Electrode-molecule energy level offsets in a gold-benzene diamine-gold single molecule tunnel junction.

One means for describing electron transport across single molecule tunnel junctions (MTJs) is to use density functional theory (DFT) in conjunction with a nonequilibrium Green's function formalism. This description relies on interpreting solutions to the Kohn-Sham (KS) equations used to solve the DFT problem as quasiparticle (QP) states. Many practical DFT implementations suffer from electron self-interaction errors and an inability to treat charge image potentials for molecules near metal surfaces. For MTJs, the overall effect of these errors is typically manifested as an overestimation of electronic currents. Correcting KS energies for self-interaction and image potential errors results in MTJ current-voltage characteristics in close agreement with measured currents. An alternative transport approach foregoes a QP picture and solves for a many-electron wavefunction on the MTJ subject to open system boundary conditions. It is demonstrated that this many-electron method provides similar results to the corrected QP picture for electronic current. The analysis of these two distinct approaches is related through corrections to a junction's electronic structure beyond the KS energies for the case of a benzene diamine molecule bonded between two gold electrodes.

Properties of homo- and hetero-Schottky junctions from first principle calculations.

Electronic structure calculations for a homo-material semimetal (thick Sn)/semiconductor (thin Sn) heterodimensional junction and two conventional metal (Ag or Pt)/silicon hetero-material junctions are performed. Charge distributions and local density of states are examined to compare the physics of junctions formed by quantum confinement in a homo-material, heterodimensional semimetal junction with that of conventional Schottky hetero-material junctions. Relative contributions to the Schottky barrier heights are described in terms of the interface dipoles arising due to charge transfer at the interface and the effects of metal induced gap states extending into the semiconducting regions. Although the importance of these physical mechanisms vary for the three junctions, a single framework describing the junction energetics captures the behaviors of both the heterodimensional semimetal junction and the more conventional metal/semiconductor junctions.

Electronic and structural properties of rhombohedral [1 1 1] and [1 1 0] oriented ultra-thin bismuth nanowires.

Structures and electronic properties of rhombohedral [1 1 1] and [1 1 0] bismuth nanowires are calculated with the use of density functional theory. The formation of an energy band gap from quantum confinement is studied and to improve estimates for the band gap the GW approximation is applied. The [1 1 1] oriented nanowires require surface bonds to be chemically saturated to avoid formation of metallic surface states, whereas the surfaces of the [1 1 0] nanowires do not support metallic surface states. It is found that the onset of quantum confinement in the surface passivated [1 1 1] nanowires occurs at larger critical dimensions than for the [1 1 0] nanowires. For the [1 1 1] oriented nanowires it is predicted that a band gap of ~0.5 eV can be formed at a diameter of approximately 6 nm, whereas for the [1 1 0] oriented nanowires a diameter of approximately 3 nm is required to achieve a similar band gap energy. The GW correction is also applied to estimates of the electron affinity, ionisation potentials and work functions for both orientations of the nanowires for various diameters below 5 nm. The magnitude of the energy band gaps that arise in bismuth at critical dimensions of a few nanometers are of the same order as for conventional bulk semiconductors.

A Semimetal Nanowire Rectifier: Balancing Quantum Confinement and Surface Electronegativity.

For semimetal nanowires with diameters on the order of 10 nm, a semimetal-to-semiconductor transition is observed due to quantum confinement effects. Quantum confinement in a semimetal lifts the degeneracy of the conduction and valence bands in a "zero" gap semimetal or shifts energy levels with a "negative" overlap to form conduction and valence bands. For semimetal nanowires with diameters less than 10 nm, the band gap energy can be significantly larger than the thermal energy at room temperature resulting in a new class of semiconductors suitable for nanoelectronics. As a nanowire's diameter is reduced, its surface-to-volume ratio increases rapidly leading to an increased impact of surface chemistry on its electronic structure. Energy level shifts to states in the vicinity of the Fermi energy with varying surface electronegativity are shown to be comparable in magnitude to quantum confinement effects arising in nanowires with diameters of a few nanometer; these two effects can counteract one another leading to semimetallic behavior at nanowire cross sections at which confinement effects would otherwise dominate. Abruptly changing the surface terminating species along the length of a nanowire can lead to an abrupt change in the surface electronegativity. This can result in the formation of a semimetal-semiconductor junction within a monomaterial nanowire without impurity doping nor requiring the formation of a heterojunction. Using density functional theory in tandem with a Green's function approach to determine electronic structure and charge transport, respectively, current rectification is calculated for such a junction. Current rectification ratios of the order of 103-105 are predicted at applied biases as low as 300 mV. It is concluded that rectification can be achieved at essentially molecular length scales with conventional biasing, while rivaling the performance of macroscopic semiconductor diodes.

Divacancies in carbon nanotubes and their influence on electron scattering.

First-principles calculations are applied to study the formation energies of various divacancy defects in armchair and zigzag carbon nanotubes of varying diameter, and the transport properties for the corresponding structures. Our explicit ab initio calculations confirm that the lateral 585 divacancy is the most stable defect in small diameter tubes, with the 555 777 divacancy becoming more stable in armchair tubes larger than (30, 30). Evaluating the electron transmission as a function of diameter and chirality for a range of defects, the strongest scattering is found for the 555 777 divacancy configuration, which is observable in electrical spectroscopy experiments. Finally, validation of an approximation relating contributions from independent scattering sites enables the study of the characteristic localization length in large diameter tubes. Despite the fixed number of channels, localization lengths increase with increasing diameter and can exceed 100 nm for typical defect densities.

Monte Carlo configuration interaction with perturbation corrections for dissociation energies of first row diatomic molecules: C2, N2, O2, CO, and NO.

Dissociation energies for the diatomic molecules C2, N2, O2, CO, and NO are estimated using the Monte Carlo configuration interaction (MCCI) and augmented by a second order perturbation theory correction. The calculations are performed using the correlation consistent polarized valence "triple zeta" atomic orbital basis and resulting dissociation energies are compared to coupled cluster calculations including up to triple excitations (CCSDT) and Full Configuration Interaction Quantum Monte Carlo (FCIQMC) estimates. It is found that the MCCI method readily describes the correct behavior for dissociation for the diatomics even when capturing only a relatively small fraction (∼80%) of the correlation energy. At this level only a small number of configurations, typically O(10(3)) from a FCI space of dimension O(10(14)), are required to describe dissociation. Including the perturbation correction to the MCCI estimates, the difference in dissociation energies with respect to CCSDT ranges between 1.2 and 3.1 kcal/mol, and the difference when comparing to FCIQMC estimates narrows to between 0.5 and 1.9 kcal/mol. Discussions on MCCI's ability to recover static and dynamic correlations and on the form of correlations in the electronic configuration space are presented.

A proposed confinement modulated gap nanowire transistor based on a metal (tin).

Energy bandgaps are observed to increase with decreasing diameter due to quantum confinement in quasi-one-dimensional semiconductor nanostructures or nanowires. A similar effect is observed in semimetal nanowires for sufficiently small wire diameters: A bandgap is induced, and the semimetal nanowire becomes a semiconductor. We demonstrate that on the length scale on which the semimetal-semiconductor transition occurs, this enables the use of bandgap engineering to form a field-effect transistor near atomic dimensions and eliminates the need for doping in the transistor's source, channel, or drain. By removing the requirement to supply free carriers by introducing dopant impurities, quantum confinement allows for a materials engineering to overcome the primary obstacle to fabricating sub-5 nm transistors, enabling aggressive scaling to near atomic limits.

Spin-polarization mechanisms of the nitrogen-vacancy center in diamond.

The nitrogen-vacancy (NV) center in diamond has shown great promise for quantum information due to the ease of initializing the qubit and of reading out its state. Here we show the leading mechanism for these effects gives results opposite from experiment; instead both must rely on new physics. Furthermore, NV centers fabricated in nanometer-sized diamond clusters are stable, motivating a bottom-up qubit approach, with the possibility of quite different optical properties to bulk.

Ballistic conductance in oxidized Si nanowires.

The influence of local oxidation in silicon nanowires on hole transport, and hence the effect of varying the oxidation state of silicon atoms at the wire surface, is studied using density functional theory in conjunction with a Green's function scattering method. For silicon nanowires with growth direction along [110] and diameters of a few nanometers, it is found that the introduction of oxygen bridging and back bonds does not significantly degrade hole transport for voltages up to several hundred millivolts relative to the valence band edge. As a result, the mean free paths are comparable to or longer than the wire lengths envisioned for transistor and other nanoelectronics applications. Transport along [100]-oriented nanowires is less favorable, thus providing an advantage in terms of hole mobilities for [110] nanowire orientations, as preferentially produced in some growth methods.

Silicon nanowire band gap modification.

Band gap modification for small-diameter (approximately 1 nm) silicon nanowires resulting from the use of different species for surface termination is investigated by density functional theory calculations. Because of quantum confinement, small-diameter wires exhibit a direct band gap that increases as the wire diameter narrows, irrespective of surface termination. This effect has been observed in previous experimental and theoretical studies for hydrogenated wires. For a fixed cross-section, the functional group used to saturate the silicon surface significantly modifies the band gap, resulting in relative energy shifts of up to an electronvolt. The band gap shifts are traced to details of the hybridization between the silicon valence band and the frontier orbitals of the terminating group, which is in competition with quantum confinement.

Tunnelling in alkanes anchored to gold electrodes via amine end groups.

For investigation of electron transport on the nanoscale, a system possessing a simple-to-interpret electronic structure is composed of alkane chains bridging two electrodes via end groups; to date, the majority of experiments and theoretical investigations on such structures have considered thiols bonding to gold electrodes. Recently experiments show that well-defined molecular conductances may be resolved if the thiol end groups are replaced by amines. In this theoretical study, we investigate the bonding of amine groups to gold clusters and calculate electron transport across the resulting tunnel junctions. We find very good agreement with recent experiments for alkane diamines and discuss differences with respect to the alkane dithiol system.

Determination of complex absorbing potentials from the electron self-energy.

The electronic conductance of a molecule making contact to electrodes is determined by the coupling of discrete molecular states to the continuum electrode density of states. Interactions between bound states and continua can be modeled exactly by using the (energy-dependent) self-energy or approximately by using a complex potential. We discuss the relation between the two approaches and give a prescription for using the self-energy to construct an energy-independent, nonlocal, complex potential. We apply our scheme to studying single-electron transmission in an atomic chain, obtaining excellent agreement with the exact result. Our approach allows us to treat electron-reservoir couplings independent of single-electron energies, allowing for the definition of a one-body operator suitable for inclusion into correlated electron transport calculations.

A force-field description of short-range repulsions for high density alkane molecular dynamics simulations.

The use of Buckingham (exp-6) van der Waals potentials in molecular dynamics (MD) simulations can quite successfully reproduce experimental thermodynamic data at low densities. However, they are less successful in producing a description of the repulsive regions of the potential energy surface (PES) that is in accord with the results of high-level ab initio computations. We show that Morse potentials can be parameterized to give excellent fits to both the attractive and repulsive regions of the PES. The best set of alkane van der Waals Morse function parameters reported to date for the description of nonbond repulsive interactions is presented, as determined by comparison with both ab initio and experimental results. C...C, H...H and C...H atom-pair potentials employing parameter sets based on the use of the geometric mean in the fitting procedure are found to be portable from methane to n-butane. Fitting to a combination of methane dimer interaction energies and forces from ab initio calculations yields parameter sets whose performance is superior to those determined from the interaction energies alone. Used in MD simulations, our newly developed parameter sets predict thermodynamic functions that show better agreement with experiment than those based on parameter sets in common use.

Bonding of atomic phosphorus to polycyclic hydrocarbons and curved graphitic surfaces.

We present a theoretical study of the bonding of atomic phosphorus to planar hydrocarbons and to curved graphite-like surfaces. We find that bonding of phosphorus to planar polycyclic hydrocarbons induces curvature away from the phosphorus atom, as defined by the pyramidalization angle. Similarly, bonding of atomic phosphorus to the [5,5] fulvalene-circulene semifullerene and buckminsterfullerene is only possible on the convex side of the carbon surface. On the other hand, we find the interaction of atomic phosphorus with the concave side of fullerene-like surfaces to be nonbonding for both quartet and doublet spin states. We find the prerequisite for stable epoxy-type bonds within these systems is the ability of the carbon atoms to maintain or induce curvature away from the P.C=C bond.