Electron Transport Calculations Research Articles

The effect of fuel oxidation on electron swarm properties and nanosecond discharge characteristics in combustible mixtures

The results of the calculations of electron transport and rate coefficients in oxidized methane:O2, propane:O2 and H2:O2 mixtures were presented for different temperatures and reduced electric fields E/N from 1 to 3 × 103 Td. The calculations were made for various oxidized fuel fractions. It was shown that fuel oxidation led to a drastic decrease in the average electron energy, electron drift velocity and attachment coefficient for E/N < 60 Td. At higher reduced electric fields, the effect of fuel oxidation on the average electron energy and electron drift velocity was small, whereas the attachment coefficient increased with increasing oxidation degree. In addition, in the hydrocarbon-containing mixtures for E/N ∼ 30 Td, the electron drift velocity varied nonmonotonously with the oxidation degree increase. The critical reduced electric field at which the average rate of electron production in the mixture was equal to the average attachment rate increased with fuel oxidation due to the increased attachment rate and decreased detachment rate. The effect of intermediate species on the electron transport and rate properties in partially oxidized mixtures was small. The calculated results were used to self-consistently simulate fuel oxidation and plasma characteristics for high-voltage nanosecond repetitive discharges in combustible mixtures. Zero-dimensional simulation in a H2:O2 mixture showed that the reduced electric field at the instant when the deposited energy peaked was close to the critical values of E/N and increased with increasing oxidized fuel fraction.

Electron Transport from Quantum Kinetic Monte Carlo Simulations

The exact quantum kinetic Monte Carlo method is proposed to calculate electron transport for one-dimensional (1D) nanostructure described by the Hubbard model. The method is directly formulated in real time and can be applied to extract time-dependent dynamics of general interacting electronic models in 1D. When coupled with density functional theory calculation and maximally localized Wannier functions analysis, our method can be used to predict electron transport in nanostructures with interfaces. We first applied our method to case study of α-quartz dielectric breakdown.

Photoconversion of an anthraquinone derivative in the presence of human serum albumin

Photconversion of an anthraquinone photochrome (AQP) from Trans to Ana forms were studied by different methods and techniques. Solution of AQP was irradiated under UV light in buffer condition, pH = 7.5, 10 mM phosphate buffer in the absence and presence of human serum albumin at 27 and 37 °C. The results showed that a new peak at higher wavelength was observed that indicative of producing the Ana form. Rate of Trans to Ana conversion increases in the presence of human serum albumin (HSA). Electron transport calculations were carried out from the first principles with a method based on non-equilibrium Green's functions (NEGF) combined with DFT. The results showed that electron transport is easier in Ana form due to increasing the resonance length and electron delocalization. Binding study by docking and spectroscopy showed that Trans form has more tendency to interact with HSA due to higher number of HSA-Trans hydrogen bond. Structural studies by circular dichroism and molecular dynamics results show that at lower concentration of AQP, percentage of helix was increased and then decreases at higher concentration. In addition structural parameters such as RMSD, accessible surface area, hydrogen bond, in associated with experimental results showed that protein folded at low concentration.

Perfectly conducting graphene electronic waveguide with curved channels

We theoretically investigate the electronic transport properties of curved graphene waveguides by employing non-equilibrium Green’s function techniques. We systematically study the dependence of the confined waveguide modes on the potential difference, the width of waveguide and side barrier. Through two-terminal electronic transport calculations, we show that the conductance of confined waveguide modes is rather robust against the bending degree of waveguide, in consistence with the band insensitivity to the side barrier. This finding of the perfectly conducting channels strongly suggests the possibility of applying the graphene waveguide in the design of low-power nanoelectronics.

Valley filters, accumulators, and switches induced in graphene quantum dots by lines of adsorbed hydrogen atoms

We present electronic structure and quantum transport calculations that predict conducting channels induced in graphene quantum dots by lines of adsorbed hydrogen atoms to function as highly efficient, experimentally realizable valley filters, accumulators and switches. The underlying physics is a novel property of graphene Dirac point resonances (DPRs) that is revealed here, namely, that an electric current passing through a DPR-mediated conducting channel in a given direction is carried by electrons of em only one of the two graphene valleys. Our predictions apply to lines of hydrogen atoms adsorbed on graphene quantum dots that are either free standing or supported on a hexagonal boron nitride substrate.

Multifunctional structural design of graphene thermoelectrics by Bayesian optimization.

Materials development often confronts a dilemma as it needs to satisfy multifunctional, often conflicting, demands. For example, thermoelectric conversion requires high electrical conductivity, a high Seebeck coefficient, and low thermal conductivity, despite the fact that these three properties are normally closely correlated. Nanostructuring techniques have been shown to break the correlations to some extent; however, optimal design has been a major challenge due to the extraordinarily large degrees of freedom in the structures. By taking graphene nanoribbons (GNRs) as a representative thermoelectric material, we carried out structural optimization by alternating multifunctional (phonon and electron) transport calculations and Bayesian optimization to resolve the trade-off. As a result, we have achieved multifunctional structural optimization with an efficiency more than five times that achieved by random search. The obtained GNRs with optimized antidots significantly enhance the thermoelectric figure of merit by up to 11 times that of the pristine GNR. Knowledge of the optimal structure further provides new physical insights that independent tuning of electron and phonon transport properties can be realized by making use of zigzag edge states and aperiodic nanostructuring. The demonstrated optimization framework is also useful for other multifunctional problems in various applications.

Contour integral method for obtaining the self-energy matrices of electrodes in electron transport calculations

We propose an efficient computational method for evaluating the self-energy matrices of electrodes to study ballistic electron transport properties in nanoscale systems. To reduce the high computational cost incurred in large systems, a contour integral eigensolver based on the Sakurai-Sugiura method combined with the shifted biconjugate gradient method is developed to solve exponential-type eigenvalue problem for complex wave vectors. A remarkable feature of the proposed algorithm is that the numerical procedure is very similar to that of conventional band structure calculations. We implement the developed method in the framework of the real-space higher-order finite difference scheme with nonlocal pseudopotentials. Numerical tests for a wide variety of materials validate the robustness, accuracy, and efficiency of the proposed method. As an illustration of the method, we present the electron transport property of the free-standing silicene with the line defect originating from the reversed buckled phases.

Multilevel Molecular Modeling Approach for a Rational Design of Ionic Current Sensors for Nanofluidics.

The complexity displayed by nanofluidic-based systems involves electronic and dynamic aspects occurring across different size and time scales. To properly model such kind of system, we introduced a top-down multilevel approach, combining molecular dynamics simulations (MD) with first-principles electronic transport calculations. The potential of this technique was demonstrated by investigating how the water and ionic flow through a (6,6) carbon nanotube (CNT) influences its electronic transport properties. We showed that the confinement on the CNT favors the partially hydrated Na, Cl, and Li ions to exchange charge with the nanotube. This leads to a change in the electronic transmittance, allowing for the distinguishing of cations from anions. Such an ionic trace may handle an indirect measurement of the ionic current that is recorded as a sensing output. With this case study, we are able to show the potential of this top-down multilevel approach, to be applied on the design of novel nanofluidic devices.

Simulation of High-Power Microwave Air Breakdown Modeled by a Coupled Maxwell–Euler System With a Non-Maxwellian EEDF

In this paper, a discontinuous Galerkin time-domain (DGTD) method is developed to simulate high-power microwave air breakdown phenomena, which is modeled by a coupled electromagnetic–plasma system. In the coupled system, the electromagnetic fields are governed by Maxwell’s equations and the plasma is modeled by the five-moment fluid equations (Euler’s equations). The non-Maxwellian electron energy distribution function is used to calculate electron transport coefficients and describe the nonequilibrium collision reactions between electrons and neutral air particles. The coupled Maxwell–Euler equations are solved by the DGTD method with high-order spatial and temporal discretization, which are able to provide a sufficient resolution for the physical quantities in both space and time. Several numerical examples are presented to investigate the physical process and demonstrate the capability of the numerical method.

Improvement of accuracy in the wave-function-matching method for transport calculations

The wave-function-matching (WFM) technique for first-principles transport-property calculations was modified by S\o{}rensen {\it et al.} so as to exclude rapidly decreasing evanescent waves [S\o{}rensen {\it et al.}, Phys. Rev. B {\bf 77}, 155301 (2008)]. However, this method lacks translational invariance of the transmission probability with respect to insertion of matching planes and consistency between the sum of the transmission and reflection probabilities and the number of channels in the transition region. We reformulate the WFM method since the original methods are formulated to include all the generalized Bloch waves. It is found that the translational invariance is destroyed by the overlap of the layers between the electrode and transition regions and by the pseudoinverses used to exclude the rapidly decreasing evanescent waves. We then devise a method that removes the overlap and calculates the transmission probability without the pseudoinverses. As a result, we find that the translational invariance of the transmission probability with respect to insertion of the extra layers is properly retained and the sum of the transmission and reflection probabilities exactly agrees with the number of channels. In addition, we prove that the accuracy in the transmission probability of this WFM technique is comparable with that obtained by the nonequilibrium Green's function method. Furthermore, we carry out the electron transport calculations on two-dimensional graphene sheets embedded with B--N line defects sandwiched between a pair of semi-infinite graphene electrodes and find the dependence of the electron transmission on the transverse momentum perpendicular to the direction of transport.

Cyanographone and isocyanographone - Two asymmetrically functionalized graphene pseudohalides and their potential use in chemical sensing.

Graphene pseudohalides are natural candidates for use in molecular sensing due to their greater chemical activity as compared to both graphene halides and pristine graphene. Though their study is still in its infancy, being hindered until recently by the unavailability of both selective and efficient procedures for their synthesis, they promise to considerably widen the application potential of chemically modified graphenes. Herein, we employ van der Waals density functional theory to study the structural and electronic properties of two selected graphene pseudohalides, namely, cyanographone and isocyanographone and investigate the potential use of the latter as a chemical sensor via electron transport calculations.

Theory of electron–phonon–dislon interacting system—toward a quantized theory of dislocations

We provide a comprehensive theoretical framework to study how crystal dislocations influence the functional properties of materials, based on the idea of a quantized dislocation, namely a ‘dislon’. In contrast to previous work on dislons which focused on exotic phenomenology, here we focus on their theoretical structure and computational power. We first provide a pedagogical introduction that explains the necessity and benefits of taking the dislon approach and why the dislon Hamiltonian takes its current form. Then, we study the electron–dislocation and phonon–dislocation scattering problems using the dislon formalism. Both the effective electron and phonon theories are derived, from which the role of dislocations on electronic and phononic transport properties is computed. Compared with traditional dislocation scattering studies, which are intrinsically single-particle, low-order perturbation and classical quenched defect in nature, the dislon theory not only allows easy incorporation of quantum many-body effects such as electron correlation, electron–phonon interaction, and higher-order scattering events, but also allows proper consideration of the dislocation’s long-range strain field and dynamic aspects on equal footing for arbitrary types of straight-line dislocations. This means that instead of developing individual models for specific dislocation scattering problems, the dislon theory allows for the calculation of electronic structure and electrical transport, thermal transport, optical and superconducting properties, etc, under one unified theory. Furthermore, the dislon theory has another advantage over empirical models in that it requires no fitting parameters. The dislon theory could serve as a major computational tool to understand the role of dislocations on multiple materials’ functional properties at an unprecedented level of clarity, and may have wide applications in dislocated energy materials.

Tunable magnetism and spin-polarized electronic transport in graphene mediated by molecular functionalization of extended defects

We use first-principles simulations to investigate the magnetic and the electronic transport properties of functionalized graphene layers upon the presence of extended linear defects (ELDs). We have considered electron-donor molecules, tetrathiafulvalene (TTF), lying on the graphene sites neighboring the ELD (TTF/ELD). Those molecules bond to the graphene sheet mediated by van der Waals interactions, giving rise to a net (molecule$\ensuremath{\rightarrow}$graphene) charge transfer. The $n$-type doping of graphene, as well as the molecule-graphene interaction, are strengthened by the presence of the ELD. There is a charge density accumulation on the C atoms along the defect sites, promoting the formation of magnetic states in TTF/ELD. We show that such a net magnetization can be modified through an external electric field (${E}_{\text{field}}$). Further electronic transport calculations reveal that the transmission coefficients exhibit a spin anisotropy which can be controlled by the ${E}_{\text{field}}$, and thus showing that TTF/ELD is a quite interesting system to realize tunable spin-polarized electronic currents on graphene.

Electronic transport calculations for CO2 adsorption on calcium-decorated graphene nanoribbons

Calcium-atom functionalization is considered to be an effective tool of enhancing CO2 uptake capacity of carbon-based materials (Cazorla et al., 2011). Here we use density functional theory calculations combined with the nonequilibrium Green’s function formalism to study electronic transport properties of Ca-decorated zig-zag and armchair graphene nanoribbons after CO2 adsorption. Sensitivity of the system to CO2 attachment is considerably increased due to the Ca-decoration: the electronic transmission near the Fermi level increases due to the formation of extended states. In addition, the variations of the electrostatic potential along the transport direction reduces due to CO2 adsorption. Enhanced electronic transport due to the CO2 adsorption is also obtained in current-voltage calculations. Since the conductivity change is one of the main properties of gas sensors, our results will be useful in developing graphene-based solid-state gas sensors.

Novel antimonene tunneling field-effect transistors using an abrupt transition from semiconductor to metal in monolayer and multilayer antimonene heterostructures.

Recently, a mono-elemental two-dimensional (2-D) material, namely antimonene, with a large band gap, decent mobility and ambient stability has been extensively researched. Interestingly, although antimonene is a semiconductor with a sizable band gap in the monolayer, it is transformed to a metal in the multilayer. Inspired by this thickness dependent semiconductor to metal transition, we propose novel antimonene tunneling field-effect transistors (TFETs) based on the lateral monolayer (semiconducting)/multilayer (metallic)/monolayer (semiconducting) heterostructure. Our antimonene TFETs consist of a semiconducting monolayer source, channel and a drain and a small metallic multilayer region between the source and the channel. The local multilayer region introduces gapless metallic states which dramatically enhance the tunneling probability and hence result in a large current. To investigate the effect of a metallic multilayer on device performances, we carried out ab-initio electronic structure and quantum transport calculations for several antimonene TFETs based on different monolayer/multilayer/monolayer heterostructures. Simulation shows that even ∼1 nm scale nanostructured multilayer significantly boosts the current and enables abrupt device switching. More extensive evaluation is performed through benchmarking with phosphorene TFETs which have been identified as the best 2-D material based TFETs so far. In terms of the main figures of merit for FETs such as the intrinsic delay time and the power delay product, antimonene heterostructure TFETs outperform phosphorene TFETs, primarily due to the elimination of the tunneling barrier by the locally constructed multilayer antimonene.

High-Throughput Screening of Sulfide Thermoelectric Materials Using Electron Transport Calculations with OpenMX and BoltzTraP

The electron transport properties of 809 sulfides have been investigated using density functional theory (DFT) calculations in the relaxation time approximation, and a material design rule established for high-performance sulfide thermoelectric (TE) materials. Benchmark electron transport calculations were performed for Cu12Sb4S13 and Cu26V2Ge6S32, revealing that the ratio of the scattering probability of electrons and phonons (κ lat τ el −1 ) was constant at about 2 × 1014 W K−1 m−1 s−1. The calculated thermopower S dependence of the theoretical dimensionless figure of merit ZT DFT of the 809 sulfides showed a maximum at 140 μV K−1 to 170 μV K−1. Under the assumption of constant κ lat τ el −1 of 2 × 1014 W K−1 m−1 s−1 and constant group velocity v of electrons, a slope of the density of states of 8.6 states eV−2 to 10 states eV−2 is suitable for high-ZT sulfide TE materials. The Lorenz number L dependence of ZT DFT for the 809 sulfides showed a maximum at L of approximately 2.45 × 10−8 V2 K−2. This result demonstrates that the potential of high-ZT sulfide materials is highest when the electron thermal conductivity κ el of the symmetric band is equal to that of the asymmetric band.

Designing topological defects in 2D materials using scanning probe microscopy and a self-healing mechanism: a density functional-based molecular dynamics study

Engineering of materials at the atomic level is one of the most important aims of nanotechnology. The unprecedented ability of scanning probe microscopy to address individual atoms opened up the possibilities for nanomanipulation and nanolitography of surfaces and later on of two-dimensional materials. While the state-of-the-art scanning probe lithographic methods include, primarily, adsorption, desorption and repositioning of adatoms and molecules on substrates or tailoring nanoribbons by etching of trenches, the precise modification of the intrinsic atomic structure of materials is yet to be advanced. Here we introduce a new concept, scanning probe microscopy with a rotating tip, for engineering of the atomic structure of membranes based on two-dimensional materials. In order to indicate the viability of the concept, we present our theoretical research, which includes atomistic modeling, molecular dynamics simulations, Fourier analysis and electronic transport calculations. While stretching can be employed for fabrication of atomic chains only, our comprehensive molecular dynamics simulations indicate that nanomanipulation by scanning probe microscopy with a rotating tip is capable of assembling a wide range of topological defects in two-dimensional materials in a rather controllable and reproducible manner. We analyze two possibilities. In the first case the probe tip is retracted from the membrane while in the second case the tip is released beneath the membrane allowing graphene to freely relax and self-heal the pore made by the tip. The former approach with the tip rotation can be achieved experimentally by rotation of the sample, which is equivalent to rotation of the tip, whereas irradiation of the membrane by nanoclusters can be utilized for the latter approach. The latter one has the potential to yield a yet richer diversity of topological defects on account of a lesser determinacy. If successfully realized experimentally the concept proposed here could be an important step toward controllable nanostructuring of two-dimensional materials.

The Effect of the Electron Tunneling on the Photoelectric Hot Electrons Generation in Metallic-Semiconductor Nanostructures

Semiconductor photonic crystals (MSPhC) were used to convert solar energy into hot electrons. An experimental model was designed by using metallic semiconductor photonic crystals (MSPhC). The designed MSPhC is based on TiO2/Au schottky contact. The model has similar nanocavity structure for broad gold absorption, but the materials on top of the cavity were changed to a metal and a semiconductor in order to collect the hot electrons. Detailed design steps and characterization have shown a broadband sub-bandgap photoresponse at a wavelength of 590 nm. This is due to the surface plasmon absorption by the wafer-scale Au/TiO2 metallic-semiconductor photonic crystal. Analytical calculation of the hot electron transport from the Au thin layer to the TiO2 conduction band is discussed. This theoretical study is based on the quantum tunneling effect. The photo generation of the hot electrons was undertaken at different wavelengths in Au absorber followed by tunneling through a schottky barrier into a TiO2 collector. The presence of a tunnel current from the absorber to the collector under illumination, offers a method to extract carriers from a hot-electron distribution at few bias voltages is presented in this study. The effects of doping different concentrations of the semiconductor on the evolution of the current characteristics were also investigated and discussed. The electrical characteristics were found to be sensitive to any change in the thickness of the barrier.

High potential thermoelectric figure of merit in ternary La3Cu3X4 (X\u2009=\u2009P, As, Sb and Bi) compounds

We investigate the thermoelectric properties of the relatively unexplored rare-earth ternary compounds La3Cu3X4 (X = Bi, Sb, As, and P) using first principles electronic structure and Boltzmann transport calculations. These compounds, of which the La3Cu3Sb4 and La3Cu3Bi4 have previously been synthesized, are all predicted to be semiconductors and present a wide range of bandgaps varying from 0.24 eV (for the Bi compound) to 0.87 eV (for the P compound). We further find a mixture of light and heavy bands, which results in a high thermoelectric power factor. In addition, as discussed in our previous study (Phys. Rev. B 95 (22), 224306, 2017) at high temperatures of 1000 K these compounds exhibit lattice thermal conductivity less than 1 W/mK. The combination of low thermal conductivity and good transport properties results in a predicted ZT as high as ~1.5 for both La3Cu3P4 and La3Cu3As4, under high p-type doping. This predicted high performance makes these compounds promising candidates for high temperature thermoelectric applications and thus merits further experimental investigation.

Electron–phonon interaction and transport properties of metallic bulk and monolayer transition metal dichalcogenide TaS2

Transition metal dichalcogenides have recently emerged as promising two-dimensional materials with intriguing electronic properties. Existing calculations of intrinsic phonon-limited electronic transport so far have concentrated on the semicondcucting members of this family. In this paper we extend these studies by investigating the influence of electron–phonon coupling on the electronic transport properties and band renormalization of prototype inherent metallic bulk and monolayer TaS2. Based on density functional perturbation theory and semi-classical Boltzmann transport calculations, promising room temperature mobilities and sheet conductances are found, which can compete with other established 2D materials, leaving TaS2 as promising material candidate for transparent conductors or as atomically thin interconnects. Throughout the paper, the electronic and transport properties of TaS2 are compared to those of its isoelectronic counterpart TaSe2 and additional informations to the latter are given. We furthermore comment on the conventional superconductivity in TaS2, where no phonon-mediated enhancement of TC in the monolayer compared to the bulk state was found.