Surface states and related quantum interference in ab initio electron transport

Herein, a highly practical first‐principles approach for studying phonon transport across various materials, ranging from low‐dimensional systems to three‐dimensional bulk structures, as well as nanostructures, is presented. This method integrates the nonequilibrium Green's function (NEGF) formalism with ab initio calculations of dynamical matrices generated by Quantum Espresso. A detailed technical approach for extracting atomistic force constants and forming dynamical matrices that adhere to the tri‐diagonal technique within the NEGF framework is provided. Additionally, an efficient method for constructing dynamical matrices of conventional cells from those of primary cells is introduced, remarkably reducing the computational costs associated with density‐functional theory calculations. The robustness and applicability of this methodology are validated through several case studies.

Zero-bias transport properties of alkanediisothiocyanate $[\text{-SCN-}{({\text{CH}}_{2})}_{n}\text{-NCS-},\text{ }n=4,6,8]$ molecules anchored to various quasi-one-dimensional Au(111) electrodes are studied by an ab initio method within a standard two-probe methodology based on the density-functional theory and the nonequilibrium Green's functions formalism. We have shown that the high-/low-conductance traces observed in STM break-junction experiments on this molecular species can be clearly related to the difference in the electronic structures between the molecular contacts with different geometric configurations of the near-interface planes of the electrodes. The difference in transport properties of alkanediisothiocyanate single-molecule junctions can be understood in terms of the distribution of the surface states, emerging in the energy gap of the alkanes due to the electrode environment. A general feature of the high-conductance junctions is a number of closely located surface states around the Fermi energy, well coupled to the scattering states of the junctions. These well-coupled states broaden the transmission peaks responsible for conductance, increasing their amplitudes at the Fermi energy, while the sparser distribution of the surface states gives sharper peaks with smaller amplitudes at the Fermi energy, naturally representing the low-conductance junctions. The effects of changing the end groups, junction distance, and electrode cross-section are also investigated and analyzed in terms of changes in the electronic structure of the molecular junctions. A comparison with the available experimental data is also provided.

This paper describes the point defect treatment in nanowire MOSFETs. The 3D Poisson equation is self-consistently coupled to the Schrodinger equation. We adopt the mode-space representation in which the 3D Schrodinger equation is divided into a 2D equation, including the confinement of the cross-section, and an ID equation describing the ballistic transport based on the nonequilibrium Green's function formalism (NEGF). We use the simplest form of the tight-binding theory, with one orbital per atom, and considering only the first-neighbor-interactions. In this framework, equivalent to the ellipsoidal energy band approximation, influence of point defects on electronic transport is discussed.

Using self-consistent calculations based on nonequilibrium Green's function formalism, the origin of negative differential resistance (NDR) in molecular junctions and quantum wires is investigated. Coupling of the molecule to electrodes becomes asymmetric at high bias due to asymmetry between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. This causes appearance of an asymmetric potential profile due to a depletion of charge and reduction of screening near the source electrode. With increasing bias, this sharp potential drop leads to an enhanced localization of the HOMO and LUMO states in different parts of the system. The reduction in overlap, caused by localization, results in a significant reduction in the transmission coefficient and current with increasing bias. An atomic chain connected to two graphene ribbons was investigated to illustrate these effects. For a chain substituting a molecule, an even-odd effect is also observed in the NDR characteristics.

The effects of surface functionalization on the electronic transport properties of the MXene compound Ti3C2 are studied using density-functional theory in combination with the nonequilibrium Green's function formalism. Fluorinated, oxidized and hydroxylated surfaces are considered and the obtained results are compared with the ones for the pristine MXene. It is found that the surface termination has a considerable impact on the electronic transport in MXene. For example, the fluorinated sample shows the largest transmission, whereas surface oxidation results in a considerable reduction of the electronic transmission. The current in the former sample can be up to 4 times larger for a given bias voltage as compared to the case of bare MXene. The increased transmission originates from the extended electronic states and smaller variations of the electrostatic potential profile. Our findings can be useful in designing MXene-based anode materials for energy storage applications, where enhanced electronic transport will be an asset.

We designed acene molecules attached to two semi-infinite metallic electrodes to explore the source-drain current of graphene and the gate leakage current of the gate dielectric material in the field-effect transistors (FETs) device using the first-principles density functional theory combined with the non-equilibrium Green's function formalism. In the acene-based molecular junctions, we modify the connection position of the thiol group at one side, forming different electron transport routes. The electron transport routes besides the shortest one are defined as the cross channels. The simulation results indicate that electron transport through the cross channels is as efficient as that through the shortest one, since the conductance is weakly dependent on the distance. Thus, it is possible to connect the graphene with multiple leads, leading the graphene as a channel utilized in the graphene-based FETs in the mesoscopic system. When the conjugation of the cross channel is blocked, the junction conductance decreases dramatically. The differential conductance of the BA-1 is nearly 7 (54.57 μS) times as large as that of the BA-4 (7.35 μS) at zero bias. Therefore, the blocked graphene can be employed as the gate dielectric material in the top-gated graphene FET device, since the leakage current is small. The graphene-based field-effect transistors fabricated with a single layer of graphene as the channel and the blocked graphene as the gate dielectric material represent one way to overcome the problem of miniaturization which faces the new generation of transistors.

Recent experimental advances probing coherent phonon and electron transport in nanoscale devices at contact have motivated theoretical channel-based analyses of conduction based on the nonequilibrium Green's function formalism. The transmission through each channel has been known to be bounded above by unity, yet actual transmissions in typical systems often fall far below these limits. Building upon recently derived radiative heat transfer limits and a unified formalism characterizing heat transport for arbitrary bosonic systems in the linear regime, we propose new bounds on conductive heat transfer. In particular, we demonstrate that our limits are typically far tighter than the Landauer limits per channel and are close to actual transmission eigenvalues by examining a model of phonon conduction in a 1-dimensional chain. Our limits have ramifications for designing molecular junctions to optimize conduction.

The effect of inelastic scattering on quantum electron transport through double-barrier resonant-tunneling structures with large cross-sectional areas is studied numerically using the approach based on the nonequilibrium Green's-function formalism of Keldysh, Kadanoff, and Baym. The Markov assumption is not made, and the energy coordinate is retained. This makes the inclusion of the phonon-energy spectrum straightforward both conceptually and in practice. The electron-phonon interaction is treated in the self-consistent first Born approximation (SCFBA). The Pauli exclusion principle is taken into account exactly within the SCFBA. The retention of the energy coordinate allows the calculation of a number of quantities that give insight into the effect of inelastic scattering on electron transport: The effect of inelastic scattering on the occupation of the energy levels, the density of states, the energy distribution of the current density, and the power density is calculated from a quantum kinetic equation for actual device structures under high bias.

In this paper, we review and extend recent work on the effect of random discrete dopants on the statistical variability in gate-all-around silicon nanowire transistors. The electron transport is described using the nonequilibrium Green's function formalism. Full 3-D real-space and coupled-mode-space repre sentations are used. Two different cross sections (i.e., 2.2 × 2.2 and 4.2 × 4.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and two different channel lengths (i.e., 6 and 12 nm) have been considered. The resistivity associated with discrete dopants can be estimated from the averaged current-voltage characteristics. The threshold-voltage variability and the sub threshold-slope variability are reduced greatly in the transistors with longer channel length. Both are smaller at equivalent channel lengths in the 2.2 × 2.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device due to better electrostatic integrity. At the same time, the ON-state-current variability associated with the varying resistance of the access regions is virtually independent of the channel length. However, it is reduced greatly in the 4.2 × 4.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> transistor due to a fourfold increase in the number of dopants in the access regions and corresponding self-averaging effects. Finally, we present results for the smallest transistor combining two sources of variability (i.e., discrete random dopants and surface roughness) and phonon scattering.

We examine the transport properties of a tunnel-coupled asymmetric double-dot structure in a parallel arrangement between leads in the presence of Aharonov-Bohm magnetic flux and an additional bias voltage applied between the dots. We solve the Schrodinger equation for the energy eigenfunctions and eigenvalues of a biased asymmetric double-dot system. The magnetic flux-induced Peierls phase factors experienced by electrons in tunneling from/to the leads to/from double-dot levels are determined and incorporated. Employing the nonequilibrium Green's function formalism we calculate the lead-to-lead current, taking account of dot-lead tunnel coupling and Coulomb repulsion. We demonstrate that, for appropriate parameter values (such as lead-to-lead bias voltage and equilibrium chemical potential of the leads), the lead-to-lead current oscillates as a function of applied magnetic field, if the dot-to-dot bias compensates the structure asymmetry and only one of the double-dot levels is conductive. We also show that when both levels become conductive, their interference is destructive and the total lead-to-lead current decreases with increasing magnetic field. This decrease is larger for larger dots.

The conductance of a family of ruthenium-octene-ruthenium molecular junctions with different pi conjugation are investigated using a fully self-consistent ab initio approach which combines the nonequilibrium Green's function formalism with density functional theory. Our calculations demonstrate that the continuity of the pi conjugation in the contact region as well as along the molecular backbone affects the junction conductance significantly, showing the advantage of using the ruthenium-carbon double bond as the linkage of conjugated organic molecules.

A comprehensive study is conducted on the electron transport in conductive polymer matrix composites (CPMCs), employing the nonequilibrium Green's function formalism. This paper provides a microscopic insight into the electron tunneling through the potential barriers existing between conducting sites. It is shown that Wentzel-Kramers-Brillouin approximation as well as other models with simple barrier shapes, which are widely used in literature, can lead to inaccurate results in comparison with the quantum mechanical approach using a hyperbolic barrier. In this paper, unlike most previous ones, percolation-related effects are disregarded for further focus on electron transport through the polymer potential barriers. It is assumed that a tunneling-conductive channel exists between the electrodes. This can be created either by applying electric field alignment or using a filler volume fraction higher than the percolation threshold. A two electrode resistive device is studied and the results indicate that a conductor-insulator transition occurs at a barrier thickness of ~1.7 nm and the barrier thickness should be larger than several angstroms. Next, a novel tunneling field-effect structure based on CPMCs is introduced and its characteristics are comprehensively investigated. This device features a remarkably simple structure, an extremely high channel to gate coupling, a large transconductance, and a high current level. Besides, it has the advantage of being based on polymers. This ensures favorable physical properties, ease of fabrication, and low-cost processing techniques.

We discuss within the nonequilibrium Green's function formalism the self-consistent Born approximation of the electronic self-energy for concurrent coupling of charge carriers to photons and phonons. The expansion of the charge carrier Green's function is shown to provide a description for a range of physical processes with relevance for the photovoltaic device operation, such as indirect optical transitions, thermally assisted photocarrier escape from quantum wells or phonon-mediated interband transfer in tunnel junctions. The nonequilibrium Green's function picture of these processes is reviewed with focus on the evolution of the system towards the steady state in the convergence of the self-consistency iteration process and on the impact of inelastic scattering on device operation.

Expanded porphyrins arise as an alternative for potential application as chromophores in dye-sensitized solar cells. (DSSCs). The modification of the core of these compounds provides remarkable changes in the photoelectronic behavior. In the present article, the improvement of its properties for a potential application as UV/vis/NIR chromophores in DSSCs has been studied, when an oxazolone moiety has replaced an imine ring in analogy to the porpholactones first synthesized by Crossley et al. ( J. Chem. Soc., Chem. Commun. 1984, 920-922). These expanded porpholactones present a noticeable red shift as well as an increase in the intensity of the Q-bands regarding the parent compounds. The photophysical properties of Sapphyrin have been explored through DFT calculations and vibrationally resolved absorption spectra simulations. Energetic parameters showed favorable electron injection from the chromophore to the TiO2 semiconductor. In addition, aromaticity was analyzed and rationalized using magnetic and delocalization criteria. Results showed qualitatively similar trends between aromaticity descriptors and Q bands giving a great opportunity to the use this property in the rational design of chromophores. Finally, the nonequilibrium Green's function formalism shows the ability of expanded porpholactones in electron transport.

Characteristics of electronic and spin-independent linear conductance in conjugated aromatic polymer based molecular device