Nonequilibrium Green Function Research Articles

Exploring spin photovoltaics in defective armchair phosphorene nanoribbons

This study explores the spin photovoltaic potential within armchair phosphorene nanoribbons (APNRs) that feature a periodic distribution of monovacancies (MVs) under the influence of light radiation. We investigate spin-semiconducting behavior induced by MV defects by utilizing both the mean-field Hubbard approximation and the self-consistent non-equilibrium Green's function model. This behavior is characterized by localized and anisotropic band structures around the Fermi energy, particularly within the antiferromagnetic phase. The existence of spin-splitting band gaps in defective APNRs not only enables the crafting of spin-optoelectronic nanodevices but also allows for the manipulation of electronic structure behavior with applied electric fields in both the vertical and transverse directions. Notably, the implementation of electric fields, offering tunability in electronic structure, results in varied spin photovoltaic responses encompassing a broad spectrum of photon energies from visible to ultraviolet. This research reveals promising avenues for advancing the field of spin-optoelectronic devices by MVs in APNRs.

Strain engineering of electronic structure and thermoelectric properties of quasi-hexagonal fullerene monolayer

As a newly synthesized two-dimensional (2D) carbon material, monolayer quasi-hexagonal phase fullerene (qHP C60) has an excellent electronic structure and low thermal conductivity. qHP C60 attracted significant attention from scientists because it has potential applications in thermoelectric materials. Thermoelectric properties of 2D materials significantly depend on the transport of carriers (such as electrons and phonons), and strain engineering is an essential method for modulating the transport of electrons and phonons in 2D materials. However, the strain engineering method for the modulation of the thermoelectric properties of monolayer qHP C60 has not been reported yet. In the present paper, the first-principles combined with the non-equilibrium Green's function method are used to investigate the ballistic transport properties of electrons and phonons in monolayer qHP C60. The effects of temperature, chemical potential, and biaxial tensile strain on the thermoelectric transport parameters (including conductivity, Seebeck coefficient, power factor, and thermal conductivity) as well as the figure of merit (ZT) of monolayer qHP C60 are presented, compared, discussed, and analyzed. We found that monolayer qHP C60 exhibits anisotropic characteristics in electron and phonon transport properties, showcasing outstanding thermoelectric properties. The distinctive quasi-hexagonal phase fullerene network structure offers a novel platform for exploring innovative 2D thermoelectric materials in research. This study provides crucial theoretical insights to guide the designing and implementation of 2D thermoelectric materials based on fullerenes.

Theoretical Investigation of Transport Properties of X-doped (X = Cl, P, N) Graphene Quantum Dots Layer Adsorbed with Ammonia Molecule Using Non-Equilibrium Green’s Function

This study investigates the various transport properties, including Density of States (DOS), Transmission Coefficient, Current-Voltage Characteristics (I-V), and HOMO-LUMO energy levels, of X-doped (X = Cl, P, N) graphene quantum dot (GQD) layers with adsorbed ammonia molecules using density functional theory and non-equilibrium Green's function techniques. Our findings revealed the profound influence of different doping and adsorption scenarios on the transport properties of GQD layers. Specifically, chlorine doping leads to notably stronger resonance peaks, resulting in enhanced conductivity of the GQD layer. This effect is less pronounced for P-doped and N-doped GQDs, which exhibit minimal changes in conductance capability. Importantly, we observed a prominent interaction between ammonia molecules and chlorine-doped GQD layers. The HOMO and LUMO energy levels exhibit double degeneracy, significantly impacting edge passivation and the finite dimensions of GQDs, consequently reducing the energy gap. As a result, chlorine-doped GQD layers with adsorbed ammonia molecules exhibit increased energy levels and stronger current characteristics compared to P-GQDs and N-GQDs. HOMO and LUMO are doubly degenerated which has a clear effect on edge passivation as well as the finite size of GQDs and reduces energy gap. Therefore, more energy level and stronger current is available for chlorine doped graphene quantum dots layer with adsorbed ammonia molecule than P-GQDs and N-GQDs. Our theoretical finding highlighted the significant features of edge passivation, doping effect and adsorption of ammonia molecule onto graphene quantum dots layer. Our theoretical analysis highlighted key features encompassing edge passivation, doping effects, and ammonia molecule adsorption on GQD layers. These insights not only develop our fundamental understanding of these systems but also hold promise for novel applications that harness their distinct electronic properties.

Effect of Ferromagnet/Organic Semiconductor Interface Defect States on Tunnel Magnetoresistance of Hybrid Magnetic Tunnel Junctions

Herein, analytical modeling of Fe3O4/x(≈1.1 nm)/Co (x = rubrene, C60, and bathocuproine (BCP)) magnetic tunnel junctions (MTJs) has been performed using rubrene, C60, and BCP as organic spacer layers. The simulation is considered as nonequilibrium Green's function assuming spin precession at ferromagnet/organic semiconductor (FM/OSC) interface defect states. The voltage‐dependent resistances for both parallel (RP) and antiparallel (RAP) orientations have been observed to be dependent on spin injection from FM/OSC defect states. Pinning well‐dependent defect state depths have been associated with band misalignment‐induced lattice distortion at FM/OSC interface of the devices. The large tunnel magnetoresistance (TMR) response for rubrene‐based MTJ device has been attributed to a higher change of FM/OSC defect state depths with voltage. High TMR may have reduced spin torque‐dependent spin precession, leading to lower spin transfer torque for the rubrene device. Hence, engineering of defect states at the FM/OSC interface may lead to the successful realization of enhanced TMR in organic spacer MTJs for high‐performance spintronic memory applications.

The study of electron mobility on ultra-scaled silicon nanosheet FET

The nanosheet FET (NSFET) is the successor to FinFET, and its mobility significantly affects device performance. In this paper, we investigate the impact of phonon (ph) and surface roughness (SR) scattering on the electron mobility of n-type silicon NSFETs. The effects of channel width, thickness, and doping concentrations on NSFETs' mobility are also analyzed. Non-Equilibrium Green’s Function (NEGF) solver which incorporates scattering mechanisms using the self-energy formulation is employed. The mobility behavior in NSFETs is strongly affected by ph scattering and SR scattering. As for ultrathin nanosheets, severe mobility degradation is dominated by SR scattering. The mobility is slightly affected by the doping concentrations. Simulation results provide guidance to researchers and industry in understanding and predicting the variation of mobility under the trend of continuous scaling.

Zigzag boron nitride nanoribbon doped with carbon atom for giant magnetoresistance and rectification behavior based nanodevices

Using the principles of density functional theory (DFT) and nonequilibrium Green’s function (NEGF), We thoroughly researched carbon-doped zigzag boron nitride nanoribbons (ZBNNRs) to understand their electronic behavior and transport properties. Intriguingly, we discovered that careful doping can transform carbon-doped ZBNNRs into a spintronic nanodevice with distinct transport features. Our model showed a giant magnetoresistance (GMR) up to a whopping 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} under mild bias conditions. Plus, we spotted a spin rectifier having a significant rectification ratio (RR) of 104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^4$$\\end{document}. Our calculated transmission spectra have nicely explained why there’s a GMR up to 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} for spin-up current at biases of -1.2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1.2$$\\end{document} V, -1.1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1.1$$\\end{document} V, and -1.0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1.0$$\\end{document} V, and also accounted for a GMR up to 103\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}–105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} for spin-down current at biases of 1.0 V, 1.1 V, and 1.2 V. Similarly, the transmission spectra elucidate that at biases of 1.0 V, 1.1 V, and 1.2 V for spin-up, and at biases of 1.1 V and 1.2 V for spin-down in APMO, the RRs reach 104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^4$$\\end{document}. Our research shines a light on a promising route to push forward the high-performance spintronics technology of ZBNNRs using carbon atom doping. These insights hint that our models could be game-changers in the sphere of nanoscale spintronic devices.

Perfect Spin Filtering, Giant Magnetoresistance, and Rectification Behavior Induced by V‐Doped Zigzag Graphene Nanoribbons

AbstractEmploying the constructs of density functional theory (DFT) and the Nonequilibrium Green's Function (NEGF), the investigation extensively explores the electronic and transport properties of zigzag graphene nanoribbons (ZGNRs) doped with vanadium (V). Notably, this inquiry unveils that strategic doping can transform V‐doped ZGNRs into spintronic nanodevices with distinctive transport attributes. Initially, the simulations showcase remarkably high spin‐filtering efficiencies (SFEs) at certain bias voltages. Furthermore, a giant magnetoresistance (GMR) peaking at 6.87 10 is detected. In conclusion, the examination discerns a spin rectifier that exhibits a significant rectification ratio (RR) of 9.62 10. This research delineates a viable trajectory for the refinement of high‐performance spintronics in ZGNRs via vanadium doping. The implications of this study indicate that the model harbors considerable promise for application in miniature spintronic devices.

Atomistic simulations of coherent thermoelectric transport in ultrathin pristine and decorated noble metals nanowires

Nanowires (NWs) are envisioned to be an important component of novel nanoscale electronic devices. Here, we perform atomistic simulations of coherent thermoelectric (TE) transport in free-standing ultrathin NWs of noble metals (viz. Au, Ag, Cu, Pt) cut along the [111] crystallographic direction using a non-equilibrium Greens function based first principles approach. Effect of decorating these NWs with Au or Pt atoms in an fcc environment, is also explored. Results of the calculated electronic structure show that all considered NWs, except the Pt and Pt-decorated NWs, are metallic and non-magnetic. Interestingly, Cu@Pt NW is found to be a magnetic semiconductor with an unequal band gap in ↑− and ↓−spin energy bands, while Pt NW and other Pt-decorated NWs (viz. Ag@Pt and Au@Pt) behave like a magnetic half-metal. To explore the TE potential of as-considered NWs, electrical conductance G, thermal conductance κT, and thermopower S are computed in the linear response regime. Notably, the metallic NWs exhibit a quantized G. Among these, pristine NWs show a quantized value 3G0 up to a temperature of 200 K, while Cu@Au NW preserves its quantized value 5G0 even at T exceeding 300 K. In thermal transport, valence electrons make a dominant contribution to the room-temperature κT of pristine NWs. The phononic contribution κph, however, goes up following decoration, with Pt-decorated NWs showing a pronounced increment (by a factor of ∼2). But, these NWs undergo a simultaneous reduction in the electronic contribution κel owing to half-metallic conduction, resulting in a net suppression of total thermal conductance κT. Moreover, Pt-decorated NWs show a manifold enhancement in zero-bias S, with Ag@Pt exhibiting the highest value ∼119μV/K. However, S can be augmented considerably by adjusting the chemical potential μ of electrodes, and an overall best S∼−890μV/K is found in Au NW for μ=0.575eV. In this way, TE figure of merit ZT is tunable in pristine NWs over the range 1−3.7. Correspondingly, the decorated NWs exhibit somewhat smaller S and ZT. Furthermore, spin-asymmetric electronic transport in Pt and Pt-decorated NWs results in net spin conductance Gs and spin thermopower Ss. It is found that the spin figure of merit ZsT, as high as 3.5, can be attained in Pt NW by controlling μ. Thus, noble metals NWs can be tailored for both thermoelectric and spin-thermoelectric applications. We have also investigated TE transport for the pristine NWs in the [110] direction, and found S and ZT to be comparatively much smaller. Such NWs may be useful as interconnects for low noise applications.

Coherent spin transport in a copper protein.

The coherent electron/spin transport in azurin, a species of copper protein, was calculated based on the Landauer model. The research is motivated by the fast electron transport and spin selectivity/polarization in azurin, which have been reported in relation to the chiral-induced spin selectivity of the peptide structure. The calculated spin polarization of copper proteins was large. This phenomenon was strongly influenced by the spin density of the atoms in the ligand group, whereas the contribution of copper was negligible. The results suggest that spin polarization in copper proteins is enhanced by that of the ligand groups. The predicted spin polarization aligns primarily with the scanning tunneling microscope-based break-junction technique to study the electronic properties of single-molecule junctions. Computational techniques employed in this study are nonequilibrium Green's functions (NEGF) and density functional theory (DFT) based on the Landauer model, implemented using the QuantumATK software (Synopsys Inc.). The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was adopted for spin-polarized generalized gradient approximation (SGGA). The valence atomic orbitals were constructed using the wavefunctions of the SIESTA package, which was based on the norm-conserving Troullier-Martins relativistic pseudopotentials for describing core electrons. The mesh used for real-space integration was 150 Ha.

Chirality-Induced Spin Selectivity: Analysis of Density Functional Theory Calculations.

Chirality-induced spin selectivity (CISS), which was demonstrated in several molecular and material systems, has drawn much interest recently. The phenomenon, described in electron transport by the difference in the transport rate of electrons of opposite spins through a chiral system, is however not fully understood. Herein, we employed density functional theory in conjunction with spin-orbit coupling to evaluate the percent spin-polarization in a device setup with finite electrodes at zero bias, using an electron transport program developed in-house. To study the interface effects and the level of theory considered, we investigated a helical oligopeptide chain, an intrinsically chiral gold cluster, and a helicene model system that was previously studied (Zöllner et al. J. Chem. Theory Comput. 2020, 16, 7357-7371). We find that the magnitude of the spin-polarization depends on the chiral system-electrode interface that is modeled by varying the interface boundary between the system's regions, on the method of calculating spin-orbit coupling, and on the exchange-correlation functional, e.g., the amount of exact exchange in the hybrid functionals. In addition, to assess the effects of bias, we employ the nonequilibrium Green's function formalism in the Quantum Atomistix Toolkit program, showing that the spin-flip terms could be important in calculating the CISS effect. Although understanding CISS in comparison to experiment is still not resolved, our study provides intrinsic responses from first-principles calculations.

Carrier transport simulation methods for electronic devices with coexistence of quantum transport and diffusive transport

In this manuscript, carrier transport simulation methods are proposed for devices with the coexistence of quantum transport and diffusive transport by combining the nonequilibrium Green's function method with the drift-diffusion transport simulation method. Current continuity between quantum transport and drift-diffusion transport is ensured by setting quantum transport current as the connection boundary condition of drift-diffusion simulation or by introducing quantum transport-induced carrier generation rates to drift-diffusion simulation. A comprehensive study of our method and the method combining the Wentzel–Kramers–Brillouin (WKB) method with the drift-diffusion transport simulation method is performed for n-type tunnel oxide passivating contact solar cell to investigate their applicable conditions and balance the accuracy and computational cost. As the oxide barrier width, barrier height, and electron effective mass increase, or the doping concentration in the electron transport layer decreases to the extent that the blocking effect of the oxide barrier on light-generated electrons becomes significant, method I is more accurate since the transmission coefficient near the conduction band edge calculated by WKB is overestimated; otherwise, method II is more suitable due to its low computational cost without the loss of accuracy. In addition, the differences between current densities, carrier densities, and Shockley–Read–Hall recombination rates simulated under the two current continuity conditions for the solar cell with different carrier mobilities are also further explored and analyzed.

Electronic and transport properties of U-cut edge patterned AGNR superlattice for RTD application

In this paper, we first characterize a superlattice structure created by repeating a heterostructure formed from two armchair graphene nanoribbon (AGNR) segments with different widths. We investigate the electronic and transport properties of this structure by varying its widths and lengths to demonstrate the tunability of its overall band gap. The plot of the local density of states shows the formation of localized states at the low band gap segments of the superlattice. The superlattice is then used as a barrier to create a double barrier quantum well (DBQW) to design a proposed resonant tunneling diode (RTD) structure. We observe this device's negative differential resistance (NDR) operation for a range of bias voltages between the contacts. We study the effect of dimensional parameters on the RTD performance. The non-equilibrium Green's function method, based on a tight-binding model, is employed for numerical computation.

Effect of Bridging Manner on the Transport Behaviors of Dimethyldihydropyrene/Cyclophanediene Molecular Devices.

A molecule-electrode interface with different coupling strengths is one of the greatest challenges in fabricating reliable molecular switches. In this paper, the effects of bridging manner on the transport behaviors of a dimethyldihydropyrene/cyclophanediene (DHP/CPD) molecule connected to two graphene nanoribbon (GNR) electrodes have been investigated by using the non-equilibrium Green's function combined with density functional theory. The results show that both current values and ON/OFF ratios can be modulated to more than three orders of magnitude by changing bridging manner. Bias-dependent transmission spectra and molecule-projected self-consistent Hamiltonians are used to illustrate the conductance and switching feature. Furthermore, we demonstrate that the bridging manner modulates the electron transport by changing the energy level alignment between the molecule and the GNR electrodes. This work highlights the ability to achieve distinct conductance and switching performance in single-molecular junctions by varying bridging manners between DHP/CPD molecules and GNR electrodes, thus offering practical insights for designing molecular switches.

Enhanced Seebeck effect in gated O-shaped Kekulé graphene junctions

The Seebeck effect in graphene with uniform Kekulé lattice distortion has been studied using the tight-binding model combined with the nonequilibrium Green's function method. The numerical results of this work show that the Seebeck coefficient of the O-shaped Kekulé graphene can be enhanced by modulating the gate voltage. At room temperature (T≈300 K), the Seebeck coefficient for the armchair nanoribbon is Sc≈1370μV/K and for the zigzag nanoribbon is Sc≈1260μV/K. At low temperature (T≈128 K), the Seebeck coefficient for the armchair nanoribbon is Sc≈2500μV/K. In addition, the nano-structuration of the O-shaped Kekulé graphene layer induces an anisotropic behavior in the Seebeck effect. Our work provides an idea for the fabrication of efficient and controllable thermoelectric effect devices.

Classical and quantum theory of fluctuations for many‐particle systems out of equilibrium

AbstractCorrelated classical and quantum many‐particle systems out of equilibrium are of high interest in many fields, including dense plasmas, correlated solids, and ultracold atoms. Accurate theoretical description of these systems is challenging both, conceptionally and with respect to computational resources. While for classical systems, in principle, exact simulations are possible via molecular dynamics, this is not the case for quantum systems. Alternatively, one can use many‐particle approaches such as hydrodynamics, kinetic theory, or nonequilibrium Green functions (NEGF). However, NEGF exhibit a very unfavorable cubic scaling of the CPU time with the number of time steps. An alternative is the G1–G2 scheme [N. Schlünzen et al., Phys. Rev. Lett. 124, 076601 (2020)] which allows for NEGF simulations with time linear scaling, however, at the cost of large memory consumption. The reason is the need to store the two‐particle correlation function. This problem can be overcome for a number of approximations by reformulating the kinetic equations in terms of fluctuations – an approach that was developed, for classical systems, by Yu.L. Klimontovich [JETP 33, 982 (1957)]. Here, we present an overview of his ideas and extend them to quantum systems. In particular, we demonstrate that this quantum fluctuations approach can reproduce the nonequilibrium GW approximation [E. Schroedter et al., Cond. Matt. Phys. 25, 23401 (2022)] promising high accuracy at low computational cost which arises from an effective semiclassical stochastic sampling procedure. We also demonstrate how to extend the approach to the two‐time exchange‐correlation functions and the density response properties. [E. Schroedter et al., Phys. Rev. B 108, 205109 (2023)]. The results are equivalent to the Bethe–Salpeter equation for the two‐time exchange‐correlation function when the generalized Kadanoff‐Baym ansatz with Hartree‐Fock propagators is applied [E. Schroedter and M. Bonitz, phys. stat. sol. (b) 2024, 2300564].

Thermoelectric transport and Rashba spin splitting of black arsenic phosphorus under strain regulation

The Rashba spin-splitting and thermoelectric transport of two-dimensional black arsenic phosphorus (2D B-AsP) under strain control are studied using non-equilibrium Green's function and first-principles calculations. Firstly, we determine the stability range of 2D B-AsP under strain conditions, with biaxial strain ranging from −2% to +10% and uniaxial strain ranging from −9% to +10%. Secondly, we analyze the spin splitting at the point Γ under strain within the range from −1% to +2%, and find that the spin-splitting coefficient decreases with increasing strain. Thirdly, we discuss the effects of applying strain and changing temperature on 2D B-AsP thermoelectric transport. When the uniaxial compression strain is −4%, the ZT coefficient increases by 14.8%, and increases by 193.6% at a temperature of 600 K. Finally, when two external conditions exist simultaneously, the ZT coefficient of the material increases by 103.4%. The research results demonstrate the potential applications of 2D B-AsP in spintronics and thermoelectricity.

Impact of Rh, Ru, and Pd Leads and Contact Topologies on Performance of WSe2 FETs: A First Comparative Ab Initio Study.

2D field-effect transistors (FETs) fabricated with transition metal dichalcogenide (TMD) materials are a potential replacement for the silicon-based CMOS. However, the lack of advancement in p-type contact is also a key factor hindering TMD-based CMOS applications. The less investigated path towards improving electrical characteristics based on contact geometries with low contact resistance (RC) has also been established. Moreover, finding contact metals to reduce the RC is indeed one of the significant challenges in achieving the above goal. Our research provides the first comparative analysis of the three contact configurations for a WSe2 monolayer with different noble metals (Rh, Ru, and Pd) by employing ab initio density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. From the perspective of the contact topologies, the RC and minimum subthreshold slope (SSMIN) of all the conventional edge contacts are outperformed by the novel non-van der Waals (vdW) sandwich contacts. These non-vdW sandwich contacts reveal that their RC values are below 50 Ω∙μm, attributed to the narrow Schottky barrier widths (SBWs) and low Schottky barrier heights (SBHs). Not only are the RC values dramatically reduced by such novel contacts, but the SSMIN values are lower than 68 mV/dec. The new proposal offers the lowest RC and SSMIN, irrespective of the contact metals. Further considering the metal leads, the WSe2/Rh FETs based on the non-vdW sandwich contacts show a meager RC value of 33 Ω∙μm and an exceptional SSMIN of 63 mV/dec. The two calculated results present the smallest-ever values reported in our study, indicating that the non-vdW sandwich contacts with Rh leads can attain the best-case scenario. In contrast, the symmetric convex edge contacts with Pd leads cause the worst-case degradation, yielding an RC value of 213 Ω∙μm and an SSMIN value of 95 mV/dec. While all the WSe2/Ru FETs exhibit medium performances, the minimal shift in the transfer curves is interestingly advantageous to the circuit operation. Conclusively, the low-RC performances and the desirable SSMIN values are a combination of the contact geometries and metal leads. This innovation, achieved through noble metal leads in conjunction with the novel contact configurations, paves the way for a TMD-based CMOS with ultra-low RC and rapid switching speeds.

Performance Projection of Vacuum Gate Dielectric Doping-Free Carbon Nanoribbon/Nanotube Field-Effect Transistors for Radiation-Immune Nanoelectronics.

This paper investigates the performance of vacuum gate dielectric doping-free carbon nanotube/nanoribbon field-effect transistors (VGD-DL CNT/GNRFETs) via computational analysis employing a quantum simulation approach. The methodology integrates the self-consistent solution of the Poisson solver with the mode space non-equilibrium Green's function (NEGF) in the ballistic limit. Adopting the vacuum gate dielectric (VGD) paradigm ensures radiation-hardened functionality while avoiding radiation-induced trapped charge mechanisms, while the doping-free paradigm facilitates fabrication flexibility by avoiding the realization of a sharp doping gradient in the nanoscale regime. Electrostatic doping of the nanodevices is achieved via source and drain doping gates. The simulations encompass MOSFET and tunnel FET (TFET) modes. The numerical investigation comprehensively examines potential distribution, transfer characteristics, subthreshold swing, leakage current, on-state current, current ratio, and scaling capability. Results demonstrate the robustness of vacuum nanodevices for high-performance, radiation-hardened switching applications. Furthermore, a proposal for extrinsic enhancement via doping gate voltage adjustment to optimize band diagrams and improve switching performance at ultra-scaled regimes is successfully presented. These findings underscore the potential of vacuum gate dielectric carbon-based nanotransistors for ultrascaled, high-performance, energy-efficient, and radiation-immune nanoelectronics.

Theoretical study on the structure, spectroscopic, and current–voltage behavior of 11-Cis and Trans retinal isomers in rhodopsin

In this study, the electronic transport properties of 11-Cis and Trans retinal, components of rhodopsin, were investigated as optical molecular switches using the nonequilibrium Green’s function (NEGF) formalism combined with first-principles density functional theory (DFT). These isomers, which can be reversibly converted into each other, were examined in detail. The structural and spectroscopic properties, including infrared (IR), Raman, nuclear magnetic resonance (NMR), and ultraviolet (UV) spectra, were analyzed using the hybrid B3LYP/6–311 + + G** level of theory. Complete vibrational assignments were performed for both forms utilizing the scaled quantum mechanical force field (SQMFF) methodology. To evaluate the conductivity of these molecules, we utilized current–voltage (I-V) characteristics, transmission spectra, molecular projected self-consistent Hamiltonian (MPSH), HOMO–LUMO gap, and second-order interaction energies (E2). The trendline extrapolation of the current–voltage plots confirmed our findings. We investigated the effect of different electrodes (Ag, Au, Pt) and various connection sites (hollow, top, bridge) on conductivity. The Ag electrode with the hollow site exhibited the highest efficiency. Our results indicate that the Cis form has higher conductivity than the Trans form.

Spin-Orbit Torque in Single-Molecule Junctions from ab Initio.

The use of electric fields applied across magnetic heterojunctions that lack spatial inversion symmetry has been previously proposed as a nonmagnetic means of controlling localized magnetic moments through spin-orbit torques (SOT). The implementation of this concept at the single-molecule level has remained a challenge, however. Here, we present first-principles calculations of SOT in a single-molecule junction under bias and beyond linear response. Employing a self-consistency scheme invoking density functional theory and nonequilibrium Green's function theory including spin-orbit interaction, we compute the change of the magnetization with the bias voltage and the associated current-induced SOT. Within the linear regime our quantitative estimates for the SOT in single-molecule junctions yield values similar to those known for magnetic interfaces. Our findings contribute to an improved microscopic understanding of SOT in single molecules.