Publications

The lifestyle of mankind has improved significantly since the start of the industrial revolution and the establishment of a science (engineering)-based society, some 250 years ago. Notwithstanding, the outcome of these advances, a major threat is looming on the future of humanity due to the greenhouse effect produced by fossil fuel effluents and the degradation of the environment on earth. Chemistry and chemical engineering are key players in confronting these challenges and establishing sustainable lifestyle on earth. In particular, the interplay between materials research; solid-state chemistry and nanoscience (nanotechnology) will be crucial for the future of sustainable life on earth. Education of the population-at-large to shift from a consumer-based society into sustainability-concerned lifestyle, is mandatory for realization of this paradigm shift. Harmonizing the interplay between entrepreneurs, financing bodies, public agencies, international organizations, legal bodies and research institutes should also play an integral part of this new equation.

Nanocomposite materials, integrating nanoscale additives into a polymer matrix, hold immense promise for their exceptional property amalgamation. This study delves into the fabrication and characterization of polyetherimide (PEI) nanocomposite strings fortified with multiwall WS₂ nanotubes. The manufacturing process capitalizes on the preferential alignment of WS₂ nanotubes along the string axis, corroborated by scanning electron microscopy (SEM). Mechanical measurements unveil a remarkable acceleration of strain hardening in the nanocomposite strings, chiefly attributed to the WS₂ nanotubes. Structural analyses via X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) reveal intriguing structural alterations during tensile deformation. Notably a semi-crystalline framework ∼100 nm in diameter surrounding the WS₂ nanotubes emerges, which is stabilized by the π-π interactions between the PEI chains. The amorphous majority phase (97% by volume) undergoes also major structural changes upon strain becoming more compact and closing-up of the distance beweeetn the PEI chains. Dynamic mechanical analysis (DMA) demonstrates improved thermal stability of the evolved semi-crystalline π-π oriented PEI molecules, characterized by delayed thermal “structural melting”, underscoring the pivotal role of the WS₂ nanotubes in reinforcing the nanocomposite. The insight gained in this study of WS₂ nanotube-reinforced PEI nanocomposite strings, could offer diverse applications for such tailor-made polymeric materials.

Tungsten suboxide W₁₈O₄₉ nanowhiskers are a material of great interest due to their potential high-end applications in electronics, near-infrared light shielding, catalysis, and gas sensing. The present study introduces three main approaches for the fundamental understanding of W₁₈O₄₉ nanowhisker growth and structure. First, W₁₈O₄₉ nanowhiskers were grown from γ-WO₃/a-SiO₂ nanofibers in situ in a scanning electron microscope (SEM) utilizing a specially designed microreactor (μReactor). It was found that irradiation by the electron beam slows the growth kinetics of the W₁₈O₄₉ nanowhisker, markedly. Following this, an in situ TEM study led to some new fundamental understanding of the growth mode of the crystal shear planes in the W₁₈O₄₉ nanowhisker and the formation of a domain (bundle) structure. High-resolution scanning transmission electron microscopy analysis of a cross-sectioned W₁₈O₄₉ nanowhisker revealed the well-documented pentagonal Magnéli columns and hexagonal channel characteristics for this phase. Furthermore, a highly crystalline and oriented domain structure and previously unreported mixed structural arrangement of tungsten oxide polyhedrons were analyzed. The tungsten oxide phases found in the cross section of the W₁₈O₄₉ nanowhisker were analyzed by nanodiffraction and electron energy loss spectroscopy (EELS), which were discussed and compared in light of theoretical calculations based on the density functional theory method. Finally, the knowledge gained from the in situ SEM and TEM experiments was valorized in developing a multigram synthesis of W₁₈O₄₉/a-SiO₂ urchin-like nanofibers in a flow reactor.

WS₂ nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS₂ nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000-5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W₅O₁₄ nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W₁₈O₄₉ nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS₂ nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS₂ nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

The dimensional limit of ferroelectricity has been long explored. The critical contravention is that the downscaling of ferroelectricity leads to a loss of polarization. This work demonstrates a zero-dimensional ferroelectricity by the atomic sliding at the restrained van der Waals interface of crossed tungsten disufilde nanotubes. The developed zero-dimensional ferroelectric diode in this work presents not only non-volatile resistive memory, but also the programmable photovoltaic effect at the visible band. Benefiting from the intrinsic dimensional limitation, the zero-dimensional ferroelectric diode allows electrical operation at an ultra-low current. By breaking through the critical size of depolarization, this work demonstrates the ultimately downscaled interfacial ferroelectricity of zero-dimensional, and contributes to a branch of devices that integrates zero-dimensional ferroelectric memory, nano electro-mechanical system, and programmable photovoltaics in one.Down-scaled ferroelectricity normally diminishes due to the arising depolarization field. Here, the authors realize a 0D ferroelectric diode device taking advantage of the sliding at the van der Waals interface by the two crossed tungsten disulfide nanotubes.

The effect of semiconducting tungsten disulfide (WS₂) nanoparticles (NPs), functionalized by either methacryloxy, glycidyl, vinyl, or amino silanes, has been studied in photocuring of acrylate and epoxy resins (the latter photocured according to a cationic mechanism). The curing time, degree of curing (DC), thermal effects, and mechanical properties of the radiation-cured resins were investigated. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses confirmed that a silane coating was formed (1\u20134 nm) on the NPs\u2019 surface having a thickness of 1\u20134 nm. Fourier transition infrared (FTIR) was used to determine the DC of the nanocomposite resin. The curing time of the epoxy resin, at 345\u2013385 nm wavelength, was 10 to 20 s, while for acrylate, the curing time was 7.5 min, reaching 92% DC in epoxy and 84% in acrylate. The glass transition temperature (Tg) of the photocured acrylates in the presence of WS₂ NPs increased. In contrast to the acrylate, the epoxy displayed no significant variations of the Tg. It was found that the silane surface treatments enhanced the DC. Significant increases in impact resistance and enhancement in shear adhesion strength were observed when the NPs were treated with vinyl silane. A previous study has shown that the addition of WS₂ NPs at a concentration of 0.5 wt.% is the optimal loading for improving the resin\u2019s mechanical properties. This study supports these earlier findings not only for the unmodified NPs but also for those functionalized with silane moieties. This study opens new vistas for the photocuring of resins and polymers in general when incorporating WS₂ NPs.

Radiation curing (photocuring) of thermosetting polymers, such as acrylate resins, is a common technology with diverse applications, such as in adhesives, coatings, advanced manufacturing, medical-related technologies, and more. Photocuring of acrylate thermosets is initiated via a radical-induced cleavage of a photocuring agent, like bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO), which activates the vinyl C═C double bond of the acrylate moiety. Here, we show that addition of semiconducting WS₂ nanoparticles (NPs) accelerates the photocuring process. Using electron paramagnetic resonance (EPR) of ethanol-water solutions, the mechanism of the radical reactions of BAPO and WS₂ NPs is investigated. It is found that the two photocuring agents operate according to entirely different mechanisms, which has been discussed in great detail. In contrast to BAPO, which is photocleaved during the curing process, the WS₂ NPs remain unchanged, leading to major mechanical reinforcements of the cured acrylate film.

The synthesis of fundamentally small MoS₂ nanotubes and nanocones(horns) that have proven elusive in prior studies has been achieved via ablation of a precursor mixture of crystallites of MoS₂ + MoO₃ by highly concentrated solar radiation. The special far-from-equilibrium conditions achieved in the solar furnace prove conducive to the generation of these singular nanostructures. Extensive electron microscopy and characterization results (transmission electron microscopy (TEM), electron diffraction (ED), X-ray diffraction (XRD), scanning TEM (STEM), and high angle annular dark field (HAADF)) reveal a range of nanoparticle shapes and sizes based on which reaction mechanisms are proposed. Molecular dynamics simulations indicate that the sizable thermal fluctuations intrinsically produced in the high-temperature solar reactor soften the MoS₂ nanostructures, yielding corrugated layers that favor nanostructures with only a few layers, in agreement with the experimental observations.

Cationic photo-initiated and polymerized epoxies are characterized by good adhesion, high modulus, zero volatiles, low shrinkage and living polymerization characteristics. Radiation—cured acrylate resins are characterized by rapid initial curing with increased initial strength. The combination of radiation-cured acrylates and epoxies may present advantageous attributes. Thus, the system investigated is a hybrid epoxy/methyl acrylate and three different initiators for cationic polymerization of epoxies, the radical reaction of acrylates and the thermal initiator. When incorporating additives like opaque WS₂ nanoparticles (NPs), absorption of the photo radiation takes place, which may lead to low photo activity. Curing kinetics measurements revealed that the absorbing/masking effect of WS₂ was insignificant, and surprisingly, the level of curing was enhanced when the WS₂ NPs were incorporated. FTIR results demonstrated that covalent bonds were formed between the inorganic fullerenes (IF-WS₂) and the crosslinked matrix. Viscosity measurements showed a surprising reduction of five to ten times in the low-shear viscosity upon NPs incorporation compared to neat resins. It was concluded that the decrease of viscosity by the inorganic NPs, in addition to the enhanced level of conversion, has profound advantages for structural adhesives and 3D printing resins. To the best of our knowledge, this investigation is the first to report on a radiation-induced curing system containing opaque WS₂ NPs that leads to an enhanced degree of curing and reduced shear viscosity.

Even though WS₂nanotubes (NTs-WS₂) have great potential as anode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) thanks to their unusual layered structure, their conductivity and cycling stability are far from satisfactory. To tackle these issues, carbon-coated WS₂(NTs-WS₂@C) nanocomposites were prepared through a facile synthesis method that involved precipitating a carbon precursor (20% sucrose) on WS₂nanotubes, followed by annealing treatment under an argon environment. Thanks to the presence of highly conductive and mechanically robust carbon on the outer surface, NTs-WS₂@C nanocomposites show improved electrochemical performance compared with bare NTs-WS₂. After 60 cycles at 80 mA g^-1current density, the cells display high capacities of 305 mAh g^-1in LIBs and 152 mAh g^-1in SIBs, respectively. As the current density increases to 600 mA g^-1, it provides specific capacities of 209 and 115 mAh g^-1, correspondingly. The enhanced electrochemical performance in LIBs and SIBs is primarily attributed to the synergistic effects of the tubular architecture of WS₂, carbon network and stable nanocomposite structure, which can effectively constrain volume variation during the metal ions intercalation/deintercalation processes.

Nanotubes of transition metal dichalcogenides such as WS2 and MoS2 offer unique quasi-1D properties and numerous potential applications. Replacing sulfur by selenium would yield ternary WS2(1\u2013x)Se2x (0 ≤ x ≤ 1; WSSe) nanotubes, which are expected to reveal strong modulation in their absorption edge as a function of selenium content, x Se. Solid WO2.72 oxide nanowhiskers were employed as a sacrificial template to gain a high yield of the nanotubes with a rather uniform size distribution. Though sulfur and selenium belong to the same period, their chemical reactivity with oxide nanowhiskers differed appreciably. Here, the closed ampoule technique was utilized to achieve the completion of the solid\u2013vapor reaction in short time scales instead of the conventional flow reactor method. The structure and chemical composition of the nanotubes were analyzed in detail. X-ray and electron diffractions indicated a systematic modulation of the WSSe lattice upon increasing the selenium content. Detailed chemical mapping showed that the sulfur and selenium atoms are distributed in random positions on the anion lattice site of the nanotubes. The optical excitonic features and absorption edges of the WSSe nanotubes do not vary linearly with the composition x Se, which was further confirmed by density functional theory calculations. The WSSe nanotubes were shown to exhibit strong light\u2013matter interactions forming exciton\u2013polariton quasiparticles, which was corroborated by finite-difference time-domain simulations. Transient absorption analysis permitted following the excited state dynamics and elucidating the mechanism of the strong coupling. Thus, nanotubes of the ternary WSSe alloys offer strong band gap tunability, which would be useful for multispectral vision devices and other optoelectronic applications.

Exploring the behavior of nanocrystals with varying shapes and sizes under high pressure is crucial to understanding the relationship between the morphology and properties of nanomaterials. In this study, we investigated the compression behaviors of WS2 nanotubes (NT−WS2) and fullerene-like nanoparticles (IF−WS2) by in situ high-pressure X-ray diffraction (XRD) and Raman spectroscopy. It was found that the bulk modulus of NT−WS2 is (81.7 GPa), which is approximately twice as large as that of IF-WS2 (46.3 GPa). This might be attributed to the fact that IF−WS2 with larger d-spacing along the c−axis and higher defect density are more compressible under isotropic pressure than NT−WS2. Thus, the slender NT−WS2 possess a more stable crystal structure than the IF-WS2. Our findings reveal that the effects of morphology and size play crucial roles in determining the high-pressure properties of WS2 nanoparticles, and provide significant insight into the relationship between structure and properties.

Among the different methods for orienting liquid crystal (LC) molecules, adding nanoparticles into the matrix of the substrate material towards modifying its surface, is actively pursued. In this context, the influence of the nanoparticle content on the texture of the surface of polymer film used as the substrate for the LC orientation is of particular interest. Thus, in the current paper, WS₂ nanotubes were used to dope the polyimide (PI) substrate-film in order to modify and control its surface morphology/roughness and properties. The modified organic surface structure is applied in order to achieve a new means for controlling the orientation of the LC molecules. This tool adds to the classical methods for controlling the orientation of the LC molecules, such as the display technique.

Radiation curing (RC) thermosets offer a variety of advantages, such as curing on demand, low viscosity, good adhesion to many surfaces, high modulus, good appearance of final coating, zero volatile organic compounds, and more. Alongside the advantages, it has some drawbacks like high brittleness, limited curing of thick and opaque formulations. This review outlines the basic chemistries and developments that were carried out through the years related to radical RC of acrylates and cationic RC of epoxies and their nanocomposites. In addition, commercially available monomers, photoinitiators will be presented, in addition to the attributes of the curing processes and future developments.

Asymmetric two-dimensional (2D) structures (often named Janus), like SeMoS and their nanotubes, have tremendous scope in material chemistry, nanophotonics, and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed, no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X = S/Se), were synthesized by high-temperature chemical vapor transport reaction in which the Se binds exclusively to the Ta atoms and La binds to S atoms rather than the anticipated random distribution.With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe = 1. These counterintuitive results shed light on the chemical selectivity and stability of misfit compounds and 2D alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.

This conference contribution is focused on decoration of WS2 nanotubes (NT-WS2) with gold and silver nanoparticles via facile routes implying direct reaction of tungsten disulfide with water-soluble AuIII and AgI species at 100°C. The underlying mechanism of these interactions will be discussed in details based on extensive studies of reaction mixtures and resulting metal–NT-WS2 nanocomposites, including thorough X-ray photoelectron spectroscopy (XPS) analysis. Surprising features in optical spectra of the designed nanocomposites would be reported, including suppression of plasmon resonance in tiny noble metal nanoparticles (

We present density functional theory calculations of the vibrational and electronic properties of the misfit-layer compound (MLC) LaS-CrS2 and its isolated sublayers to identify the vibrational modes of the compound. From the comparison of two model systems, (i) the fully relaxed misfit-layer compound supercell and (ii) its sublayers as isolated systems, we extract the charges, which are transferred between the two sublayers and the concomitant changes of the structural parameters upon formation of the supercell. The deformation within each sublayer indicates a strong influence of the charge transfer, confirming the important role of the interlayer interaction between the two sublayers. By comparing the vibrational properties of the two model systems, we assign several frequency regimes within the vibrations of the supercell to phonon branches of the sublayers. From polarized Raman spectra, we find that some of the Raman modes are directly inherited from the Δ-point phonons of the hypothetical isolated 1T-CrS2 layer; other Raman modes are backfolded from the A1g-associated phonon branch of the 1T-CrS2 onto the misfit-layer compound MLC Δ-point, which is a direct consequence of the supercell formation in LaS-CrS2.

The non-stoichiometric misfit layered compounds (MLC) of the general formula ((MX)(1+y))(m)(TX2)(n) (abbreviated herein as MX-TX2) have been investigated quite extensively over the last 30 years. Here MX is a atomic slab of a material with distorted rocksalt structure and TX2 is a layered compound with hexagonal (octahedral) coordination between the metal T atom and the chalcogen X atom. Recognizing the mismatch between the two (MX and TX2) sublattices, nanotubes from the MLC of different compositions were described in the past. In particular, semimetallic nanotubes belonging to the family LnX-TaX2 with Ln = rare earth atom and X = S, Se, Te have been studied in the past. While some of them, like LaS-TaS2 were obtained with moderately high yields, others like YbS-TaS2 were scarce. In the present study, a new strategy for promoting the yield of such MLC nanotubes by alloying the LaS sublattice with another Ln atom is proposed. Detailed transmission electron microscopy investigation of the (mixed) Ln(x)La((1-x))S-TaS2 (Ln = Pr, Sm, Ho, Yb) nanotubes show clearly that the substituting Ln atom resides in the rocksalt LaS sublattice of the nanotubes. Raman measurements show distinct differences between mixed tubes with open-shell (Pr, Sm, Ho) and close-shell (La, Yb) rare-earth atoms. Density functional calculations show that the interplay between two important factors determine the enhanced stability of the mixed nanotubes- the size and electronic structure of the substituting rare-earth atom. The smaller is the substituting rare-earth atom (larger Z number), the more dissimilar it is to the original La atom. This dissimilarity enhances the incommensurability between the Ln(x)La((1-x))5 and the TS2 subunits, promoting thereby the stability of the mixed MLC. However, the electronic structure of the Ln atom was found to play a more significant role. The MLC lattice of the LaS-TaS2 is electron-rich and consequently the 4d(z)(2) level of Ta is full. The unoccupied 4f levels of the substituent open-shell atoms (Pr, Sm, Ho), which are positioned below the Fermi level, serve as electron acceptors. Consequently, the Ln substitution is found to enhance the stability of the mixed lattice and nanotubes thereof. This strategy can be employed for enhancing the yield of these and other misfit nanotubes using different substituents of the right size and energy profile.

Due to their high porosity, aerogels can be efficiently used as host matrices for functional materials. The solid matrix is advantageous over liquid suspensions because it maintains the nanoparticles static and inhibits agglomeration and precipitation. The current paper reports on the controlled addition of less than 0.1 wt% of WS2 nanotubes (WS2 NTs) to aerogels, retaining the aerogel's mesoporous structure, and demonstrates how increasing nanotubes' concentration influences the optical properties of the composite aerogel. The absorption spectrum of WS2 NTs consists of two peaks, attributed to the direct gap transition and referred to as excitons A and B and is preserved in the aerogel. WS2 NTs' extinction spectrum, on the other hand, is dominated by exciton-polaritons and is modified in the aerogel, with respect to the NTs dispersed in liquid. This occurs due to scattering effects, resulting in broadening with increased NT content, washing out the excitonic transitions. Furthermore, femtosecond optical pump-probe measurements carried out on NTs dispersed in both ethanol and the silica aerogel suggest that the electronic processes underlying the overall optical behaviour of the nanotubes, and hence also their optoelectronic and photochemical properties are preserved in the aerogel matrix. These findings make the obtained nanocomposites interesting for use in modern optical and optoelectronic devices.

We present the analysis of a family of nanotubes (NTs) based on the quaternary misfit layered compound (MLC) YxLa1-xS-TaS2. The NTs were successfully synthesized within the whole range of possible compositions via the chemical vapor transport technique. In-depth analysis of the NTs using electron microscopy and spectroscopy proves the in-phase (partial) substitution of La by Y in the (La,Y)S subsystem and reveals structural changes compared to the previously reported LaS-TaS2 MLC-NTs. The observed structure can be linked to the slightly different lattice parameters of LaS and YS. Raman spectroscopy and infrared transmission measurements reveal the tunability of the plasmonic and vibrational properties. Density-functional theory calculations showed that the YxLa1-xS-TaS2 MLCs are stable in all compositions. Moreover, the calculations indicated that substitution of La by Sc atoms is electronically not favorable, which explains our failed attempt to synthesize these MLC and NTs thereof.

Proton exchange membranes with high through-plane proton conductivity are a critical component of high-performance fuel cells, electrolyzers, and batteries. However, isotropically distributed proton-conducting channel structures of current membranes present a limitation. Herein, a proton exchange membrane with straight proton-conducting channels aligned in the thickness direction is fabricated, achieved by magnetic field-induced alignment of proton-conductive, paramagnetic, and one-dimensional (1D) tungsten disulfide nanotubes (pms-WS2) distributed in a perfluorinated sulfonic acid (Nafion) membrane. The pms-WS2 nanotubes feature straight WS2 nanotubes as a core, a polystyrenesulfonate (PSS) skin layer, and surface-decorated Fe3O4 nanoparticles. A molecular dynamics simulation suggests that straight proton-conducting channels are constructed at the interface of Nafion/pms-WS2 due to densely populated sulfonic acids. Spectroscopic investigation and magnetization measurements verify the through-plane alignment of pms-WS2 under a weak through-plane magnetic field (0.035 T) during the removal of solvent from the membrane cast. Compared with a recast Nafion membrane with the same thickness, the through-plane aligned composite membrane exhibits 69% higher proton conductivity and 51% higher power performance in a proton exchange membrane fuel cell, demonstrating its efficacy. The through-plane alignment of a proton-conductive inorganic 1D material promises improved power performance of advanced electrochemical devices.

Multiwall WS2 nanotubes (and fullerene-like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi-1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion-fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite-difference time-domain simulations, which shed new light on the cavity mode-exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size-fractionated nanotubes fully confirm the steady-state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light-matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.

Strong coupling of electric transition dipoles with optical or plasmonic resonators modifies their light-matter interaction and, therefore, their optical spectra. Semiconducting WS2 nanotubes intrinsically provide the dipoles through their excitonic resonances, and the optical cavity via their cylindrical shape. We investigate the nonequilibrium light-matter interaction in WS2 nanotubes in the time domain using femtosecond transient extinction spectroscopy. We develop a phenomenological coupled oscillator model with time-dependent parameters to describe the transient extinction spectra, allowing us to extract the underlying nonequilibrium electron dynamics. We find that the exciton and trion resonances shift due to many-body effects of the photogenerated charge carriers and their population dynamics on the femto- A nd picosecond timescale. Our results show that the time-dependent phenomenological model quantitatively reproduces the nonequilibrium optical response of strongly coupled systems.

The photovoltaic effect in traditional p\u2013n junctions\u2014where a p-type material (with an excess of holes) abuts an n-type material (with an excess of electrons)\u2014involves the light-induced creation of electron\u2013hole pairs and their subsequent separation, generating a current. This photovoltaic effect is particularly important for environmentally benign energy harvesting, and its efficiency has been increased dramatically, almost reaching the theoretical limit1. Further progress is anticipated by making use of the bulk photovoltaic effect (BPVE)2, which does not require a junction and occurs only in crystals with broken inversion symmetry3. However, the practical implementation of the BPVE is hampered by its low efficiency in existing materials4\u201310. Semiconductors with reduced dimensionality2 or a smaller bandgap4,5 have been suggested to be more efficient. Transition-metal dichalcogenides (TMDs) are exemplary small-bandgap, two-dimensional semiconductors11,12 in which various effects have been observed by breaking the inversion symmetry inherent in their bulk crystals13\u201315, but the BPVE has not been investigated. Here we report the discovery of the BPVE in devices based on tungsten disulfide, a member of the TMD family. We find that systematically reducing the crystal symmetry beyond mere broken inversion symmetry\u2014moving from a two-dimensional monolayer to a nanotube with polar properties\u2014greatly enhances the BPVE. The photocurrent density thus generated is orders of magnitude larger than that of other BPVE materials. Our findings highlight not only the potential of TMD-based nanomaterials, but also more generally the importance of crystal symmetry reduction in enhancing the efficiency of converting solar to electric power.

Advanced nanomaterials play a prominent role in nanoscience and nanotechnology developments, opening new frontiers in these areas. Among these nanomaterials, due to their unique characteristics and enhanced chemical and physical properties, inorganic nanotubes have been considered one of the most interesting nanostructures. In recent years, important progress has been achieved in the production and study of these nanomaterials, including boron nitride, transition metal dichalcogenide nanotubular structures, misfit-based nanotubes and other hybrid/doped nanotubular objects. This review is devoted to the in-depth analysis of recent studies on the synthesis, atomic structures, properties and applications of inorganic nanotubes and related nanostructures. Particular attention is paid to the growth mechanism of these nanomaterials. This is a crucial point for the challenges ahead related to the mass production of high-quality defect-free nanotubes for a variety of applications.

Orthodontic teeth movement that is performed by a sliding archwire-bracket system is always affected by an inevitable friction (FR) force created at the wire-bracket interface. Overcoming the frictional impediment requires application of increased orthodontic load. Excessive force can cause undesirable teeth movement, increase root resorption depending on individual response, as well as inefficient treatment. To date, attempts have mostly been directed at reducing FR by altering the bracket design. These attempts have been successful to some extent, although no satisfactory solution for FR reduction of both stainless steel and nickel titanium wires was achieved. Therefore, the development of wires with low FR coefficients is of great importance. This has been successfully achieved by coating the orthodontic wires with inorganic fullerene-like tungsten disulfide nanoparticles (IF-WS₂ NP). The IF-WS₂ NP was first described by Prof. Reshef Tenne in 1992. It was shown that under certain reducing and sulfidizing conditions at elevated temperatures, tungsten oxide (WO₃) nanoparticles could form nested IF-WS₂ nanostructures creating layers that resemble an onion or a nanotube. The layers are weakly connected through van der Waals forces only. The IF-WS₂ NPs provide excellent lubricity and allow remarkable improvement of FR and wear properties under different contact conditions. Coating the wires by using an adhesion matrix (nickel or cobalt) into which IF-WS₂ NP were impregnated, brought about a significant reduction of the orthodontic force required to move the coated wire along the bracket. The toxicity of the IF-WS₂ NP was tested at various modes of exposure in animals and in vitro in human cell cultures. The results obtained showed no toxic effect of IF-WS₂ NP in all the performed tests. Obviously, biocompatibility approval from the relevant regulatory agencies is mandatory for future clinical use of IF-WS₂ NP-coated orthodontic appliances.

Transition metal dichalcogenide nanotubes are fascinating platforms for the research of superconductivity due to their unique dimensionalities and geometries. Here we report the diameter dependence of superconductivity in individual WS2 nanotubes. The superconductivity is realized by electrochemical doping via the ionic gating technique in which the diameter of the nanotube is estimated from the periodic oscillating magnetoresistance, known as the LittleParks effect. The critical temperature of superconductivity displays an unexpected linear behavior as a function of the inverse diameter, that is, the curvature of the nanotube. The present results are an important step in understanding the microscopic mechanism of superconductivity in a nanotube, opening up a new way of superconductivity in crystalline nanostructures.

Herein, the use of highly concentrated sunlight for materials science research is reviewed. Specific research directions include: (1) the generation of inorganic nanostructures, some of which had eluded experimental realization with conventional synthetic processes, and (2) elucidating the processes governing the degradation of organic and perovskite-based photovoltaic materials and devices, along with accelerated assessment of their stability. Both approaches employ solar concentrators capable of producing flux densities exceeding those of terrestrial solar radiation by up to three orders of magnitude, and are geared toward either creating extensive ultrahot reactor conditions conducive to the rapid, safe synthesis of unusual nanomaterials or judiciously interrogating photovoltaic devices.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain'' (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of similar to 280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

In this paper, we demonstrate the formation of hybrid nanostructures consisting of two distinctive components mainly in a one-to-one ratio. Thermolysis of inorganic nanotubes (INT) and closed-cage, inorganic fullerene-like (IF) nanoparticles decorated with a dense coating of metallic nanoparticles (M = Au, Ag, Pd) results in migration of relatively small NPs or surface-enhanced diffusion of atoms or clusters, generating larger particles (ripening). AuNP growth on the surface of INTs has been captured in real time using in situ electron microscopy measurements. Reaction of the AuNP-decorated INTs with an alkylthiol results in a chemically induced NP fusion process at room temperature. The NPs do not dissociate from the surfaces of the INTs and IFs, but for proximate IFs we observed fusion between AuNPs originating from different IFs.

Screening effect in finite-length carbon nanotubes (CNT) and their agglomerates hinders significantly the electromagnetic interaction in composite materials. Screening effect is strong in the microwave range, and it decreases with increasing frequency resulting in a strong frequency dependence of the effective conductivity of the composite. Since screening effect is rather small in the terahertz range, the effective conductivity in this range is determined directly by the intrinsic conductivity of the inclusions. The ratio of the microwave to terahertz effective conductivities was proposed as a parameter to estimate how effectively carbon nanotube inclusions contribute to the electromagnetic performance of composite materials in the microwave range. CNT film was considered as a material where maximal possible interaction of the CNTs with EM field occurs. Single-walled CNT films and CNT-based composite materials, as well as hybrid film comprising mixtures of WS
₂ nanotubes and CNTs were fabricated and measured in the microwave and terahertz ranges. The electromagnetic field interaction with the inclusions has been estimated for all the samples fabricated.

We report here a unique and efficient methodology for the surface functionalization of closed-cage inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INTs) composed of two-dimensional nanomaterials of transition-metal chalcogenides (MS
₂; M = W or Mo). The first step is the physical coverage of these robust inorganic materials with monodispersed and dense monolayers of gold, silver, and palladium nanoparticles. The structural continuity at the interface between the IF/INT and the metallic nanoparticles is investigated. Lattice matching between these nanocrystalline materials and strong chemical affinity lead to efficient binding of the metallic nanoparticles onto the outer sulfide layer of the MS
₂-based structures. It is shown that this functionalization results in narrowing of the IF/INT optical band gap, increased work function, and improved surface-enhanced Raman scattering. In the second step, functionalization of the surface-bound nanoparticles is carried out by a ligand-exchange reaction. This ligand exchange involving the tetraoctylammonium bromide capping layer and an alkyl thiol enhances the solubility (∼10×) of the otherwise nearly insoluble materials in organic solvents. The scope of this method is further demonstrated by introducing a ruthenium(II) polypyridyl complex on the surface of the surface-bound AuNPs to generate fluorescent multicomponent materials.

We report here a new methodology for the formation of freestanding nanotubes composed of individual gold nanoparticles (NPs) cross-linked by coordination complexes or porphyrin molecules using WS
₂ nanotubes (INT-WS
₂) as a template. Our method consists of three steps: (i) coverage of these robust inorganic materials with monodispersed and dense monolayers of gold NPs, (ii) formation of a molecular AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, and (iii) removal of the INT-WS
₂ template with a hydrogen peroxide solution. Nanoindentation of the template-free AuNP tubes with atomic force microscopy indicates a radial elastic modulus of 4 GPa. The template-free molecular AuNP tubes are characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular AuNP tubes with a defined length and diameter, depending on the dimensions of the template.

The review concerns the development and the state-of-the-art in studies on the surface modification methods aimed at fabricating promising nanocomposites based on multilayer inorganic tubular and onion (fullerene-like) MoS2 and WS2 nanostructures. The synthetic details and structural features of these materials are considered. Considerable attention is paid to targeted functionalization of molybdenum and tungsten disulfide nanostructures and to fundamental principles that underlie their ability to chemical interactions. The functional properties and applications of the obtained materials are described.

Owing to their unique properties such as mechanical, optical, magnetic, nanomaterials attracted a great interest over the last two decades. Inorganic nanotubes, e.g. WS2, make an important class of nanomaterials with numerous potential applications. In the current work, a new synthetic strategy is developed to decorate the surface of WS2 nanotubes with FeWO4 nanoparticles. The FeWO4 nanoparticles were produced by first depositing amorphous iron oxide film onto the WS2 nanotubes' surface and, subsequently, high-temperature annealing (600 C-degrees). Careful analysis by electron microscopy; X-ray diffraction and other techniques were carried out. Based on these analyses, the growth mechanism of the hybrid nanostructures was elucidated. Magnetic measurements were employed to shed light on the magnetic behavior of the hybrid nanostructures. The orientation and position of the WS2 nanotubes decorated with the FeWO4 nanoparticles could be partially affected by applying a magnetic field using non-viscous solvents, like ethanol.

Multi‐walled carbon (MWCNT) and tungsten disulfide (INT‐WS2) nanotubes are materials with excellent mechanical properties, high electrical and thermal conductivity. These special properties make them excellent candidates for high strength and electrically conductive polymer nanocomposite applications. In this work, the possibility of the improvement of mechanical, thermal and electrical properties of poly(trimethylene terephthalate) (PTT) by the introduction of MWCNT and INT‐WS2 nanotubes was investigated. The PTT nanocomposites with low loading of nanotubes were prepared by in situ polymerization method. Analysis of the nanocomposites' morphology carried out by SEM and TEM has confirmed that well‐dispersed nanotubes in the PTT matrix were obtained at low loading (

Recently, polyhedral; octahedral fullerene-like structures and cylindrical nanotubes of gallium sulfide have been produced in far from equilibrium conditions using a high power solar synthesis and Pb as growth promoter. Their exact atomistic structure and their formation path remain unknown. Here, the models of fullerenes and nanotubes are designed for both stable β- and metastable γ-polymorphs of GaS. Their stability and electronic properties were investigated as a function of both the morphology and size using density-functional tight-binding method. The results manifest that, construction principles for GaS fullerenes are different from those for “classical” dichalcogenide fullerenes. In contrast to the bulk, the kinetic stability of fullerene-like structures of the γ-GaS phase is found to be larger, than that for β-GaS which is ascribed to a sterical specificity of intersected γ-GaS layers. We predict that, the γ-GaS phase should arise at least as an intermediate state during the synthesis of polyhedral GaS nanoparticles. Notwithstanding the polymorphic type, the size and the morphology of GaS nanostructures, they are all semiconductors. Finally, the possible composition and electronic properties of Pb||GaS interface within the Pb@GaS nanoparticles are discussed.

Nanoparticles, and more specifically gold nanoparticles (AuNPs), have attracted much scientific and technological interest in the last few decades. Their popularity is attributed to their unique optical, catalytic, electrical and magnetic properties when compared with the bulk. However, one of the main problems with AuNPs is their long-term stability. Two-dimensional materials like MoS₂ (WS₂) are semiconductors that exhibit a combination of properties which make them suitable for electronic, optical and (photo)catalytic devices. Few-layer MoS₂ (WS₂) nanoparticles (NPs), and in particular single-layer ones, show intriguing optical and electrical properties which are very different from those of the bulk compounds. Here we demonstrate the synthesis of AuNPs sheathed by a single layer of MoS₂ (WS₂), i.e. a core-shell nanostructure (AuNP@1L-MoS₂). The hybrid NPs exhibit optical properties that are different from those of either constituent and are amenable for modulation via their chemistry, offering a myriad of applications.

New types of core-shell nanoparticles are reported: Pb@GaS fullerene-like and nanotubular structures, achieved via the continuously high reactor temperatures and ultra-hot strong-gradient annealing environments created by highly concentrated sunlight. Structural and chemical characterizations suggest a formation mechanism where vaporized Pb condenses into nanoparticles that are stabilized as they become covered by molten GaS, the ensuing crystallization of which creates the outer layers. Hollow-core GaS fullerene-like nanoparticles and nanotubes were also observed among the products, demonstrating that a single solar procedure can generate a variety of core-shell and hollow nanostructures. The proposed formation mechanisms can account for their relative abundance and the characterization data.

Inorganic fullerene (IF)-like nanoparticles and inorganic nanotubes (INT) form a relatively new class of nanomaterials. They are generically produced from layered (2D) materials, which enable formation of stable, closed, hollow structures in the nanodomain. Taking advantage of the structural analogy between graphite and inorganic 2D compounds, researchers demonstrated that nanoparticles of WS₂ (Tenne et al. 1992), MoS₂ (Feldman et al. 1995, Margulis et al. 1993), BN (Chopra et al. 1995, Golberg et al. 1998), NiCl₂ (Rosenfeld Hacohen et al. 1998), and the like become unstable in the platelet (bulk) form and spontaneously assemble into hollow seamless structures, such as multiwall quasi-spherical spheres, nanooctahedra (Parilla et al. 1999), or nanotubes. The research on these nanostructures brought about major advances in chemistry and nanotechnology, and pointed to many possible applications.

In this report, a systematic study of the effects of p- and n-type doping of the inorganic fullerene (IF) MoS₂ on the efficiency of the hydrogen evolution reaction (HER) is described. Active edge site enriched IF-MoS₂, promoted by strategically introducing Nb (p-type) and Re (n-type) dopants (below 500 ppm), enables facile HER over a range of pH values. Experimental results suggest that although Nb-doping on IF-MoS₂ leads to better electrocatalytic HER activity in an alkaline medium with an onset potential difference of 80 mV, Re-doping gives excellent activity in an acidic medium. The present work presents a systematic study of HER activity by finely tuning the activity in different electrolyte media with varied pH values through deliberate doping of the parent catalyst with p- and n- type materials. The doped IF-MoS₂ catalysts exhibit excellent catalytic activity even with sea water as an electrolyte.

The relationship between structure and properties has been followed for different nanoscale forms of tungsten disulfide (2H-WS₂) namely exfoliated monolayer and few-layer nanoplatelets, and nanotubes. The similarities and differences between these nanostructured materials have been examined using a combination of optical microscopy, scanning and high-resolution transmission electron microscopy and atomic force microscopy. Photoluminescence and Raman spectroscopy have also been used to distinguish between monolayer and few-layer material. Strain induced phonon shifts have been followed from the changes in the positions of the A_1g and E_2g¹ Raman bands during uniaxial deformation. This has been modelled for monolayer using density functional theory with excellent agreement between the measured and predicted behaviour. It has been found that as the number of WS₂ layers increases for few-layer crystals or nanotubes, the A_1g mode hardens whereas the E_2g¹ mode softens. This is believed to be due to the A_1g mode, which involves out of plane atomic movements, being constrained by the increasing number of WS₂ layers whereas easy sliding reduces stress transfer to the individual layers for the E_2g¹ mode, involving only in-plane vibrations. This finding has enabled the anomalous phonon shift behaviour in earlier pressure measurements on WS₂ to be resolved, as well as similar effects in other transition metal dichalcogenides, such as molybdenum disulfide, to be explained.

We present Raman spectra of misfit layer (PbS)(1.14)NbS2 nanotubes and lead intercalated NbS2 nanotubes. They represent interesting model systems to investigate the nature of interlayer interaction in layered materials. A direct correlation to the Raman modes of the parent 2H-NbS2 compound exists, but some modes are seen drastically upshifted in frequency in the misfit layer and intercalated compound while others remain almost unchanged. On the basis of the Raman spectroscopic investigations and with the help of supporting calculations we examine different interlayer bonding mechanisms and contribute to the discussion as to why these frequency shifts occur.

Polyvinyl alcohol (PVA) is a biocompatible, semi-crystalline and water soluble polymer with moderate tensile properties. To improve the thermal and mechanical properties of PVA, as well as to reduce water uptake, structural modification by glutaric acid (GA) and nanoparticle reinforcement by tungsten disulphide nanotubes (WSNTs) were used to prepare PVA based composites. We observed a significant drop in the water uptake of GA crosslinked PVA, an indication of the formation of a network. Fourier transform infrared spectroscopy was applied to confirm the presence of covalent bonds formed during the crosslinking. Crosslinked PVA and composites are found to have higher thermal stability and mechanical properties compared to their un-crosslinked counterparts. Tensile tests show that the presence of WSNTs increases the strength (up to 25%), modulus (up to 120%) and toughness (up to 80%) of the pristine as well as the crosslinked PVA. (C) 2016 Elsevier Ltd. All rights reserved.

DC and AC electrical properties of hybrid films, consisting of carbon nanotubes and tungsten disulfide nanotubes (and fullerene like nanoparticles) were studied within the 2 - 300 K temperature range and over the 20 Hz - 1 MHz frequency range. The temperature dependences of the resistance R(T) exhibit behavior typical for the fluctuation-induced tunneling model in the intermediate temperature range. Analysis of the dependences of real and imaginary components of the impedance on the frequency (Z(f ) and Z(f )) demonstrates the rising role of the contact barriers between carbon nanotubes inside hybrid films, consisting of the carbon nanotubes and inorganic tungsten disulfide nanotubes as the temperature was decreased. The active component of the impedance was found to prevail in the AC electrical properties of the hybrid films, consisting of multiwall carbon nanotubes and WS₂ nanoparticles over the entire available temperature range.

Silica aerogels are unique solids with extremely high porosity (>99.5% air by volume), transparency and low density. With their high surface area and thermal resistivity they make excellent heat insulators. However, due to their fine structure they are fragile and therefore impractical for structural applications. To improve their mechanical properties we suggest incorporation of minute amounts of tungsten disulfide nanotubes. Nanotubes of tungsten disulfide are known for their high mechanical strength, strain and thermal stability. Adding some 0.1–1 wt% of tungsten disulfide nanotubes to a variety of polymers clearly lead to substantial improvement in their mechanical properties. The current study reports the preparation of silica aerogels compounded with small amounts of tungsten disulfide nanotubes. Three-point bending and uniaxial compression tests of the composite aerogel revealed 85% and 23% improvement in the composite material toughness, respectively.

Palladium nanoparticles were deposited on multiwall WS₂ nanotubes. The composite nanoparticles were characterized by a variety of techniques. The Pd nanoparticles were deposited uniformly on the surface of WS₂ nanotubes. An epitaxial relationship between the (111) plane of Pd and the (013) plane of WS₂ was mostly observed. The composite nanoparticles were found to perform efficiently as catalysts for cross-coupling (Heck and Suzuki) reactions. The role of the nanotubes\u2032 support in the catalytic process is briefly discussed.

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS₂, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS₂, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS₂ layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds. (Figure Presented).

Inorganic, fullerene-like (IF) nanoparticles of tungsten disulfide (WS2) and molybdenum disulfide (MoS2) have been synthesised in the past and their useful tribological properties have been studied quite extensively. Rhenium doping of such nanoparticles was also reported and studied to some extent. Herein, further studies of the physico-chemical properties are reported in connection to lubrication under mild loads. Due to their self-repelling character, the doped nanoparticles form tessellated monolayers on certain substrates. When mixed in small amounts, the IF nanoparticles lead to a reduction in the viscosity of water-based gels. Furthermore, the added nanoparticles lead to a precipitous reduction in traction force (friction) under mild loads. The lubrication mechanism of such nanoparticles is briefly discussed.

Impaired salivary gland (SG) function leading to oral diseases is relatively common with no adequate solution. Previously, tissue engineering of SG had been proposed to overcome this morbidity, however, not yet clinically available. Multiwall inorganic (tungsten disulfide [WS₂]) nanotubes (INT-WS₂) and fullerene-like nanoparticles (IF-WS₂) have many potential medical applications. A yet unexplored venue application is their interaction with SG, and therefore, our aim was to test the biocompatibility of INT/IF-WS₂ with the A5 and rat submandibular cells (RSC) SG cells. The cells were cultured and subjected after 1 day to different concentrations of INT-WS₂ and were compared to control groups. Growth curves, trypan blue viability test, and carboxyfluorescein succinimidyl ester (CFSE) proliferation assay were obtained. Furthermore, cells morphology and interaction with the nanoparticles were observed by light microscopy, scanning electron microscopy and transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy. The results showed no significant differences in growth curves, proliferation kinetics, and viability between the groups compared. Moreover, no alterations were observed in the cell morphology. Interestingly, TEM images indicated that the nanoparticles are uptaken by the cells and accumulate in cytoplasmic vesicles. These results suggest promising future medical applications for these nanoparticles.

The synthesis of nanotubes from layered compounds has generated substantial scientific interest. "Misfit" layered compounds (MLCs) of the general formula [(MX)_1+x]_m[TX₂]_n, where M can include Pb, Sb, rare earths; T=Cr, Nb, and X=S, Se can form layered structures, even though each sub-system alone is not necessarily a layered or a stable compound. A simple chemical method is used to synthesize these complex nanotubes from lanthanide-based misfit compounds. Quaternary nanotubular structures formed by partial substitution of the lanthanide atom in nanotubes by other elements are also confirmed. The driving force and mechanism of formation of these nanotubes is investigated by systematic temperature and time-dependent studies. A stress-inducement mechanism is proposed to explain the formation of the nanotubes. The resulting materials may find applications in fields that include thermoelectrics, light emitters, and catalysis and address fundamental physical issues in low dimensions. When two systems meet: Interactions between two systems comprising different in-plane periodicities can result in misfit layered compounds even if each subsystem alone is not a layered or stable compound. A combination of two independent stimuli, namely the incommensurability of the misfit lattice (often leading to folding and scrolling) and the reactivity of the layer rim atoms, provides a new strategy to synthesize misfit nanotubes.

Theoretical analysis of experimental data on unzipping multilayered WS ₂ nanotubes by consequent intercalation of lithium atoms and 1-octanethiol molecules [C. Nethravathi, et al., ACS Nano, 2013, 7, 7311] is presented. The radial expansion of the tube was described using continuum thin-walled cylinder approximation with parameters evaluated from ab initio calculations. Assuming that the attractive driving force of the 1-octanethiol molecule is its reaction with the intercalated Li ions ab initio calculations of a 1-octanethiol molecule bonding with Li⁺ were carried out. In addition, the non-chemical interactions of the 1-octanethiol dipole with an array of positive point charges representing Li⁺ were taken into account. Comparing between the energy gain from these interactions and the elastic strain energy of the nanotube allows us to evaluate a value for the tube wall deformation after the implantation of 1-octanethiol molecules. The ab initio molecular dynamics simulation confirmed our estimates and demonstrated that a strained WS₂ nanotube, with a decent concentration of 1-octanethiol molecules, should indeed be unzipped into the WS₂ nanoribbon.

Tubular structures of the MS-TaS₂ with (M = Pb, Sn, Sb, Bi) misfit layered compounds are reported. The lattice mismatch between the alternating MS and TaS₂ layers leads to a variety of chiral tubular structures. Such tubular structures are studied via scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). For the PbS-TaS₂ and SnS-TaS ₂ tubules, relative in-plane orientations as well as folding vectors of the two subsystems can be determined. However, almost ring-like SAED patterns are obtained for SbS-TaS₂ nanotubes precluding exact determination of the relative in plane orientation. Also, very complex diffraction patterns were obtained for BiS-TaS₂ nanotubes.

A new two-step procedure for the synthesis of MoS₂ nanotubes using lead as a growth promoter is reported. In the first step, molybdenum suboxide nanowhiskers containing a small amount of lead atoms were created by exposing a pressed MoS₂+Pb mixture to highly compressed shock-heated argon gas, with estimated temperatures exceeding 9900 K. In the second step, these molybdenum suboxide nanowhiskers served as templates for the sulfidization of the oxide into MoS₂ nanotubes (by using H₂S gas in a reducing atmosphere at 820 °C). Unlike the case of WS₂ nanotubes, the synthesis of a pure phase of MoS₂ nanotubes from molybdenum oxide has proven challenging, due mostly to the volatile nature of the latter at the high requisite reaction temperatures (>800 °C). In contrast, the nature and apparent reaction mechanism of the method reported herein are amenable to future scale-up. The high-temperature shockwave system should also facilitate the synthesis of new nanostructures from other layered materials.

The synthesis of inorganic nanotubes (INT) from layered compounds of a small size (

Carbon fullerenes and nanotubes revolutionized understandingof the reactivity of nanoscale compounds. Subsequently, our group and others discovered analogous inorganic compounds with hollow, closed nanostructures. Such inorganic nanostructures offer many applications, particularly in the energy and electronics industries.One way to create inorganic nanostructures is via misfit layer-ed compounds (MLC), which are stacks of alternating two-dimensional molecular slabs, typically held together via weak van der Waals forces. They contain "misfits" in their a-b plane structures that can make them unstable, leading to collapse of the slabs into tubular nanostructures. For example, metal chalcogenide MLCs of the general formula (MX)_1+y/ TX₂ (M = Sn, Pb, Bi, Sb, and other rare earths; T = Sn, Ti, V, Cr, Nb, Ta, etc.; X = S or Se) consist of a superstructure of alternating layers where the MX unit belongs to a (distorted NaCl) orthorhombic symmetry group (O), the TX₂ layer possesses trigonal (T) or octahedral symmetry, and the two layers are held together via both van der Waals and polar forces. A misfit in the a axis or both a and b axes of the two sublattices may lead to the formation of nanostructures as the lattices relax via scrolling. Previous research has also shown that the abundance of atoms with dangling bonds in the rims makes nanoparticles of compounds with layered structure unstable in the planar form, and they tend to fold into hollow closed structures such as nanotubes.This Account shows that combining these two triggers, misfits and dangling bond annihilation in the slab rims, leads to new kinds of nanotubes from MLCs. In particular, we report the structure of two new types of nanotubes from misfits, namely, the SnS/SnS₂ and PbS/NbS₂ series. To decipher the complex structures of these nanotubes, we use a range of methods: high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) analyses, scanning electron microscopy (SEM), and Cs-corrected scanning transmission electron microscopy (STEM) in the high-angle annular dark-field mode (HAADF). In both new types, the lattice mismatch between the two alternating sublayers dictates the relative layer-stacking order and leads to a variety of chiral tubular structures. In particular, the incommensuration (a type of misfit) of the SnS₂/SnS system in both the (in plane) a and b directions leads to a variety of relative in-plane orientation and stacking orders along the common c-axis. Thus the SnS/SnS₂ nanotubes form superstructures with the sequence O-T and O-T-T, and mixtures thereof. We also report nanotubes of the misfit layered compound (PbS)_1.14NbS₂, and of NbS ₂ intercalated with Pb atoms, with the chemical formula PbNbS ₂. Thus, the possibility to use two kinds of folding mechanisms jointly offers a new apparatus for the synthesis of unique 1-D nanostructures of great complexity and a potentially large diversity of physicochemical properties.

Closed-cage (fullerene-like) nanoparticles (NPs) of WS₂ are currently produced in large amounts and were investigated as additives to thermoplastics and thermosetting polymers. The nanoinduced morphology and the resulting enhanced fracture toughness of epoxy/WS₂ nanocomposites were studied. The morphology of the epoxy nanocomposites was induced by controlled WS₂ surface chemistry. The WS₂ NPs used were either untreated or chemically treated with acryloxy, which is compatible, and alkyl silane, which is incompatible, respectively, with the epoxy matrix. In the case where the acryloxy silane was used to treat the WS₂ particles, good dispersion and compatibility were obtained in the epoxy resin_. Moreover, a distinct nodular morphology was induced on fracture as a result of nucleation by the compatible NPs. In the case where the alkyl silane treatment was used cavitation morphology was induced, following mechanical loading, which is the result of incompatibility with the epoxy resin. The fracture toughness results showed an increase of 70% for nanocomposites contains alkyl-treated WS₂ compared with the neat epoxy. Modeling of the nodular morphology enabled the determination of optimal concentration of the WS₂ in epoxy (0.3% by weight). Two main fracture mechanisms were observed, crack bowing around the nodular boundaries in the case of compatibility between the nanoparticle and the epoxy and particle-induced cavitation in the case of incompatibility, respectively. These results are of significant importance both for epoxy-based adhesives and fiber composites. POLYM. ENG. SCI., 53:2624-2632, 2013. © 2013 Society of Plastics Engineers

In this paper the synthesis of inorganic nanotubes (INT) and to a lesser extent, inorganic fullerene-like nanoparticles (IF) of WS₂, which have been recently scaled-up, is discussed in some detail. Subsequently the mechanical properties of IF/INT are summarized, in reference to their remarkable possibility to reinforce polymer matrices and serve as superior solid lubricants. The effect of adding minute amounts of such nanoparticles to various polymer matrices is reviewed with reference to thermoplastic and thermosetting polymers. Possible different applications of such nanocomposites are also briefly discussed.

Polymer-inorganic nanoparticle composites are an area of great research interest. Inorganic fullerene-like (IF) structures and inorganic nanotubes (INT) are used for producing composite materials with specific goals of reinforcement of the polymer and ameliorating its thermal stability. Here, a composite material containing INT and conducting polymer (polyaniline (PANI)) is synthesized by an in situ oxidative polymerization of the monomer in the presence of WS₂ nanotubes. The structure and electrical behavior of PANI/INT composite is studied and compared with the results of pristine PANI (synthesized under the same conditions without INT). Most remarkably, INT-WS₂ are found to play the role of an active doping agent. The highest conductivity is obtained for 0.85 wt% (ca. 0.06 at%) nanotubes content, two orders of magnitude higher than that of pristine PANI. This work suggests a new approach to control the host-guest interaction in the polymer nanocomposite via (Lewis) acid-base equilibrium. A composite material containing WS ₂ nanotubes (INT-WS₂) and conducting polymer (polyaniline (PANI)) is obtained by an in situ oxidative polymerization of the monomer in the presence of the INT-WS₂. Electron transfer from PANI to the INT-WS₂ is shown to produce a peak in the conductivity at 0.85 wt% (0.2 at%) of the nanotubes.

WS₂ nanoribbons have been synthesized by chemical unzipping of WS₂ nanotubes. Lithium atoms are intercalated in WS₂ nanotubes by a solvothermal reaction with n-butyllithium in hexane. The lithiated WS₂ nanotubes are then reacted with various solvents - water, ethanol, and long chain thiols. While the tubes break into pieces when treated with water and ethanol, they unzip through longitudinal cutting along the axes to yield nanoribbons when treated with long chain thiols, 1-octanethiol and 1-dodecanethiol. The slow diffusion of the long chain thiols reduces the aggression of the reaction, leading to controlled opening of the tubes.

We report a reasonably high yield (∼50%) synthesis of silicon carbide (SiC) nanowires from silicon oxides and carbon in vacuum, by novel solar and lamp photothermal ablation methods that obviate the need for catalysis, and allow relatively short reaction times (∼10 min) in a nominally one-step process that does not involve toxic reagents. The one-dimensional core/shell β-SiC/SiO_x nanostructures - characterized by SEM, TEM, HRTEM, SAED, XRD and EDS - are typically several microns long, with core and outer diameters of about 10 and 30 nm, respectively. HRTEM revealed additional distinctive nanoscale structures that also shed light on the formation pathways.

We investigated the infrared vibrational properties of pristine and Re-substituted MoS₂ nanoparticles and analyzed the extracted phonon lifetimes in terms of multiple scattering events. Our measurements reveal both size- and doping-dependent changes that we attribute to grain boundary scattering and charge and mass effects, respectively. By contrast, Born charge is affected only by size. These findings illustrate the utility of reaching beyond traditional bulk semiconductors and quantum dots to explore how doping and confinement impact carrier-phonon interactions in low-dimensional semiconducting nanomaterials.

Mechanical properties and fracture behaviors of multiwalled WS₂ nanotubes produced by large scale fluidized bed method were investigated under uniaxial tension using in situ transmission electron microscopy probing; these were directly correlated to the nanotube atomic structures. The tubes with the average outer diameter ∼40 nm sustained tensile force of ∼2949 nN and revealed fracture strength of ∼11.8 GPa. Surprisingly, these rather thick WS₂ nanotubes could bear much higher loadings than the thin WS ₂ nanotubes with almost "defect-free" structures studied previously. In addition, the fracture strength of the "thick" nanotubes did not show common size dependent degradation when the tube diameters increased from ∼20 to ∼60 nm. HRTEM characterizations and real time observations revealed that the anomalous tensile properties are related to the intershell cross-linking and geometric constraints from the inverted cone-shaped tube cap structures, which resulted in the multishell loading and fracturing.

The tribological properties of rhenium-doped inorganic fullerene-like MoS₂ nanoparticles were studied in poly alpha olefin oil, and compared to inorganic fullerene-like (IF) MS₂ (M=Mo,W) nanoparticles and 2H-MoS₂ platelets. For this purpose a rotational disc tribometer was used. Different regimes in the Stribeck curve, i.e., boundary, mixed and elasto-hydrodynamic lubrication conditions were studied. A precipitous reduction in friction and wear of oil suspensions with nanoparticles were observed for the entire IF samples. However, Re-doped IF exhibited the best results, presumably due to their reduced tribocharging and agglomeration. The various tribological mechanisms of solid state additives are discussed.

A reactivity screening of new nano-hydrodesulfurization (HDS) catalysts was conducted using an ambient pressure flow reactor as well as ultra-high vacuum kinetics techniques. Thiophene was used as a probe molecule. Clean multiwall WS₂ nanotubes (INT-WS₂ ) as well as Ni- and Cocoated INT-WS₂ were considered. In addition, undoped MoS₂ and Re-doped nanoparticles with fullerene-like structures were studied. Commercial Ni and Co HDS catalysts from Haldor Topsoe (Denmark) as well as "nano MoS₂" from Impex Corp. (USA) were considered as reference materials. The lab-synthesized and commercial systems broke down thiophene into quite similar non-sulfur containing products, as identified by a gas chromatograph. The Ni and Co promoted catalysts showed similar thiophene conversion rates. Although the commercial catalysts had larger thiophene conversion rates than the laboratory-synthesized systems, the Re-doped nano-HDS catalyst showed quite low rates of formation of H₂S, an undesirable by-product.

The electrical properties of WS ₂ nanotubes (NTs) were studied through measuring 59 devices. Important electrical parameters, such as the carrier concentration, mobility, and effective barrier height at the contacts, were obtained through fitting experimental non-linear I-V curves using a metal-semiconductor-metal model. The carrier mobility was found to be several orders of magnitude higher than that have been reported previously for WS ₂ NTs. Water absorption was found to decrease the conductivity and carrier mobility of the NTs, and could be removed when the sample was dried. Oxygen absorption also slightly decreased the conductivity of WS ₂ NTs.

WS ₂ nanostructures hold structural characteristics which suggest they will be suitable for heterogeneous catalysis in the hydrodesulfurization (HDS) process. In this work, WS ₂ nanotubes (INT-WS ₂) were coated with cobalt nanoparticles using electroless plating method. Prior to cobalt deposition, the nanotubes surface was activated using palladium seeding process. The deposited cobalt nanoparticles had hcp crystal structure and formed non-uniform layer on the nanotubes surface. The catalytic reactivity of the produced cobalt coated nanotubes toward thiophene decomposition was characterized by an atmospheric flow reactor. The coated nanotubes revealed good catalytic reactivity toward thiophene mineralization. Further, the adsorption kinetics of thiophene on coated INT-WS ₂ was studied by thermal desorption spectroscopy (TDS). The cobalt coated system was found to be more catalytically active than the pristine INT-WS ₂ system. This result is promising since further optimization of the nanofabrication process of the catalyst should increase the conversion rates even further.

A synthetic route for preparation of inorganic WS ₂ nanotube (INT)-colloidal semiconductor quantum dot (QD) hybrid structures is developed, and transient carrier dynamics on these hybrids are studied via transient photoluminescence spectroscopy utilizing several different types of QDs. Measurements reveal efficient resonant energy transfer from the QDs to the INT upon photoexcitation, provided that the QD emission is at a higher energy than the INT direct gap. Charge transfer in the hybrid system, characterized using QDs with band gaps below the INT direct gap, is found to be absent. This is attributed to the presence of an organic barrier layer due to the relatively long-chain organic ligands of the QDs under study. This system, analogous to carbon nanotube-QD hybrids, holds potential for a variety of applications, including photovoltaics, luminescence tagging and optoelectronics. This journal is

Individual WS₂ nanotubes (NTs) were bended and released in-situ inside a transmission electron microscope. Kinks appeared in the NTs suddenly during the bending process. Shell distortion, shell breaking and rotation of broken shells were observed in these kinks. Spontaneous self-healing and focused electron beam-assisted healing of the structure were observed after the load was removed. More importantly, the strength and electrical conductivity of the NTs were also restored to their original level when the structure recovered.

The phenomenon of a partial 2H→1T phase transition within multiwalled WS ₂ nanotubes under substitutional rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional tight-binding calculations for the related MoS ₂ compound, we consider a possible origin of this phase transition, which was known formerly only for WS ₂ and MoS ₂ intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity.

It is now possible to synthesize inorganic nanotubes (INT) of WS2 in a pure phase and substantial amounts. Due to their crystalline perfection, they are characterized by excellent mechanical behavior. This study is dedicated to the investigation of the effect of INT-WS2 on the mechanical, thermal, adhesion and tribological properties of epoxy based nanocomposites. Various concentrations up to 1.0 wt% of the INT were added to the epoxy matrix. First a method was developed to mix the nanotubes in the epoxy resin matrix. A combination of both magnetic stirring and ultrasonic mixing was used. The adhesion, fracture toughness and strain energy release rate were studied. The INT-WS2 were found to significantly improve all these properties. The wear of the nanotubes-reinforced epoxy was eight-times lower than that of pure epoxy. These results suggest numerous applications.

In this work chromium-rich coatings impregnated with fullerene-like (IF)-WS₂ nanoparticles were deposited on stainless steel substrates. The coatings were obtained from a trivalent chromium bath at pH 2 by galvanostatic electrodeposition. Zinc and cobalt salts were added to the aqueous solution in small amounts serving as cationic growth promoters. Photodeposition of tin-palladium nanoparticles was used as seeding enhancer for the co-deposition of the fullerene-like nanoparticles. The coatings were characterized by a number of techniques and were found to show a decreasing gradient of the IF nanoparticles towards the film-substrate interface. Tribological tests showed that in contrast to the substrate and the pure metal coating, the IF-containing films exhibit low friction and wear.

We report on the synthesis of inorganic fullerene-like molybdenum disulfide (MoS₂) nanoparticles by pulsed laser ablation (PLA) in water. The final products were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and resonance Raman spectroscopy, etc. Cell viability studies show that the as-prepared MoS₂ nanoparticles have good solubility and biocompatibility, which may show a great potential in various biomedical applications. It is shown that the technique of PLA in water also provides a green and convenient method to synthesize novel nanomaterials, especially for biocompatible nanomaterials.

The ability to form blends of polymers offers the opportunity of creating a new class of materials with enhanced properties. In addition to the polymer components, recent advances in nanoengineering have resulted in the development of nanosized inorganic particles that can be used to improve the properties of the blend, such as the flammability and the mechanical properties. While traditional methods using copolymer compatibilizers have been used to strengthen polymer blends, here, we show that the inorganic nanosized filler additive can also serve as a compatibilizer as it can localize to the interface between the polymers. We use experimental and theoretical studies to show the fundamental mechanisms by which inorganic fillers with large aspect ratio and at least one-dimension in the nanometer range, can act as non-specific compatibilizers for polymer blends. We examine a series of nanosized fillers, ranging from nanotubes to nanoclays (with varying aspect ratios) in a model polystyrene (PS)/poly(methylmethacyralate) (PMMA) blend. Using a number of experimental techniques such as transmission electron microscopy (TEM), scanning tunneling X-ray microscopy (STXM), and atomic force microscopy (AFM) we postulate that the mechanism of compatibilization occurs as a result of the fillers forming in situ grafts with the immiscible polymers. We also use theoretical studies to show that the aspect ratio and the bending energy of the fillers play a key role in the compatibilization process. Our results indicate that the compatibilization is a general phenomenon, which should occur with all large aspect ratio nanofiller additives to polymer blends.

A track record of generating novel fullerene-like and nanotubular inorganic nanostructures from Cs₂O, SiO_2-x, WS₂, and MoS₂ by photothermal ablation with highly concentrated sunlight and ultra-bright lamp light is reviewed, and augmented with new results for exfoliated MoS₂ and carbon as well as carbon nanotubes.

A nanostructure, being either an Inorganic Fullerene-like (IF) nanostructure or an Inorganic Nanotube (INT), A1-x-Bx-chalcogenide are described. A being a metal or transition metal or an alloy of metals and/or transition metals, B being a metal or transition metal B different from that of A and x being ≤0.3. A process for their manufacture and their use for modifying the electronic character of A-chalcogenide are described.

New materials and techniques pertaining to the synthesis of inorganic nanotubes have been ever increasing since the initiation of the field in 1992. Recently, WS2 nanotubes, which are produced now in large amounts, were filled with molten lead iodide salt by a capillary wetting process, resulting in PbI₂@WS₂ core-shell nanotubes. This work features progress in the synthesis of new core-shell nanotubes, including Bil₃@WS ₂ nanotubes produced in a similar same manner. In addition, two new techniques for obtaining core-shell nanotubes are presented. The first is via electron-beam irradiation, i.e., in situ synthesis within a transmission electron microscope. This synthesis results in SbI₃ nanotubes, observed either in a hollow core Of WS₂ ones (Sbl₃@WS ₂ nanotubes), or atop of them (WS₂@Sbl₃ nanotubes). The second technique involves a gaseous phase reaction, where the layered product employs WS₂ nanotubes as nucleation sites. In this case, the MoS₂ layers most often cover the WS₂ nanotube, resulting in WS₂@MoS₂ core-shell nanotubes. Notably, superstructures of the form MoS₂@WS₂ are occasionally obtained. Using a semi-empirical model, it is shown that the Pbl₂ nanotubes become stable within the core of MoS2 nanotubes only above a critical core diameter of the host (>12 nm); below this diameter the PbI₂ crystallizes as nanowires. These model calculations are in agreement with the current experimental observations, providing further support to the growth mechanism of such core-shell nanotubes.

In this work the effect of inorganic fullerene-like (closed cages) nanoparticles of tungsten disulfide (IF-WS₂) on the mechanical properties and especially on the toughness of epoxy resins, was studied. The epoxy resin used was the well-known DGEBA (di-glycidyl ether of bis-phenol A) cured with polyamidoamine. The epoxy/IF-WS₂ nanocomposites were prepared by applying a high shear mixing to obtain a uniform dispersion and homogeneous distribution of the IF nanoparticles in the epoxy matrix. Two mixing procedures were used - a low shear of short duration and high shear with a long mixing time. The resulting epoxy nanocomposites were first characterized for their shear and peel strength using appropriate bonded joints. The experimental results demonstrate that enhanced shear strengths and shear moduli were achieved, together with a significant increase in the peel strengths at low concentrations of the IF-WS₂ nanoparticles (more than 100% increase at 0.5 wt% IF-WS₂). Above the threshold value of 0.5% IF-WS ₂ the peel strength decreased sharply. The fractured surfaces of the bonded joints were examined by transmission and scanning electron microscopy in order to characterize the fracture mechanisms and analyze the dispersion level of the nanoparticles within the polymer. The electron micrographs indicated that the presence of the nanoparticles in the epoxy matrix induced fracture mechanisms which were different from those observed in the pristine epoxy phase. These mechanisms included: crack deflection; crack bowing; and crack pinning. Evidence for a chemical interaction between the nanoparticles and the epoxy were obtained by infrared measurements in the attenuated total transmittance mode. The data suggests the formation of new carbon-oxygen-sulfur bonds, which are most likely due to the reaction of the outermost sulfur layer of the IF nanoparticles with the reactive epoxy groups. The observed simultaneous increase in both shear and peel strengths at very low IF-WS₂ concentrations, found in this work, could lead to the development of high performance adhesives and to new types of structural and ballistic fiber nanocomposites.

WS₂ belongs to a family of layered metal dichalcogenide compounds that are known to form cylindrical (inorganic nanotubes-INT) and polyhedral nanostructures - onion or nested fullerene-like (IF) particles. The outermost layers of these IF nanoparticles can be peeled under shear stress, thus IF nanoparticles have been studied for their use as solid lubricants. However, the IF nanoparticles tend to agglomerate, presumably because of surface structural defects induced by elastic strain and curvature, a fact that has a deleterious effect on their tribological properties. In the present work, chemical modification of the IF-WS₂ surface with alkyl-silane molecules is reported. The surface-modified IF nanoparticles display improved dispersion in oil-based suspensions. The alkyl-silane coating reduces the IF-WS₂ nanoparticles' tendency to agglomerate and consequently improves the long-term tribological behavior of oil formulated with the IF additive.

WS₂ inorganic nanotubes (INT) and inorganic fullerene-like nano-particles (IF) are well-known for their high mechanical strength and as superior solid lubricants. The outermost WS₂ layer is considered to be fully bonded; thus, it was suggested that the interactions of these WS₂ nanostructures with their surroundings are governed by purely van der Waals (vdW) interactions. However, in the case of IF-WS₂ nanoparticles, the faceted surface may contain sites with nonsaturated coordination, which, in turn, react with the surrounding media. Gold nanoparticles(GNP) wereusedas probesforthe IF-WS₂ surfacedefects,mapped by both scanning and transmission electron microscopy. The interaction between the GNP and the reactive surface was investigated using INT-WS₂ as a model and was characterized by atomic force microscopy (AFM).

Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS ₂ nanoparticles were synthesized by reacting MoO ₃ vapor with H ₂S in a reducing atmosphere. The nanoparticles were found to be of high crystalline order; with an average size of 70 nm and consist of more than 30 closed shells. Extensive tribological testing of the nanoparticles in two types of synthetic oils- poly-alpha olefins (PAO)- was carried out and compared to that of bulk (2H platelets) MoS ₂ and IF-WS ₂. These tests indicated that under high pressure and relatively low humidity, the IF-MoS ₂ exhibited a friction coefficient as low as 0.03 and the smallest wear rate of the measured systems. However, its performance was found to be lower in comparison to IF-WS ₂ after 2500 cycles, due probably to its inferior chemical stability. This study indicates that the tribological performance of the IF nanoparticles depends strongly on their crystalline order and size.

Inorganic fullerene-like WS₂ and MoS₂ nanoparticles have been synthesized using exclusively solid precursors, by reaction of the corresponding metal oxide nanopowder, sulfur and a hydrogen-releasing agent (NaBH₄ or LiAlH₄), achieved either by conventional furnace heating up to a 900 °C or by photothermal ablation at far higher temperatures driven by highly concentrated white light. In contrast to the established syntheses that require toxic and hazardous gases, working solely with solid precursors permits relatively safer reactor conditions conducive to industrial scale-up.

In this article a comparison between inorganic nanoparticles with hollow closed structure and the carbon fullerenes and nanotubes is undertaken. First, the structural evolution of inorganic fullerene-like (IF) nanoparticles of MoS₂ as a function of their size is examined in some detail and compared to that of carbon and BN fullerenes. It is shown that hollow closed structures of MoS₂ are stable above 3 nm (app 103 atoms). In the range of 3-8 nm (103-105) nanooctahedra with metallic character are the most stable form of MoS₂ Semiconducting nanotubes and quasispherical IF nano-particles become the stable-most form beyond that size and the bulk (platelets) are stable above about 0.2 μm. The stability of inorganic nanotubes is also discussed. The scaling-up of the synthesis of IF-WS ₂ and the very recent successful synthesis of large, amounts of pure WS₂ nanotubes are briefly described. The stability of IF and INT of MoS₂ (WS₂) under pressure and that of carbon is also discussed. Applications of the IF-WS₂ as superior solid lubricants, which lead to their recent commercialization, is demonstrated.

The growth mechanism of WS 2 nanotubes in the large-scale fluidized-bed reactor is studied in greater detail. This study and careful parameterization of the conditions within the reactor lead to the synthesis of large amounts (50-100 g/batch) of pure nanotubes, which appear as a fluffy powder, and (400-500 g/batch) of nanotubes/ nanoplatelets mixture (50:50), where nanotubes usually coming in bundles. The two products are obtained simultaneously in the same reaction but are collected in different zones of the reactor, in a reproducible fashion. The characterization of the nanotubes, which grow catalyst-free, by a number of analytical techniques is reported. The majority of the nanotubes range from 10 to 50 micron in length and 20-180 nm in diameter. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior with numerous applications.

With the emergence and commercialization of nanoparticles, new opportunities have emerged for toughening of epoxy adhesives using nanoparticles without sacrificing strength, rigidity and glass transition temperature, as is the case with conventional elastomeric tougheners. Inorganic Fullerene-like tungsten disulfide (IF-WS₂) nanoparticles and functionalized nano-POSS (Polyhedral-Oligomeric-Sil-Sesquioxane) were used to study the effects of nanoparticles on the toughening and mechanical properties of low and high temperature curing epoxy systems. Experimental results indicated that IF-WS ₂ increased the fracture toughness by more than 10 fold in both epoxy systems at very low concentrations (0.3-0.5 wt%) while increasing its storage modulus and preserving its glass transition temperature. Epoxy functionalized POSS demonstrated an increase in toughness in addition to preserving rigidity and thermal properties at higher concentrations (3 wt%). It was postulated that chemical interaction of the sulfide and the epoxy matrix and the inherent properties of WS₂ were the decisive factors with respect to the outstanding nano-effect in the case IF-WS₂.

We experimentally observed atomic-scale torsional stick-slip behavior in individual nanotubes of tungsten disulfide (WS₂). When an external torque is applied to a WS₂ nanotube, all its walls initially stick and twist together, until a critical torsion angle, at which the outer wall slips and twists around the inner walls, further undergoing a series of stick-slip torque oscillations. We present a theoretical model based on density-functional-based tight-binding calculations, which explains the torsional stick-slip behavior in terms of a competition between the effects of the in-plane shear stiffness of the WS₂ walls and the interwall friction arising from the atomic corrugation of the interaction between adjacent WS₂ walls.

The characterization of nanostructures down to the atomic scale is essential to understand some physical properties. Such a characterization is possible today using direct imaging methods such as aberration-corrected high-resolution transmission electron microscopy (HRTEM), when iteratively backed by advanced modeling produced by theoretical structure calculations and image calculations. Aberration-corrected HRTEM is therefore extremely useful for investigating low-dimensional structures, such as inorganic fullerene-like particles and inorganic nanotubes. The atomic arrangement in these nanostructures can lead to new insights into the growth mechanism or physical properties, where imminent commercial applications are unfolding. This article will focus on two structures that are symmetric and reproducible. The first structure that will be dealt with is the smallest stable symmetric closed-cage structure in the inorganic system, a MoS₂ nanooctahedron. It is investigated by means of aberration-corrected microscopy which allowed validating the suggested DFTB-MD model. It will be shown that structures diverging from the energetically most stable structures are present in the laser ablated soot and that the alignment of the different shells is parallel, unlike the bulk material where the alignment is antiparallel. These findings correspond well with the high-energy synthetic route and they provide more insight into the growth mechanism. The second structure studied is WS ₂ nanotubes, which have already been shown to have a unique structure with very desirable mechanical properties. The joint HRTEM study combined with modeling reveals new information regarding the chirality of the different shells and provides a better understanding of their growth mechanism.

The recent progress in intercalation of nanotubes and fullerene-like nanoparticles with various metals was reviewed. Nanotubes from layered materials and also those from non layered compounds were discussed. While nanotubes from layered compounds are generically crystalline those of non layered compounds have in most cases a polycrystalline structure. Different intercalation routes were presented and their relative merits and pitfalls were discussed. Furthermore, the changes in the structural and physical properties of the nanoparticles which accompany the intercalation reaction were described. Special attention was paid to the implementation of the nanotubes as a potential electrode material in lithium based batteries.

Inorganic fullerene-like (IF) Mo_1-xRe_xS₂ and W_1-xRe_xS₂ nanoparticles have been synthesized by a gasphase reaction involving the respective metal halides with H₂S. The IF-Mo(W)_1-xRe_xS₂ nanoparticles, contain - ing up to 5% Re, were characterized by a variety of experimental techniques. Analyses of the X-ray powder diffraction and different electron microscopy techniques show that the Re is doped in the MoS₂ host lattice. Interestingly, Re-dop ed MoS₂ nanotubes are present as well, although in small quantities (∼5%). XPS results confirm the nanoparticles to be more n-type arising from the effect of Re doping. Additionally, density-functional tight-binding (DFTB) calculations support the observed n-type behavior.

WS₂ inorganic fullerene-like (IF) nanoparticles were subjected to intercalation with potassium, sodium, and rubidium atoms in heated sealed ampules. The product of the intercalation process was not pure and was composed of both intercalated and nonintercalated phases. X-ray diffraction measurements under inert conditions of the intercalated powders showed that the interlayer expansion was correlated with the alkali metal radius. Small increase of the a-axis was observed as well and was explained on the grounds of the WS₂ band structure. The XPS analysis of the rubidium intercalated material showed a rise in the Fermi energy as a result of the intercalation, endowing the originally p-type nanoparticles an n-type character.

Inorganic fullerene-like (IF) nanoparticles and nanotubes of WS₂ were synthesized by a gas phase reaction starting from WCl_n (n = 4, 5, 6) and H₂S. The effect of the various metal chloride precursors on the formation of the products was investigated during the course of the study. Various parameters have been studied to understand the growth and formation of the IF-WS₂ nanoparticles and nanotubes. The parameters that have been studied include flow rates of the various carrier gases, heating of the precursor metal chlorides and the temperature at which the reactions were carried out. The best set of conditions wherein maximum yields of the high quality pure-phase IF-WS₂ nanoparticles and nanotubes are obtained have been identified. A detailed growth mechanism has been outlined to understand the course of formation of the various products of WS₂.

Following the discovery of fullerenes (C₆₀) and carbon nanotubes, it was shown that nanoparticles of inorganic layered compounds, like WS₂ and MoS₂, are unstable in the planar form and they form closed cage structures with polyhedral or nanotubular shapes. Although initially the method of synthesis for the formation of such closed caged structures and nanotubes involved starting from the respective oxides, it is now well established that the gas-phase synthetic route (using metal chlorides, carbonyls etc) provides an alternative which is suitable for the synthesis of very many closed caged structures and nanotubes hitherto unknown. Various issues with this method of synthesis, including its fundamentals, mechanism, and the properties of the inorganic fullerene-like structures produced are reviewed, together with some possible applications.

In this paper, we report on the synthesis and applications of semiconducting nanostructures. Nanostructures of interest were zinc oxide (ZnO) nanowires and tungsten disulfide WS₂ nanotubes where transistors/phototransistors and photovoltaic (PV) energy conversion cells have been fabricated. ZnO nanowires were grown with both high- and low-temperature approaches, depending on the application. Individual ZnO nanowire side-gated transistors revealed excellent performance with a field-effect mobility of 928 cm² V·s. ZnO networks were proposed for large-area macroelectronic devices as a less lithographically intense alternative to individual nanowire transistors where mobility values in excess of 20 cm² /V·s have been achieved. Flexible PV devices utilizing ZnO nanowires as electron acceptors and for photoinduced charge separation and transport have been presented. Phototransistors were fabricated using individual WS₂ nanotubes, where clear sensitivity to visible light has been observed. The results presented here simply reveal the potential use of inorganic nanowires/tubes for various optoelectronic devices.

Fullerene-like WS ₂ (MoS ₂) nanoparticles (IF) have been studied in the past as solid lubricants. Using the tribological ball-on-flat experiments, it was shown that the size of the aggregates and their distribution determine the penetration and entrapping of the IF nanoparticles at the interface. It is expected that the wedge clearance at the inlet of the contact, i.e., the oblique-angle entrance to the contact zone between the two mating tribological surfaces, as well as the average surface roughness, can limit the supply of the lubricant into the interface in, e.g., the block-on-ring experiment. In the present series of experiments, the Stribeck curve was designed first using a linear loading scheme and pure oil. It was concluded that a wedge clearance (oblique-angle) in the inlet of the contact zone leads to entrapment of the IF nanoparticles and their compaction, which hamper the supply of the fluid lubricant into the interface. A ball-on-flat and flat block-on-ring friction devices with wedge clearance in the inlet of the contact can distort the efficacy of IF. Procedures for improving the supply of the IF nanoparticles to the contact zone and improving thereby their efficacy are considered.

It is already well established today that numerous materials form closed-cage structures, of which carbon fullerenes and nanotubes are a special case [1]. Inorganic fullerene-like nanoparticles (designated IF) and inorganic nanotubes (INT) have been produced by different routes and experimental techniques, achieving persistent growth of a variety of materials and structural wealth within them. The research in this area has focused on synthesizing new IF and INT materials and understanding their different properties as well as scaling up the synthetic process in order to make it suitable for industrial applications. In this review, the main synthetic procedures to obtain inorganic fullerene-like nanoparticles and nanotubes will be discussed alongside with the different mechanisms that affect the morphology of the final product. The main differences between the morphologies will be presented. Some general considerations relating the properties of the parent compound with the morphology of the product will be mentioned.

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS ₂. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T³ dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T³ dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T³ phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.

Composite coatings of Co + fullerene-like WS₂ nanoparticles on stainless steel substrate were obtained through electroless deposition, using DMAB (dimethyl borane complex, 97%) as the reducing agent, and by electroplating in acidic solution. Phase analysis results show that the coatings consist of Co and the fullerene-like WS₂ nanoparticles alone. Tribological measurements show reduced wear and friction of the composite coatings as compared with the pure cobalt film or the stainless steel substrate.

Inorganic fullerene-like (IF) solid lubricant nanoparticles with extremely useful tribological characteristics have been realized, offering a plethora of new applications for these nanomaterials. The IF nanoparticles were found to be in the aggregated state. The main goal of this work is to elucidate the effect of the mixing time of IF-WS₂ nanomaterial in the oil on the size of the IF aggregates and their influence on the friction and wear. The fraction of small aggregate increases and that of the large aggregates decreases with longer mixing time. Consequently, the spread of the tribological results diminishes with the lengthening of the mixing time. The reproducibility of the friction results for the pairs lubricated with oil +IF nanoparticles is determined by distribution of the IF aggregates in the lubricant and the size of the solid lubricant aggregates.

A simple, inexpensive, and reproducible photothermal procedure for synthesizing IF-Cs₂O nanoparticles with immensely concentrated sunlight was demonstrated. The solar-driven synthesis of IF-Cs₂O was performed in evacuated quartz ampoules that contained 3R-Cs₂-O crystallites under continuous irradiation with a concentrated solar power of 2.0-7.7 W, and periods ranging from 30 to 840 s. Samples were collected from the deposit that gathered on the quartz ampoule during the solar irradiation for TEM analysis. The interaction of the beam with the materials led to thermal motion, charging, desorption of trace quantities, or possible chemical changes. The results of the electron energy loss spectroscopy and imaging with a Gatan imaging filters proved that only cesium and oxygen were present in the nanoparticles.

Inorganic fullerene-like ( IF) nanoparticles of MoS2 were synthesized using gas-phase reaction starting from MoCl5 and H2S. The IF-MoS2 nanoparticles are spherical and in some cases faceted with diameters in general ranging between 20 and 80 nm. The IF-MoS2 nanoparticles have large hollow cores, filled in some cases with amorphous material. Various parameters have been investigated to understand the growth and formation of the IF-MoS2 nanoparticles. The parameters that have been studied include flow rates of the various carrier gases, temperature at which the reaction was carried out, time of the reaction and heating of the precursor material. The best set of conditions wherein maximum yields of the IF-MoS2 nanoparticles are obtained have been identified. Additionally, annealing the as-obtained samples or heating them in a mixture of H-2 along with H2S improves the crystallinity and reduces the amorphous material filling in the core. Apart from the fullerene-like nanoparticles under certain experimental conditions nanotubes of MoS2 have also been obtained nonetheless in small yields.

Electrical resistivity and Hall effect measurements of pellets compacted from fullerene-like WS2 nanoparticles (IF-WS2) and bulk 2H-WS2 powder were carried out using the van der Pauw method over a wide temperature range. In addition IF-WS2 pellets were annealed at elevated temperatures under vacuum in a specially designed system. Arrhenius plots for the conductivities of the WS2 samples (2H, IF and IF+annealing) exhibit marked uprise of partial derivative In (sigma T-1)/partial derivative T-1 with temperature. The resistivity of the non-annealed IF-WS2 pellets is higher by 2-8 orders of magnitude than that of 2H-WS2 pellets, whereas the resistivity of the annealed IF pellets is higher than that of the non-annealed ones. Hall Effect measurements at 300 K show p-type conductivity and similar carrier concentration for both types of materials. The carrier mobility of 2H-WS2 platelets is found to be in the range of the reported values. However, IF-WS2 pellets have shown an unusually low mobility for a semiconducting material. The experimental data was found to be in a good agreement with a model used for analyzing the conductivity of polycrystalline semiconductors, which takes into consideration fluctuations of the barrier heights among the different nanoparticles as well as within a single nanoparticle boundary. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Bi₂Se₃ nanorods have been synthesized through a simple hydrothermal reduction approach. The nanorods formed were ≈10nm in diameter and 100-200nm in length. XRD characterization suggested that the product consisted of the hexagonal phase of pure Bi₂Se₃. EDX and XPS studies further confirmed the composition and purity of the product. A possible mechanism for the reaction is proposed, where Bi₂Se ₃ microsheets are presumed to be the intermediate for the formation of the nanorods. The effect of solvent on the morphology of the final product is discussed, where, in the presence of aprotic solvent DMF, nanoparticle formation is observed. A bandgap of 2.25eV is observed from the UV-visible absorption spectra.

Although graphite, with its anisotropic two-dimensional lattice, is the stable form of carbon under ambient conditions, on nanometre length scales it forms zero- and one-dimensional structures, namely fullerenes and nanotubes, respectively. This virtue is not limited to carbon and, in recent years, fullerene-like structures and nanotubes have been made from numerous compounds with layered two-dimensional structures. Furthermore, crystalline and polycrystalline nanotubes of pure elements and compounds with quasi-isotropic (three-dimensional) unit cells have also been synthesized, usually by making use of solid templates. These findings open up vast opportunities for the synthesis and study of new kinds of nanostructures with properties that may differ significantly from the corresponding bulk materials. Various potential applications have been proposed for the inorganic nanotubes and the fullerene-like phases. Fullerene-like nanoparticles have been shown to exhibit excellent solid lubrication behaviour, suggesting many applications in, for example, the automotive and aerospace industries, home appliances, and recently for medical technology. Various other potential applications, in catalysis, rechargeable batteries, drug delivery, solar cells and electronics have also been proposed.

We report the rapid high-yield generation of inorganic fullerene-like cesium oxide (IF-CS2O) nanoparticles, activated by highly concentrated sunlight. The solar process represents an alternative to the only reported method for synthesizing IF-CS2O nanostructures: laser ablation. IF-CS2O formed at solar irradiation >= 6W, confirmed by high resolution transmission electron microscopy. These closed-cage Cs2O nanostructures are stable under electron microscope conditions, and also when exposed temporarily to air - of significance for their use in a variety of photonic devices.

TiS2 nanoparticles with nested fullerene-like structure (IF) 60-120 nm in size consisting of up to 100 concentric molecular layers and having quite a perfectly spherical shape were obtained by reacting TiCl4 and H2S using first a horizontal and subsequently a vertical reactor. The proposed growth mechanism of these nanoparticles, i.e. nucleation and growth, is radically different from the one proposed for the growth of the fullerene-like WS2 from the respective oxide nanoparticles. It was found that adding 1-2% IF-TiS2 improves the behavior of lubricating oil substantially. The improved performance of the additive was attributed to the nearly spherical shape of the nanoparticles which promotes rolling friction.

The excellent shock-absorbing performance of WS₂ and MoS ₂ nanoparticles with inorganic fullerene-like structures (IFs) under very high shock wave pressures of 25 GPa is described. The combined techniques of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, thermal analysis, and transmission electron microscopy have been used to evaluate the diverse, intriguing features of shock recovered IFs, of interest for their tribological applications, thereby allowing improved understanding of their antishock behavior and structure-property relationships. Two possible failure mechanisms are proposed and discussed. The supershock-absorbing ability of the IF-WS₂ enables them to survive pressures up to 25 GPa accompanied with concurrent temperatures of up to 1000 °C without any significant structural degradation or phase change making them probably the strongest cage molecules now known.

We present the results from an experimental study of the magnetotransport of superconducting wires of amorphous indium-oxide having widths in the range 40-120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those that underlie the operation of a superconducting quantum interference device.

Fullerene-like WS₂ (MoS₂) nanoparticles (IF) have been studied in the past. It was shown that the IF nanopaticles appear to form a protective film allowing increased load capacity of the rubbed pairs under mixed lubrication. The main objective of the present work is to evaluate the behavior of the IF nanoparticles under severe contact conditions - the transition to seizure. The effect of the IF nanoparticles burnished to porous alumina matrix on friction and wear of alumina-Si₃N₄ pair under high contact pressure is also considered. It was shown that when the gap between the contact surfaces is smaller than the size of the IF nanoparticles, there is no effect of the nanopaprticles on the friction force. With load increasing, the IF nanoparticles penetrate into the interface, protecting the rubbed surfaces from a direct contact and thus increase the critical point of transition to seizure. Burnishing of the porous alumina surfaces by the IF solid lubricant nanoparticles provides very low friction coefficient and wear loss under high contact pressure. The IF-WS₂ nanoparticles are found to be preserved in the rough summits, fill the valleys, and the pores of the sintered alumina and thus limit the straight contact between the ceramic surfaces. Although the external sheets of the outermost layers of the IF-WS₂ nanoparticles were peeled-off, the pristine IF nanoparticles remain in the valleys and pores of the alumina supplying the solid lubricant sheets to the contact area during a long-term test.

Recently, the behavior of inorganic fullerene-like (IF) WS₂ nanoparticles in the interface of steel-on-steel pair has been analyzed. It was shown that originally when the gap between the contact surfaces is smaller than the size of the IF nanoparticles, there is no effect of the nanoparticles on the friction force. During the test stiff IF nanoparticles can plough the surface of hard steel samples and penetrate into the interface under friction. Molecular sheets of WS₂ from the delaminated IF nanoparticles, which reside in the valleys of the rough surfaces cover the contact spots and thus decrease the number of adhered spots at the transition to seizure. The goal of the present work was to study the behavior of IF nanoparticles in the interface of ceramic surfaces. The friction tests were performed using a ball-on-flat device. A silicon nitride ball was slid against an alumina flat with maximum contact pressure close to 2 GPa. SEM, TEM and AFM techniques have been used in order to assess the behavior of IF nanoparticles in the interface. The behavior of IF nanoparticles in the much harder ceramic interfaces was found to be appreciably different from the steel pair. The pristine IF nanoparticles are damaged in the inlet of the contact during the first few cycles and thin shells of broken nanoparticles gradually cover the middle range of the contact surface. Different modes of deformation and destruction of the IF nanoparticles are exhibited when going from the middle to edge area of the contact. While aggregates of the pristine nanoparticles are formed at the edge of the contact, thin shells of broken IF nanoparticles are observed in the middle area where contact pressure is maximum. Mechanical stability and damage of IF nanoparticles in the ceramic interface are discussed.

Nanoparticles of WS₂ and MoS₂ with a closed cage structure (fullerene-like) that are termed IF phases are synthesized in large amounts in a pure form. These nanoparticles were shown to play a favorable role as solid lubricants under severe conditions where fluids are unable to support the heavy load and are squeezed away from the contact area. Various tribological scenarios are presented for these superior solid lubricants, demonstrating the large scale potential for applications of these materials. The mechanism of action of these solid lubricants is briefly discussed. Various other potential applications of IF phases for nanocomposites with high impact resistance; in rechargeable batteries and in optical devices are discussed in short.

The Young's modulus of WS2 nanotubes is an important property for various applications. Measurements of the mechanical properties of individual nanotubes are challenging because of the small size of the tubes. Lately, measurements of the Young's modulus by buckling of an individual nanotube using an atomic force microscope(1) resulted in an average value of 171GPa. Tensile tests of individual WS2 nanotubes were performed experimentally using a scanning electron microscope and simulated tensile tests of MoS2 nanotubes were performed by means of a density-functional tight-binding (DFTB) based molecular dynamics (MD) scheme. Preliminary results for WS2 nanotubes show Young's modulus value of ca. 162GPa, tensile strength value of ca. 13GPa and average elongation of ca. 12%. MD simulations resulted in elongation of 19% for zigzag and 17% for armchair MoS2 single wall nanotubes. Since MoS2 and WS2 nanotubes have similar structures the same behavior is expected for both, hence there is a good agreement regarding the elongation of WS2 nanotubes between experiment and simulation.

Adding guest atoms to inorganic nanotubes, known as 'doping', influences their room-temperature magnetic properties - properties that could be exploited in 'spintronic' devices and computer memory.

Cesium oxides are materials of great interest to the photodetection industry because of their relatively low work function (∼1 eV). Used mainly as coating films for photoemissive devices, they provide high wavelength thresholds and high photocurrents. However, they are unstable, air-sensitive, and hygroscopic, rendering them short-lived and limiting their applications. Although the technology of these devices is highly developed, their characterization on the micro- and nanoscale suffers from their poor chemical stability and poor crystallinity. In the present study, cesium oxides were synthesized from the elements and were characterized using a combination of chemical and structural analysis techniques. Because the reaction products were extremely sensitive to humidity, sample analysis without atmospheric exposure was essential, and techniques were developed for the transfer of the samples to the measurements systems. Extensive data obtained from X-ray energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and Raman microscopy were obtained. Raman spectra with bands at 103, 742, and 1134 cm^-1 strongly confirmed the presence of the oxide, peroxide, and Superoxide ions, respectively, as well as the absence of carbonate as an impurity. The A_1g mode of Cs₂O was detected as an anti-Stokes band at 103 cm^-1. This study provides further insight into the reactivity of the various cesium oxides.

Inorganic fullerene-like materials have been identified as being of potentially utmost importance for many industrial applications. MoS₂ and WS₂ hollow nanoparticles have been identified as strong candidates for tribological applications such as solid lubricants. The main goal of this work was to evaluate the mechanical properties of solid lubricant particles in ensemble under hydrostatic pressure. The behavior of nanopowders under compression has been described on the basis of constitutive models of continuum mechanics. The model will be applied to an isotropic compaction of copper (well-studied medium), fullerene-like (IF-WS₂) nanoparticles and a natural powder of 2H-WS₂ platelets. The morphology of individual nanoparticles and nanoparticle ensembles will be examined and discussed. Another aspect of this work was to study the applicability and limitations of the proposed constitutive model for the understanding of the tribological behavior of solid lubricant nanoparticles. Compression with the maximal pressure (500 MPa) showed that the shape of the IF nanoparticles is preserved. The dominant mechanism of damage was found to be the delamination or peeling-off of the external sheets of hollow nanoparticles. Strong destruction of 2H-WS₂ platelets was observed under compression.

Pronounced surface diffusion is observed during x-ray photoelectron spectroscopy measurements of 2H platelets and inorganic fullerene-like (IF) MS2 (M=WMo) powders, intercalated with alkaline (A=K,Na) elements. Using controlled surface charging the intercalants migrate towards the surface, where they oxidize. This dry deintercalation is controllable via external charging parameters, yet showing that internal chemical and structural parameters play an important role in the process. Diffusion rates out of 214 matrixes are generally higher than in corresponding IF samples. Clear differences are also found between Mo and W-based systems. Application of this approach into surface modification and processing is, proposed. (C) 2003 American Vacuum Society.

It was shown previously that nanoparticles of layered comppunds form closed-cage and nanotubular structures. These nanostructures received the generic name of inorganic fullerene-like (IF) materials. In particular nanoparticles of metal dihalides, like NiCl₂ and CdCl₂, were shown to form such closed-cage structures in the past. In the present report faceted IF-CdI₂ nanoparticles encapsulating a Cd core are reported. These nanoparticles were obtained using electron beam irradiation of the source powder of CdI₂. The faceted nanoparticles exhibit one of two morphologies: hexagonal or elongated rectangular characters. The nanoparticles can be considered as core (Cd)-shell (CdI₂) nanostructures. Consistent with previous observations, this study shows that the seamless structure of the IF materials can stabilize phases, which are otherwise unstable Under the electron-beam irradiation. The growth mechanism of these nanostructures is briefly discussed.

Laser ablation has been extensively used for the synthesis of nanoparticles of various sorts, and in particular single wall carbon nanotubes and C₆₀ molecules. NiCl₂ nanotubes were recently also produced using this technique. While fullerene-like NiCl₂ structures can be obtained through regular ablation, vapor phase enriched with CCl₄ gas (reactive ablation) is necessary for the synthesis of the nanotubes. The experimental results indicate that the synthesis of such nanotubes is much more difficult than the synthesis of say MoS₂ or WS₂ nanotubes. Moreover, the NiCl₂ nanotubes are of larger diameter and consist on the average of more layers than their MoS₂ predecessors. First principle calculations show that single layer NiCl₂ nanotubes of diameter smaller than 54 nm are unstable and lose their outer chlorine atoms. In contrast, MoS₂ nanotubes with diameter of 2 nm and larger are found to be stable using the same kind of calculations. To gain better understanding of the differences between the materials, a review of the mechanical properties of layered metal dihalide and metal dichalcogenide compounds is undertaken. First principle calculations show that the Young's and bending moduli of NiCl₂ are almost twice larger than those of MoS₂. The large ionicity of NiCl₂ entails much larger shear and stacking fault energies for this compound as compared to MoS₂, which explains its smaller propensity to bend and fold. These observations are supported by analysis of the corresponding Raman modes. Furthermore, metal dihalide compounds are very hygroscopic making their handling, and especially their analysis more difficult. This analysis explains the greater difficulties to grow NiCl₂ nanotubes or fullerene-like nanoparticles, as compared to their MoS₂ analogues.

Non-carbon inorganic fullerene-like (IF) nanoscale materials have recently attracted intense interest due to their nested hollow and nanotube structures. In this letter, IF-WS₂ nanoparticles prepared by solid-gas reaction were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that the IF-WS₂ nanoparticles have a nested hollow closed spherical structure with diameter of 100-150 nm.

The shock-wave resistance of WS₂ nanotubes has been studied and compared to that of carbon nanotubes. Detailed structural features of post-shock samples were investigated using HRTEM, XRD, and Raman spectroscopy. WS₂ nanotubes are capable of withstanding shear stress caused by shock waves of up to 21 GPa, although some nanotube tips and nanoparticles containing multiple structural defects in the bending regions are destroyed. Small WS₂ species, consisting of only a few layers, are extruded from the nanotubes. Well-crystallized tube bodies were found to exhibit significant stability under shock, indicating high tensile strength. XRD and Raman analyses have confirmed this structural stability. Under similar shock conditions, WS₂ tubes are more stable than carbon nanotubes, the latter being transformed into a diamond phase. WS₂ nanotubes containing small concentrations of defects possess significantly higher mechanical strength, and, as a consequence, hollow WS₂ nanoparticles are expected to act as excellent lubricants under much higher loading than was previously thought.

Friction and wear of powder materials impregnated with commercially available layered (platelets) WS₂ (2H) and inorganic fullerene-like WS₂ nanoparticles (IF) were studied. Bronze-graphite, iron-graphite and iron-nickel-graphite samples were used in this experiment. The linear wear of powder materials (in situ) was measured. It was shown that the IF nanoparticles impregnated into the pores improve the tribological properties of powder materials in comparison to a reference sample or the sample impregnated with 2H solid lubricant particles. The mechanisms of friction and wear of the IF nanoparticles have been considered. The tribological role of the wear particles and nanoparticles of solid lubricants has been analyzed in the framework of a third body lubrication model. The state of the IF nanoparticles before and after the wear test was studied. It was found that the shape of the IF nanoparticles is preserved during the friction tests under high loads. Thin wear debris surrounded by spherical IF nanoparticles appear to be formed and provide easily sheared lubrication film (low friction coefficient) during friction experiments of powder materials containing IF nanoparticles.

Inorganic Fullerene-like (IF)-MoS2 nanoparticles were tested under boundary lubrication and ultra-high vacuum (UHV) and were found to give an ultra-low friction coefficient in both cases compared to hexagonal (h)-MoS2 material. Previous works made by Rapoport et al. with IF-WS2 revealed that the benefit effect of the inorganic fullerene-like materials decreases at high loads and sliding velocities. Nevertheless, under the conditions used in our experiments using high contact pressure (maximum pressure above 1.1 GPa in oil and 400 MPa in high vacuum) and slow sliding velocities (1.7 mm/s in oil test and 1 mm/s in hi-h vacuum), friction always decreases and stabilizes at about 0.04 for 800 cycles in both cases. Therefore, IF-MoS2 material appears to be a good candidate for use in various environments in regard to other MoS2 crystal structures. Wear mechanisms were investigated using both High Resolution TEM and surface analyses (XPS) on the wear tracks. Wear particles collected from the flat wear scar show several morphologies, suggesting at least two lubricating mechanisms. As spherical particles are found in the wear debris, rolling may be a possible event. However, flattened and unwrapped IF-MoS2 particles are often observed after friction. In this case, low friction is thought to be due either to sliding between IF-MoS2 external flattened planes or to slip between individual unwrapped MoS2 sheets. (C) 2002 Elsevier Science B.V. All rights reserved.

Laser ablation is a powerful technique for producing nanoparticles such as C₆₀. In comparison to high-temperature gas-phase or solid/gas-phase reactions, the laser ablation technique is found to be suitable for the growth of nano-crystals of a specific shape and size. This paper reports on the synthesis, by means of a modified laser ablation technique, of inorganic fullerene-like structures (IF) of NiCl₂, in the form of polyhedral and nanotubes.

The Raman spectrum of Cs₂O₂ excited at 632.8 nm shows a single O₂^-2 fundamental band at wavenumber of 742.5 cm^-1 and second and third overtones at 1468.5 and 2176.8 cm^-1, respectively. The harmonic wavenumber ω_e = 759.5 ± 0.7 cm^-1 and anharmonicity constant ω_eχ_e = 8.4 ± 0.3 cm^-1 were calculated directly from the measurements. These results are compared with ω_e = 1151.0 ± 1.5 cm-^-1 and ω_eχ_e = 9.6 ± 0.5 cm^-1 recently found for O₂^- ion in CsO₂ by others. The lower ω_eχ_e of O₂^-2 relative to that of O₂^- indicates a broader (smaller curvature) anharmonic oscillator potential well.

Layered metal disulfides-MS₂ (M = Mo, W) in the form of fullerene-like nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer and X-ray photoelectron spectroscopy (XPS). The alkali metal concentration in the host lattice was found to depend on the kind of sample and the experimental conditions. Furthermore, an inhomogeneity of the intercalated samples was observed. The product consisted of both nonintercalated and intercalated phases. X-ray diffraction analysis and transmission electron microscopy of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 Å) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. Deintercalation of the hydrated alkali atoms and restacking of the MS₂ layers was observed in all the samples after prolonged exposure to the atmosphere. Electric field induced deintercalation of the alkali metal atoms from the host lattice was also observed by means of the XPS technique. Magnetic moment measurements for all the samples indicate a diamagnetic to paramagnetic transition after intercalation. Measurements of the transport properties reveal a semiconductor to metal transition for the heavily K intercalated 2H-MOS₂. Other samples show several orders of magnitude decrease in resistivity and two- to five-fold decrease in activation energies upon intercalation. These modifications are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Recovery of the pristine compound properties (diamagnetism and semiconductivity) was observed as a result of deintercalation.

The impregnation of inorganic fullerene-like nanoparticles of WS₂ (IF) allows to improve effectively the tribological properties of powdered materials in comparison to the impregnation of oil or commercially available layered WS₂ (2H) particles. The main goal of this work was to determine the dominant lubrication regimes under friction of the bronze, iron, and iron-nickel porous matrixes impregnated with 2H and IF solid lubricants. The tribological tests were performed at laboratory atmosphere (humidity Ο50%) using a ring-on-block tester at the sliding speed of V = 1 m/s, and the loads of 150-1000 N. The wear of the metal bodies was measured by an eddy current probe system and by weighting of the samples before and after the test. In order to evaluate the radial clearance, the profiles of wear blocks were analyzed by profile projector. Than these data were used in the calculation of the Sommerfeld reciprocal values. Friction and wear results were presented as the Stribeck curves. The critical Sommerfeld reciprocals were evaluated from these curves. The Stribeck curves were compared with the Morgan's curves for different ranges of non-dimensional permeability, ψ. These results are used then in calculation of the permeability, Φ. Three lubrication regimes as: quasi-hydrodynamic, boundary or mixed, and dry friction were revealed under friction of porous samples in the load-range studied. It was found that the impregnation of IF nanoparticles provides the regime of quasi-hydrodynamic lubrication in the widest range of loads in comparison to the reference sample and the sample impregnated with 2H-WS₂. Fe-Ni samples exhibited the highest wear resistance and provided the widest range of quasi-hydrodynamic lubrication in comparison to bronze and iron powdered composites. The effect of IF on the regimes of lubrication is explained on the base of the third-body model. It is expected that the sliding/rolling of the IF nanoparticles in the boundary of the first bodies and in between the wear particles (third-body) facilitate the shear of the lubrication film and thus provide the quasi-hydrodynamic regime of friction. It is supposed that the roll shape of IF nanoparticles allows to release the IF from the pores to contact surfaces.

Recent experimental results demonstrate that it is possible to grow a variety of different multiphase, nested nanotube structures. This paper predicts the structure and energetics of such multiphase nanotubes. There are several distinct contributions to the energetics: the internal and external surface energies, the energy of the interface between the different phases, the long-range (van der Waals) interactions between interfaces, and the elastic bending energy. We perform energy minimizations to compare the energies of two- and three-layer films and nanotubes. We present physical guidelines, quantitative theory, and structure maps that show how materials and geometric parameters influence the stability of competing structures.

Ni-P composite coating containing inorganic fullerene-like WS₂ nanosize particles was prepared by electroless codeposition. Its tribological performances were evaluated by a ring-on-block wear tester. It was found that the Ni-P-(IF-WS₂) composite coating exhibited both higher wear resistance and lower friction coefficient than Ni-P, Ni-P-(layer 2H-WS ₂) and Ni-P-graphite electroless coating. The favorable effects of inorganic fullerene-like nanomaterials on the tribological properties of the composite coating were discussed.

In the past few years, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide WS₂ and MoS₂ with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. Recent experiments showed that IF added to oil and impregnated into the porous matrixes possess lubricating properties superior to those of layered WS₂ and MoS₂ (2H platelets). The main goal of this work was to analyze the mechanism of friction of fullerene-like nanoparticles. Friction and wear behavior of IF in different contact conditions is studied. Third body model is considered. Sliding/rolling of the IF nanoparticles in the boundary of the first bodies and in between the wear particles (third body) is supposed to facilitate the shear of the lubrication film. Broken and oxidized 2H-WS₂ small pieces adhered to wear debris do not provide high tribological properties especially under high loads.

Inorganic fullerene-like (hollow onion-like) nanoparticles (IF) and nanotubes are of significant interest over the past few years due to their unusual crystallographic morphology and their interesting physical properties. The synthesis of inorganic fullerene-like spherical MoS2 nanoparticles (IF-MoS2 of 5-300nm in diameter was studied in the present work. This process is based on the previous formation of suboxide (MoO3-x) 5-300nm nanoparticles and their subsequent sulfidization. During the sulfidization process the overall geometrical parameters of the suboxide nanoparticles are preserved. The oxide nanoparticles were obtained in-situ by the condensation of the evaporated MoO3 powder precursor. The condensation was provoked not by cooling (conventional method for nano-size particle formation), but by a chemical reaction (partial reduction of the MoO3 vapor by hydrogen). hi this case the vapor pressure of the product (MoO2) was much lower than that of the precursor WOOD. Based on the comprehensive understanding of the IF-MoS2 growth mechanism from MoO3 powder, a gas phase reactor, which allowed reproducible preparation of a pure IF-MoS2 phase (up to 100mg/batch) with controllable sizes, is demonstrated. Further scale-up of this production is underway.

Inorganic nanotubes of WS₂ have been investigated by high-resolution transmission electron microscopy, and scanning tunneling microscopy, providing support for the theoretical prediction of correlation of bandgap with diameter.

The tribological behavior of platelet and nested colloidal particles of tungsten disulfide was studied using the surface forces apparatus, atomic force microscopy, lateral force microscopy, and Auger spectroscopy. Shear-induced material transfer from the colloidal particles to the surfaces was shown to be a dominant factor in the tribological behavior observed for both structures. An ultrathin, ordered layer was observed when nested particles were sheared, while WS₂ platelets produced a rough and disordered transfer layer, with substantially inferior lubricating properties.

We explore the possibility of controlling electronic properties along an inorganic nanotube (INT) through the influence of nanometer-scale features in the underlying substrate. We examined single multi-walled WS₂ INTs using scanning tunneling microscopy (STM) in high vacuum. As long as the INTs lie flat on MoS₂ (0001) or graphite (0001) surfaces, they appear semimetallic. However, when the INT is suspended above the surface due to crossing steps or other nanotubes, a band gap opens up. We discuss this observation in terms of either a potential drop under the INT, or a change in its electronic properties due to its distortion when it lies flat on a surface.

Densified iron-nickel-graphite blocks were prepared by powder metallurgy processes and subsequently impregnated with a few percent of fullerene-like WS₂ nanoparticles (

The inorganic analogs of carbon fullerenes and nanotubes, like MoS(2) and BN, are reviewed. It is argued that nanoparticles of 2D layered compounds are inherently unstable in the planar configuration and prefer to form closed cage structures. The progress in the synthesis of these nanomaterials. and, in particular. the large-scale synthesis of BN, WS(2) and V(2)O(5) nanotubes, are described. Some of the electronic, optical and mechanical properties of these nanostructures are reviewed. The red-shift of the energy gap with shrinking nanotube diameter is discussed as well as the suggestion that zigzag nanotubes exhibit a direct gap rather than an indirect gap, as is prevalent in many of the bulk 2D materials. Some potential applications of these nanomaterials are presented as well, most importantly the superior tribological properties of WS(2) and MoS(2) nested fullerene-like structures (onions).