Nonequilibrium Green Function Research Articles

Band Gap Engineering and Photoelectronic Properties of Novel Pentagonal Materials penta-XN2 (X = Ni, Pt)： A first principle calculations

Abstract Abstract:Two-dimensional (2D) pentagonal materials as the next-generation nanoelectronic devices are promising candidates due to their interesting structures and novel electronic, mechanical, optical and other properties. Penta-NiN2, a newly synthesized material with pentagonal atomic arrangement under high pressure (ACS Nano 15 (2021), 13539), has also sparked considerable interest. This study systematically investigates the effects of the biaxial strain on monolayer PtN2 (penta-NiN2) electronic structure applying first-principles computational approaches. Furthermore, combining the non-equilibrium Green's function (NEGF) approach, we research the optoelectronic and transport properties of penta-NiN2 (PtN2). The results indicate that biaxial strain can effectively modulate the bandgap of penta-NiN2 (PtN2), particularly achieving a semiconductor-to-metal transition under compressive strain. Moreover, tensile and compressive strains effectively enhance the optical characteristics of penta-NiN2 (PtN2) in visible light range. Under tensile and compressive strains, the absorption peak of penta-NiN2 shows a red shift and a blue shift in visible region, respectively. The pin-junction photodiode of penta-NiN2 (PtN2) exhibit significant photocurrent under illumination. The strongest photocurrent is observed in penta-NiN2 photodiodes under -3% compressive strain, showing the highest response to yellow light. Under the tensile stress of 7% and compressive stress of -3%, the photocurrent of the Penta-PtN2 photodiode is enhanced in the yellow and green light regions. Additionally, applying compressive strain reduces the bandgap of penta-NiN2 (PtN2), significantly enhancing its transport properties and thereby inducing a switch effect in devices. In summary, our study demonstrates that penta-XN2 (X=Ni, Pt) is a promising material in the fields of nanoelectronics and optoelectronic devices.

Application of the shift-invert Lanczos algorithm to a nonequilibrium Green's function for transport problems

Application of the shift-invert Lanczos algorithm to a nonequilibrium Green's function for transport problems

First principle atomistic modelling of magnetic tunnel junction (MTJ) to enhance its tunnel magneto-resistance (TMR) ratio

Abstract This paper investigates the impact of electrode materials on the Tunnel Magneto-Resistance (TMR) ratio of Magnetic Tunnel Junction (MTJ) device. Four different structures of MTJ have been simulated by using cobalt (Co), Nickel (Ni), Iron (Fe) and two alloy materials of nickel-iron (NiFe) and cobalt-iron (CoFe). These materials have been used as ferromagnetic electrodes. Mulliken population and transmission spectrum observed in both parallel and antiparallel configurations of these devices to understand the spin transport properties and Tunnel Magneto-Resistance (TMR) ratio has been estimated. The first principal study was performed based on density function theory (DFT) and Non-equilibrium Green’s Function (NEGF) computational methods using the QuantumATK simulation tool to study properties such as band structure, the density of states (DOS), Spin Transfer Torque (STT), I-V characteristics and TMR. This study explores how different electrode materials affect the Tunnel Magneto-Resistance (TMR) ratio in Magnetic Tunnel Junction (MTJ) devices. With these results, it is observed that cobalt-based MTJ devices (that is Co-MgO-Fe and CoFe-MgO-CoFe) exhibit higher TMR ratio as compared to Nickel- and Iron-based MTJ devices (that is NiFe-MgO-NiFe and Ni-MgO-Fe). As Cobalt has a high spin polarization this property makes it suitable for use in spintronics devices like MTJs, where the manipulation of electron spins is essential for data storage and information processing. These findings can be employed to improve the performance characteristics of the MTJ-based Random Access Memory (MRAM) in the field of spintronics.

Theoretically design of 2D penta-BN2/penta-Graphene wdW heterostructure for NO gas sensing implications: a first-principles study

As air pollution becomes more and more serious, the detection of harmful gas of NO has become an urgent need in daily life. In this paper, the electronic and gas-sensing properties of penta-BN2/penta-Graphene heterojunction (P-BN2/P-G) are studied by density functional theory and non-equilibrium Green's function method. The analysis results indicate that the P-BN2/P-G heterojunction has metallic properties. The electronic structures in combination with the adsorption behavior shows that the P-BN2/P-G heterojunction can chemically adsorb NO gas molecules, and there is obvious charge transfer, indicating that it has high gas sensitivity to NO gas. In addition, the sensing performance of the gas sensor constructed by NO adsorption on upper, middle and lower layers of the P-BN2/P-G heterojunction is deeply studied. Transport calculations show that when NO is adsorbed on the upper and middle layers of the P-BN2/P-G heterojunction-based gas sensor, the current of the device decreases significantly. When adsorbed on the lower layer, the current shows a trend of first decreasing and then increasing. Under a bias of 0.1 V, the gas sensitivity can reach up to 49% when NO is adsorbed in the middle layer, which is substantially higher than that of the P-BN2 monolayer device. It can be seen that the gas sensitivity is significantly improved after forming the heterojunction. In addition, the bias transport spectrum and scattering state can reveal the change in device current caused by the adsorption of NO gas molecules, thereby clarifying its microscopic mechanism. Finally, all the findings indicate that penta-based materials can be used as a very promising gas-sensitive material for detecting NO.

Large magnetoresistance and spin negative differential resistance in photoisomers-based molecular spin valves

By using non-equilibrium Green's functions in combination with density functional theory, the spin transport properties of photoisomers-based molecular spin valves constructed by salicylidene methylamine connected with two cobalt- dibenzotetraaza[14]annulene (Co-DBTAA) complex molecules via acetylene linkers are explored. A large magnetoresistance (MR) effect with a MR ratio larger than 104 can be observed in the molecular spin valves whether salicylidene methylamine is in the cis-enol form or the trans-keto form. Moreover, spin negative differential resistance (NDR) effects are also observed in the photoisomers-based molecular spin valves. The spin transport mechanisms are explored for MR and spin NDR effects observed in the molecular spin valves.

σ Interference: Through-Space and Through-Bond Dichotomy.

Dividing orbital interactions into through-space (TS) and through-bond (TB) modes is valuable for understanding various molecular properties. In this paper, we elucidate how the quantum interference phenomenon known as σ interference in electron transport through σ systems arises from TS and TB interactions. We performed electron transport calculations using a combination of density functional theory and nonequilibrium Green's function methods, focusing on ethylenediamine, a classical molecule that effectively highlights the contrast between TS and TB interactions. Our results confirm that destructive σ interference occurs in the syn and gauche conformers of this molecule. To further investigate both TS and TB interactions, we employed two analytical methods: the fragment molecular orbital (FMO) method, which captures the effects of both TS and TB interactions, and the chemical graph theory method, which specializes in TB interactions. The FMO analysis demonstrated that TB interactions lead to the characteristic distribution and energy level alignment of the frontier orbitals. Additionally, it was clarified that a change in TS interaction, due to a variation in the dihedral angle of the molecule, alters the energy gap between these orbitals, resulting in the manifestation of σ interference in the syn and gauche conformers, but not in the trans conformer. The chemical graph theory analysis based on the ladder C model, aimed at exploring the topological origin of σ interference from the network of TB interactions, revealed that σ interference is caused by the cancellation between the walk associated with geminal interactions (σ-conjugation) and the one related to vicinal interaction (σ-hyperconjugation). Notably, it was found that the vicinal interaction, which changes sign with the dihedral angle, has a decisive influence on whether this cancellation occurs. These findings clarify that σ interference arises from the interplay between TS and TB interactions. This insight will be valuable for designing molecular systems that utilize σ interference.

Improving the ON/OFF Current Ratio and Ambipolarity of Doping-Less Tunneling Carbon-Nanotube FET Using Drain Engineering Technique

This paper presents a novel structure utilizing a doping-less (DL) tunneling carbon nanotube field-effect transistor (T-CNTFET) with dual drain work functionality aimed at enhancing the ON/OFF current ratio. The proposed design features a zigzag carbon nanotube (CNT) of type (13, 0) with a diameter of 1 nm. It employs HfO2 as the gate oxide, which is 2 nm thick and has a dielectric constant of 16. The CNT serves as an intrinsic semiconductor for the source and drain regions, which are composed of metals with appropriate work functions. By selecting appropriate metals for the source and drain regions, the necessity for doping as a fabrication step is avoided, simplifying construction and reducing costs for the nanoscale device. This design includes two drain electrodes, each measuring 15 nm in length and having different work functions (DWFs). The work function of the drain positioned closest to the channel (DWFAC) is set at 3.9 eV, which is 0.5 eV lower than that of the CNT, while the other drain section has a work function of 3.4 eV, 1 eV lower than the CNT. The electrical properties of the device were examined through quantum numerical simulations using the non-equilibrium Green's function (NEGF) method. The results indicate that the proposed structure significantly decreases the OFF-state current and improves the ON/OFF current ratio. Additionally, the leakage current is substantially lowered, resulting in favorable changes to the device's ambipolarity, along with a reduction in hot carrier effects and subthreshold swing (SS).

Random nonequilibrium Green's function method for large-scale quantum transport simulation

Random nonequilibrium Green's function method for large-scale quantum transport simulation

Thermomagnetic Anomalies by Magnonic Criticality in Ultracold Atomic Transport.

We investigate thermomagnetic transport in an ultracold atomic system with two ferromagnets linked via a magnetic quantum point contact. Using the nonequilibrium Green's function approach, we show a divergence in spin conductance and a slowing down of spin relaxation that manifest in the weak effective-Zeeman-field limit. These anomalous spin dynamics result from the magnonic critical point at which magnons become gapless due to spontaneous magnetization. Our findings unveil untapped dynamics in ultracold atomic systems, opening new avenues in thermomagnetism.

Simulating graphene-based single-electron transistor: incoherent current effects due to the presence of electron–electron interaction

Carbon-based nanostructures have unparalleled electronic properties. At the same time, using an allotrope of carbon as the contacts can yield better device control and reproducibility. In this work, we simulate a single-electron transistor composed of a segment of a graphene nanoribbon coupled to carbon nanotubes electrodes. Using the non-equilibrium Green's function formalism we atomistically describe the electronic transport properties of the system including electron-electron interactions. Using this methodology we are able to recover experimentally observed phenomena, such as the Coulomb blockade, as well as the corresponding Coulomb diamonds. Furthermore, we separate the different contributions to transport and show that incoherent effects due to the interaction play a crucial role in the transport properties depending on the region of the stability diagram being considered.

Strain and External Electric Field Engineering of S-Terminated MXene on Selective and Sensitive Detection of N-Containing Compound Gases: A Computational Study.

Gas sensors are used for monitoring hazardous gases and vapors. With the emergence of S-terminated MXene synthesis, herein, we explore the sensing ability of Ti3C2S2 toward N-containing gases, including NH3, NO, NO2, trimethylamine (TMA), and nicotine (NT), using first-principles calculations and a statistical thermodynamic model. We find that Ti3C2S2 exhibits high selectivity to TMA, NO, and NT with moderate adsorption energies of -0.610, -0.490, and -0.476 eV, respectively, minimizing environmental noise from ambient gases. The electronic structure of Ti3C2S2 subtly alters upon adsorption of TMA, NO, and NT, facilitating detectable signals in the sensing device. However, the adsorption of NO2 and NH3 is less pronounced due to weak physisorption (<0.3 eV). Employing engineering strategies including biaxial strain and an external electric field greatly enhances the selectivity and sensitivity of NO2 (NH3) detection by boosting adsorption strength up to -0.351 eV with ε = 5% (-0.370 eV with |E⃗| = 0.6 V/ Å). In addition, the moderate adsorption energies of the gases result in a suitable recovery time in the range of milliseconds, leading to high reusability of the sensing device. The estimated adsorption densities suggest potential coverage of these N-containing molecules even at low concentrations and room temperature. Computational analysis of the sensing capability of Ti3C2S2 using the nonequilibrium Green's function method indicates that it is a promising gas-sensing material. In addition, mechanical modifications, electric field adjustments, and gate voltage alterations could be used to obtain effective sensing materials for N-containing gas detection.

Rational Design of High-Performance Photocontrolled Molecular Switches Based on Chiroptical Dimethylcethrene: A Theoretical Study.

The reversible photo-induced conformation transition of a single molecule with a [5]helicene backbone has garnered considerable interest in recent studies. Based on such a switching process, one can build molecular photo-driven switches for potential applications of nanoelectronics. But the achievement of high-performance reversible single-molecule photoswitches is still rare. Here, we theoretically propose a 13,14-dimethylcethrene switch whose photoisomerization between the ring-closed and ring-open forms can be triggered by ultraviolet (UV) and visible light irradiation. The electronic structure transitions and charge transport characteristics, concurrent with the photo-driven electrocyclization of the molecule, are calculated by the non-equilibrium Green's function (NEGF) in combination with density functional theory (DFT). The electrical conductivity bears great diversity between the closed and open configurations, certifying the switching behavior and leading to a maximum on-off ratio of up to 103, which is considerable in organic junctions. Further analysis confirms the evident switching behaviors affected by the molecule-electrode interfaces in molecular junctions. Our findings are helpful for the rational design of organic photoswitches at the single-molecule level based on cethrene and analogous organic molecules.

Floquet nonequilibrium Green's functions with fluctuation-exchange approximation: Application to periodically driven capacitively coupled quantum dots

Floquet nonequilibrium Green's functions with fluctuation-exchange approximation: Application to periodically driven capacitively coupled quantum dots

Mechanical and Electrical Conductivity Properties of Graphene/Cu Interfaces: A Theoretical Insight.

For graphene/copper (Gr/Cu) composites, achieving high-quality interfaces between Gr and Cu (strong interfacial bonding strength and excellent electron transport performance) is crucial for enabling their widespread applications in electronic devices. This study employs first-principles calculations and the nonequilibrium Green's function method to systematically investigate the mechanical and electrical conductivity properties of Cu(111)/Gr/Cu(111) interfaces with various stacking sequences and different forms of Gr. For these interface systems, the binding energy, separation work, charge transfer, and electrical conductivity across the interface were obtained. The results show that the top-fcc interface exhibits superior interfacial properties, characterized by relatively high binding energy (-3.00 eV/C atom) and separation work (≥0.78 J/m2), a small interfacial distance (2.85 Å), and enhanced electron transport capacity (2.12 G0/nm2). A bilayer form of Gr significantly reduces electronic conductance across the Gr/Cu interface by nearly 2.46 orders of magnitude. Furthermore, point defects in Gr, especially single-vacancy defects, disrupt the traditional trade-offs between mechanical and electrical performance, simultaneously enhancing mechanical performance by 7.50-124.36% and electrical performance by 33.02%. Additionally, stress mechanisms have been proposed to further enhance the interfacial electrical conductivity of Gr/Cu composites. The present study provides a theoretical basis for exploring the engineering applications of Gr/Cu composite materials.

Theoretical study of point defects on transport properties in metallic interconnections

During the line width reduction, electron scattering caused by various defects in metal interconnects increases dramatically, which causes leakage or short circuit problems in the device, reducing device performance and reliability. Point defects are one of the important factors. Here, using density functional theory and non-equilibrium Green's function methods, we systematically study the effects of point defects on the transport properties of metals Al, Cu, Ag, Ir, Rh, and Ru, namely vacancy defects and interstitial doping of C atom. The results show that the conductivity of all systems decreases compared to perfect systems, because defects cause unnecessary electron scattering. Since the orbital hybridization of the C atom with the Al, Cu and Ag atoms is stronger than that metals Ir, Rh and Ru, the doping of C atom significantly reduces the conductivity of metals Al, Cu and Ag compared to vacancy defects. In contrast, vacancy defects have a greater impact than doping on the transport properties of metals Ir, Rh and Ru, which is mainly attributed to the larger charge transfer of the host atoms around the vacancies caused by lattice distortion. In addition, metal Rh exhibits excellent conductivity in all systems. Therefore, in order to optimize the transport properties of interconnect metals, our work points out that the doping of impurity atoms should be avoided for metals Al, Cu and Ag, while the presence of vacancy defects should be avoided for metals Ir, Rh and Ru, and Rh may be an excellent candidate material for future metal interconnects.

Interface defect state induced spin injection in organic magnetic tunnel junctions

This article analytically explores defect assisted spin injection in organic magnetic tunnel junctions (MTJs) [x/rubrene/Co, x = La2O3, LaMnO3, La0.7Ca0.3MnO3 (LCMO), La0.7Sr0.3MnO3 (LSMO)] employing nonequilibrium Green’s function (NEGF). Spin precession at ferromagnet (FM)/organic semiconductor (OSC) interface defect states have been considered while modeling the MTJ devices. Variations in voltage dependent parallel (RP) and antiparallel (RAP) resistances have been attributed to modified spin dependent scattering at modified spin resolved density of states of magnetic electrodes. Moreover, change in distribution of defect state depths at a spin injection interface has also been observed to modify RP/RAP, and hence, tunnel magnetoresistance (TMR) across the devices. Localization of defect state distribution due to a high spin split band may have resulted in large TMR for La2O3 devices. Nonlinear spin transfer torque (STT) in devices other than LSMO indicates compensation of spin damping, resulting in a high TMR response across the devices. Hence, the localization of defect state distribution and the choice of magnetic electrodes with high spin split bands may be exercised to realize spintronic devices for low power spin memory applications.

Design of spintronic devices based on adjustable half-metallicity induced by electric field in A-type antiferromagnetic bilayer NiI2

Exploring the attainment of half-metallic behavior in two-dimensional (2D) materials through external perturbations is a popular area of current research. In this work, we demonstrate, using first-principles calculations, that bilayer NiI2 (bi-NiI2) is an A-type antiferromagnetic (AFM) semiconductor with an indirect bandgap of 0.86 eV, with the most stable configuration being the AB stacking mode. Upon the application of a vertical electric field, the material transforms from its original semiconducting state into a half-metallic state. Moreover, the spin polarization reverses its orientation whenever the direction of the electric field is altered. This intriguing behavior has inspired us to design a spintronic device based on the A-type AFM bi-NiI2. By employing nonequilibrium Green's function (NEGF) combined with density functional theory (DFT) calculations, we find that the device achieves ON/OFF switching by applying vertical electric fields in parallel or anti-parallel configurations in the two leads. The device displays 100 % spin polarization in the parallel configuration (PC) scenario, driven by bias voltage or temperature differences. Utilizing either the parallel or antiparallel configuration (APC) for ON/OFF switching enables the device to exhibit tunneling magnetoresistance (TMR) of up to 1.45 × 1010 % due to bias voltage and up to 1011 % thermal TMR arising from temperature differences between the leads. These findings highlight the potential of NiI2 and A-type AFM bilayers in the design of spintronic devices.

Current-driven mechanical motion of double stranded DNA results in structural instabilities and chiral-induced-spin-selectivity of electron transport.

Chiral-induced-spin-selectivity of electron transport and its interplay with DNA's mechanical motion are explored in a double stranded DNA helix with spin-orbit-coupling. The mechanical degree of freedom is treated as a stochastic classical variable experiencing fluctuations and dissipation induced by the environment as well as force exerted by nonequilibrium, current-carrying electrons. Electronic degrees of freedom are described quantum mechanically using nonequilibrium Green's functions. Nonequilibrium Green's functions are computed along the trajectory for the classical variable taking into account dynamical, velocity dependent corrections. This mixed quantum-classical approach enables calculations of time-dependent spin-resolved currents. We showed that the electronic force may significantly modify the classical potential, which, at sufficient voltage, creates a bistable potential with a considerable effect on electronic transport. The DNA's mechanical motion has a profound effect on spin transport; it results in chiral-induced spin selectivity, increasing spin polarization of the current by 9% and also resulting in temperature-dependent current voltage characteristics. We demonstrate that the current noise measurement provides an accessible experimental means to monitor the emergence of mechanical instability in DNA motion. The spin resolved current noise also provides important dynamical information about the interplay between vibrational and spin degrees of freedom in DNA.

Resonant tunneling properties of laser dressed hyperbolic Pöschl-Teller double barrier potential

We examine the resonant tunneling properties of the laser-dressed hyperbolic Pöschl-Teller double quantum barrier structure. We use the non-equilibrium Green's function method to investigate structure parameters and electric field bias on the transmission properties of the system. The transmission probabilities and resonance energy levels are significantly influenced by the well widths and barrier heights. The barrier height increases, resonance energy levels shift toward higher values, and the resonance peak width narrows, leading to sharper and more selective tunneling behavior. Our results show that increasing the electric field bias leads to a decrease in the transmission probability at the first resonance peak, but this effect is not as strong for the subsequent peaks. Moreover, we find that changes in the laser field's parameter and structure parameters allow for fine control over the electronic spectra, allowing for modifications like red or blue shifts based on particular needs. The significance of comprehending the interaction among structural factors, external fields, and transmission qualities in quantum barrier structures is highlighted by our research, providing valuable information for the development and enhancement of electronic and optoelectronic systems with customized functionality. Our findings show the laser field has a considerable impact on resonant tunneling properties, opening the door to new device applications.

Designing High-Performance Nanoscale Spin-MOSFET Devices by Using Two-Dimensional Half-Metallic Cr2Se3.

Constructing nanoscale spin devices has been a crucial pursuit in the field of nano spintronics. Here, by using the density functional theory (DFT) and nonequilibrium Green's function (NEGF) method, high-performance nanoscale spin-MOSFET devices using half-metallic 2D Cr2Se3 as electrodes are theoretically designed. Specifically, seven typical two-dimensional (2D) semiconductors, Sb, Bi, BP, BAs, MoTe2, WTe2, and WSeTe (with two different contacting surfaces), are considered here as the channel materials. The properties of contact interfaces between these 2D semiconductors and half-metallic 2D Cr2Se3 are first investigated. It is found that except BP and BAs (having Schottky contacts with Cr2Se3), the other 2D semiconductors have vertical Ohmic contacts with Cr2Se3 among which Cr2Se3/Sb, Cr2Se3/MoTe2, Cr2Se3/WTe2, Cr2Se3/WSeTe-Se, and Cr2Se3/WSeTe-Te retain the half-metallic characteristic. Then, these 2D semiconductors with Ohmic vertical contacts are further used to construct spin-MOSFET devices. The results show that devices constructed by half-metallic vertical contacting systems have nearly 100% SIE and therefore giant MR (>107%) when the gate voltage varies. Furthermore, four designed spin-MOSFET devices, namely, Cr2Se3/MoTe2, Cr2Se3/WTe2, Cr2Se3/WSeTe-Se, and Cr2Se3/WSeTe-Te spin-MOSFET have high efficient gate modulations on the magnitude of completely spin-polarized source-drain current with Cr2Se3/WTe2 having the smallest SS value of 134.1 mV/dec. The calculations suggest that Cr2Se3 is a good candidate for constructing spin-MOSFET devices. Our study sheds light on the design of high-performance nanoscale spin-MOSFET devices by using two-dimensional half-metallic electrodes.