Nonequilibrium Green Function Research Articles

Negative Differential Resistance in Spin-Crossover Molecular Devices.

We demonstrate, based on low-temperature scanning tunneling microscopy (STM) and spectroscopy, a pronounced negative differential resistance (NDR) in spin-crossover (SCO) molecular devices, where a FeII SCO molecule is deposited on surfaces. The STM measurements reveal that the NDR is robust with respect to substrate materials, temperature, and the number of SCO layers. This indicates that the NDR is intrinsically related to the electronic structure of the SCO molecule. Experimental results are supported by density functional theory (DFT) with nonequilibrium Green's function (NEGF) calculations and a generic theoretical model. While the DFT+NEGF calculations reproduce NDR for a special atomically sharp STM tip, the effect is attributed to the energy-dependent tip density of states rather than the molecule itself. We, therefore, propose a Coulomb blockade model involving three molecular orbitals with very different spatial localization as suggested by the molecular electronic structure.

The tunnel-current and photon-induced current in nanoscale photovoltaics

In this work, the photo-induced current in a nanoscale photovoltaic device consisting of a quatum dot (QD) that is coupled to two metallic electrodes is derived by using the nonequilibrium Green function (NEGF) technique. The QD is assumed to have two levels and only one of them is within the bias window. The dependence of photocurrent on the photon’s energy and the level widths is analysed. This study shows the photocurrent could be maximized for a given range of photons’ energy which relates to the gap between levels and the level widths.

Atomistic simulation of phonon heat transport across metallic vacuum nanogaps

The understanding and modeling of the heat transport across nanometer and sub-nanometer gaps where the distinction between thermal radiation and conduction become blurred remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon non-equilibrium Green's function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic nanogaps are generated from MD as inputs into harmonic phonon NEGF. Phonon tunneling across gold-gold and copper-copper nanogaps is quantified, and is shown to be a significant heat transport channel below gap size of 1nm. We demonstrate that lattice anharmonicity contributes to within 20-30% of phonon tunneling depending on gap size, whereas electrostatic interactions turn out to have negligible effect for the small bias voltage typically used in experimental measurements. This work provides detailed information of the heat current spectrum and interprets the recent experimental determination of thermal conductance across gold-gold nanogaps. Our study contributes to deeper insight into heat transport in the extremely near-field regime, as well as hints for the future experimental investigation.

DFT Analysis of Hydrogenated Zigzag Aluminum Nitride Nanoribbons for Spintronic Devices

Density functional theory (DFT) and nonequilibrium Green’s function (NEGF) framework are used to explore the structural, spin-polarized electronic, and spin-based transport properties of edge-hydrogenated zigzag aluminum nitride nanoribbons (ZAlNNRs). The proposed ZAlNNR is observed to be structurally stable and exhibits half-metallic nature in the magnetic state. The quantum transport property of the proposed two-terminal device model of 1H-AlN-1H demonstrates the bipolar spin-filter characteristics along with giant magnetoresistance (GMR), spin-based peak to valley current ratio (spin-PVCR), and spin-based rectification ratio (spin-RR) of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> , 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> , and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> , respectively. The calculated GMR and spin-RR of the 1H-AlN-1H device are 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> times higher than zigzag silicene nanoribbon (ZSiNR) and doped-zigzag graphene nanoribbon (doped ZGNR), respectively. The observed GMR, spin-PVCR, and spin-rectifying behavior of the reported ZAlNNR device could be deployed for multifunctional spintronic device applications.

Impact of Metal Hybridization on Contact Resistance and Leakage Current of Carbon Nanotube Transistors

Carbon nanotube field effect transistors (CNFETs) have potential applications in future logic technology as they display good electrostatic control and excellent transport properties. However, contact resistance and leakage currents could limit scaling of CNFETs. Non-equilibrium Green’s function (NEGF) simulation investigates that coupling between contact metal and CNT impacts both contact resistance and leakage current. The physical mechanisms underlying the effects are analyzed. A model with calibrated metal coupling strength from experimental data projects I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> -I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> design space to understand the trade-off between shrinking contact and extension lengths. For CNT with diameter of 1 nm, both contact and extension lengths greater than 8 nm are a good compromise between I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> and I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> for digital logic in advanced technology nodes.

σ-σ Stacked supramolecular junctions.

Intermolecular charge transport plays an essential role in organic electronic materials and biological systems. To date, experimental investigations of intermolecular charge transport in molecular materials and electronic devices have been restricted to conjugated systems in which π-π stacking interactions are involved. Herein we demonstrate that the σ-σ stacking interactions between neighbouring non-conjugated molecules offer an efficient pathway for charge transport through supramolecular junctions. The conductance of σ-σ stacked molecular junctions formed between two non-conjugated cyclohexanethiol or single-anchored adamantane molecules is comparable to that of π-π stacked molecular junctions formed between π-conjugated benzene rings. The current-voltage characteristics and flicker noise analysis demonstrate the existence of stacked molecular junctions formed between the electrode pairs and exhibit the characteristics of through-space charge transport. Density functional theory calculations combined with the non-equilibrium Green's function method reveal that efficient charge transport occurs between two molecules configured with σ-σ stacking interactions.

Performance study of tunneling field effect transistors based on the graphene and phosphorene nanoribbons

The present work deals with the simulation and comparison of two tunneling field effect transistors (TFETs) based on the single layers of armchair graphene nanoribbons (AGNRs) and armchair phosphorene nanoribbons (APNRs). The transmission properties of the transistor have been obtained by solving the Schrödinger-Poisson equations self-consistently using the non-equilibrium Green's function (NEGF) formalism and the real-space approach. In addition, the energy band structure of the two materials have been obtained using tight-binding approximation method. The simulation results in each bias include the energy band structure, energy-resolved current spectrum, transmission function and drain current versus gate voltage diagrams. The results indicate that the structure of phosphorene TFETs meets a better ION to IOFF ratio and sub-threshold swing than the graphene TFETs despite a relative decrease in the amount of ION.

Classical and quantum harmonic mean-field models coupled intensively and extensively with external baths

We study the nonequilibrium steady-state of a fully-coupled network of N quantum harmonic oscillators, interacting with two thermal reservoirs. Given the long-range nature of the couplings, we consider two setups: one in which the number of particles coupled to the baths is fixed (intensive coupling) and one in which it is proportional to the size N (extensive coupling). In both cases, we compute analytically the heat fluxes and the kinetic temperature distributions using the nonequilibrium Green's function approach, both in the classical and quantum regimes. In the large N limit, we derive the asymptotic expressions of both quantities as a function of N and the temperature difference between the baths. We discuss a peculiar feature of the model, namely that the bulk temperature vanishes in the thermodynamic limit, due to a decoupling of the dynamics of the inner part of the system from the baths. At variance with the usual case, this implies that the steady-state depends on the initial state of the bulk particles. We also show that quantum effects are relevant only below a characteristic temperature that vanishes as 1/N. In the quantum low-temperature regime the energy flux is proportional to the universal quantum of thermal conductance.

Light-matter interactions in van der Waals photodiodes from first principles

Strong light-matter interactions in van der Waals heterostructures (vdWHs) made of two-dimensional (2D) transition metal dichalcogenides (TMDs) provide a fertile ground for optoelectronic applications. Of particular interest are photoexcited interlayer electron-hole pairs, where electrons and holes are localized in different monolayers. Here, we present an ab initio quantum transport framework relying on maximally localized Wannier functions and the nonequilibrium Green's functions to explore light-matter interactions and charge transport in 2D vdWHs from first principles. Electron-photon scattering is accurately taken into account through dedicated self-energies. As testbed, the behavior of a ${\mathrm{MoSe}}_{2}\text{\ensuremath{-}}{\mathrm{WSe}}_{2}$ PIN photodiode is investigated under the influence of a monochromatic electromagnetic signal. Interlayer electron-hole pair generations are observed even in the absence of phonon-assisted processes. The origin of this phenomenon is identified as the delocalization of one valence band state over both monolayers composing the vdWH.

Effect of Nitrogen Atoms on Structures and Electron Transport of N-heteropentacene Devices.

Pentacene polycyclic aromatic hydrocarbons are the most promising organic semiconductor materials among aromatic compounds due to their potential optoelectronic properties. Introducing nitrogen atoms into the main chain of pentacene is supposed to tune the electronic structure and develop new high-performance organic molecular devices. Herein, we have investigated the electron transport properties of N-heteropentacenes consisting of different numbers, positions, and valence states of N atoms using density functional theory (DFT) and nonequilibrium Green's function (NEGF) method. The results show that the transport properties of N-heteropentacenes are strongly dependent on whether the C-N is a single or double bond. For devices with C-N double bonds, the change of current with voltage is consistent, and its electron transport properties are independent of the number and position of N atoms. In comparison, C-N single-bond devices exhibit an early negative differential resistance (NDR) and significant rectification. Moreover, the threshold voltages exist within certain bias voltages for different numbers of N atoms and might even show a second NDR. These studies would be useful to design performance-enhancing molecular devices by manipulating the molecular structure of N-heteropentacenes.

Ab Initio Properties of Hybrid Cove-Edged Graphene Nanoribbons as Metallic Electrodes for Peptide Sequencing via Transverse Tunneling Current.

Recently synthesized metallic cove-edged graphene nanoribbons are considered for use as one-dimensional (1D) electrodes for ideal atomistically resolved recognition of amino acids. To this purpose a narrow nanogap device is considered, and the transversal tunneling current flowing across it is calculated during the translocation of a model Gly homopeptide using the nonequilibrium Green function scheme, based on density functional theory. We show that the signal collected from the metallic spin states is characterized by a double peak per residue in analogy with the results obtained with 1D graphene nanoribbon template electrodes. The presented results pave the way for experimentally feasible atomistically resolved tunneling current recognition using metallic edge engineered graphene electrodes obtained by bottom-up fabrication strategies.

A computational study of mechanical and electrical properties of zigzag graphene nanoribbon force sensor

Force sensors have many applications in new technologies such as robotic systems, medical tools and electronics. Therefore, it is necessary to make high sensitivity force sensors. Nanomaterials are very useful for making these sensors. In this paper, we study the mechanical and electrical properties of zigzag graphene nanoribbon force sensor by classical molecular dynamics (MD) with reactive force field (Reaxff) and density functional based tight binding (DFTB) combined with non-equilibrium Green's function (NEGF) method. Our results show that the bending angle, potentials energy, bond energy and deflection distance increase with increasing external force, while distance between tip and nanoribbon and electrical conductivity decrease. In addition, to study the electrical properties of force sensor, we calculated the local density of states (LDOS), density of states (DOS) and transmission spectrum. Our results show that these quantities change by external force. So we can measure them to judge the operation of sensor.

Influence of contact interface on electric transport in in-plane graphene/MoSSe heterojunction

Janus transition-metal dichalcogenides (JTMDS) have attracted much attention due to their obvious potential for electronic and optical applications. The electronic transport properties of in-plane graphene/MoSSe heterojunctions are studied using density functional theory and nonequilibrium Green's function method. It is found that different contact patterns lead to different accumulation and depletion on the contact interface and exhibit a P-type Schottky barrier. Moreover, the Schottky barrier can be modulated by doping. The ohmic contact is formed when the doping concentration is 3 × 1013 cm−2, which is proved by the approximately linear characteristics of the current-voltage curve. These results would provide an insight into designing a high-performance device.

Proposal for All-Electrical Spin Manipulation and Detection for a Single Molecule on Boron-Substituted Graphene.

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S=1 and S=3/2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fe-d_{z^{2}} and B-p_{z} orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (>10% for typical coverage). Our work paves the way to realize all-electrical spintronics devices using molecular spinterfaces.

Ultrahigh on/off-current ratio γ-graphyne-1 nanotube-based sub-10-nm TFET modeling and simulation

The use of γ-graphyne-1 nanotubes (GyNTs) in tunneling field effect transistors (TFETs) suppresses ambipolarity and enhances the subthreshold swing (\(\mathrm{SS}\)) of TFETs, due to the large energy band gap and high electron effective mass of GyNTs. In this research, the analysis of the structural, electronic and thermoelectric properties of the γ-graphyne-1 family under the deformation potential (DP) approach reveals that the electron–phonon mean free path (MFP) of an armchair GyNT (3AGyNT) and zigzag GyNT (2ZGyNT) are \(24\) and \(279\) nm, respectively. Therefore, ballistic transport of sub-10-nm 3AGyNT-TFETs and 2ZGyNT-TFETs in different channel lengths is investigated utilizing the non-equilibrium Green’s function (NEGF) formalism in the DFTB platform. An ultrahigh \(\mathrm{on}/\mathrm{off}\)-current ratio (\(\mathrm{OOCR}\)) value of 1.6 × 1010 at \(V_{DD} = 0\). \(2 \;{\text{V}}\) and very low point \({\text{SS}}\) of \(5 \;{\text{mV}}/{\text{dec}}\) were demonstrated by the 3AGyNT-TFET with a channel length of 9.6 nm. 2ZGyNT-TFETs show higher on-state current and \({\text{SS}}\) and lower \({\text{OOCR}}\) than those of 3AGyNT-TFETs. A linear relationship was found between channel length and logarithmic off-state current that is consistent with the WKB approximation. The obtained results along with the ultralow power consumption of the proposed GyNT-TFETs make them candidates to replace digital silicon MOSFETs in next-generation nanoelectronic devices.

Zero sound and plasmon modes for non-Fermi liquids

We derive the quantum Boltzmann equation (QBE) by using generalized Landau-interaction parameters, obtained through the nonequilibrium Green's function technique. This is a generalization of the usual QBE formalism to non-Fermi liquid (NFL) systems, which do not have well-defined quasiparticles. We apply this framework to a controlled low-energy effective field theory for the Ising-nematic quantum critical point, in order to find the collective excitations of the critical Fermi surface in the collisionless regime. We also compute the nature of the dispersion after the addition of weak Coulomb interactions. The zero angular momentum longitudinal vibrations of the Fermi surface show a linear-in-wavenumber dispersion, which corresponds to the zero sound of Landau's Fermi liquid theory. The Coulomb interaction modifies it to a plasmon mode in the long-wavelength limit, which disperses as the square-root of the wavenumber. Remarkably, our results show that the zero sound and plasmon modes exhibit the same behaviour as in a Fermi liquid, although an NFL is fundamentally different from the former.

Bilayer armchair graphene nanoribbon photodetector with Stone-Wales defect: A computational study

Bilayer armchair graphene nanoribbons (BAGNRs), due to their low sensitivity to low frequency noise and high controllability of their bandgap have drawn a lot of attention as building blocks of photodetectors in recent years. In this work, using tight binding (TB) method and non-equilibrium Green's function formalism (NEGF) a computational study has been done on the BAGNR photodetector with perfect and Stone-Wales (SW) defected channel and the effect of defects' number and position on the device performance has been investigated. In order to validate the TB parameters used in this work, the bandstructures of both perfect and SW defected BAGNRs are compared with the results of DFT method and a good matching is obtained. Simulation results show that perfect BAGNR photodetector has quantum efficiency (QE) of 19.77% at 0.12 eV photon energy. Among defected photodetectors, highest QE of 21.77% belongs to the structure with single SW defect near the right contact (higher bias voltage) at 0.14 eV. This structure also possesses the highest Iph/Idark of 64.45 which is 1.8 times larger than Iph/Idark of perfect BAGNR photodetector. Moreover, the structure with two SW defects located at left and right sides of the channel exhibits a drastically low Idark which results in a high Iph/Idark. In addition, this structure yields the high QE (19.72%) at blue-shifted photon energy of 0.2 eV which makes it a suitable choice for mid-wavelength infrared applications.

Perfect spin filtering of T-shaped device based on the zigzag silicon carbide nanoribbons

How to generate stable and pure spin-polarized current in nanodevice based on low-dimensional magnetic materials and systems has attracted extensive attention. In this work, using the non-equilibrium Green’s function (NEGF) combined with density-functional theory (DFT), we predict highly conductive transport and perfect spin filtering of the novel T-shaped device, which is constructed by a finite armchair silicon carbide nanoribbon (ASiCNR) connected to one edge of an infinite zigzag silicon carbide nanoribbon (ZSiCNR). This particular performance is due to the fact that there are always two edge states contributing to electron transport around the Fermi level, no matter whether the ZSiCNR is in the ferromagnetic or anti-ferromagnetic state. As a consequence, the T-shaped device would ideally yield 100% spin-polarized current with one spin channel blocked by the ASiCNR part. Moreover, it is found that such blocking effect is independent of the shape of the ASiCNR part. Based on the above analysis, it is easily predicted that such pure spin-polarized current could also be obtained in the device constructed by ZSiCNRs, as long as one pristine edge of ZSiCNRs is defected by doping, absorbing, vacancies and so on, which opens new possibility of two dimensional silicon carbide in spintronic device applications.

Atomistic Modeling of the Electrical Conductivity of Single‐Walled Carbon Nanotube Junctions

Carbon nanotubes (CNTs) have many interesting properties that make them a focus of research in a wide range of technological applications. In CNT films, the bottleneck in charge transport is typically attributed to higher resistance at CNT junctions, leading to electrical transport characteristics that are quite different from individual CNTs. Previous simulations confirm this; however, a systematic study of transport across junctions is still lacking in the literature. Herein, density functional tight binding (DFTB) theory combined with the nonequilibrium Green's functions (NEGF) method is used to systematically calculate current across a range of CNT junctions. A random sampling approach is used to sample an extensive library of junction structures. The results demonstrate that the conductivity of CNT contacts depends on the overlap area between nanotubes and exponentially on the distances between the carbon atoms of the interacting CNTs. Two models based solely on the atomic positions of carbon atoms within the nanotubes are developed and evaluated: a simple equation using only the smallest C–C separation and a more sophisticated model using the positions of all C atoms. These junction current models can be used to predict transport in larger‐scale simulations where the CNT fabric structure is known.

A statistical quasi-particles thermofield theory with Gaussian environments: System-bath entanglement theorem for nonequilibrium correlation functions.

For open quantum systems, the Gaussian environmental dissipative effect can be represented by statistical quasi-particles, namely, dissipatons. We exploit this fact to establish the dissipaton thermofield theory. The resulting generalized Langevin dynamics of absorptive and emissive thermofield operators are effectively noise-resolved. The system-bath entanglement theorem is then readily followed between an important class of nonequilibrium steady-state correlation functions. All these relations are validated numerically. A simple corollary is the transport current expression, which exactly recovers the result obtained from the nonequilibrium Green's function formalism.