Nonequilibrium Green Function Research Articles

Controllable gas adsorption via inter-coupled ferroelectricity in In2Se3 monolayer

Recently, two-dimensional ferroelectrics with spontaneous electric polarization have attracted growing interest for application in gas sensors and other electronic devices. This study proposes an in-plane polarization-induced sensor based on the α-In2Se3 ferroelectric material with reversible gas capture or release. A comprehensive quantum mechanical analysis on the characteristics of the device is presented by employing density functional theory calculations, ab initio molecular dynamic simulations, and the non-equilibrium Green's function formalism. The ferroelectricity of α-In2Se3 results in different surface polarizations, leading to different adsorption behaviors on surfaces with opposite polarizations. Based on the adsorption energy, charge transfer, and recovery time, the negatively polarized surface is better suited for gas capture than the positively polarized surface. Consequently, by applying a vertical electric field, which changes the out-of-plane polarization, reversible capture or release of NH3 and NO2 molecules can be achieved. However, applying vertical electric fields in transistor-based gas sensors is rather challenging. Meanwhile, the unique interlocking feature of electric dipoles in α-In2Se3 monolayer can be used to control gas adsorption or desorption by an in-plane bias voltage (electric field). The sensitivities of the positive and negative surfaces to the NH3 molecule are 107.6% and 11.3%, respectively, when an appropriate in-plane bias voltage is applied. The proposed sensor that allows manipulating the out-of-plane polarization by an in-plane bias is of special importance and opens up a new pathway for sensing applications.

Orthogonal Spin Current Injected Magnetic Tunnel Junction for Convolutional Neural Networks

We propose that a spin Hall effect (SHE) driven magnetic tunnel junction (MTJ) device can be engineered to provide a continuous change in the resistance across it when injected with orthogonal spin currents. Using this concept, we develop a hybrid device-circuit simulation platform to design a network that realizes multiple functionalities of a convolutional neural network (CNN). At the atomistic level, we use the Keldysh nonequilibrium Green’s function (NEGF) technique that is coupled self-consistently with the stochastic Landau-Lifshitz-Gilbert–Slonczewski (LLGS) equations, which in turn is coupled with the HSPICE circuit simulator. We demonstrate the simultaneous functionality of the proposed network to evaluate the rectified linear unit (ReLU) and max-pooling functionalities. We present a detailed power and error analysis of the designed network against the thermal stability factor of the free ferromagnets (FMs). Our results show that there exists a nontrivial power-error trade-off in the proposed network, which enables an energy-efficient network design based on unstable free FMs with reliable outputs. The static power for the proposed ReLU circuit is 0.56 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> W and whereas the energy cost of a nine-input ReLU -max-pooling network with an unstable free FM ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sf\Delta$</tex-math> </inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$=$</tex-math> </inline-formula> 15) is 3.4 pJ in the worst case scenario. We also rationalize the magnetization stability of the proposed device by analyzing the vanishing torque gradient points.

Order amidst disorder for two-dimensional nanoribbons with various boundary conditions

We show quantum systems with disordered Hamiltonians may exhibit order in commonly measured quantities. This counter-intuitive situation is demonstrated using a conventional tight binding model for two-dimensional nanoribbons with various boundary conditions. The analysis uses the traditional non-equilibrium Green's function (NEGF) methodology for electron transport. We study quantum dragon nanodevices that exhibit order amidst disorder. Each disordered Hamiltonian nanodevice shows order in both the bond currents and the local density of states (LDOS) of the propagating electrons.

SnS monolayers based heavy metal sensors: DFT and NEGF analysis

Monitoring heavy metals helps prevent heavy metal contamination from affecting human health. However, existing monitoring technologies encounter challenges in real-time detection. Using density functional theory and non-equilibrium Green's function method, sensing capacities of SnS monolayer towards heavy metals are analyzed by adsorption energy, charge transfer, energy band structure, the density of states, differential charge density, recovery time, and transport performance. SnS monolayers exhibit high selectivity and good sensitivity of 13527% and 368860% for detecting As and Pb at the coverage of 1, respectively. With recovery times of 2.99 s for As and 76.44 s for Pb at 598 K, SnS monolayers can detect As or Pb at ambient temperature and be reused at high temperatures. SnS monolayers can selectively detect As and Pb through their response peaks at different bias voltages. SnS monolayers can detect Cd with a sensitivity of 442% in the concentration as low as coverage 1, when desorb at a recovery time of 31.2 μs at 298 K, showing strong potential for ambient temperature sensors for Cd. SnS monolayers require Hg in high concentration, as high as coverage of more than 7, to reach the detection limit. The highest sensitivity of SnS monolayers towards Hg is 326% at the condition of a 0.6-V bias voltage and 8 coverage. Overall, our research provides fresh insight into low-cost, environmentally friendly materials for online heavy metal detection.

Spin Transport Properties of MnBi2Te4-Based Magnetic Tunnel Junctions

The van der Waals heterojunctions, stacking of different two-dimensional materials, have opened unprecedented opportunities to explore new physics and device concepts. Here, combining the density functional theory with non-equilibrium Green's function technique, we systematically investigate the spin-polarized transport properties of van der Waals magnetic tunnel junctions (MTJs), Cu/MnBi2Te4/MnBi2Te4/Cu and Cu/MnBi2Te4/h-BN/n⋅MnBi2Te4/Cu (n = 1, 2, 3). It is found that the maximum tunnel magnetoresistance of Cu/MnBi2Te4/h-BN/3⋅MnBi2Te4/Cu MTJs can reach 162.6%, exceeding the system with only a single layer MnBi2Te4. More interestingly, our results indicate that Cu/MnBi2Te4/h-BN/n⋅MnBi2Te4/Cu (n = 2, 3) MTJs can realize the switching function, while Cu/MnBi2Te4/h-BN/3⋅MnBi2Te4/Cu MTJs exhibit the negative differential resistance. The Cu/MnBi2Te4/h-BN/3⋅MnBi2Te4/Cu in the parallel state shows a spin injection efficiency of more than 83.3%. Our theoretical findings of the transport properties will shed light on the possible experimental studies of MnBi2Te4-based van der Waals magnetic tunneling junctions.

Assessing electron conduction mechanisms in highly organized surface water on a model neuronal lipid using quantum chemical methods

A comprehensive characterization of key mechanisms underlying signal transmission within the human brain remains an unsolved problem. The neuronal surface, composed principally of phosphatidylcholine (POPC), has a known ordering effect on water. This produces highly organized water layers (OWLs) at the neuron–water interface—the neurochemical implications of which are not currently understood. The human brain is 75% water by volume, with folds and grooves to maximize surface area, suggesting that characterization of neuronal OWLs may contribute to an understanding of neuronal signal transmission. Previous experimental work has measured enhanced conductivity of POPC OWLs relative to bulk water. The mechanism underlying this conductivity is still debated. Using quantum chemical methods on a POPC–water interface model system, we present data characterizing OWL conductance. Non-equilibrium Green's function calculation results demonstrate that there is negligible electron transfer-based conductivity through the OWL at biological temperatures. This is consistent with existing studies suggesting the Grotthuss mechanism as the most likely explanation for experimentally observed enhanced conductivity at the POPC–water interface. The broader implications of enhanced proton conductivity at the neuron–water interface are discussed.

Dicke effect driven thermoelectric properties modified by Majorana bound state in triple quantum dot structure

Using the non-equilibrium Green's function technique, thermoelectric transport through a quantum dot connected to two side quantum dots and a topological superconductor is theoretically investigated. Our findings indicate that for weak coupling between quantum dot and Majorana zero mode (QD-MZM), due to the existence of the Dicke effect, the thermoelectric effect in the considered structure can be enhanced by adjusting the structural parameters. However, under strong QD-MZM coupling condition, Majorana zero mode leakage into the QD2 destroys the Dicke effect in the structure, which in turn leads to a severe suppression of the thermoelectric efficiency of the system. These findings, in our opinion, can serve as potential thermoelectric device for detecting Majorana bound states and shed light on the significance of Majorana zero mode in thermoelectricity.

Modeling and simulation of molecular armchair graphene nanoribbons as a gas detector

Optical detectors are essential for gas detection using optical technology. This study introduces an armchair graphene nanoribbon (AGNR) molecular optical gas detector with two conjugated molecular elements and a non-conjugated interface for gas detection using optical technology. The detector absorbs light and generates peaks in the current-voltage curve that correspond to gas concentration and the Raman frequency spectrum. The detector exhibits negative resistances controlled by input light frequency, suitable for high-frequency oscillators. Multiple gases can be simultaneously identified and monitored using the detector. The detector was analyzed using the non-equilibrium Green function method and offers high-speed, accurate selectivity, precise gas detection, and reproducibility with multiple electrical outputs.

Spin-polarized electronic/transport properties of iron-benzene complex-adsorbed graphene sheets

The electronic and transport properties of a series of covalently functionalized graphene sheets m(FeBz)n-G (m, n = 1, 2) are calculated by density functional theory (DFT) and the nonequilibrium Green's function (NEGF) methods. All the studied systems are chemically stable due to the d–π interaction between Fe and graphene. Addition of (FeBz)n (n = 1, 2) introduces a half-metallic behavior: the up-spin state degradates into a semiconductor channel while the down-spin state behaves as a metallic channel. Particularly, two-fold anisotropy magnetism could be obtained, with the ferromagnetic ordering extending either parallel to the graphene plane originated from the Fe-d up-spin state or perpendicular due to the d-electron coupling along the Fe-Bz-Fe-Bz chain in the down-spin state. Because of the trapping effect, attaching (FeBz)n decreases the conductivity. Anisotropy inherited from graphene after functionalized with (FeBz)n (n = 1, 2): zigzag direction has higher conductivity than armchair direction. The number and alignment style of (FeBz)n (n = 1, 2), and the spin-polarized character, all have impacts on the transport property. Our findings provide useful guidelines for achieving graphene-based electronic and spintronic nanodevices by attaching other similar long multi-decker metal-arene systems.

RSDFT-NEGF transport simulations in realistic nanoscale transistors

The paper presents a device simulator for computing transport characteristics from first principles. The developed computer program effectively performs large-scale parallel calculation of quasi-one-dimensional quantum transport in realistic nanoscale devices with thousands of atoms in the cross section area of the device channel. Our simulator is based on the real-space Kohn–Sham Hamiltonian in the density functional theory and improved numerical algorithms for reducing computational burden in non-equilibrium Green’s function (NEGF) method. Several computational improvements have been introduced in constructing a reduced quantum transport model from the original Kohn-Sham Hamiltonian and implementing the R-matrix computational scheme in the NEGF simulations.

Tuning the transport properties of tetracene-based single-molecule junctions with chemical or structural variation of side and anchoring groups.

This study aims to tune the transport properties of tetracene single-molecule junctions with the proper choice and placement of side and anchoring groups. For the operationalization of the molecule that was anchored with thiol or isocyanide groups, two different side groups, amine and nitro, in two different positions, were taken into consideration. For unperturbed tetracene molecule, a prominent negative differential resistance (NDR) feature at 1.8 V was observed with the isocyanide anchoring group while the thiol anchoring group exhibits a plateau region over a bias voltage of 2.2 to 3.2 V. At a bias voltage that is dependent on the chemical or structural change of side or anchoring groups, NDR feature of varying degree was seen in all configurations. Results show that the current flowing through the thiol-anchored molecule perturbed with the amine group at S' position is relatively larger than other configurations because of the smaller HOMO-LUMO gap and broader transmission peaks resulting in a peak to valley current ratio (PVCR) of 1.22. In addition, multiple NDR regions were realized in nitro-perturbed isocyanide-anchored molecule at S position. These results suggest their promising applications in switches, logic cells, and storage devices. The modeling and simulation of side-group mediated anchored tetracene molecule through two electrodic systems were studied using density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) in Virtual NanoLab-AtomistixToolkit (ATK). The electron transport properties were calculated using Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation function. To optimize computing time, gold electrodes were single zeta polarized whereas the molecule, anchor groups, and side groups were double zeta polarized.

Effects of Inorganic Substitutions and Different Metal Electrode Materials on Electronic Transport Properties of Organic Molecular Devices.

Incorporating inorganic components into organic molecular devices offers one novel alternative to address challenges existing in the fabrication and integration of nanoscale devices. In this study, using a theoretical method of density functional theory combined with the nonequilibrium Green's function, a series of benzene-based molecules with group III and V substitutions, including borazine molecule and XnB3-nN3H6 (X = Al or Ga, n = 1-3) molecules/clusters, are constructed and investigated. An analysis of electronic structures reveals that the introduction of inorganic components effectively reduces the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, albeit at the cost of reduced aromaticity in these molecules/clusters. Simulated electronic transport characteristics demonstrate that XnB3-nN3H6 molecules/clusters coupled between metal electrodes exhibit lower conductance compared to prototypical benzene molecule. Additionally, the choice of metal electrode materials significantly impacts the electronic transport properties, with platinum electrode devices displaying distinct behavior compared to silver, copper, and gold electrode devices. This distinction arises from the amount of transferred charge, which modulates the alignment between molecular orbitals and the Fermi level of the metal electrodes by shifting the molecular orbitals in energy. These findings provide valuable theoretical insights for the future design of molecular devices incorporating inorganic substitutions.

A physically realizable molecular motor driven by the Landauer blowtorch effect.

We propose a model for a molecular motor in a molecular electronic junction driven by a natural manifestation of Landauer's blowtorch effect. The effect emerges via the interplay of electronic friction and diffusion coefficients, each calculated quantum mechanically using nonequilibrium Green's functions, within a semiclassical Langevin description of the rotational dynamics. The motor functionality is analyzed through numerical simulations where the rotations exhibit a directional preference according to the intrinsic geometry of the molecular configuration. The proposed mechanism for motor function is expected to be ubiquitous for a range of molecular geometries beyond the one examined here.

Photoconductive metal-organic frameworks based on 10,20-meso-substituted Zn-porphyrin and fullerene C60

Metal-organic frameworks (MOFs) with electrical conductivity have potential applications as electrochemical sensors, electrocatalysis, and energy storage. We present the design of a new class of conductive MOFs based on porphyrin, substituted in the 10,20 meso-positions (1 amine, 2 hydroxy, 3 methoxy, 4 nitro, and 5 dimethylamine). Furthermore, these MOFs are doped with fullerene molecules (C60) within their pores, resulting in 1@C60, 2@C60, 3@C60, 4@C60, and 5@C60. To study the electronic and optical properties of these MOFs before and after the incorporation of C60, theoretical calculations based on density functional theory (DFT) (PBE+D3/DZP) and time-dependent DFT (TPSSh/6-31G) were performed. Incorporating C60 causes a decrease in the bandgap of MOFs due to its unoccupied crystalline p orbitals (LUCOs). It also promotes the Q and Soret (B) bands of porphyrin in the MOF to absorb at longer wavelengths. Charge transport properties were calculated using the Keldysh nonequilibrium Green's function (NEGF) formalism and molecular junctions with Au(111)-based electrodes for 1 and 1@C60. When a bias voltage of 0.2 V is applied, an increase in current is obtained for 1@C60 concerning 1. Our results suggest that MOF@C60 is a promising alternative for designing new photoconductive materials because of the donor-acceptor interactions generated.

Small-Signal Modeling of Dissipative Carrier Transport in Nanodevices with Nonequilibrium Green’s Functions

Emerging applications in the field of finite-frequency mesoscopic physics require accurate modeling tools for the evaluation of carrier-transport dynamics, modulation bandwidth, and frequency conversion effects in nanodevices. With foundations in advanced concepts of many-body and quantum field theories, the nonequilibrium Green's function technique is widely adopted in the calculation of steady-state carrier-transport properties of nanostrucures, while the evaluation of the frequency response is so far largely unexplored by genuine quantum models. Guided by the connection with drift-diffusion solvers within a local description of carrier-phonon scattering, we propose an accurate, yet computationally efficient nonequilibrium Green's function model of dissipative carrier transport to study the small-signal properties of semiconductor nanostructures. From the numerical evaluation of steady-state Green's functions and their functional derivatives, we compute spectrally resolved observables expressed in terms of familiar microscopic quantities germane to the drift-diffusion framework, the most prevalent tool in semiclassical device simulation. Besides drastically improving the convergence properties, the exact Jacobian, complemented with the contribution of the displacement current, gives access to the small-signal admittance. Current-conserving boundary conditions suitable for small-signal analysis provide the correct physical behavior near the contacts. Numerical examples show the accuracy and flexibility of the proposed model.

DC and RF Performance of an N-channel Monolayer Black Phosphorus Nanoribbon Transistor

Two-dimensional black phosphorus is a relatively new discovery. There are numerous studies on black phosphorus two-dimensional transistors that focus on analog and RF performance. However, the RF performance of black phosphorus nanoribbon transistors is yet to be explored. We use a four-band tight binding Hamiltonian in conjunction with a non-equilibrium Green's function quantum transport simulator to investigate both the DC and RF performance of a monolayer black phosphorus nanoribbon transistor. We found that electron intra-band tunneling is responsible for current flow in the off-state, while in the on-state, the electrons flow over the top of the channel barrier potential. With a VDD of 0.4 volt and a gate length of 5 nm, our black phosphorus nanoribbon transistor has DC performance metrics of 510 µA/µm on-state current, 105 on/off current ratio, and 65 mV/dec inverse subthreshold slope. The device's RF performance characteristics are as follows: cut-off (unity current gain) frequency of 772.84 GHz, maximum oscillation (unity power gain) frequency of 1.15 THz, and open circuit voltage gain of 26.7 dB with transistor operating in the on-state. The RF performance of the device is found to be significantly impacted by the source and drain contact resistances. With source and drain resistances set to zero, the cut-off frequency increases to 995.23 GHz and the unity power gain frequency increases to 4.16 THz. The device shows unconditional stability above 893 GHz and it is conditionally stable below this frequency.

Performance Boost of Si TFETs by Insertion of III–V Dipole Formation Layer: A First Principle Study

Silicon channel tunnel field-effect transistors (TFETs) are known to exhibit low ON-state current due to the wide indirect band gap. In this work, to achieve high ON-state current, we propose a Si TFET with an ultra-thin dipole formation layer (DFL) inserted at the source/channel junction. The inserted DFL forms charge transfer dipoles at the interface, resulting in a staggered band gap. As the consequence, the effective tunnel barrier for electrons is significantly lowered so that ON-state current increases by orders of magnitude. To analyze the characteristics of DFL-inserted Si TFETs, we have solved nonequilibrium Green’s function (NEGF) and Poisson’s equation using density functional theory (DFT) Hamiltonians. We demonstrate that DFL-inserted Si TFETs show boosted performance over conventional TFETs, exhibiting high ON-state current and steep subthreshold swing (SS).

Enhancement of the thermoelectric performance of a single-walled carbon nanotube via Al, Si, P, and S impurities

Using the tight-binding approach based on density functional theory (DFT) and nonequilibrium Green's function (NEGF) computations, the potential use of single-walled carbon nanotubes (SWCNTs) as high-performance thermoelectric materials is studied. Consequently, the structural, electrical, and thermoelectric characteristics of pristine and X (Al, Si, P, and S) -doped SWCNTs are studied. It has been observed that Al, Si, P, and S impurities open the band structure of SWCNTs. In Boltzman theory, transport factors including the Seebeck coefficient (S), thermal conductivity (k), and figure of merit (ZT) are dependent on the band structure. Therefore, band structure has a significant impact on thermoelectric properties. The findings reveal that the thermoelectric performance of doped SWCNTs surpasses that of their pristine SWCNTs counterparts, and the figure of merit value of doped CNTs is increased to high as ∼2, which implies their very attractive thermoelectric applications.

Nonequilibrium quantum transport properties of tetragonal zinc chalcogenide monolayers

Tetragonal zinc chalcogenide monolayers (TZCM) are emerging as interesting electronic materials with a direct band gap and relatively high carrier mobility. In this work, we report a theoretical investigation of electronic transport properties and photoelectric response properties of TZCM with gold contacts by density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. When there is no gate voltage applied, the current increases nonlinearly as bias voltage increases. Among the four proposed devices, the Au(100)/ZnS/Au(100) device has the best electronic transport performance and is most sensitive to the adjustment of bias voltage and gate voltage. The photocurrent calculation results indicate that the low-frequency oscillatory photocurrent of the Au(100)/ZnSe/Au(100) device in the high photon energy region may have potential applications in ultraviolet light-emitting diodes. The Au(100)/Zn2SeS/Au(100) device has more stable photoelectric response and polarization sensitivity than the Au(100)/Zn2SSe/Au(100) device. The Au(100)/TZCM/Au(100) devices exhibit considerable photocurrent and good extinction ratios. This work could pave the way for the future application of TZCM in the field of optoelectronics and so on.

Ab Initio Investigations of Gallium Nitride Nanoribbons for Spin Filter and Negative Differential Behavior

This work demonstrates the influence of alternative edge passivation via H/F on the electronic and magnetic characteristics of gallium nitride nanoribbons (GaNNRs). For this study, the first-principles density functional theory (DFT) and non-equilibrium green function (NEGF) frameworks are deployed. Our study demonstrates that the selective edge passivation with F/H zigzag GaN nanoribbon (NR) is a strong contender for spintronics due to its half-metallic property under specific conditions. Various nano-configurations for the H/F ZGaNNRs passivation are investigated here. Thermodynamically, most stable configuration is alternate fluorinated F-GaN-F NR. Further, the negative differential resistance (NDR) behavior is also reported along with the perfect spin-filtering properties. This study paves the way to employ these 2-D nanostructures-based devices for spin filters, spin logic switches, and steep switching nanoelectronic devices.