Nonequilibrium Green Function Research Articles

A first-principles study of the ultra-high spin rectification effect based on nitride MXenes (Sc2NO2, Ti2NO2).

Nitride MXenes exhibit inherent strong chemical stability and ferromagnetic properties, which are significant for their application in nanoscale spintronic devices. To demonstrate the potential of nitride MXenes in spintronics, we have designed a Sc2NO2/Ti2NO2 heterojunction and investigated its spin transport properties using first principles calculations combined with the non-equilibrium Green's function. The results show that the Sc2NO2/Ti2NO2 heterojunction has a stable negative differential resistance effect and a great spin rectification effect with a rectification ratio of 1.73 × 1011. The corresponding energy band structures and the transport behavior establish the mechanism for such properties. In addition, the perfect two-dimensional morphology provides a suitable nanostructured material for developing spintronic devices.

Optically controlled silicene and germanene transistors driven by spin-bias

The transistor is the core unit of digital integrated circuits, and its performance and integration are the main determinants of chip performance. With the continuous progress of nano-manufacturing technology and process, high power and heat consumption have become a major problem restricting the development of integrated circuits. Using topological insulators instead of traditional semiconductors, and the spin and valley degrees of freedom instead of charge as information carriers, to design and fabricate transistors, and the use of optical interconnections to replace metal interconnections between functional units, are effective solutions of the thermal power consumption problem of nano-integrated circuits. Based on the two-dimensional topological insulators-silicene and germanene, we theoretically propose a kind of optically controlled transistor suitable for spin bias. The effects of off-resonant circularly polarized light on the output currents of silicene and germanene transistors are calculated by using the non-equilibrium Green's function method. It is shown that the topological properties of silicene (germanene) and the output characteristics of drain current are controlled by the chirality and intensity of incident light. Under the coaction of weak left circularly polarized light and spin bias, the silicene transistors output pure spin current and fully polarized spin-up current. Under the action of strong field, the phase transition of the edge state of the silicene forms a band gap, the transistor is cut off, and the output current is almost zero. Different from the silicene transistor, the germanene transistor can obtain stable pure spin current under a weak light field, and output 100% polarized spin-down current under a strong field. By simultaneously applying off-resonant circularly polarized light of different chirality to the central device region, the germanene transistor can be turned off effectively by using the edge state phase transition induced by the polarized light field and the energy band mismatch caused by the local light field. The output poles of spin-dependent currents are almost equal for silicene and germanene transistors in the ON state, however, the breakdown voltage of the germanene transistor is significantly higher than that of the silicene transistor, and optically controlled germanene transistor can maintain effective operation at higher temperatures.

Design of multifunctional spin logic gates based on manganese porphyrin molecules connected to graphene electrodes.

The spin-resolved transport properties of molecular logic devices composed of two Mn porphyrin molecules connected to each other via a six-carbon atomic chain were studied using the non-equilibrium Green's function combined with density functional theory. The molecules were symmetrically connected to armchair graphene nanoribbon electrodes through four-carbon atomic chains on the left- and right-hand sides. Our calculations revealed that the spin-resolved current-voltage curves depend on the initial spin setting of the transition metal Mn atoms and carbon atoms on the zigzag edges where the electrodes come in contact with the molecule. By simultaneously regulating the spin orientations of the intermediate functional molecules and the zigzag edges of the armchair graphene nanoribbon electrodes, seven spin polarization configurations were obtained. These configurations were examined in this study considering the spin-related symmetry of molecular junctions. By meticulously selecting different combinations according to the specific input and output signals, YES, NOT, OR, NOR, and XOR multifarious spin logic devices were created. The findings of this study are expected to contribute toward the extension of molecular junction functions in future spintronic integrated circuit design and further miniaturization.

Comprehensive understanding of intrinsic mobility and sub-10 nm quantum transportation in Ga2SSe monolayer.

Two-dimensional chalcogenides could play an important role in solving the short channel effect and extending Moore's law in the post-Moore era due to their excellent performances in the spintronics and optoelectronics fields. In this paper, based on theoretical calculations combining density functional theory and non-equilibrium Green's function, we have systematically explored the intrinsic mobility in the Ga2SSe monolayer and quantum transport properties of sub-10 nm Ga2SSe field-effect transistors (FET). Interestingly, the Ga2SSe monolayer presents high intrinsic electron mobility up to 104 cm2 (V s)-1. It is highlighted that the intrinsic mobility in the Ga2SSe monolayer is significantly restrained by phonon scattering, where the out-of-plane acoustic mode and high-frequency optic phonon mode are found predominantly coupled with the electrons. As a result, the n-type doping sub-10 nm Ga2SSe FETs represent distinguished transport properties. In particular, even the gate length is shortened to 3 nm, the on-state current, delay time and power consumption of the n-type doping Ga2SSe FET along the armchair direction can reach the International Technology Roadmap for Semiconductor industry standards for high-performance requirements. Our present study paves the way for the application of Ga2SSe monolayers in ultra-small sized FETs in the post-silicon era.

Theoretical study of anisotropy and ultra-low thermal conductance of porous graphene nanoribbons

The thermal transport properties of porous graphene nanoribbons are studied by the non-equilibrium Green's function method. The results show that owing to the existence of nano-pores, the thermal conductance of porous graphene nanoribbons is much lower than that of graphene nanoribbons. At room temperature, the thermal conductance of zigzag porous graphene nanoribbons is only 12% of that of zigzag graphene nanoribbons of the same size. This is due to the phonon localization caused by the nano-pores in the porous graphene nanoribbons. In addition, the thermal conductance of porous graphene nanoribbons has remarkable anisotropy. With the same size, the thermal conductance of armchair porous graphene nanoribbons is about twice higher than that of zigzag porous graphene nanoribbons. This is because the phonon locality in the zigzag direction is stronger than that in the armchair direction, and even part of the frequency phonons are completely localized.

Giant rectification of ferromagnetic zigzag SiC nanoribbons connecting anthradithiophene molecules

Using non-equilibrium Green's function combined with density functional theory, we investigate the spin-resolved transport properties of the zigzag SiC nanoribbon (zSiCNR) connecting anthradithiophene (ADT) molecules and obtain the giant spin current rectification in the presence of a ferromagnetic field. The dual-hydrogenation on edge C atoms or Si atoms can change the initial metallicity of the pristine zSiCNR with the edge mono-hydrogenation into semiconductivity in the presence of a ferromagnetic field. The up-spin current-voltage characteristic of the cis-ADT device and the trans-ADT device can present the significant rectification, and the corresponding giant spin current rectification ratios are close to 10<sup>11</sup> and 10<sup>10</sup> respectively. In addition, the current-voltage characteristics of two devices both perform a perfect spin filtering behavior in the positive bias region due to the huge difference between the up-spin current value and the down-spin current value. These findings are of great significance in the functional applications of spin-resolved molecular devices in the future.

Transport properties of MoS2/V7(Bz)8 and graphene/V7(Bz)8 vdW junctions tuned by bias and gate voltages.

The MoS2/V7(Bz)8 and graphene/V7(Bz)8 vdW junctions are designed and the transport properties of their four-terminal devices are comparatively investigated based on density functional theory (DFT) and the nonequilibrium Green's function (NEGF) technique. The MoS2 and graphene nanoribbons act as the source-to-drain channel and the spin-polarized one-dimensional (1D) benzene–V multidecker complex nanowire (V7(Bz)8) serves as the gate channel. Gate voltages applied on V7(Bz)8 exert different influences of electron transport on MoS2/V7(Bz)8 and graphene/V7(Bz)8. In MoS2/V7(Bz)8, the interplay of source and gate bias potentials could induce promising properties such as negative differential resistance (NDR) behavior, output/input current switching, and spin-polarized currents. In contrast, the gate bias plays an insignificant effect on the transport along graphene in graphene/V7(Bz)8. This dissimilarity is attributed to the fact that the conductivity follows the sequence of MoS2 < V7(Bz)8 < graphene. These transport characteristics are examined by analyzing the conductivity, the currents, the local density of states (LDOS), and the transmission spectra. These results are valuable in designing multi-terminal nanoelectronic devices.

Breakdown Voltage and Linear Temperature Drift in a Single-Molecule Junction

Using first-principles calculations based on density functional theory combined with the non-equilibrium Green's function approach, the transport behaviors of a single-molecule junction formed by benzenedithiol connected to gold electrodes are investigated. The breakdown voltage for the model of benzenedithiol plus gold electrodes is 0.7 V, which is close to the experimental value. A linear response between the conductance and temperature (known as linear temperature drift) is found in the molecular device, which indicates that it could be used to maintain the stability of molecular circuits. Meanwhile, input and output with the same accuracies would be useful for designing multi-level circuits, which would be used to improve the resolution ratio in analog-to-digital converters. The present findings indicate that benzenedithiol-based single-molecule junctions would be promising functional units for molecular sensors.

Ab Initio Study of Carbon Nanotube Field Effect Transistor Gas Sensor for Detection of Ammonia and Nitrogen Dioxide Gas

Lebel-free sensors are capable for sensing low concentration of gas molecules. In this article, the importance of Carbon Nanotube Field Effect Transistor (CNFET) is described for gas sensing application. The first principal study to investigate the CNFET to detection of low concentration of ammonia (NH3) and nitrogen dioxide (NO2) gas molecules. By discussing the electronic and transport properties of CNFET, we find that CNFET can be used for gas sensing applications. Detailed analysis of binding energy, e-k diagram, density of state (DOS), device density of state (DDOS), transmission pathways and current-voltage (I-V) characteristics configuration have been performed using density functional theory (DFT) and non-equilibrium green function (NEGF) method. It has been observed that CNFET can be used for the potential application of gas sensor at room temperature. Out theoretical findings are corroborated with experimental data and this virtual device structure can be converted into physical device to get nano dimensions integrated gas sensor.

Time-Dependent Landau-Ginzburg Equation-Based Ferroelectric Tunnel Junction Modeling With Dynamic Response and Multi-Domain Characteristics

Overcoming the drawbacks of the existing ferroelectric tunnel junction (FTJ) models which ignore the dynamic or multi-domain switching behaviors, we develop a more comprehensive FTJ model by combining the Time-Dependent Landau-Ginzburg (TDLG) equations to solve the multi-domain dynamic switching of ferroelectric layer and the Non-Equilibrium Green Function (NEGF) to solve the tunneling current. The model successfully reproduces the experimental results of our fabricated metal-ferroelectrics-insulator-semiconductor (MFIS) FTJ. This model empowers us to predict both the dynamic and multi-state switching of FTJ, showing its promise for applications in the high-density data storage and analog computing.

Giant Magnetoresistance and Rectification Behavior in Fluorinated Zigzag Boron Nitride Nanoribbon for Spintronic Nanodevices

In the present work, the spin-polarized structural and electronic properties of fluorine (F) passivated zigzag boron nitride nanoribbons (ZBNNRs) at the selective boron (B) and nitrogen (N) edge atoms are investigated. This study is based on the density functional theory (DFT) along with non-equilibrium Green function (NEGF) formalism. Our study predicts that half-metallic property can be obtained in ZBNNRs via F passivation at selective edges. The F-passivated ZBNNRs are found to be structurally stable in both non-magnetic as well as magnetic ground states irrespective of their width. Hence, the transport properties of F-passivated ZBNNRs are also studied as fluorinated structures are reported to be more stable. The current-voltage characteristic of F-passivated ZBNNRs based devices exhibit the perfect spin-filter characteristics with magnificently high spin-filtering efficiency (SFE) even under a low bias. It is worth mentioning here that giant magnetoresistance resistance (GMR), and rectification ratio of the order of 10⁸and 10⁵ have also been observed for F-BN-F devices. This is because dangling bonds break the edge states' symmetry and induce some localized states, which suppress the electron transmission and reduce the current. The observed perfect spin-filtering characteristics, GMR, and rectifying characteristics suggest that Fpassivated ZBNNRs have immense potentials to be deployed for nanoscale spintronic devices.

Variation of Transmission Probability Spectrum with the Number of Dots in T-shaped Topology of Quantum Dot System

We analyze the transmission coefficient for T-shaped configuration for quantum dot systems by increasing the number of dots side coupled to the active dot. Analytical expressions for the transmission coefficient have been calculated, in the presence of on-dot Coulomb interaction, using non-equilibrium Green function techniques. The higher-order Green functions containing Coulomb interaction terms have been decoupled using the mean-field approximation. Numerical results show a systematic variation in the transmission peak structure as the number of quantum dots side coupled to the active dot increases. The energy levels of dots available for electron transport have been identified in terms of the internal parameters of quantum dot systems. It is found that the number of transmission probability peaks is directly related to the number of dots in the T-shaped quantum dot system.

Edge Effect in Electronic and Transport Properties of 1D Fluorinated Graphene Materials.

A systematic examination of the electronic and transport properties of 1D fluorine-saturated zigzag graphene nanoribbons (ZGNRs) is presented in this article. One publication (Withers et al., Nano Lett., 2011, 11, 3912–3916.) reported a controlled synthesis of fluorinated graphene via an electron beam, where the correlation between the conductivity of the resulting materials and the width of the fluorinated area is revealed. In order to understand the detailed transport mechanism, edge-fluorinated ZGNRs with different widths and fluorination degrees are investigated. Periodic density functional theory (DFT) is employed to determine their thermodynamic stabilities and electronic structures. The associated transport models of the selected structures are subsequently constructed. The combination of a non-equilibrium Green’s function (NEGF) and a standard Landauer equation is applied to investigate the global transport properties, such as the total current-bias voltage dependence. By projecting the corresponding lesser Green’s function on the atomic orbital basis and their spatial derivatives, the local current density maps of the selected systems are calculated. Our results suggest that specific fluorination patterns and fluorination degrees have significant impacts on conductivity. The conjugated system is the dominate electron flux migration pathway, and the edge effect of the ZGNRs can be well observed in the local transport properties. In addition, with an asymmetric fluorination pattern, one can trigger spin-dependent transport properties, which shows its great potential for spintronics applications.

Electronic and transport property of two-dimensional boron phosphide sheet

Using density functional theory (DFT) approach, we have investigated the effect of strain on the electronic properties of two-dimensional (2D) boron phosphide (BP) sheet. With the increase in uniaxial and biaxial tensile strain band gap increases while band gap decreases and becomes metallic with the increase in uniaxial and biaxial compressive strain. Electrical and thermal transport properties of zigzag and armchair 2D BP sheets have been explored using nonequilibrium Green's function formalism (NEGF) and the changes in the nature of I–V characteristics with the application of strain have been reported. The magnitude of the current decreases with the increase of strain value along transport direction for both zigzag and armchair 2D BP sheets. For unstrained systems, the magnitude of current is nearly same for both zigzag and armchair 2D BP sheets. However, for a particular strain value, magnitude of current is more for zigzag sheet compared to armchair sheet. Though both zigzag and armchair 2D BP sheets have reasonably high ZTe which confirms its potentiality for designing efficient thermoelectric material but zigzag sheet is more preferable for thermoelectric application compared to armchair sheet due to its higher ZTe in comparison to armchair sheet.

Bandstructure and Size-Scaling Effects in the Performance of Monolayer Black Phosphorus Nanodevices.

Nanodevices based on monolayer black phosphorus or phosphorene are promising for future electron devices in high density integrated circuits. We investigate bandstructure and size-scaling effects in the electronic and transport properties of phosphorene nanoribbons (PNRs) and the performance of ultra-scaled PNR field-effect transistors (FETs) using advanced theoretical and computational approaches. Material and device properties are obtained by non-equilibrium Green’s function (NEGF) formalism combined with a novel tight-binding (TB) model fitted on ab initio density-functional theory (DFT) calculations. We report significant changes in the dispersion, number, and configuration of electronic subbands, density of states, and transmission of PNRs with nanoribbon width (W) downscaling. In addition, the performance of PNR FETs with 15 nm-long channels are self-consistently assessed by exploring the behavior of charge density, quantum capacitance, and average charge velocity in the channel. The dominant consequence of W downscaling is the decrease of charge velocity, which in turn deteriorates the ON-state current in PNR FETs with narrower nanoribbon channels. Nevertheless, we find optimum nanodevices with W > 1.4 nm that meet the requirements set by the semiconductor industry for the “3 nm” technology generation, which illustrates the importance of properly accounting bandstructure effects that occur in sub-5 nm-wide PNRs.

A tight-binding study of the electron transport through single-walled carbon nanotube-graphene hybrid nanostructures.

Hybrid carbon nanostructures based on the sp2 hybridized allotropes of carbon, such as graphene and single-walled carbon nanotubes (SWCNTs), hold vast potential for applications in electronics of various forms. Electronic properties of such hybrid structures are modified due to the interaction between atoms of the components, which can be utilized to tailor the properties of the hybrid structures to suite the application. In this study, we have explored charge (electron) transport through the hybrid structures of single-layer graphene (SLG) and SWCNTs (both metallic and semiconducting) using the nonequilibrium Green's function formalism within the framework of tight-binding density functional theory. Our calculations show that the electronic transport in hybrid nanostructures is affected by the interactions between SWCNT and SLG in comparison to the individual components. The changes in the electronic structure and the transport properties with increasing interaction in hybrids (captured by decreasing the separation between SWCNT and SLG) are discussed, and it is demonstrated from this analysis that the hybrids with semiconducting SWCNTs and metallic SWCNTs show different behavior in the low bias regime while they show similar behavior at higher biases. The difference in the transport properties of hybrids with semiconducting and metallic SWCNTs is explained in terms of changes in the electronic structure, the local density of states, and the energy dispersion for electrons due to the interaction between atoms of the two components.

Half-metallic porphyrin-based molecular junctions for spintronic applications

Single molecular magnets (SMMs) have become promising paradigms to develop novel spintronics for futuristic information technologies such as high-density information and quantum computing. The efficiency and characteristics of SMM devices are determined by the intrinsic nature of the molecular magnets placed in the spin transport pathway. In this work, to understand the role of the central magnetic ion on the performance of SMM devices, we screen the spin-conductance properties of whole $3d$ and $4d$ metalloporphyrins using the nonequilibrium Green's function formalism in conjunction with density functional theory. Our results show that the investigated SMMs according to spintronic conductance behavior can be categorized into three groups: type I non-spin-polarized, type ${\mathrm{II}}^{\ensuremath{'}}$ minor spin-polarized current, and type ${\mathrm{II}}^{\ensuremath{''}}$ major spin-polarized current devices. Type-${\mathrm{II}}^{\ensuremath{'}}$ and type-${\mathrm{II}}^{\ensuremath{''}}$ molecular systems show perfect spin filtering and spin-dependent negative differential resistance. The optimal energy alignment of spin-polarized molecular orbitals with gold electrodes results in one-channel spin transport (minor for type ${\mathrm{II}}^{\ensuremath{'}}$ and major for type ${\mathrm{II}}^{\ensuremath{''}}$). Thus type-II junctions are half-metal. The type-${\mathrm{II}}^{\ensuremath{''}}$ junctions also show a voltage-induced spin switchability at low bias voltages. In this regard, type-II molecular systems are promising candidates for a low-power consumption spin filter, spin switch, memory, to name just a few. Our results highlight the practical applications of metalloporphyrin for the development of multipurpose miniature spintronic devices.

All-electric electron spin resonance studied by means of Floquet quantum master equations

We present a theoretical framework to describe experiments directed to controlling single-atom spin dynamics by electrical means using a scanning tunneling microscope. We propose a simple model consisting of a quantum impurity connected to electrodes where an electrical time-dependent bias is applied. We solve the problem in the limit of weak coupling between the impurity and the electrodes by means of a quantum master equation that is derived by the non-equilibrium Green's function formalism. We show results in two cases. The first case is just a single atomic orbital subjected to a time-dependent electric field, and the second case consists of a single atomic orbital coupled to a second spin-1/2. The first case reproduces the main experimental features Ti atoms on MgO/Ag (100) while the second one directly addresses the experiments on two Ti atoms. These calculations permit us to explore the effect of different parameters on the driving of the atomic spins as well as to reproduce experimental fingerprints.

Controllable spin filtering and half-metallicity in β12 -borophene nanoribbons

The experimental observation of the Dirac fermion states in $\beta_{12}$-borophene sheets and the discovery of their novel topological properties properties have made them a promising candidate for spintronic applications. Here, by combining non-equilibrium Green's function (NEGF) and tight-binding (TB) approximation, we study the charge and spin transport properties through a $\beta_{12}$-borophene nanoribbon (BNR) with the different edge shapes. We show when a BNR exposed to a nonlocal exchange magnetic field, the spin filtering occurs for both spin-up and spin-down so that the spin direction of transmitted electrons could be controlled by adjusting the energy of incoming electrons with the help of an external backgate voltage. It is found that an armchair BNR (ABNR) in the simultaneous presence of a transverse electric field and a nonlocal exchange field indicates a half-metallic nature which is electrically controllable. Moreover, the influence of local exchange field fields is evaluated by exposing the edges of ABNR to ferromagnetic strips with parallel and antiparallel configurations. Our findings show that the edge manipulations in ABNRs lead to the emergence of a giant magnetoresistance and a perfect spin filter. Finally, we studied the effects of edge vacancies and Anderson disorder on the spin-dependent conductance of an ABNR and find that the perfect spin polarization is not destroyed in the presence of Anderson disorder and various single vacancies. Our results reveal the outstanding spin transport properties of ABNRs for future spintronic devices.

Thermal boundary conductance across Co/Cu interfaces with spin–lattice interactions

This work combines first-principles calculations, spin–lattice dynamics, and the non-equilibrium Green’s function (NEGF) method to compute thermal boundary conductance at a three-dimensional Co–Cu interface, considering spin–lattice interactions. Spin–lattice interactions are quantified through exchange interactions between spins, and the exchange constants are obtained from first-principles calculations. Equilibrium molecular dynamics is used to calculate the heat flux across the interface after the spin and lattice subsystems are in equilibrium. Because of the weak interaction between Co and Cu layers adjacent to the interface, spin-wave transmission is low. Spins are scattered by phonons inside the Co contact, and interfacial thermal conductance is reduced. We also compare the results to the NEGF method. Phonon and magnon scattering rates are incorporated into Büttiker probes attached to the device. The NEGF method shows a similar trend in thermal boundary conductance with spins included. Green’s function is solved recursively; therefore, it can be applied to large devices.