Molecular junctions based on ferromagnetic electrodes allow the study of electronic spin transport near the limit of spintronics miniaturization. However, these junctions reveal moderate magnetoresistance that is sensitive to the orbital structure at their ferromagnet-molecule interfaces. The key structural parameters that should be controlled in order to gain high magnetoresistance have not been established, despite their importance for efficient manipulation, of spin transport at the nanoscale. Here, we show that single-molecule junctions based on nickel electrodes and benzene molecules can yield a significant anisotropic magnetoresistance of up to similar to 200% near the conductance quantum Go. The measured magnetoresistance is mechanically tuned by changing the distance between the electrodes, revealing a nonmonotonic response to junction elongation. These findings are ascribed with the aid of first-principles calculations to variations in the metal-molecule orientation that can be adjusted to obtain highly spin-selective orbital hybridization. Our results demonstrate the important role of geometrical considerations in determining the spin transport properties of metal-molecule interfaces.
Generating highly spin-polarized currents at the nanoscale is essential for spin current manipulations and spintronic applications. We find indications for up to 100% spin-polarized currents across nickel oxide atomic junctions formed between two nickel electrodes. The degree of spin polarization is probed by analyzing the shot noise resulting from the discrete statistics of spin-polarized electron transport. We show that spin filtering can be significantly enhanced by local chemical modifications at the single-atom level. This approach paves the way for effective manipulations of spin transport at the fundamental limit of miniaturization.
The vibration-mediated Kondo effect attracted considerable theoretical interest during the last decade. However, due to lack of extensive experimental demonstrations, the fine details of the phenomenon were not addressed. Here, we analyze the evolution of vibration-mediated Kondo effect in molecular junctions during mechanical stretching. The described analysis reveals the different contributions of Kondo and inelastic transport.
We investigate periodical oscillations in the conductance of suspended Au and Pt atomic chains during elongation under mechanical stress. Analysis of conductance and shot noise measurements reveals that the oscillations are mainly related to variations in a specific conduction channel as the chain undergoes transitions between zigzag and linear atomic configurations. The calculated local electronic structure shows that the oscillations originate from varying degrees of hybridization between the atomic orbitals along the chain as a function of the zigzag angle. These variations are highly dependent on the directionally and symmetry of the relevant orbitals, in agreement with the order-of-magnitude difference between the Pt and Au oscillation amplitudes observed in experiment. Our results demonstrate that the sensitivity of conductance to structural variations can be controlled by designing atomic-scale conductors in view of the directional interactions between atomic orbitals.
Using a break junction technique, we find a dear signature for the formation of conducting hybrid junctions composed of a single organic molecule (benzene, naphthalene, or anthracene) connected to chains of platinum atoms. The hybrid junctions exhibit metallic-like conductance (similar to 0.1-1G(0)), which is rather insensitive to further elongation by additional atoms. At low bias voltage the hybrid junctions can be elongated significantly beyond the length of the bare atomic chains. Ab initio calculations reveal that benzene based hybrid junctions have a significant binding energy and high structural flexibility that may contribute to the survival of the hybrid junction during the elongation process. The fabrication of hybrid junctions opens the way for combining the different properties of atomic chains and organic molecules to realize a new class of atomic scale interfaces.
The effect of electron-vibration interaction in atomic-scale junctions with a single conduction channel was widely investigated both theoretically and experimentally. However, the more general case of junctions with several conduction channels has received very little attention. Here we study electron-vibration interaction in multichannel molecular junctions, formed by introduction of either benzene or carbon dioxide between platinum electrodes. By combining shot noise and differential conductance measurements, we analyze the effect of vibration activation on conductance in view of the distribution of conduction channels. Based on the shift of vibration energy while the junction is stretched, we identify vibration modes with transverse and longitudinal symmetry. The detection of different vibration modes is ascribed to efficient vibration coupling to different conduction channels according to symmetry considerations. While most of our observations can be explained in view of the theoretical models for a single conduction channel, the appearance of conductance enhancement, induced by electron-vibration interaction, at high conductance values indicates either unexpected high electron-vibration coupling or interchannel scattering.
For the study of junctions formed by single molecules shot noise offers interesting new information that cannot be easily obtained by other means. At low bias it allows, for some cases of interest, determining the transmission probability and the number of current carrying conductance channels. By this method it is possible to identify the cross-over in sign of the inelastic scattering signal in the differential conductance. This is a first step towards the study of inelastic scattering signals in shot noise, as the second moment of the current.
We report on high-resolution electronic measurements of doped organic thin-film transistors using Kelvin probe force microscopy. Measurements conducted on field effect transistors made of N,N-I-diphenyl- N, N-I-bis(1-naphthyl)-1,1(I)-biphenyl-4,4(I)-diamine p-doped with tetrafluoro-tetracyanoquinodimethane have allowed us to determine the rich structure of the doping-induced density of states. In addition, the doping process changes only slightly the Fermi energy position with respect to the highest occupied molecular orbital level center. The moderate change is explained by two counter-acting effects on the Fermi energy position: the doping-induced additional charge and the broadening of the density of states.
The potential across an organic thin-film transistor is measured by Kelvin probe force microscopy and is used to determine directly the pinch-off voltage at different gate voltages. These measurements lead to the determination of a generalized threshold voltage, which corresponds to molecular level shift as a function of the gate voltage. A comparison between measured and calculated threshold voltage reveals a deviation from a simple Gaussian distribution of the transport density of states available for holes. (c) 2006 American Institute of Physics.
Kelvin probe force microscopy was used for extraction of the threshold and the pinch off voltages in organic thin film transistors. The first was determined by direct detection of the charge accumulation onset and the latter by a direct observation of the pinch off region formation. In addition, an effective threshold voltage shift can be extracted from the pinch-off voltage as a function of charge concentration. The dependence of the effective threshold voltage on the gate voltage must be considered when calculating charge carrier concentrations in organic thin film transistors.
We report on high-resolution potential measurements across complete metal/organic molecular semiconductor/metal structures using Kelvin probe force microscopy in inert atmosphere. It is found that the potential distribution at the metal/organic interfaces is in agreement with an interfacial abrupt potential changes and the work function of the different metals. The potential distribution across the organic layer strongly depends on its purification. In pure Alq(3) the potential profile is flat, while in nonpurified layers there is substantial potential bending probably due to the presence of deep traps. The effect of the measuring tip is calculated and discussed. (C) 2004 American Institute of Physics.
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 He flat on MoS2 (0 0 0 1) or graphite (0 0 0 1) 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. (C) 2001 Elsevier Science B.V. All rights reserved.