Nonequilibrium Green Function Research Articles

Strain-Modulated Electronic Transport Properties in Two-Dimensional Green Phosphorene with Different Edge Morphologies

Based on two-dimensional green phosphorene, we designed two molecular electronic devices with zigzag (Type 1) and whisker-like (Type 2) configurations. By combining density functional theory (DFT) and non-equilibrium Green’s function (NEGF), we investigated the electronic properties of Types 1 and 2. Type 1 exhibits an interesting negative differential resistance (NDR), while the current characteristics of Type 2 show linear growth in the current–voltage curve. We studied the electronic transport properties of Type 1 under uniaxial strain modulation and find that strained devices also exhibit a NDR effect, and the peak-to-valley ratio of device could be controlled by varying the strain intensity. These results show that the transport properties of green phosphorene with different edge configuration are different, and the zigzag edge have adjustable negative differential resistance properties.

Temperature-dependent electronic, optical, and solar cell device properties of AlAs and AlSb semiconductors and their p-n homojunctions

AlSb and AlAs have attracted renewed interest in recent years as promising photovoltaic (PV) materials due to significant light absorption, eco-friendliness, and low cost. Here, density functional theory (DFT) in combination with the non-equilibrium Green's function method (NEGF) has been used to compute the temperature-dependent PV parameters of nanoscale AlSb and AlAs p-n junction solar cell devices. In particular, the influence of electron-phonon coupling (EPC) and spin-orbit coupling (SOC) on the PV properties of these devices is investigated. Our results highlight the importance of inclusion of EPC in evaluating the ab initio PV properties of p-n junctions at finite temperatures. At non-zero temperatures, the PV parameters can differ significantly compared to those at zero temperature due to phonon-assisted tunnelling of charge carriers across the p-n junction. In particular, the mobility of low-effective-mass carriers at the p-n junction is found to be enhanced by phonon scattering, which in turn results in improved PV transport properties. The short-circuit current (Jsc) at 300 K proved to be 21.7% (16.2%) higher for AlSb (AlAs) than that at 0 K. On the other hand, the open-circuit voltage (Voc) at 300 K proved to be 19.1% (16.7%) lower for AlSb (AlAs) than that at 0 K. Jsc is higher in AlSb than in AlAs due to higher phonon-influenced mobility of low-effective-mass carriers. Overall, our study strongly suggests the importance of inclusion of EPC and hence phonon scattering in the first-principles quantitative determination of PV parameters of solar cells. The estimates obtained in this study may be utilized as input ab initio parameters in continuum-model-based multi-scale simulations of AlSb and AlAs p-n homo-junction solar cells.

Theoretical Study of Electronic and Thermoelectric Properties of Ultra-Thin Silicene and Germanene

In this study, we used the Non-Equilibrium Green Function (NEGF) and Density Functional Theory (DFT) to analyze the nanoscale electronic properties of silicene and germanene structures, the band structure and Density of State (DOS) for the unit cell of silicene and germanene are found that the energy gap is (0 eV) in both. A comparison was made between silicene and germanene in terms of electrical and thermal properties, by calculating the transmission coefficient T(E) for silicene patterns (Si1, Si2, Si3, and Si4) and germanene patterns (Ge1, Ge2, Ge3, and Ge4). The results show that T(E) decreases with increasing etching depth, which also leads to a decrease in electronic conductivity. Moreover, the value of the Seebeck coefficient (S) increases with increasing drilling depth, and also the sign of S varies from a positive value for (Si1, Si4, Ge1, Ge2, Ge4) to a negative value for (Si2, Si3, Ge3) at Fermi level (Ef) equal to (0 eV). The highest value of the figure of merit is about (1.8) for the Ge4 structure.

Unveiling the temperature-dependent thermoelectric properties of the undoped and Na-doped monolayer SnSe allotropes: a comparative study

Motivated by the excellent thermoelectric (TE) performance of bulk SnSe, extensive attention has been drawn to the TE properties of the monolayer SnSe. To uncover the fundamental mechanism of manipulating the TE performance of the SnSe monolayer, we perform a systematic study on the TE properties of five monolayer SnSe allotropes such as α-, β-, γ-, δ-, and ε-SnSe based on the density functional theory and the non-equilibrium Green’s functions. By comparing the TE properties of the Na-doped SnSe allotropes with the undoped ones, the influences of the Na doping and the temperature on the TE properties are deeply investigated. It is shown that the figure of merit ZT will increase as the temperature increases, which is the same for almost all the Na-doped and undoped cases. The Na doping can enhance or suppress the ZT in different SnSe allotropes at different temperatures, implying the presence of the anomalous suppression of the ZT. The Na doping induced ZT suppression may be caused basically by the sharp decrease of the power factor and the weak decrease of the electronic thermal conductance, rather than by the decrease of the phononic thermal conductance. We hope this work will be able to enrich the understanding of the manipulation of TE properties by means of dimensions, structurization, doping, and temperature.

Influence of temperature on bandgap shifts, optical properties and photovoltaic parameters of GaAs/AlAs and GaAs/AlSb p–n heterojunctions: insights from ab-initio DFT + NEGF studies

The III–V group semiconductors are highly promising absorbers for heterojunctions based solar cell devices due to their high conversion efficiency. In this work, we explore the solar cell properties and the role of electron–phonon coupling (EPC) on the solar cell parameters of GaAs/AlSb and GaAs/AlAs p–n heterojunctions using non-equilibrium Green function method (NEGF) in combination of ab-initio density functional theory (DFT). In addition, the band offsets at the heterointerfaces, optical absorption and bandgap shifts (BGSs) due to temperature are estimated using DFT + NEGF approach. The interface band gaps in heterostructures are found to be lower than bulk band gaps leading to a shift in optical absorption coefficient towards lower energy side that results in stronger photocurrent. The temperature dependent electronic BGS is significantly influenced by the phonon density and phonon energy via EPC. The phonon influenced BGS is found to change the optical absorption, photocurrent density and open-circuit voltage. In case of GaAs/AlSb junction, the interface phonons are found to have significantly higher energies as compared to the bulk phonons and thereby may have important implications for photovoltaic (PV) properties. Overall, the present study reveals the influence of EPC on the optical absorption and PV properties of GaAs/AlSb and GaAs/AlSb p–n heterojunctions. Furthermore, the study shows that the DFT + NEGF method can be successfully used to obtain the reasonable quantitative estimates of temperature dependent BGSs, optical absorption and PV properties of p–n heterojunctions.

Performance analysis of carbon nanotube and graphene nanoribbon based biochemical sensors at atomic scale

Advancements in fabrication technologies have led to the possibility of synthesizing atomic-scale graphene nanoribbon (GNR) and carbon nanotube (CNT) based nanodevices. The purpose of this study was to model the electronic properties and electrical characteristics of these devices by atomistic modeling using density functional theory and the non-equilibrium Green’s function and compare the effects of molecular functionalization and sensing. The potential profile of the device was computed using the three-dimensional Poisson equation for smaller applied bias within one voltage range. Simulations showed a bandgap of 1 eV for armchair GNRs (AGNRs), which were insensitive to functionalized amine molecules, resulting in fewer alterations in the density of states (DoS), transmission spectra and the device current (ΔI). The bandgap further increased to 2 eV upon rolling the GNR into a armchair CNT (ACNT), which further decreased sensitivity. However, changing the configuration of the AGNR to a zigzag GNR (ZGNR) led to remarkable changes in the DoS and transmission spectra and a significant improvement in sensitivity. This improvement increased by 1.5–2 times upon rolling the ZGNR into a zigzag CNT (ZCNT). Thus, at lower dimensions in atomic scale, we found an alteration in device current of the carbon structures that was directly proportional to sensitivity in the following order: ΔI ACNT < ΔI AGNR < ΔI ZGNR < ΔI ZCNT. However, the same was found to fall for ZGNR and ZCNT with an increase in width to length (W/L) ratio. This highlights the importance of smaller atomic structures and this work provides a guideline for effective utilization of these structures for biochemical sensing.

Nonequilibrium Dynamics of the Hubbard Dimer

Electron dynamics in a two‐sites Hubbard model is studied using the nonequilibrium Green's function approach. The study is motivated by the empirical observation that a full solution of the integro‐differential Kadanoff–Baym equation (KBE) is more stable and often accompanied by artificial damping [Marc Puig von Friesen, C. Verdozzi, and C.‐O. Almbladh (2009)] than its time‐linear reformulations relying on the generalized Kadanoff–Baym ansatz (GKBA). Additionally, for conserving theories, numerical simulations suggest that KBE produces natural occupations bounded by one and zero in agreement with the Pauli exclusion principle, whereas, in some regimes, GKBA‐based theories violate this principle. As the first step for understanding these issues, the electron dynamics arising in the adiabatic switching scenario is studied. Many‐body approximations are classified according to the channel of the Bethe–Salpeter equation in which electronic correlations are explicitly treated. They give rise to the so‐called second Born, T‐matrix, and GW approximations. In each of these cases, the model is reduced to a system of ordinary differential equations, which resemble equations of motion for a driven harmonic oscillator with time‐dependent frequencies. A more complete treatment of electronic correlations is achieved by combining different correlation channels, with parquet theory serving as a starting point.

Proposal for All-Electrical Skyrmion Detection in van der Waals Tunnel Junctions.

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in two-dimensional vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a graphite/Fe3GeTe2/germanene/graphite vdW tunnel junction. For atomic-scale skyrmions, the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin mixing between spin-up and -down states of pz and dz2 character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

Investigation of novel two-dimensional B2CO monolayer and its high sensing performance in detecting nitrogen-containing gases: A combined density functional theory calculation and non-equilibrium Green's function study

Two-dimensional (2D) materials have attracted widespread attention because of their excellent optical and electrical properties and have been extensively studied as gas sensors. A new type of 2D material, the B2CO monolayer, has recently been reported to possess significant structural, energetic, and thermodynamic stability. In this study, density functional theory (DFT) was used to explore the sensing properties of CO, CO2, NH3, NO, and NO2 gas molecules on the B2CO monolayer. After adsorption, the NH3, NO, and NO2 molecules formed chemical bonds with the B2CO monolayer. Among these, the NO2 molecule has the highest muscular adsorption energy in the B2CO monolayer. Based on electronic analysis, the B2CO monolayer can provide robust electron interactions with NO and NO2 molecules. Furthermore, the simulated current–voltage curves showed that the increasing amplitude of the current response for the adsorption of the NO2 molecule is more significant than that for NO. Thus, the calculated sensitivity for the NO2 molecules was much higher than that for the NO molecules in the B2CO monolayer. Based on these results, we predict that the B2CO monolayer will have a much higher sensing performance for detecting nitrogen-containing gases (NCGs) molecules than CO and CO2 molecules, especially NO2 molecules.

Transport Properties with Multiple Functions of Fe@C60-GNR Single Molecule.

Encouraged by the successful fabrication of C60-GNR (GNR=graphene nanoribbon) single-molecule transistors in experiments, four Fe-containing derived double-layered devices of Fe@C60-GNR are designed by employing different electrode linkages and their transport properties are investigated by using density functional theory (DFT) and nonequilibrium Green's function (NEGF) methods. Regardless of electrode connection, all these devices give rise to a smaller negative differential resistance (NDR) peak at V=0.2 and a higher peak at 1.2 V, suggesting their stable maneuverability as molecular devices and good candidates for developing on(off)-off(on)-on(off) current switches. The macroscopic NDR performance depends on the delocalization character and the crossing mechanism of the frontier orbitals. The peak-to-valley current ratios (Rmax) range from 454 to 2737, determined by the electrode linkage. Such a large Rmax-value is necessary for developing dynamic random-access memory (DRAM) cells. Encapsulating the Fe atom inside C60 not only improves the conductivity but also introduces the spin-polarized transport property. The spin-filtering efficiency (SFE) of almost all devices oscillates up and down in response to the bias voltage, indicating the possibility of designing on(off)-off(on)-on(off) spin switches and up-down spin switches. All these fascinating properties provide an important clue for designing similar molecular devices with multiple functions by trapping magnetic transition metal atoms inside fullerenes.

SHE-MTJ based ReLU-max pooling functions for on-chip training of neural networks

We present a detailed investigation of various routes to optimize the power consumption of the spintronic-based devices for implementing rectified linear activation (ReLU) and max-pooling functions. We examine the influence of various spin Hall effect layers, and their input resistances on the power consumption of the ReLU-max pooling functions, we also access the impact of the thermal stability factor of the free-ferromagnet layer on the power consumption and accuracy of the device. The design for ReLU-max pooling relies on the continuous rotation of magnetization, which is accomplished by applying orthogonal spin current to the free-FM layer. We also demonstrate the non-trivial power-resistance relation, where the power consumption decreases with an increase in SHE resistance. We utilize the hybrid spintronic-CMOS simulation platform that combines Keldysh non-equilibrium Green’s function (NEGF) with Landau-Lifshitz-Gilbert-Slonzewski (LLGS) equations and the HSPICE circuit simulator to evaluate our network. Our design takes 0.343 μW of power for ReLU emulation and 17.86 μW of power for ReLU-max pooling network implementation at a thermal stability factor of 4.58, all while maintaining reliable results. We validate the efficiency of our design by implementing a convolutional neural network that classifies the handwritten-MNIST and fashion-MNIST datasets. This implementation illustrates that the classification accuracies achieved are on par with those attained using the ideal software ReLU-max pooling functions, with an energy consumption of 167.31 pJ per sample.

Electronic characteristics of the two-dimensional van der Waals ferroelectric α-In2Se3/Cs3Bi2I9 heterostructures

Two-dimensional (2D) van der Waals (vdWs) ferroelectric heterostructures provide a platform to design multifunctional electronic devices. In this Letter, the 2D vdWs ferroelectric α-In2Se3/Cs3Bi2I9 heterostructures were constructed to investigate the appropriate band alignment, effective masses, charge transfer, and polarization switching barriers by employing first-principle calculation. The results show that the polarization reversal of α-In2Se3 engineers band alignment transition from type-II (α-In2Se3-P↑/Cs3Bi2I9) to type-I (α-In2Se3-P↓/Cs3Bi2I9), resulting in an indirect bandgap changing from 0.09 to 0.75 eV. Calculated electron effective masses are isotropic, which is smaller than those of anisotropic hole effective masses. Interfacial charges transfer from Cs3Bi2I9 to α-In2Se3, which can be ascribed to work function difference. By combining non-equilibrium Green's functions, the current–voltage characteristics of α-In2Se3/Cs3Bi2I9 based ferroelectric tunnel junction were investigated, and the current on/off ratio of 103 is resulted by spatial charge carrier separation and recombination. The proposed electron–hole separation transport design strategy provides a comprehensive theoretical support for the development of microelectronic storage devices.

Liouvillian exceptional points of an open driven two-level system.

We study the applicability of the Liouvillian exceptional points (LEPs) approach to nanoscale open quantum systems. A generic model of the driven two-level system in a thermal environment is analyzed within the nonequilibrium Green's function (NEGF) and Bloch quantum master equation formulations. We derive the latter starting from the exact NEGF Dyson equations and highlight the qualitative limitations of the LEP treatment by examining the approximations employed in its derivation. We find that the non-Markov character of evolution in open quantum systems does not allow for the introduction of the concept of exceptional points for a description of their dynamics. Theoretical analysis is illustrated with numerical simulations.

Ti3C2O2 MXene single-layer as a nanoscale transport device

We considered Ti3C2O2 MXene single-layers with stepped edges as a nanoscale field effect transistor (FET) device. Our model device contains stepped edges at the interface of Ti3C2O2 and Ti2CO2 segments, and a top gate. We suggest that Ti2CO2 semiconducting device region can be obtained by etching the central part of a Ti3C2O2 single-layer. We determined the device characteristic of the proposed device in non-equilibrium Green’s function (NEGF) calculations and observed the transistor behavior. The current through the device is controllable by the total amount of accumulated charge on the gate electrode. Our findings should be applicable to a large number of MXenes: Starting from M3C2O2 MXene single-layers, nanoscale FETs could be produced using conventional mask and etching lithography techniques.

Spin-polarized electrical transport properties of organic radicals in presence of zigzag-graphene nanoribbon leads.

The present work delves into the spin-polarized transport property of organic radicals sandwiched between two zigzag-graphene nanoribbon (ZGNR) electrodes by employing density functional theory and nonequilibrium Green's function technique. We demonstrated that the magnetic center(s) of the radical can manipulate the localized edge states of the ZGNR in the scattering region, causing ferromagnetic coupling. Such manipulation of the magnetic edges results in a high spin-filter effect in molecular junctions, and even the antiferromagnetic diradicals serve as nearly perfect spin filters. We have confirmed that this is a general phenomenon of ZGNR by analyzing two antiferromagnetic diradicals and a doublet. The spin-polarized density of states, transmission spectra, and current vs voltage curves of the systems provide strong evidence for our findings. This research strongly suggests that ZGNRs attached with organic radicals could be the perfect building blocks for spintronic materials.

Microscopic theory of current-induced skyrmion transport and its application in disordered spin textures

Introduction: Magnetic skyrmions hold great promise for realizing compact and stable memory devices that can be manipulated at very low energy costs via electronic current densities.Methods: In this work, we extend a recently introduced method to describe classical skyrmion textures coupled to dynamical itinerant electrons. In this scheme, the electron dynamics is described via nonequilibrium Green’s function (NEGF) within the generalized Kadanoff–Baym ansatz, and the classical spins are treated via the Landau–Lifshitz–Gilbert equation. Here, the framework is extended to open systems by the introduction of a non-interacting approximation to the collision integral of NEGFs. This, in turn, allows us to perform computations of the real-time response of skyrmions to electronic currents in large quantum systems coupled to electronic reservoirs, which exhibit linear scaling in the number of time steps. We use this approach to investigate how electronic spin currents and dilute spin disorder affect skyrmion transport and the skyrmion Hall drift.Results: Our results show that the skyrmion dynamics is sensitive to a specific form of the spin disorder, such that different disorder configurations lead to qualitatively different skyrmion trajectories for the same applied bias.Discussion: This sensitivity arises from the local spin dynamics around the magnetic impurities, a feature that is expected not to be well-captured by phenomenological or spin-only descriptions. At the same time, our findings illustrate the potential of engineering microscopic impurity patterns to steer skyrmion trajectories.

Electrostatically Doped Junctionless Graphene Nanoribbon Tunnel Field-Effect Transistor for High-Performance Gas Sensing Applications: Leveraging Doping Gates for Multi-Gas Detection.

In this paper, a new junctionless graphene nanoribbon tunnel field-effect transistor (JLGNR TFET) is proposed as a multi-gas nanosensor. The nanosensor has been computationally assessed using a quantum simulation based on the self-consistent solutions of the mode space non-equilibrium Green's function (NEGF) formalism coupled with the Poisson's equation considering ballistic transport conditions. The proposed multi-gas nanosensor is endowed with two top gates ensuring both reservoirs' doping and multi-gas sensing. The investigations have included the IDS-VGS transfer characteristics, the gas-induced electrostatic modulations, subthreshold swing, and sensitivity. The order of change in drain current has been considered as a sensitivity metric. The underlying physics of the proposed JLGNR TFET-based multi-gas nanosensor has also been studied through the analysis of the band diagrams behavior and the energy-position-resolved current spectrum. It has been found that the gas-induced work function modulation of the source (drain) gate affects the n-type (p-type) conduction branch by modulating the band-to-band tunneling (BTBT) while the p-type (n-type) conduction branch still unaffected forming a kind of high selectivity from operating regime point of view. The high sensitivity has been recorded in subthermionic subthreshold swing (SS < 60 mV/dec) regime considering small gas-induced gate work function modulation. In addition, advanced simulations have been performed for the detection of two different types of gases separately and simultaneously, where high-performance has been recorded in terms of sensitivity, selectivity, and electrical behavior. The proposed detection approach, which is viable, innovative, simple, and efficient, can be applied using other types of junctionless tunneling field-effect transistors with emerging channel nanomaterials such as the transition metal dichalcogenides materials. The proposed JLGNRTFET-based multi-gas nanosensor is not limited to two specific gases but can also detect other gases by employing appropriate gate materials in terms of selectivity.

Armchair zinc chalcogenide nanoribbons for high-performance photodetectors: A computational study

In this study, we employ density functional theory and non-equilibrium Green's function methods to investigate the electronic and photoelectric response properties of one-dimensional hexagonal ZnX (X = S, Se) nanoribbons (NRs). Remarkably, armchair (ARM) NRs demonstrate superior photoelectric response in the visible and near-ultraviolet regions. It is worth noting that for all considered photon energies, the photocurrent exhibits a cosine dependence on the polarization angle, which is consistent with the photogalvanic effect. when the width of NR is eight atoms, the zinc blende ZnSe/ARM NR-based device achieves a maximum extinction ratio of 63.9 at the photon energy of 3.0 eV. The significant photocurrent and high polarization sensitivity suggest the potential of ZnX NRs for applications in optoelectronic devices, particularly in visible and near-ultraviolet light detection.

Atomistic simulation of thermoelectric properties in cove-edged graphene nanoribbons

We present an atomistic simulation of thermoelectric properties in cove-edged graphene nanoribbons (CGNRs) via the nonequilibrium Green's function. Different from gapless zigzag graphene nanoribbons (ZGNRs), CGNRs exhibit a noticeable bandgap. Such a bandgap can be modulated by varying three structural parameters (namely, the width N, the distance between adjacent coves m, as well as the shortest offset n) of CGNRs, which can give rise to the transition from semiconducting to semi-metallic. Due to the less dispersive phonon bands and the decrease in the number of phonon channels of CGNRs, they are found to have the lower phonon thermal conductance than ZGNRs. Modulation of CGNRs can produce over tenfold improvement of the maximum of ZT compared to ZGNRs. This improvement is due to the promotion of the Seebeck coefficient together with the degradation of the phonon thermal conductance of CGNRs compared to ZGNRs.

Electronic properties, interface contact and transport properties of strain-modulated MS2/borophosphene and MSeS/borophosphene (M = Cr, Mo, W) heterostructure: Insights from first-principles

Controlling the interface contact performance to form low-resistance ohmic contact is of great importance in designing low-power two-dimensional (2D) devices. Here, we explore a 2D semi-metal with Dirac cones, borophosphene, for different types of contact with MS2, MSeS/borophosphene (M = Cr, Mo, W) using density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. The results indicate that under no strain, different van der Waals heterojunctions (vdWHs) exhibit an n-type (p-type) Schottky contact type, with the CrS2/borophosphene vdWHs exhibiting a lower barrier height (Φn = 0.08 eV). Next, we find that the in-plane strain is more efficient than the vertical strain. Under in-plane strain modulation, the contact type will change from Schottky to Ohmic contacts as the tensile strain increases. Similarly, the contact type will change from Schottky to semiconductor as the compressive strain increases. Under the influence of vertical strain, the CrS2/borophosphene vdWHs transition into ohmic contact only when the interlayer spacing increases to above 0.4 Å. Finally, we study the ohmic contact formation and transport characteristics based on the device angle. These findings demonstrate that interface contact types can be effectively regulated by strain, which is crucial for the design and manufacturing of novel nano electronic devices composed of borophosphene based vdWHs.