Transport studies of $$\hbox {CeO}_{2}$$ molecular device and adsorption behavior of CO on $$\hbox {CeO}_{2}$$ device: a first-principles investigation

Using density functional theory and the non-equilibrium Green’s function formalism, the transport and CO adsorption properties of \(\hbox {CeO}_{2}\) molecular device are studied. The band structure shows that \(\hbox {CeO}_{2}\) nanostructure exhibits semiconducting nature. The electron density is found to be more in oxygen sites rather than in cerium sites along \(\hbox {CeO}_{2}\) nanostructure. The density of states spectrum shows the variation in density of charge upon adsorption of CO on CeO\(_2\) device. The transmission spectrum provides the insights on the transition of charge in \(\hbox {CeO}_{2}\) molecular device upon adsorption of CO along the scattering region. I–V characteristics confirm the adsorption of CO with the variation of current along \(\hbox {CeO}_{2}\) molecular device. The findings show that \(\hbox {CeO}_{2}\) two probe molecular device can be efficiently used for CO detection in the atmosphere.     

Mobility Variations in Ultra Small Devices due to Random Discrete Dopants

Scaling of conventional MOSFETs to channel lengths of 20 nm and below will require channel doping in the range of 10¹⁹ cm^?3. At such doping concentrations ionised impurity scattering may start to dominate over interface roughness scattering within the channel. Additionally, discrete doping variations, both in number and position, will become increasingly important in such small devices resulting in mobility variations from device to device. Such mobility variations will be in addition to the random dopant induced fluctuations due to the electrostatics. Here we report results from 3-D Ensemble Monte Carlo simulations that include ab-initio ionised impurity scattering, and has been developed to investigate mobility variations in small volumes comparable to the channel region of nano-scale MOSFETs.     

First-principles versus semi-empirical modeling of global and local electronic transport properties of graphene nanopore-based sensors for DNA sequencing

Using first-principles quantum transport simulations, based on the nonequilibrium Green function formalism combined with density functional theory (NEGF+DFT), we examine changes in the total and local electronic currents within the plane of graphene nanoribbon with zigzag edges (ZGNR) hosting a nanopore which are induced by inserting a DNA nucleobase into the pore. We find a sizable change of the zero-bias conductance of two-terminal ZGNR + nanopore device after the nucleobase is placed into the most probable position (according to molecular dynamics trajectories) inside the nanopore of a small diameter \(D=1.2\) nm. Although such effect decreases as the nanopore size is increased to \(D=1.7\) nm, the contrast between currents in ZGNR + nanopore and ZGNR + nanopore + nucleobase systems can be enhanced by applying a small bias voltage \(V_b \lesssim 0.1\) V. This is explained microscopically as being due to DNA nucleobase-induced modification of spatial profile of local current density around the edges of ZGNR. We repeat the same analysis using NEGF combined with self-consistent charge density functional tight-binding (NEGF+SCC-DFTB) or self-consistent extended Hückel (NEGF+SC-EH) semi-empirical methodologies. The large discrepancy we find between the results obtained from NEGF+DFT vs. those obtained from NEGF+SCC-DFTB or NEGF+SC-EH approaches could be of great importance when selecting proper computational algorithms for in silico design of optimal nanoelectronic sensors for rapid DNA sequencing.     

Transport in silicon nanowires: role of radial dopant profile

We consider the electronic transport properties of phosphorus (P) doped silicon nanowires (SiNWs). By combining ab initio density functional theory (DFT) calculations with a recursive Green’s function method, we calculate the conductance distribution of up to 200 nm long SiNWs with different distributions of P dopant impurities. We find that the radial distribution of the dopants influences the conductance properties significantly: surface doped wires have longer mean-free paths and smaller sample-to-sample fluctuations in the cross-over from ballistic to diffusive transport. These findings can be quantitatively predicted in terms of the scattering properties of the single dopant atoms, implying that relatively simple calculations are sufficient in practical device modeling.     

First principles modeling of disorder scattering in graphene

It is important to investigate impurity scattering phenomena when modeling graphene nanoscale devices, as impurities are invariably present in any realistic system and can significantly influence graphene carrier transport. We present a short review of quantum transport where density functional theory (DFT) is carried out within the nonequilibrium Green’s function formalism (NEGF), focusing on a recent extension of this framework in the form of nonequilibrium vertex correction (NVC) that captures random graphene impurity scattering in a systematic fashion. Our results show that disorder effects significantly alters the electronic and transport properties of graphene devices. We argue that disorder effects should not be ignored if one were to model graphene nanoscale devices in realistic situations, including arriving at fundamental electronic properties such as Ohm’s law.     

Band structure effect on the electron current oscillation in ultra-scaled GaSb Schottky MOSFET: tight-binding approach

This paper explores band structure effect on the quantum transport of a low-dimensional GaSb Schottky MOSFET (SBFET) for the implementation of III–V transistor with a low series resistance. Precise treatment of the full band structure is employed applying sp ³ d ⁵ s ^? tight-binding (TB) formalism. A remarkable distinction between the thickness dependent effective masses extracted from the TB and the bulk values imply that the quantum confinement modifies the device performance. Strong transverse confinement leads to the effective Schottky barrier height increment. Owing to the adequate enhanced Schottky barriers at low drain voltages, a double barrier gate modulated potential well is formed along the channel. The double barrier profile creates a longitudinal quantum confinement and induces drain current oscillation at low temperatures. Significant factors that may affect the current oscillation are thoroughly investigated. Current oscillation is gradually smoothed out as the gate length shrinks down in ultra scaled structure. The results in this paper are paving a way to clarify the feasibility of this device in nanoscale regime.     

A Quantum Mechanical Approach for the Simulation of Si/SiO2 Interface Roughness Scattering in Silicon Nanowire Transistors

In this work, we present a quantum mechanical approach for the simulation of Si/SiO₂ interface roughness scattering in silicon nanowire transistors (SNWTs). The simulation domain is discretized with a three-dimensional (3D) finite element mesh, and the microscopic structure of the Si/SiO₂ interface roughness is directly implemented. The 3D Schrödinger equation with open boundary conditions is solved by the non-equilibrium Green’s function method together with the coupled mode space approach. The 3D electrostatics in the device is rigorously treated by solving a 3D Poisson equation with the finite element method. Although we mainly focus on computational techniques in this paper, the physics of SRS in SNWTs and its impact on the device characteristics are also briefly discussed.     

Theoretical Evidence of Spontaneous Spin Polarization in GaAs/AlGaAs Split-Gate Heterostructures

The spontaneous spin polarization of a quantum point contact (QPC) formed by the lateral confinement of a high-mobility two-dimensional electron gas in a GaAs/AlGaAs heterostructure is investigated. We present self consistent calculations of the electronic structure of the QPC using the spin-polarized density functional formalism of Kohn and Sham. Spin polarization occurs at low electron densities and exchange potential is found to be the dominant mechanism driving the local polarization within the QPC. We compute the conductance using the cascading scattering matrix approach and observe the conductance anomaly at ∼ 0.7 (2e²/h).     

Dissipative transport in CNTFETs

Based on the non-equilibrium Green’s function formalism the performance of carbon nanotube field-effect transistors has been studied. The effects of elastic scattering and the impact of parameters of inelastic scattering, such as electron-phonon coupling strength and phonon energy, on the device performance are analyzed.     

A new F(ast)-CMS NEGF algorithm for efficient 3D simulations of switching characteristics enhancement in constricted tunnel barrier silicon nanowire MuGFETs

In this paper, we present 3D quantum simulations based on Non-Equilibrium Green’s Function (NEGF) formalism using the Comsol Multiphysics^? software and on the implementation of a new Fast Coupled Mode-Space (FCMS) approach. The FCMS algorithm allows one to simulate transport in nanostructures presenting discontinuities, as the normal Coupled Mode-Space (CMS) algorithm does, but with the speed of a Fast Uncoupled-Mode Space (FUMS) algorithm (a faster algorithm that cannot handle discontinuities). We then use this new algorithm to explore the effect of local constrictions on the performance of nanowire MultiGate Field-Effect Transistors (MuGFETs). We show that cross-section variations in a nanowire result in the formation of energy barriers that can be used to improve the on/off current ratio and switching characteristics of transistors: (1) A small constriction resulting in a barrier of the order of a 0.1 eV can be used as an effective means to improve the subthreshold slope and minimize the on/off current ratio degradation resulting from SD tunneling in ultra scaled transistor, and (2) We also report a new variable barrier transistor (VBT) device concept that is able to achieve sub-kT/q subthreshold slope without using impact ionization or band-to-band tunneling. Intra-band tunneling through constriction barriers is used instead. The device is, therefore, fully symmetrical and can operate at very low supply voltages. A subthreshold slope as low as 56.5 mV/decade is reported at T=300 K. The VBT reported here breaks the 60 mV/dec barrier over more than five decades of subthreshold current, which is the widest current range reported so far.     

Oscillator strengths of the intersubband electronic transitions in the multi-layered nano-antidots with hydrogenic impurity

In this study, we have obtained the exact solutions of the Schr?dinger equation for a multi-layered quantum antidot (MLQAD) within the effective mass approximation and dielectric continuum model for the spherical symmetry. The MLQAD is nano-structured semiconductor system that consists of a spherical core (e.g. Ga_1?x Al _x As) and a coated spherical shell (e.g. Ga_1?y Al _y As) as the whole anti-dot is embedded inside a bulk material (e.g. GaAs). The dependence of the electron energy spectrum and its radial probability density on nano-system radius are studied. The numeric calculations and analysis of oscillator strength of intersubband quantum transition from the ground state into two first allowed excited states at the varying radius, for both the finite and infinite confining potential (CP) as well as constant shell thickness, are performed. It is shown that, in particular, the binding energy and the oscillator strength of the hydrogenic impurity of a MLQAD behave differently from that of a single-layered quantum antidot (SLQAD). For a MLQAD with finite core and shell CPs, the state energies and the oscillator strengths of the impurity are found to be dependent on the shell thickness. At the large core radius and very small shell thickness, our results are closer to respective values for a SLQAD that previously reported.     

Quantum kinetics approach to calculation of the low field mobility in the hole inversion layers of silicon MOSFET’s

Analytic expressions for low field mobility have been obtained in the quantized p-type inversion layers. The confining potential is approximated by a triangular quantum well. Main attention is paid to study the dependence of the hole mobility on transverse effective field at different temperatures and concentrations of the ionized impurities. Acoustic and optical phonons, charged impurities, and surface roughness have been adopted as scattering system. Theoretical considerations are based on the quantum kinetic equation and special form of the non-equilibrium distribution function (shifted Fermi distribution). Calculations show that the acoustic phonon limited mobility does not depend on the transverse effective electrical field \(E_\mathrm{eff} \) and has a temperature dependence closer to experiment than known expression for the universal mobility. At the same time, the mobility limited by scattering with optical phonons and surface roughness is proportional to \(E_\mathrm{eff} ^{-1/3}\) and \(E_\mathrm{eff} ^{-2}\), respectively. The mobility limited by scattering by ionized impurities is a weak function of the transverse effective field. Results of the calculations are compared with known experimental data.     

Investigating the behavior of impurities in a multicomponent heat carrier in the flow path of a geothermal power station

Results are presented from experimental and theoretical investigations into the behavior of a heat carrier in the flow path of geothermal power stations and the buildup of corrosive impurities in the liquid phase during steam expansion with crossing of the saturation line x = 1. Data are presented on the improvement in the reliability and efficiency of geothermal power stations when octadecylamine is added to the station’s flow path.     

复杂土壤结构对水电站接地装置散流机理影响分析

准确计算复杂土壤结构中接地装置接地参数及其散流机理是水电站接地装置合理优化设计的基础。提出考虑大范围复杂土壤结构的接地装置有限元计算方法,针对有限元数值计算方法中大范围求解区域和接地导体截面尺寸的差异导致的计算量过大问题,采用二维接地导体与三维散流土壤耦合的有限元数值计算方法;基于有限元模型分别建立垂直三层土壤模型、块状土壤模型以及落差土壤模型,对接地装置散流过程中土壤中的电流密度、电场强度分布规律进行了详尽分析;并对河道两岸土壤电阻率、河水深度对水电站接地网散流过程及接地电阻的影响规律进行了定量分析。对比分析了三种不同土壤模型对水电站接地网散流过程及接地电阻的影响规律,结果表明河水深度是影响接地装置散流过程的重要因素,复杂土壤结构中水电站接地网设计中宜采用考虑实际河水深度的块状土壤模型和落差土壤模型。     

Atomistic deconstruction of current flow in graphene based hetero-junctions

We describe the numerical modeling of current flow in graphene heterojunctions, within the Keldysh Landauer Non-equilibrium Green’s function (NEGF) formalism. By implementing a k-space approach along the transverse modes, coupled with partial matrix inversion using the Recursive Green’s function Algorithm (RGFA), we can simulate on an atomistic scale current flow across devices approaching experimental dimensions. We use the numerical platform to deconstruct current flow in graphene, compare with experimental results on conductance, conductivity and quantum Hall, and deconstruct the physics of electron ‘optics’ and pseudospintronics in graphene pn junctions. We also demonstrate how to impose exact open boundary conditions along the edges to minimize spurious edge reflections.     

Self-consistent time-dependent boundary conditions for static and dynamic simulations of small electron devices

A quasi-analytical self-consistent and time-dependent boundary conditions algorithm for micro- and nanoscale electron transport simulators is presented and discussed. The algorithm is the result of imposing overall charge neutrality and current conservation along the reservoirs, the leads and the active region. Only an explicit numerical simulation of the active region is required. By means of analytical solutions of the spatial distribution for the charge density, electric field and potential energy along the leads, the algorithm is able to self-consistently translate standard “metallic” boundary conditions in the reservoirs into intricate constraints at the borders of the active region. The algorithm is robust, requires small computational effort and it is suitable for any (classical or quantum) electronic device simulator for stationary (DC) and dynamic (transients, noise and AC) regimes up to several THz. The algorithm is specially welcomed for dynamical regime simulations, where the predictions of the time-evolution of the electrostatic potential, electric field and charge density at the borders of the active region are rather complicated.     

Random phonon model of dissipative electron transport in nanowire MOSFETs

A method for quantum transport simulations of nanowire (NW) field-effect transistors (FETs) with inelastic electron–phonon scattering processes incorporated is presented in this paper. The microscopic device Hamiltonian with realistic phonon spectrum and electron–phonon interaction is transformed into an equivalent low-dimensional transport model with discrete random phonon modes. The electron–phonon coupling constants are optimized in order to reproduce the inelastic scattering effects. Small size of the model and special form of the inelastic self-energy terms in the NEGF formalism make it a powerful tool to study dissipative transport in realistic NW transistors. The utility of the method is demonstrated by computing inelastic transport characteristics in Si NW FETs.     

Detection of volatile impurities in turbine oils by the heat-pulse testing method

The research is aimed at development and implementation of methods and devices to control critical sections of the oil system of the power equipment that operates in the real time mode. The task was to develop a method for rapid detection of volatile impurities in turbine oils. The approach to the study is based on quantitative assessment of the short-term thermal stability of the substance that is formally associated with the content of the volatile impurity. The approach was selected on the basis of the results of search experiments taking into consideration the formulation of requirements for the method and the device, viz., (1) the method should reliably determine the moisture content in the range of 10–150 g of the impurity per ton of oil and (2) the device is to be applicable “in situ.” For this purpose, a variant of the method of the controlled pulse heating of a wire probe, a resistance thermometer, has been developed. The advantages of the method are its speed, sensitivity to small contents of volatile impurities regardless of the nature of the impurity, and smallness of methodologically contributed perturbation. The heating conditions of the probe most sensitive to the appearance of moisture— including its trace amounts—in the system, has been defined. The duration of the measurement is on the order of milliseconds; the heat flux density through the surface of the probe reaches 1 MW/m². The essence of the method consists in measuring, in the characteristic time interval, the temperature of the thermal instability onset associated with the content of the volatile impurity. The approach proposed by the authors is aimed at increasing the lifetime of the oil and preventing unpredictable failures of the operating equipment.     

Computational study of transport properties of graphene upon adsorption of an amino acid: importance of including –$$\hbox {NH}_{2}$$ and –COOH groups

The effects of histidine and its imidazole ring adsorption on the electronic transport properties of graphene were investigated by first-principles calculations within a combination of density functional theory and non-equilibrium Greens functions. Firstly, we report adsorption energies, adsorption distances, and equilibrium geometrical configurations with no bias voltage applied. Secondly, we model a device for the transport properties study: a central scattering region consisting of a finite graphene sheet with the adsorbed molecule sandwiched between semi-infinite source (left) and drain (right) graphene electrode regions. The electronic density, electrical current, and electronic transmission were calculated as a function of an applied bias voltage. Studying the adsorption of the two systems, i.e., the histidine and its imidazole ring, allowed us to evaluate the importance of including the carboxyl (–COOH) and amine (–\(\hbox {NH}_{2}\)) groups. We found that the histidine and the imidazole ring affects differently the electronic transport through the graphene sheet, posing the possibility of graphene-based sensors with an interesting sensibility and specificity.