Nonequilibrium Green Function Research Articles

Ultra-low voltage adenine based gas sensor to detect H2 and NH3 at room temperature: First-principles paradigm

The molecular system-level detection of H2 and NH3 gas using an electrically doped Adenine bio-molecular gas sensor has been proposed and investigated using Density Functional Theory (DFT) combined with Non-Equilibrium Green's Function (NEGF) formalisms. First-principles calculations were applied and the structures and electronic properties of the Adenine gas sensor have calculated. This sensor reveals that the current-voltage response and conductivity of the bio-molecules increased evidently after the adsorption of these gas molecules. The Adenine sensor offers approximately 1800 times and 3300 times better current response during H2 and NH3 adsorption respectively. The significant gap between Highest Occupied Molecular Orbital (HOMO) and Lowest Un-occupied Molecular Orbital (LUMO) indicates the system's thermodynamic stability. Therefore, we hope that the Adenine monolayer could be a room temperature H2 and NH3 sensor with high selectivity and sensitivity and fast response and recovery time. Therefore, we hope that the Adenine monolayer will be a good candidate forH2 and NH3 work-function-type gas sensors.

Spin-Polarized Resonant Tunneling in Antiferromagnetic Heterojunctions of Graphene Nanoribbons with 3d Adatoms

Binding, magnetic, and electron transport properties of $3d$ transition-metal (TM) adatoms ($\mathrm{Cr}$-$\mathrm{Cu}$) on an antiferromagnetic armchair graphene nanoribbon heterojunction are studied by means of density-functional-theory (DFT) calculations and nonequilibrium Green function (NEGF) formalism. The heterojunction emulates a system of two symmetric potential barriers and one well with resonances (quasibound states). Binding energies show a strong interaction between $3d$ TM adatoms and heterojunction hollow sites, where $3d$ TM adatoms prefer the edges rather than the middle of the graphene heterojunction. As for magnetic properties, heterojunctions with Cr, $\mathrm{Mn}$, and $\mathrm{Fe}$ have a ferromagnetic behavior with a total magnetic moment $\ensuremath{\ge}4{\ensuremath{\mu}}_{B}$, while $\mathrm{Co}$, $\mathrm{Ni}$, and $\mathrm{Cu}$ adatoms induce antiferromagnetism in the full system with total magnetizations $<3{\ensuremath{\mu}}_{B}$. Transmission curves show that electronic resonances undergo a spin-splitting effect induced by $3d$ TM adatoms, and current versus bias-voltage curves show spin currents from which spin-polarized ratios (SPRs) $>75\mathrm{%}$ for $\mathrm{Co}$ and approximately $100\mathrm{%}$ for $\mathrm{Ni}$ adatoms are calculated. Finally, an oscillatory behavior is observed in SPR curves at low bias due to a combined effect of resonant tunneling and spin filtering. Therefore, these results suggest that the present antiferromagnetic graphene heterojunction with $\mathrm{Co}$ and $\mathrm{Ni}$ adatoms is suitable for future spin and low-power nanoelectronics as a spin-dependent resonant tunneling device.

Hierarchical modeling for TCAD simulation of short-channel 2D material-based FETs

An integrated hierarchical modeling flow for fast analysis and prototyping of 2D material-based field-effect transistors (FETs) is presented. Advanced transport simulators using ab initio atomistic density functional theory (DFT) and continuum effective mass non-equilibrium Green’s Functions (NEGF) consistently provide TCAD tools with device material and component parameters and reference curves for physical model selection and calibration. We validate each step of the hierarchical flow, and we show that the simulations performed with the resulting TCAD setup accurately predict 2D-FETs device characteristics.

First-principles investigation of polytypic defects in InP

In this paper we study polytypic defects in Indium Phosphide (InP) using the complementary first-principles methods of density functional theory and non-equilibrium Greens functions. Specifically we study interfaces between the ground state Zincblende crystal structure and the meta-stable Wurtzite phase, with an emphasis on the rotational twin plane defect, which forms due to the polytypic nature of InP. We found that the transition of the band structure across the interface is anisotropic and lasts 7 nm (3.5 nm). Due to this, a crystal-phase quantum well would require a minimal width of 10 nm, which eliminates rotational twin planes as possible quantum wells. We also found that for conducting current, the interfaces increase conductivity along the defect-plane ([11bar{2}]), whereas due to real growth limitations, despite the interfaces reducing conductivity across the defect-plane ([111]), we found that a high degree of polytypic defects are still desirable. This was argued to be the case, due to a higher fraction of Wurtzite segments in a highly phase-intermixed system.

Parallel-Self-Assembling Stack, Center-Capture Effect, and Reactivity-Enhancing Effect of N-Layer (N = 1, 2, 3) Cyclo[18]carbon.

Cyclo[18]carbon (C18) is an captivating allotrope of carbon synthesized recently, which has drawn the attention among scientists. There are still few studies on the dynamic behaviors of C18. To gain knowledge in this area, we systematically explored the stacking behaviors and the oxidation kinetics of C18, as well the electronic transport behaviors of C18 oxides, by density functional theory and nonequilibrium Green's function calculations combined with reactive force field molecular dynamics simulations. The parallel-self-assembling behaviors were observed in the stack of two- or three-layer C18. During the oxidation process of C18, we found an evident center-capture effect in which the hollow rings would preferentially attract an O2 molecule into their centers. Moreover, the adsorption of O2 on the O2-doped rings was dramatically enhanced by the O2 at the center of the ring, showing the reactivity-enhancing effect. The excellent electron transport property of central-O2-doped C18 among 13 types of C18 oxides demonstrates the potential of C18 oxides as promising molecular devices for various applications. This study reveals the dynamic behaviors of C18 and provides theoretical guidance for use of C18 and C18 oxides in molecular devices.

Giant effective electron-magnon coupling in a nonmagnetic metal–ferromagnetic insulator heterostructure

Magnon-mediated spin transport across nonmagnetic metal (NM) and ferromagnetic insulator (FI) interface depends critically on electron-magnon coupling. We propose a novel route to enhance electron-magnon coupling dynamically from transport viewpoint. Using non-equilibrium Green's function a theoretical formalism for magnon-mediated spin current is developed. In the language of transport, the effective electron-magnon coupling at NM/FI interface is determined by self-energy of FI lead, which is proportional to density of states (DOS) at NM/FI interface due to nonlinear process of electron-magnon conversion. By modifying interfacial DOS, the spin conductance of 2D and 3D NM/FI systems can be increased by almost three orders of magnitude, setting up a new platform of manipulating dynamical electron-magnon coupling.

Emergence of negative viscosities and colored noise under current-driven Ehrenfest molecular dynamics

Molecules in molecular junctions are subject to current-induced forces that can break chemical bonds, induce reactions, destabilize molecular geometry, and halt the operation of the junction. Theories behind current-driven molecular dynamics simulations rely on a perturbative time-scale separation within the system with subsequent use of nonequilibrium Green's functions (NEGF) to compute conservative, non-conservative, and stochastic forces exerted by electrons on nuclear degrees of freedom. We analyze the effectiveness of this approximation, paying particular attention to the phenomenon of negative viscosities. The perturbative approximation is directly compared to the nonequilibrium Ehrenfest approach. We introduce a novel time-stepping approach to calculate the forces present in the Ehrenfest method via exact integration of the equations of motion for the nonequilibrium Green's functions, which does not necessitate a time-scale separation within the system and provides an exact description for the corresponding classical dynamics. We observe that negative viscosities are not artifacts of a perturbative treatment but also emerge in Ehrenfest dynamics. However, the effects of negative viscosity have the possibility of being overwhelmed by the predominantly positive dissipation due to the higher-order forces unaccounted for by the perturbative approach. Additionally, we assess the validity of the white-noise approximation for the stochastic forces, finding that it is justifiable in the presence of a clear time-scale separation and is more applicable when the current-carrying molecular orbital is moved outside of the voltage window. Finally, we demonstrate the method for molecular junction models consisting of one and two classical degrees of freedom.

Multifunctional 2D g-C4N3/MoS2 vdW Heterostructure-Based Nanodevices: Spin Filtering and Gas Sensing Properties.

Two-dimensional (2D) magnetic materials are the key to the development of the new generation in spintronics technology and engineering multifunctional devices. Herein, the electronic, spin-resolved transmission, and gas sensing properties of the 2D g-C4N3/MoS2 van der Waals (vdW) heterostructure have been investigated by using density functional theory with non-equilibrium Green's function method. First, the g-C4N3/MoS2 vdW heterostructure demonstrates ferromagnetic half-metallicity and superior adsorption capacity for gas molecules. The spin-dependent electronic transport of the g-C4N3/MoS2-based nanodevice is obviously regulated by parallel or anti-parallel spin configuration in electrodes, leading to perfect single-spin conduction behavior with a nearly 100% spin filtering efficiency, a negative differential resistance effect, and other interesting electrical transport phenomena. Moreover, g-C4N3/MoS2 exhibits directional dependency and strong transport anisotropic behavior under bias windows, indicating that the electric current propagates more easily through the vertical direction than the horizontal direction. The physical mechanisms are revealed and analyzed by presenting the bias-dependent transmission spectra in combination with the projected local device density of states. Finally, the g-C4N3/MoS2-based gas sensor is more sensitive to CO, NO, NO2, and NH3 molecules with the chemisorption type. The strong chemical adsorption leads to the formation of electrons on the local scattering center and ultimately affects the transport properties, resulting in the maximum gas sensitivity reaching 6.45 for NO at the bias of 0.8 V. This work not only reveals that the g-C4N3/MoS2 vdW heterostructure with high anisotropy, perfect spin filtering, and outstanding gas sensitivity is a promising 2D material but also provides an insight into the further application in futuristic electronic nanodevices.

Phononic heat conductance of gold atomic contacts: Coherent versus incoherent transport

We present here a theoretical method to determine the phononic contribution to the thermal conductance of nanoscale systems in the phase-coherent regime. Our approach makes use of classical molecular dynamics (MD) simulations to calculate the temperature-dependent dynamical matrix, and the phononic heat conductance is subsequently computed within the Landauer-B\"uttiker formalism with the help of nonequilibrium Green's function techniques. Tailored to nanostructures, crucial steps of force constant and heat transport calculations are performed directly in real space. As compared to conventional density functional theory (DFT) approaches, the advantage of our method is two-fold. First, interatomic interactions can be described with the method of choice. Semiempirical potentials may lead to large computational speedups, enabling the study of much larger systems. Second, the method naturally takes into account the temperature dependence of atomic force constants, an aspect that is ignored in typical static DFT-based calculations. We illustrate our method by analyzing the temperature dependence of the phononic thermal conductance of gold (Au) chains with lengths ranging from 1 to 12 atoms. Moreover, in order to evaluate the importance of anharmonic effects in these atomic-scale wires, we compare the phase-coherent approach with nonequilibrium MD (NEMD) simulations. We find that the predictions of the phase-coherent method and the classical NEMD approach largely agree above the Debye temperature for all studied chain lengths, which shows that heat transport is coherent and that our phase-coherent approach is well suited for such nanostructures.

Increasing the output power of a heavily doped terahertz quantum cascade laser by avoiding the subband misalignment

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. This enabled us to avoid the subband misalignment induced by the strong band bending effect, to provide efficient carrier transport even at high doping concentrations, and to improve the output power. The maximum output power of a GaAs/Al0.16Ga0.84As 3.5 THz-band QCL at 10 K was enhanced from 390 to 600 mW by increasing the doping concentration at the depopulation layers from 6 × 1016 to 1.2 × 1017 cm−3. The average power of 21 mW was achieved by implementing the appropriate pulse drive conditions.

First-principle study of the current–voltage on the β-diketones with alkyl and methoxy groups at the beta position as molecular switches

In this study, the nonequilibrium green's function (NEGF) combination with density functional theory (DFT) was used to investigate the molecular switching properties in some β-diketones with different alkyl and methoxy groups in their beta positions. The compounds used in this research are: 1,7-dimercapto-2,2,6,6-tetramethyl-3 5-heptanedione (STMHDS), 1,7-dimercapto3,5-Heptanedione (SHPDS), 1,5-dimercaptopentane-2,4-dione (SAAS), and mercaptomethyl 4-mercapto-3-oxobutanoate (SMOBS). In these four compounds, in most cases, the current in keto form is lower than in the enol form. We thoroughly examined the STMHDS molecule, including different electrodes (Pt, Au, and Ag), the types of connection states (top, bridge, and hollow), the on–off ratios, the HOMO-LUMO gaps, the I-V, and the transmission spectra. For this molecule, the best connection is hollow, so the same type of connection is considered for all calculations. With changing the enol form to keto form, the conductivity changed from the on-state (low resistance) to the off-state (high resistance). The best electrodes with the highest current ratios in STMHDS, SHPDS, SAAS, and SMOBS are Au, Au, Pt, and Ag, respectively. Because the ratios in SHPDS, SAAS, and SMOBS molecules are much smaller than that in STMHDS molecule, the transmission spectra and the HOMO-LUMO gaps cannot explain which form is more conductive.

Charge filling induced quasi-two dimensional charge/spin density wave channel in a ballistic transport device

Charge filling controlled mean field metal–insulator phase transition is examined in the context of two dimensional Fermi surface nesting and van Hove singularity induced charge density wave (CDW), spin density wave (SDW) condensates. In the framework of a coherent ballistic transport model utilizing the Non-Equilibrium Green Function approach (NEGF), a three terminal device with metallic gate, source, drain and CDW/SDW channel is simulated and studied. Within the validity of mean field approximation, we exposit the commensurability and boundary effects. The efficacy of the Hubbard model for (quasi) two dimensional Charge and Spin Density Wave materials is discussed. A two orbital generalization of the effective Hamiltonian is proposed for transport calculations in rare earth Tellurides RTe 3.

Effect of strain on quantum transport in fully-hydrogenated silicene based field effect transistor

In this work, the impact of strain on the quantum transport in fully hydrogenated silicene based field effect transistor (FET) has been comprehensively studied by using the multi-scale approach consisting of density functional theory (DFT), Wannier function based tight binding and the non-equilibrium Green's Function (NEGF) formalism. It has been observed that, the buckling height of the fully hydrogenated silicene increases with the compressive strain and it reduces with the tensile strain. Therefore, the application of the strain on the fully hydrogenated silicene tunes the bandgap and for the top surface fully hydrogenated silicene it is found to be 1.34 eV. Further, it has been noticed that, in comparison to uniaxial compressively strained hydrogenated silicene, the bandgap reduction in the biaxial compressively strained hydrogenated silicene is higher. In addition to this, as compared to uniaxial tensile strained hydrogenated silicene, the increase in effective mass of the electron is more dominant in the biaxial tensile strained fully hydrogenated silicene. It has also been observed that, due to increase in compressive strain, the on-state drive current (ION) of the FET improves whereas due to reduction in bandgap, the off-state leakage current (IOFF) increases. On the other hand, the variations in the tensile strain present in the channel of the device improves the ION. Unlike the effect of variations in compressive strain on transfer characteristics, the variations in the tensile strain does not affect both sub-threshold slope and the IOFF of the device.

Investigation and comparison of graphene nanoribbon and carbon nanotube based SARS-CoV-2 detection sensors: An ab initio study

The rapid detection of SARS-CoV-2, the pathogen of the Covid-19 pandemic, is obviously of great importance for stopping the spread of the virus by detecting infected individuals. Here, we report the ab initio analysis results of graphene nanoribbon (GNR) and carbon nanotube (CNT) based SARS-CoV-2 detection sensors which are experimentally demonstrated in the literature. The investigated structures are the realistic molecular models of the sensors that are employing 1-pyrenebutyric acid N-hydroxysuccinimide ester as the antibody linker. Density functional theory in conjunction with non-equilibrium Green's function formalism (DFT-NEGF) is used to obtain the transmission spectra, current-voltage and resistance-voltage characteristics of the sensors before and after the attachment of the SARS-CoV-2 spike protein. The operation mechanism of the GNR and CNT based SARS-CoV-2 sensors are exposed using the transmission spectrum analysis. Moreover, it is observed that GNR based sensor has more definitive detection characteristics compared to its CNT based counterpart.

First-Principles-Based Quantum Transport Simulations of High-Performance and Low-Power MOSFETs Based on Monolayer Ga2O3.

The electronic properties of monolayer (ML) Ga2O3 and transport properties of ML Ga2O3-based n-type metal-oxide-semiconductor field-effect transistors (MOSFETs) are investigated by first-principles calculations under the framework of density functional theory (DFT) coupled with the nonequilibrium Green's function (NEGF) formalism. The results show that ML Ga2O3 has a quasi-direct band gap of 4.92 eV, and the x- and y-directed electron mobilities are 1210 and 816 cm2 V-1 s-1 at 300 K, respectively, under the full consideration of phonon scattering. The electron-phonon scattering mechanism shows a temperature-dependent behavior, with the acoustic modes dominating below 300 K and optical modes dominating above 300 K. At a gate length of Lg = 5 nm, the on-current of ML Ga2O3 n-MOSFET for high-performance (HP) application is 2890 μA/μm, which is more than those of the most reported two-dimensional (2D) materials. The delay time as well as the power delay product of ML Ga2O3 MOSFETs can meet the demands of the latest International Technology Roadmap for Semiconductors (ITRS) for HP and low-power (LP) applications until Lg is less than 4 and 5 nm, respectively. Through underlap structure and doping optimization strategies, ML Ga2O3 n-MOSFET can further fulfill the ITRS requirements for 1 nm. At last, we compare the performance of the 32-bit arithmetic logic unit (ALU) built on ML Ga2O3 MOSFETs with the recently reported beyond-CMOS devices. Our results indicate that ML Ga2O3 can serve as a promising channel material in the post-silicon era.

Switchable single-molecule electronic and thermoelectric device induced by light in a designed diarylethene molecule

Tremendous efforts have been devoted to studying ordinary and novel properties in molecular-scale. Especially the reversible and controlled electrical and thermoelectric switches, which have been extensively studied experimentally and theoretically. They possess two distinct electronic and thermoelectric transport states actuated by external triggers, such as light, temperature, and electric field. Here, a photochromic diarylethene, linked to a nontriazatriangulene platform covalently, is studied by first-principles calculation. By means of density functional theory, we calculate the density of states and scanning tunneling microscope (STM) images of the designed diarylethene adsorbed on a Au(111) surface. Moreover, the thermoelectric transport properties of diarylethene-based molecular junction with a Au(111) electrode are studied systematically by using nonequilibrium Green's function transport formalism. We illustrate that due to the structure chang e of the designed diarylethene induced by light, the electronic and thermoelectric transport properties of the junction are completely different. Those differences can be visualized by the calculated electronic transmission eigenchannel and transport pathway. Furthermore, we demonstrate that the structure change of the designed diarylethene can be detected by STM experiments. The present study shows that the designed diarylethene could be a desirable candidate for electronic and thermoelectric molecular switch driven by light.

Impact analysis of vacancy defects on Analog/RF performance parameters of GNR FET

The effects of vacancy defects, such as single vacancy defect (SVD) and di-vacancy defect (DVD) for low voltage, on the performance parameters of a dual-gated graphene nanoribbon (GNR) field-effect transistor (FET) are investigated and compared to an ideal GNR FET with no vacancy defect (zero vacancy). Self-consistent atomistic simulations are used to evaluate the performance parameters of the device based on the non-equilibrium Green's function (NEGF) formalism. Initially, the impact of vacancy defects on the analog and RF performance parameters is examined, and then a trade-off analysis is conducted between transistor efficiency and frequency. The results indicate that the di-vacancy device has the highest transconductance generation factor (TGF), output resistance, intrinsic gain, gate capacitance, power delay product, gain frequency product (GFP), and gain transfer frequency product (GTFP). In contrast, the highest transconductance value, transconductance frequency product (TFP) and cut-off frequency are obtained with the device without any vacancy defect.

Full quantum simulation of Shockley–Read–Hall recombination in p-i-n and tunnel diodes

We have included Shockley–Read–Hall (SRH) generation/recombination in Non-equilibrium Green’s function (NEGF) calculations via a multiphonon relaxation model. The model has been used to study how the presence of defects affects the current–voltage characteristics of GaAs p-i-n diodes, and of an InGaAs tunnel diode. Regarding p-i-n diodes, we show that SRH generation/recombination is responsible for ideality factors approaching the theoretical value of two in the forward bias regime, while in reverse bias the recombination current density varies slowly with the applied voltage. In all the considered cases, the defects located in the center of the active region proved to be the most effective in allowing trap-assisted tunneling from the valence to the conduction band. Finally, the inclusion of SRH recombination in NEGF simulations of Esaki tunnel diodes permits to predict a realistic degradation of the peak-to-valley current ratio.

Quantized charge-pumping in higher-order topological insulators

We study the quantized charge pumping of higher-order topological insulators (HOTIs) with edge-corner correspondences based on the combination of the rotation of in-plane magnetic field and the quantum spin Hall effect. A picture of a specific charge pumping process is uncovered with the help of the non-equilibrium Green's function method. Significantly, we demonstrate that the quantized charge pumping current is achieved without the participation of bulk states, and the charges move along the boundary of the sample. Furthermore, the effects of external parameters on the pumping current is also studied. We find that the magnitude and direction of the pumping current can be manipulated by adjusting the coupling strength between the leads and sample. Our work deepens the understanding of the charge pumping in HOTIs and extends the study of their transport properties.

Tunneling FET Based on Monolayer Antimonene: The Role of Vacancy

In this work, the electrical performance of the antimonene TFET is investigated using nonequilibrium Green’s function (NEGF) through a tight-binding approach. In the following, the effect of atom vacancy on the electrical behavior of the TFET is explored. The creation of the mid-gap state due to the vacancy is shown using the local density of states (LDOS) for the TFET and the density of states (DOS) for a ribbon. Furthermore, the effects of these mid-gap states on the ON-current, OFF-current, ON– OFF ratio, and subthreshold swing (SS) are discussed. Finally, the effect of the scaling in presence of vacancy is explored. The results show that a small vacancy percentage declines the SS while the variation of the ON– OFF ratio is negligible.