NEGF Method Research Articles

Defect-Induced Sensitivity Improvement in HfNBr Monolayers for Ammonia Detection.

Two-dimensional materials, owing to their unique physical properties and high surface area, play a crucial role in intelligent sensing, particularly in the domain of atmospheric pollutant monitoring. In this work, we have extensively investigated the gas-sensing capabilities of the HfNBr monolayer for ammonia detection by introducing point defects, utilizing density functional theory and nonequilibrium Green's function calculations. Upon the introduction of point defects, the adsorption energy of HfNBr monolayers for ammonia significantly increased (from -0.162 to -1.257 eV), indicating a markedly strengthened affinity. To further elucidate the sensing mechanism, we conducted an in-depth investigation into the charge transfer dynamics, the density of states, and the charge density difference between the adsorbent and the adsorbate. Besides, we employed the NEGF method to evaluate the changes in the current-voltage characteristics of the HfNBr monolayer before and after adsorption, which revealed a remarkable change in the apparent resistance, thereby demonstrating excellent sensitivity. The exceptional performance of the HfNBr monolayer toward NH3 demonstrates its significant value in practical applications for ammonia detection.

A tutorial on the NEGF method for electron transport in devices and defective materials

A tutorial on the NEGF method for electron transport in devices and defective materials

Electronic transport on the two state “ON–OFF” of 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran as a light-driven molecular optical switch: A first-principle study

Here, the electronic transport characteristics of the 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran were studied by using NEGF and DFT methods. By being exposed to UV or visible light, this molecule can change from its MC to SP states. The titled structure was optimized at B3LYP/6–311++G(d,p) level of theory. To obtain conductivity results, the electrons of metal and organic atoms were approached by the single-plus polarization (SZP) for single-zeta and double-plus polarization (DZP) for double zeta basis sets, respectively. We examined some factors in this study, including various molecule geometries, the type of surface materials (Platinum, gold, and silver), the switching ratio, the gap between HOMO and LUMO levels, and the transmission spectra, at different bias voltages. The outcomes show that conductivity transforms from an off-state (high resistance) to an on-state as the molecule switches from MC to SP form (low resistance).

Improving the HCHO Sensing Selectivity on Ag-Doped Graphene by Oxygen Functionalization: A First-Principles Study.

Graphene-based sensors typically fail in the selectivity of target gas detection when exposed to complex and multicompound atmospheres. We have thoroughly compared the adsorptions of various interfering gases (CO, NH3, CH4, C2H2, C2H4, CH3OH, and CH3Cl) with target HCHO on AgG and AgOG by first-principles simulations. The results demonstrate that AgG shows a poor selectivity for HCHO detection and an oxygen functionalized one can improve the selectivity by enhancing the adsorption strength of HCHO and weakening those of other gas molecules. Moreover, the sensing properties of the AgOG sensors are evaluated by the NEGF method, and the predicted HCHO sensing responses are 76 and 32% along the armchair and zigzag directions, respectively. The present work helps shed some light on designing graphene-based sensing materials with high selectivity.

Photo Effect Studies in Graphene/N-GaAs Schottky Junction Using NEGF Method for Photovoltaic Application

In this paper, we present the photovoltaic characteristics of nanoscale Schottky junction solar cell consisting of graphene and GaAs using numerical simulation based on non-equilibrium Green’s function formalism. In our model, light-matter interaction is formulated by the coupling and scattering self-energy matrices whereas heterostructure is designed with a Hamiltonian matrix. An efficiency of 2.36% is obtained for monolayer graphene on GaAs; the efficiency is later enhanced to 5.40% by increasing both the number of graphene layers and the doping concentration of GaAs. The parameters to calculate power conversion efficiency, series and shunt resistances are extracted from the J-V characteristic. The I-V characteristic is also numerically simulated to extract reverse saturation current, ideality factor, and rectification ratio. Moreover, the power density is calculated for the optimized structure; the maximum power density of 7.46 mW/cm2 is obtained for four layers of graphene and a doping concentration of 1017/cm3 in GaAs.

Exploring the enhancement of the thermoelectric properties of bilayer graphyne nanoribbons

The electronic and thermoelectric properties of bilayer α-graphyne nanoribbons are investigated using density functional theory and the NEGF method. The power factor (σS2) is increased for the bilayer nanoribbons in comparison with isolated ribbons.

Examining conductance values in the biphenyl molecular switch with reduced density matrices

AbstractElectronic switches built from single molecules such as biphenyl dithiol are promising replacements for traditional electronic devices. To support experimental investigations of molecular switches, charge transport values are typically predicted using non‐equilibrium Green's functions constructed using density functional theory (NEGF‐DFT). Previous studies of biphenyl dithiol, however, have usually overestimated experimental conductance values and have not reproduced a drop in conductance at low torsional angles. In this paper, we employ a methodology that constructs the non‐equilibrium Green's functions from 2‐electron reduced density matrix theory (NEGF‐RDM). This approach has previously predicted lower conductance values than NEGF‐DFT for systems where NEGF‐DFT is known to overestimate transport. In the first direct comparison of the NEGF‐RDM method and experimental results, we examine the biphenyl dithiol at a range of torsional angles. We evaluate both the quantitative accuracy of multiple NEGF methods and provide insights into the experimentally observed drop in conductance at small torsional angles.

Half-Metal–Spin-Gapless-Semiconductor Junctions as a Route to the Ideal Diode

The ideal diode is a theoretical concept that completely conducts the electric current under forward bias without any loss and that behaves like a perfect insulator under reverse bias. However, real diodes have a junction barrier that electrons have to overcome and thus they have a threshold voltage $V_T$, which must be supplied to the diode to turn it on. This threshold voltage gives rise to power dissipation in the form of heat and hence is an undesirable feature. In this work, based on half-metallic magnets and spin-gapless semiconductors we propose a diode concept that does not have a junction barrier and the operation principle of which relies on the spin-dependent transport properties of the HMM and SGS materials. We show that the HMM and SGS materials form an Ohmic contact under any finite forward bias, while for a reverse bias the current is blocked due to spin-dependent filtering of the electrons. Thus, the HMM-SGS junctions act as a diode with zero threshold voltage $V_T$, and linear $I-V$ characteristics as well as an infinite on:off ratio at zero temperature. However, at finite temperatures, non-spin-flip thermally excited high-energy electrons as well as low-energy spin-flip excitations can give rise to a leakage current and thus reduce the on:off ratio under a reverse bias. Furthermore, a zero threshold voltage allows one to detect extremely weak signals and due to the Ohmic HMM-SGS contact, the proposed diode has a much higher current drive capability and low resistance, which is advantageous compared to conventional semiconductor diodes. We employ the NEGF method combined with DFT to demonstrate the linear $I-V$ characteristics of the proposed diode based on two-dimensional half-metallic Fe/MoS$_2$ and spin-gapless semiconducting VS$_2$ planar heterojunctions.

Gas sensors Based on U-shaped Graphene Nanoribbons: A First-Principles Study

In this paper, I present first-principle calculations which are performed by using combination of DFT and NEGF methods to investigate the adsorption of CO2 and NH3 molecules on straight and U-shaped GNRs junctions. The density of state, molecular orbital, adsorption energy, transmission spectrum, and current-voltage characteristics were analyzed. It is found show that the adsorption of CO2 and NH3 on straight GNRs at the dangling bond edges exhibit chemical interaction behavior with high adsorption energy. Besides, for U-shaped ZGRNs without adsorption, the states around Fermi level are quasi-bound states localized at the zigzag edges. The electrons at these localized states are almost confined and do not contribute on conductance. However, for U-shaped GRNs with CO2 and NH3 adsorbed on dangling bond armchair edges, these states become delocalized on whole of the U-shaped junction. The current-voltage characteristics show that the sensitivity and selectivity of the U-shaped GNRs gas sensors are significantly improved in comparison with the pristine GNRs ones.

Single-Shell Carbon Nanotubes Covered with Iron Nanoparticles for Ion-Lithium Batteries: Thermodynamic Stability and Charge Transfer

The mathematical model which describes interaction between maghemite particles and CNTs was built on the base of experimental data. It was revealed by the combination of SCC DFTB and NEGF methods that addition of γ-Fe2O3 to sidewalls of semi-conductive CNT leads to sharp decrease in the resistance from 4 MΩ to 13.9 kΩ. It was also found that the growth of the maghemite’s mass ratio in composite γ-Fe2O3/CNT increases the charge transfer between objects. Amount of this charge transfer depends only on the contact area between CNTs and maghemite particles and doesn’t depend on a diameter of maghemite particles. The developed mathematical model can be an effective tool for modification of γ-Fe2O3/CNT synthesis technology with the goal to improve the charge capacity of lithium-ion batteries.

Thermal Engineering in Low‐Dimensional Quantum Devices: A Tutorial Review of Nonequilibrium Green's Function Methods

AbstractThermal engineering of quantum devices has attracted much attention since the discovery of quantized thermal conductance of phonons. Although easily submerged in numerous excitations in macrosystems, quantum behaviors of phonons manifest in nanoscale low‐dimensional systems even at room temperature. Especially in nanotransport devices, phonons move quasiballistically when the transport length is smaller than their bulk mean free paths. It has been shown that the phonon nonequilibrium Green's function method (NEGF) is effective for the investigation of nanoscale quantum transport of phonons. Here, two aspects of thermal engineering of quantum devices are discussed using NEGF methods. One covers transport properties of pure phonons; the other concerns caloritronic effects, which manipulate other degrees of freedom, such as charge, spin, and valley, via the temperature gradient. For each part, the basic theoretical formalisms are outlined first, then a survey of related investigations on models or realistic materials is presented. Particular attention is given to phonon topologies and the generalized phonon NEGF method. Finally, conclusions are offered and the study is summarized with an outlook.

Electronic transport properties of PbSi Schottky-clamped transistors with a surroundingmetal-insulator gate.

Sustaining Moore's law requires the design of new materials and the construction of FET. Herein, we investigated theoretically the electronic transport properties of PbSi nanowire Schottky-clamped transistors with a surrounding metal–insulator gate by employing MD simulations and the NEGF method within the extended Hückel frame. The conductance of PbSi nanowire transistors shows ballistic and symmetrical features because of the Schottky contact and the resonance transmission peak, which is gate-controlled. Interestingly, the PbSi(8,17) nanowire FET shows a high ON/OFF ratio and proves to be a typical Schottky contact between atoms as described by the EDD and EDP metrics.

Quantum Transport in AlGaSb/InAs TFETs With Gate Field In-Line With Tunneling Direction

Quantum transport simulations are performed in tunneling FETs (TFETs) with the gate electric field in-line with the tunneling junction direction (in-line TFETs). Charge self-consistency and thermalization effects are included in a semiclassical Poisson solution to compute the electrostatic potential. The obtained potential is then used for current calculation with the ballistic nonequilibrium Green’s function method (NEGF) in the tight binding basis. It is shown that the NEGF method predicts a higher subthreshold swing than the often-used dynamic nonlocal (DNL) path band-to-band method. The NEGF method accounts for the direct source–drain tunneling, which is underestimated in the DNL path approach in the studied geometries. Undercut is shown to be essential to obtain switching slope below 60 mV/decade in the in-line TFETs.

Improving performance of armchair graphene nanoribbon field effect transistors via boron nitride doping

Device performance of 10nm length armchair graphene nanoribbon field effect transistors with 1.5nm and 4nm width (13 and 33 atoms in width respectively) are compared in terms of Ion/Ioff, trans-conductance, and sub-threshold swing. While narrow devices suffer from edge roughness wider devices are subject to more substrate surface roughness and reduced bandgap. Boron Nitride doping is employed to compensate reduced bandgap in wider devices. Simultaneous effects of edge and substrate surface roughness are considered. Results show that in the presence of both the edge and substrate surface roughness the 4nm wide device with boron nitride doping shows improved performance with respect to the 1.5nm one (both of which incorporate the same bandgap AGNR as channel material). Electronic simulations are performed via NEGF method along with tight-binding Hamiltonian. Edge and surface roughness are created by means of one and two dimensional auto correlation functions respectively. Electronic characteristics are averaged over a large number of devices due to statistic nature of both the edge and surface roughness.

Electronic and transport properties of porous graphene sheets and nanoribbons: benzo-CMPs and BN codoped derivatives

The electronic and electron transport properties of a series of 2D porous graphene sheets (n-benzo-CMPs) with different pore sizes n and their BN codoped derivatives, BN-n-benzo-CMPs, as well as 1D porous graphene nanoribbons (p-GNRs) tailored from n-benzo-CMPs and BN-n-benzo-CMPs are studied using DFT and NEGF methods.

LESSONS OF NANOELECTRONICS: HALL EFFECT AND MEASUREMENT OF ELECTROCHEMICAL POTENTIALS WITHIN «BOTTOM – UP» APPROACH

Classical and quantum Hall effects, measurement of electrochemical potentials, the Landauer formulas and Buttiker formula, measurement of Hall potential, an account of magnetic field in the NEGF method, quantum Hall effect, Landau levels, and edge states in graphene are discussed in the frame of the «bottom – up» approach of modern nanoelectronics in continuation to the previous tutorial review articles of the authors.

Electronic and spin transport properties of graphene nanoribbon mediated by metal adatoms: a study by the QUAMBO–NEGF approach

Structural and electronic properties, including deformation, magnetic moment, Mulliken population, bond order, as well as electronic transport properties, of zigzag graphene nanoribbon (ZGNR) with Co adatoms on hollow sites are investigated by quasi-atomic minimal basis orbits (QUAMBOs), a first-principles tight binding (TB) scheme based on density functional theory (DFT), combined with a non-equilibrium Green’s function. For electronic transport, below the Fermi level the transmission is strongly suppressed and spin dependent as a result of magnetism by Co adatom adsorption, while above the Fermi level the transmission is slightly distorted and spin independent. Due to the local environment dependence of QUAMBOs–TB parameters, we construct QUAMBOs–TB parameters of ZGNR leads and ZGNR with Co adatoms on hollow center sites by a divide-and-conquer approach, and accurately reproduce the electronic transmission behavior. Our QUAMBO–NEGF method is a new and promising way of examining electronic transport in large-scale systems.

A theoretical model for metal–graphene contact resistance using a DFT–NEGF method

The contact resistance (R(c)) between graphene and metal electrodes is of crucial importance for achieving potentially high performances for graphene devices. However, previous analytical models based on Landauer's approach have failed to include the Fermi velocity difference between the graphene under the metal and the pure graphene channel. Hereby we report a theoretical model to estimate the R(c) using density-functional theory and non-equilibrium Green's function methods. Our model not only presents a clear physical picture of the metal-graphene contacts, but also generates R(c) values which are in good agreement with the experimental results: 210 Ω μm for double-sided Pd contacts compared with 403 Ω μm for single-sided Pd contact.

SCATTERING IN NANO-FILMS

In this communication, a quantum mechanical technique for treatment of effects of scattering transport at nanoscale in thin films is discussed. We implemented a rigorous treatment of scattering within the NEGF simulation platform. Results obtained by applying the rigorous scattering model to simulate the devices were used as a benchmark to validate a simple computationally-efficient, phenomenological treatment of scattering. The NEGF method is used to study the effect of electron confinement on silicon nano-films and wires. Electron confinement results in almost a factor of 3 decreases in the electrical conductivity of the 5 nm silicon film compared to the 10 nm film. Increase in the amount of confinement also leads to a 35% decrease in the conductivity of a 5 nm × 5 nm wire compared to the 5 nm film. Our simple model provides an excellent tradeoff between increased computational cost and the physics of scattering that needs to be captured in devices of the future.

(Invited) Boosting ION/IOFF Ratio in Ultimate Sub-Decananometer Transistors: The Case of FDSOI Multi-gate MOSFETs and Multi-barrier Enhanced Gate Modulated Resonant Tunneling-FETs

Using efficient NEGF methods, we explore and optimize here ION/IOFF ratios and slope characteristics of ultra scaled nanotransistors. Those depend on the gate-to-channel coupling and carrier statistics that dictate the way carriers are made available to drive a current when increasing VG. We first investigate the gateto- channel coupling. Quantum effects and their impact on ultrascaled nanowires are enlightened when optimizing ION/IOFF ratios vs. the cross-section size in a 10nm nanowire. It is shown that an optimum cross-section of about 3x3 nm2 exist due to a trade-off between electrostatic and confinement. Also the fundamental limit of improving gate control by thinning gate oxide is shown and leads us to switch from EOT to the CET concept. As the density of state is about constant with VG in a standard transistor, when varying the gate voltage the current varies at a rate dictated by Fermi-Dirac statistics only. In order to further increase ION/IOFF ratios, we propose a new device concept, the gate-modulated resonant tunneling FET (RT-FET), that uses a multi-barrier gate controlled architecture to render this DoS non-constant with VG.