Spin-Crossover and Fragmentation of Fe(neoim)2 on Silver and Gold.

The neutral spin crossover complex Fe(neoim)2, neoim being the deprotonated form of the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline (neoimH), is investigated on the (111) surfaces of Au and Ag using scanning tunneling microscopy and density functional theory calculations. The complex sublimates and adsorbs intact on Ag(111), where it exhibits an electron-induced spin crossover. However, it fragments on Au. According to density functional theory calculations, the adsorbed complex is drastically distorted by the interactions with the substrates, in particular by van der Waals forces. Dispersion interaction is also decisive for the relative stabilities of the low- and high-spin states of the adsorbed complex. The unexpected instability of the complex on the gold substrate is attributed to enhanced covalent bonding of the fragments to the substrate.

Resonance-Enhanced Vibrational Spectroscopy of Molecules on a Superconductor.

Molecular vibrational spectroscopy with the scanning tunneling microscope is feasible but usually detects few vibrational modes. We harness sharp Yu-Shiba-Rusinov states observed from molecules on a superconductor to significantly enhance the vibrational signal. From a lead phthalocyanine molecule 46 vibrational peaks are resolved enabling a comparison with calculated modes. The energy resolution is improved beyond the thermal broadening limit and shifts induced by neighbor molecules or the position of the microscope tip are determined. Vice versa, spectra of vibrational modes are used to measure the effect of an electrical field on the energy of Yu-Shiba-Rusinov states. The method may help to further probe the interaction of molecules with their environment and to better understand selection rules for vibrational excitations.

Electron-Induced Spin-Crossover in Self-Assembled Tetramers.

The spin crossover compound Fe(H2B(pyrazole)(pyridylpyrazole))2 was investigated in detail on Ag(111) with scanning tunneling microscopy (STM). A large fraction of the deposited molecules condenses into gridlike tetramers. Two molecules of each tetramer may be converted between two states by current injection. We attribute this effect to a spin transition. This interpretation is supported by control experiments on the analogous, magnetically passive Zn compound that forms virtually identical tetramers but exhibits no switching. The switching yields were studied for various electron energies, and the resulting values exceed those reported from other SCO systems by 2 orders of magnitude. The other two molecules of a tetramer were immutable. However, they may be used as contacts for current injection that leads to conversion of one of their neighbors. This "remote" switching is fairly efficient with yields reduced by only one to two orders of magnitude compared to direct excitation of a switchable molecule. We present a model of the tetramer structure that reproduces key observations from the experiments. In particular, sterical blocking prevents spin crossover of two molecules of a tetramer. Density functional theory calculations show that the model indeed represents a minimum energy structure. They also reproduce STM images and corroborate a remote-switching mechanism that is based on electron transfer between molecules.

QuantumATK: an integrated platform of electronic and atomic-scale modelling tools.

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.

Stacked Janus Device Concepts: Abrupt pn-Junctions and Cross-Plane Channels.

Janus transition metal dichalcogenides with a built-in structural cross-plane (cp) asymmetry have recently emerged as a new class of two-dimensional materials with a large cp dipole. Using first-principles calculations, and a tailored transport method, we demonstrate that stacking graphene and MoSSe Janus structures result in record high homogeneous doping of graphene and abrupt, atomically thin, cross-plane pn-junctions. We show how graphene in contrast to metals can act as electrodes to Janus stacks without screening the cp dipole and predict a large photocurrent response dominated by a cp transport channel in a few-layer stacked device. The photocurrent is above that of a corresponding thin-film silicon device illustrating the great potential of Janus stacks, for example, in photovoltaic devices.

Cross-conjugation and quantum interference: a general correlation?

We discuss the relationship between the π-conjugation pattern, molecular length, and charge transport properties of molecular wires, both from an experimental and a theoretical viewpoint. Specifically, we focus on the role of quantum interference in the conductance properties of cross-conjugated molecules. For this, we compare experiments on two series of dithiolated wires. The first set we synthesized consists of three dithiolated oligo(phenylene ethynylene) (OPE) benchmark compounds with increasing length. The second series synthesized comprises three molecules with different π-conjugation patterns, but identical lengths, i.e. an anthracene (linear conjugation), an anthraquinone (cross-conjugation), and a dihydroanthracene (broken conjugation) derivative. To benchmark reliable trends, conductance experiments on these series have been performed by various techniques. Here, we compare data obtained by conductive-probe atomic force microscopy (CP-AFM) for self-assembled monolayers (SAMs) with single-molecule break junction and multi-molecule EGaIn data from other groups. For the benchmark OPE-series, we consistently find an exponential decay of the conductance with molecular length characterized by ß = 0.37 ± 0.03 Å(-1) (CP-AFM). Remarkably, for the second series, we do not only find that the linearly conjugated anthracene-containing wire is the most conductive, but also that the cross-conjugated anthraquinone-containing wire is less conductive than the broken-conjugated derivative. We attribute the low conductance values for the cross-conjugated species to quantum interference effects. Moreover, by theoretical modeling, we show that destructive quantum interference is a robust feature for cross-conjugated structures and that the energy at which complete destructive interference occurs can be tuned by the choice of side group. The latter provides an outlook for future devices in this fascinating field connecting chemistry and physics.

Energy level alignment and quantum conductance of functionalized metal-molecule junctions: density functional theory versus GW calculations.

We study the effect of functional groups (CH3*4, OCH3, CH3, Cl, CN, F*4) on the electronic transport properties of 1,4-benzenediamine molecular junctions using the non-equilibrium Green function method. Exchange and correlation effects are included at various levels of theory, namely density functional theory (DFT), energy level-corrected DFT (DFT+Σ), Hartree-Fock and the many-body GW approximation. All methods reproduce the expected trends for the energy of the frontier orbitals according to the electron donating or withdrawing character of the substituent group. However, only the GW method predicts the correct ordering of the conductance amongst the molecules. The absolute GW (DFT) conductance is within a factor of two (three) of the experimental values. Correcting the DFT orbital energies by a simple physically motivated scissors operator, Σ, can bring the DFT conductances close to experiments, but does not improve on the relative ordering. We ascribe this to a too strong pinning of the molecular energy levels to the metal Fermi level by DFT which suppresses the variation in orbital energy with functional group.

Phonon interference effects in molecular junctions.

We study coherent phonon transport through organic, π-conjugated molecules. Using first principles calculations and Green's function methods, we find that the phonon transmission function in cross-conjugated molecules, like meta-connected benzene, exhibits destructive quantum interference features very analogous to those observed theoretically and experimentally for electron transport in similar molecules. The destructive interference features observed in four different cross-conjugated molecules significantly reduce the thermal conductance with respect to linear conjugated analogues. Such control of the thermal conductance by chemical modifications could be important for thermoelectric applications of molecular junctions.

Surface disordered Ge-Si core-shell nanowires as efficient thermoelectric materials.

Ge-Si core-shell nanowires with surface disorder are shown to be very promising candidates for thermoelectric applications. In atomistic calculations we find that surface roughness decreases the phonon thermal conductance significantly. On the contrary, the hole states are confined to the Ge core and are thereby shielded from the surface disorder, resulting in large electronic conductance values even in the presence of surface disorder. This decoupling of the electronic and phonon transport is very favorable for thermoelectric purposes, giving rise to promising room temperature figure of merits ZT > 2. It is also found that the Ge-Si core-shell wires perform better than pure Si nanowires.

Observation of quantum interference in molecular charge transport.

As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and it becomes possible to control the conductance through direct manipulation of the electron wavefunction. Such control has been demonstrated in various mesoscopic devices at cryogenic temperatures, but it has proved to be difficult to exert control over the wavefunction at higher temperatures. Molecules have typical energy level spacings (∼eV) that are much larger than the thermal energy at 300 K (∼25 meV), and are therefore natural candidates for such experiments. Previously, phenomena such as giant magnetoresistance, Kondo effects and conductance switching have been observed in single molecules, and theorists have predicted that it should also be possible to observe quantum interference in molecular conductors, but until now all the evidence for such behaviour has been indirect. Here, we report the observation of destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. We studied five different rigid π-conjugated molecular wires, all of which form self-assembled monolayers on a gold surface, and find that the degree of interference can be controlled by simple chemical modifications of the molecular wire.

Controlling the transmission line shape of molecular t-stubs and potential thermoelectric applications.

Asymmetric line shapes can occur in the transmission function describing electron transport in the vicinity of a minimum caused by quantum interference effects. Such asymmetry can be used to increase the thermoelectric efficiency of molecular junctions. So far, however, asymmetric line shapes have been only empirically found for just a few rather complex organic molecules where the origins of the line shapes relation to molecular structure were not resolved. In the present, work we introduce a method to analyze the structure dependence of the asymmetry of interference dips from simple two site tight-binding models, where one site corresponds to a molecular π orbital of the wire and the other to an atomic p(z) orbital of a side group, which allows us to characterize analytically the peak shape in terms of just two parameters. We assess our scheme with first-principles electron transport calculations for a variety of t-stub molecules and also address their suitability for thermoelectric applications.

Robust conductance of dumbbell molecular junctions with fullerene anchoring groups.

The conductance of a molecular wire connected to metallic electrodes is known to be sensitive to the atomic structure of the molecule-metal contact. This contact is to a large extent determined by the anchoring group linking the molecular wire to the metal. It has been found experimentally that a dumbbell construction with C(60) molecules acting as anchors yields more well-defined conductances as compared to the widely used thiol anchoring groups. Here, we use density functional theory to investigate the electronic properties of this dumbbell construction. The conductance is found to be stable against variations in the detailed bonding geometry and in good agreement with the experimental value of G=3×10(-4) G(0). Electron tunneling across the molecular bridge occurs via the lowest unoccupied orbitals of C(60) which are pinned close to the Fermi energy due to partial charge transfer. Our findings support the original motivation to achieve conductance values more stable towards changes in the structure of the molecule-metal contact leading to larger reproducibility in experiments.

Graphical prediction of quantum interference-induced transmission nodes in functionalized organic molecules.

Quantum interference (QI) in molecular transport junctions can lead to dramatic reductions of the electron transmission at certain energies. In a recent work [Markussen et al., Nano Lett., 2010, 10, 4260] we showed how the presence of such transmission nodes near the Fermi energy can be predicted solely from the structure of a conjugated molecule when the energies of the atomic p(z) orbitals do not vary too much. Here we relax the assumption of equal on-site energies and generalize the graphical scheme to molecules containing different atomic species. We use this diagrammatic scheme together with tight-binding and density functional theory calculations to investigate QI in linear molecular chains and aromatic molecules with different side groups. For the molecular chains we find a linear relation between the position of the transmission nodes and the side group π orbital energy. In contrast, the transmission functions of functionalized aromatic molecules generally display a rather complex nodal structure due to the interplay between molecular topology and the energy of the side group orbital.

The relation between structure and quantum interference in single molecule junctions.

Quantum interference (QI) of electron pathways has recently attracted increased interest as an enabling tool for single-molecule electronic devices. Although various molecular systems have been shown to exhibit QI effects and a number of methods have been proposed for its analysis, simple guidelines linking the molecular structure to QI effects in the phase-coherent transport regime have until now been lacking. In the present work we demonstrate that QI in aromatic molecules is intimately related to the topology of the molecule's π system and establish a simple graphical scheme to predict the existence of QI-induced transmission antiresonances. The generality of the scheme, which is exact for a certain class of tight-binding models, is proved by a comparison to first-principles transport calculations for 10 different configurations of anthraquinone as well as a set of cross-conjugated molecular wires.

Electrochemical control of quantum interference in anthraquinone-based molecular switches.

Using first-principles calculations we analyze the electronic transport properties of a recently proposed anthraquinone-based electrochemical switch. Robust conductance on/off ratios of several orders of magnitude are observed due to destructive quantum interference present in the anthraquinone but absent in the hydroquinone molecular bridge. A simple explanation of the interference effect is achieved by transforming the frontier molecular orbitals into localized molecular orbitals thereby obtaining a minimal tight-binding model describing the transport in the relevant energy range in terms of hopping via the localized orbitals. The topology of the tight-binding model, which is dictated by the symmetries of the molecular orbitals, determines the amount of quantum interference.

Surface-decorated silicon nanowires: a route to high-ZT thermoelectrics.

Based on atomistic calculations of electron and phonon transport, we propose to use surface-decorated silicon nanowires for thermoelectric applications. Two examples of surface decorations are studied to illustrate the underlying ideas: nanotrees and alkyl functionalized silicon nanowires. For both systems we find (i) that the phonon conductance is significantly reduced compared to the electronic conductance leading to high thermoelectric figure of merit ZT, and (ii) for ultrathin wires, surface decoration leads to significantly better performance than surface disorder.

Heat conductance is strongly anisotropic for pristine silicon nanowires.

We compute atomistically the heat conductance for ultrathin pristine silicon nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature thermal conductance is found to be highly anisotropic: wires oriented along the <110> direction have 50-75% larger conductance than wires oriented along the <100> and <111> directions. We show that the anisotropies can be qualitatively understood and reproduced from the bulk phonon band structure. Ab initio density functional theory (DFT) is used to study the thinnest wires, but becomes computationally prohibitive for larger diameters, where we instead use the Tersoff empirical potential model (TEP). For the smallest wires, the thermal conductances obtained from DFT and TEP calculations agree within 10%. The presented results could be relevant for future phonon-engineering of nanowire devices.

Modeling transport in ultrathin Si nanowires: charged versus neutral impurities.

At room temperature dopants in semiconducting nanowires are ionized. We show that the long-range electrostatic potential due to charged dopants has a dramatic impact on the transport properties in ultrathin wires and can virtually block minority carriers. Our quantitative estimates of this effect are obtained by computing the electronic transmission through wires with either charged or neutral P and B dopants. The dopant potential is obtained from density functional theory (DFT) calculations. Contrary to the neutral case, the transmission through charged dopants cannot be converged within a supercell-based DFT scheme, because the system size implied by the long-ranged electrostatic potential becomes computationally unmanagable. We overcome this problem by modifying the DFT potential with finite element calculations. We find that the minority scattering is increased by a factor of 1,000, while majority transmission is within 50% of the neutral dopant results.

Scaling theory put into practice: first-principles modeling of transport in doped silicon nanowires.

We combine the ideas of scaling theory and universal conductance fluctuations with density-functional theory to analyze the conductance properties of doped silicon nanowires. Specifically, we study the crossover from ballistic to diffusive transport in boron or phosphorus doped Si nanowires by computing the mean free path, sample-averaged conductance G, and sample-to-sample variations std(G) as a function of energy, doping density, wire length, and the radial dopant profile. Our main findings are (i) the main trends can be predicted quantitatively based on the scattering properties of single dopants, (ii) the sample-to-sample fluctuations depend on energy but not on doping density, thereby displaying a degree of universality, and (iii) in the diffusive regime the analytical predictions of the Dorokhov-Mello-Pereyra-Kumar theory are in good agreement with our ab initio calculations.