Stability of silicon-tin alloyed nanocrystals with high tin concentration synthesized by femtosecond laser plasma in liquid media.

Nanocrystals have a great potential for future materials with tunable bandgap, due to their optical properties that are related with the material used, their sizes and their surface termination. Here, we concentrate on the silicon-tin alloy for photovoltaic applications due to their bandgap, lower than bulk Si, and also the possibility to activate direct band to band transition for high tin concentration. We synthesized silicon-tin alloy nanocrystals (SiSn-NCs) with diameter of about 2-3 nm by confined plasma technique employing a femtosecond laser irradiation on amorphous silicon-tin substrate submerged in liquid media. The tin concentration is estimated to be [Formula: see text], being the highest Sn concentration for SiSn-NCs reported so far. Our SiSn-NCs have a well-defined zinc-blend structure and, contrary to pure tin NCs, also an excellent thermal stability comparable to highly stable silicon NCs. We demonstrate by means of high resolution synchrotron XRD analysis (SPring 8) that the SiSn-NCs remain stable from room temperature up to [Formula: see text] with a relatively small expansion of the crystal lattice. The high thermal stability observed experimentally is rationalized by means of first-principle calculations.

Deep-Learning Approach to First-Principles Transport Simulations.

Large-scale first-principles transport calculations, while essential for device modeling, remain computationally demanding. To overcome this bottle neck, we combine first-principles transport calculations with machine learning-based nonlinear regression. We calculate the electronic conductance through first-principles based nonequilibrium Green's function techniques for small systems and map the transport properties onto local properties using local descriptors. We show that using the local descriptor as input features for deep learning-based nonlinear regression allows us to build a robust neural network that can predict the conductance of large systems beyond that of the current state-of-the-art first-principles calculation algorithms. Our protocol is applied to alkali metal nanowires, i.e., potassium, which have unique geometrical and electronic properties and hence nontrivial transport properties. We demonstrate that within our approach we can achieve qualitative agreement with experiment at a fraction of the computational effort as compared to the direct calculation of the transport properties using conventional first-principles methods.

How To Probe the Limits of the Wiedemann-Franz Law at Nanoscale.

While the Wiedemann-Franz law is known to be robust for many bulk materials, possible violations have been actively discussed for certain classes of bulk materials such as heavy Fermion materials. At nanoscale on the other hand the limits of the Wiedemann-Franz law and how to probe and control them remains an open question. Therefore, we propose here a systematic way to elucidate the limits of the Wiedemann-Franz law at nanoscale. Using first-principles calculations, we examine the Wiedemann-Franz law in nanoscale conductors, namely in gold and platinum-based atomic wires. We explain the recently observed experimental evidence of the Wiedemann-Franz law in atomic-point contacts, but conversely we show that in regimes not discussed in these experiments notable violations of the Wiedemann-Franz law emerge. Depending on the temperature and gate potential as well as chemical properties and conformation, the violations reach up to 30% for gold and for platinum they can even exceed 350%.

The Orbital Selection Rule for Molecular Conductance as Manifested in Tetraphenyl-Based Molecular Junctions.

Using two tetraphenylbenzene isomers differing only by the anchoring points to the gold electrodes, we investigate the influence of quantum interference on the single molecule charge transport. The distinct anchor points are realized by selective halogen-mediated binding to the electrodes by formation of surface-stabilized isomers after iodine cleavage. Both isomers are essentially chemically identical and only weakly perturbed by the electrodes avoiding largely parasitic effects, which allows us to focus solely on the relation between quantum interference and the intrinsic molecular properties. The conductance of the two isomers differs by over 1 order of magnitude and is attributed to constructive and destructive interference. Our ab initio based transport calculations compare very well with the accompanying scanning tunneling microscope break junction measurements of the conductance. The findings are rationalized using a two level model, which shows that the interorbital coupling plays the decisive role for the interference effects.

Toward Multiple Conductance Pathways with Heterocycle-Based Oligo(phenyleneethynylene) Derivatives.

In this paper, we have systematically studied how the replacement of a benzene ring by a heterocyclic compound in oligo(phenyleneethynylene) (OPE) derivatives affects the conductance of a molecular wire using the scanning tunneling microscope-based break junction technique. We describe for the first time how OPE derivatives with a central pyrimidine ring can efficiently link to the gold electrode by two pathways presenting two different conductance G values. We have demonstrated that this effect is associated with the presence of two efficient conductive pathways of different length: the conventional end-to-end configuration, and another with one of the electrodes linked directly to the central ring. This represents one of the few examples in which two defined conductive states can be set up in a single molecule without the aid of an external stimulus. Moreover, we have observed that the conductance through the full length of the heterocycle-based OPEs is basically unaffected by the presence of the heterocycle. All these results and the simplicity of the proposed molecules push forward the development of compounds with multiple conductance pathways, which would be a breakthrough in the field of molecular electronics.

Single-molecule conductance of a chemically modified, π-extended tetrathiafulvalene and its charge-transfer complex with F4TCNQ.

We describe the synthesis and single-molecule electrical transport properties of a molecular wire containing a π-extended tetrathiafulvalene (exTTF) group and its charge-transfer complex with F4TCNQ. We form single-molecule junctions using the in situ break junction technique using a homebuilt scanning tunneling microscope with a range of conductance between 10 G0 down to 10(-7) G0. Within this range we do not observe a clear conductance signature of the neutral parent molecule, suggesting either that its conductance is too low or that it does not form a stable junction. Conversely, we do find a clear conductance signature in the experiments carried out on the charge-transfer complex. Due to the fact we expected this species to have a higher conductance than the neutral molecule, we believe this supports the idea that the conductance of the neutral molecule is very low, below our measurement sensitivity. This idea is further supported by theoretical calculations. To the best of our knowledge, these are the first reported single-molecule conductance measurements on a molecular charge-transfer species.

Characteristics of amine-ended and thiol-ended alkane single-molecule junctions revealed by inelastic electron tunneling spectroscopy.

A combined experimental and theoretical analysis of the charge transport through single-molecule junctions is performed to define the influence of molecular end groups for increasing electrode separation. For both amine-ended and thiol-ended octanes contacted to gold electrodes, we study signatures of chain formation by analyzing kinks in conductance traces, the junction length, and inelastic electron tunneling spectroscopy. The results show that for amine-ended molecular junctions no atomic chains are pulled under stretching, whereas the Au electrodes strongly deform for thiol-ended molecular junctions. This advanced approach hence provides unambiguous evidence that the amine anchors bind only weakly to Au.

Single-molecule junctions based on nitrile-terminated biphenyls: a promising new anchoring group.

We present a combined experimental and theoretical study of the electronic transport through single-molecule junctions based on nitrile-terminated biphenyl derivatives. Using a scanning tunneling microscope-based break-junction technique, we show that the nitrile-terminated compounds give rise to well-defined peaks in the conductance histograms resulting from the high selectivity of the N-Au binding. Ab initio calculations have revealed that the transport takes place through the tail of the LUMO. Furthermore, we have found both theoretically and experimentally that the conductance of the molecular junctions is roughly proportional to the square of the cosine of the torsion angle between the two benzene rings of the biphenyl core, which demonstrates the robustness of this structure-conductance relationship.

Influence of conformation on conductance of biphenyl-dithiol single-molecule contacts.

The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle phi between two phenyl rings; twisting the biphenyl system from flat (phi = 0 degrees ) to perpendicular (phi = 90 degrees ) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos(2) phi correlation between the junction conductance G and the torsion angle phi. The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the pi-pi coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy.