heterojunction and is shown to yield the commonly accepted band offset value. (C) 1996 American Institute of Physics.

A simple method for in-situ distinction between the effect of dipole formation/annihilation and charge transfer to/from surface gap states on the semiconductor work function is described. The technique is based on simultaneous monitoring of the work function and photovoltage at the semiconductor surface. The approach is illustrated by experiments performed on single crystalline InP(100) surfaces and polycrystalline Cu(In,Ga)Se-2.

The effect of air annealing on state-of-the-art, solar-cell-quality CdS/Cu(In,Ga)Se₂ heterojunctions has been studied using contact potential difference and surface photovoltage measurements. The annealing treatment is shown to have no significant effect on the band lineup of the heterojunction. However, the surface photovoltage spectral response increases markedly upon air annealing. These results can be reconciled if air annealing of the junctions leads mainly to elimination of recombination centers, rather than to changes in the built-in voltage or in the band lineup. We also show that ZnO deposition has an effect on the surface photovoltage that is similar to that of air annealing.

The surface photovoltage (SPV) spectrum due to subband-gap illumination of thin films is theoretically studied. It is shown that this SPV is inherently sensitive to buried interfaces just as it is sensitive to the external semiconductor surface. The different contributions to the SPV from all the optically active gap states present within a sample, consisting of a bulk substrate covered by a thin film, are analyzed. Analytical expressions are obtained in the low illumination intensity and the depletion approximation regime. The evolution of the SPV spectrum with film thickness is examined and is found to depend on both site and population of the gap states. Three modes of evolution are found, according to the relative importance of gap state population changes with film thickness. These modes are confirmed by a numerical simulation of a thin film of pseudomorphic InAlAs on InP substrates and by experiments conducted on the same system. The approach is also applied to the InP/In2O3 system, revealing gap state formation, followed by filling with electrons, thereby explaining previous observations of nearly ideal I-V behavior at this junction. (C) 1996 American Institute of Physics.

Surface photovoltage spectroscopy (SPS) has been employed to monitor optical transitions in quantum well and superlattice structures at room temperature. Excellent agreement is found between theoretical predictions of heavy hole and electron energy level positions and the observed transitions. The results show that using this technique, the complete band diagram of the quantum structure may be constructed. SPS emerges as a powerful tool capable of monitoring optical transitions above the lowest one in a simple to interpret, contactless, and nondestructive way.

A corona discharge photography system using a transparent electrode, which allowed removing the film from the discharge, was constructed, Color photographs were produced from the discharge on various object electrodes. It is shown that the color is dependent on the material within the discharge, and that the ''engineering'' of the color pattern is feasible.

Using the optical methods of reflection anisotropy spectroscopy, surface photovoltage spectroscopy, and contactless electroreflectance, we have conducted an ex situ investigation of (a) the InP/In0.53Ga0.47As(001) heterojunction interface as a function of InP overlayer thickness (50-1000 nm) and (b) the surfaces of n- and p-doped In0.53Ga0.47As(001). All samples were fabricated by organometallic vapor phase epitaxy. The results from these optical probes make it possible to form a comprehensive quantitative picture of the InP/InGaAs heterojunction, including conduction and valence band offsets of 275 and 325 meV, respectively, as well as the (001) surface of InGaAs(surface Fermi level = 200 mV from the conduction band edge). (C) 1996 American Vacuum Society.

An original approach for studying the formation of semiconductor heterojunctions and their electronic properties is discussed and illustrated. Monitoring the changes in the surface potential during the heterojunction formation lends itself to direct measurement of the band discontinuities, Debye length, and the width of the space-charge region at heterojunction interfaces. The contribution of an interface dipole is considered. The technique is demonstrated by a technologically significant experimental example.

A comprehensive and quantitative method for extracting the important parameters of interface states is presented. The method is based on wavelength-, intensity-, and time-resolved surface photovoltage spectroscopy, as well as on measurements as a function of the thickness of an overlayer. Data analysis provides detailed information about interface state properties, including their energy position and distribution, density, and the transition probabilities, i.e. their thermal and optical cross sections. It is also possible to distinguish between surface and bulk states, and determine the spatial site of the states in the case of a heterostructure. Experimental examples for various III-V and II-VI compound semiconductors are given.

A novel approach for constructing the band diagrams of semiconductor heterojunctions is discussed and illustrated. It is based on a simple measurement of band discontinuities, Debye length and the width of the space-charge region at the heterojunction interface. Monitoring the changes in the surface potential during heterojunction formation makes it possible to identify the contributions of the interface states and dipole. The approach is illustrated by the results of experiments performed on the InP/In₂O₃ heterojunction.

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔE _c) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se₂. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band (ΔE_c=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.

The effect of localized electron states on the photovoltage at a free semiconductor surface is analyzed. The analysis shows that surface-photovoltage spectroscopy (SPS) is inherently more sensitive to surface states than to bulk states. Moreover, a fundamental difference between the effect of surface and bulk states on the surface photovoltage (SPV) is shown. The analysis demonstrates that the same illumination-induced variation of the population at a surface and a bulk state may result in a significantly different dependence of the SPV on the illumination intensity. Under certain conditions, this difference makes it possible to distinguish between surface and bulk states by means of SPS. Analytical expressions for these relations are obtained under the depletion approximation, and are compared with the results of a numerical simulation. Experimental results obtained from InP samples demonstrate an application of the theory to practical distinction between surface and bulk states.

The effect of mirror instability and jitter in Q-switched lasers is investigated. Pulse shapes and timing are significantly disturbed by these effects. A simple model is presented based on a state-space approach. Several types of pulses are obtained, which are compared with experimental traces of a CO2 rotating mirror Q-switched laser. Loss modulation as a global means of pulse shaping is also discussed.

A new experimental technique, which utilizes a tunable laser as the illumination source for surface photovoltage spectroscopy measurements, is presented. The data obtained by this technique make it possible to determine the distribution function of gap states observed at semiconductor interfaces. An outline of the approach together with experimental results obtained using a Ti:sapphire laser on InAlAs and CdTe crystals is given.

A new approach is presented for determining surface state parameters at semiconductor interfaces using time resolved surface photovoltage spectroscopy. The analysis provides precise determination of surface state energies, densities, occupancies, and cross sections for electrons and photons (including the wave-length dependence of the photonic cross sections). Unlike previous methods, this approach does not require prior knowledge of the absolute band bending, but rather determines it among the other parameters. The algorithm is extended for any number of discrete surface energy levels.