Electronic Transmission Coefficient Research Articles

Quantum transport of electrons through axially symmetric junctions of zigzag and armchair nanotubes

An analytic approach is developed in the nearest neighbor approximation for describing electron transport through axially symmetric junctions of zigzag and armchair nanotubes. This method yields an analytic expression for the electron transmission probability and can be used to establish selection rules for electron scattering. Oscillations in the gap width in the electron transmission spectrum are predicted as a function of the nanotube diameter and the minimum gap size is determined. The thresholds for the appearance of steps in the electron transmission coefficient are also found and it is shown that, except in its initial section, the origin of the steps is associated with the electronic characteristics of armchair nanotubes, alone.

First-principles scheme for spectral adjustment in nanoscale transport

We implement a general method for correcting the low-bias transport properties of nanoscale systems within an ab initio methodology based on linear combinations of atomic orbitals. This method consists of adjusting the molecular spectrum, i.e. shifting the position of the occupied and unoccupied molecular orbitals to match the experimental highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO (HL)) gap. Thus we show how the typical problem of an underestimated HL gap can be corrected, leading to quantitative and qualitative agreement with experiments. We show that an alternative method based on calculating the position of the relevant transport resonances and fitting them to Lorentzians can significantly underestimate the conductance and does not accurately reproduce the electron transmission coefficient between resonances. We compare this simple method in an ideal system of a benzene molecule coupled to featureless leads to more sophisticated approaches, such as GW, and find rather good agreement between both. We also present the results of a benzenedithiolate molecule between gold leads, where we study different coupling configurations for straight and tilted molecules, and show that this method yields the observed evolution of two-dimensional conductance histograms. We also explain the presence of low-conductance zones in such histograms by taking into account different coupling configurations.

Sodium dodecyl sulfate sensitized electrochemical method for subnanomole level determination of ortho-phenylphenol at a novel disposable electrode

The electrochemical behavior of ortho-phenylphenol (OPP) at a disposable electrode (an improved wax-impregnated graphite electrode) in the presence of sodium dodecyl sulfate (SDS) was studied for the first time. The results demonstrated that the electrocatalytic oxidation process of OPP was accompanied with two-charge-two-proton transference. The electronic transmission coefficient (α) and diffusion coefficient (DR) for OPP were calculated to be 0.8126 and 3.61 × 10−2 cm2/s, respectively. The electrochemical signal was apparently improved by SDS at the disposable electrode and the oxidative peaks current was proportional to the concentration of OPP over the range from 1.0 × 10−9 to 4.0 × 10−6 mol/L with the detection limit of 8.7 × 10−10 mol/L. This novel and highly sensitive method can be successfully applied to detect OPP in the orange rind sample.

Quantum interference in single molecule electronic systems

We present a general analytical formula and an ab initio study of quantum interference in multi-branch molecules. Ab initio calculations are used to investigate quantum interference in a benzene-1,2-dithiolate (BDT) molecule sandwiched between gold electrodes and through oligoynes of various lengths. We show that when a point charge is located in the plane of a BDT molecule and its position varied, the electrical conductance exhibits a clear interference effect, whereas when the charge approaches a BDT molecule along a line normal to the plane of the molecule and passing through the centre of the phenyl ring, interference effects are negligible. In the case of olygoynes, quantum interference leads to the appearance of a critical energy $E_c$, at which the electron transmission coefficient $T(E)$ of chains with even or odd numbers of atoms is independent of length. To illustrate the underlying physics, we derive a general analytical formula for electron transport through multi-branch structures and demonstrate the versatility of the formula by comparing it with the above ab-initio simulations. We also employ the analytical formula to investigate the current inside the molecule and demonstrate that large counter currents can occur within a ring-like molecule such as BDT, when the point charge is located in the plane of the molecule. The formula can be used to describe quantum interference and Fano resonances in structures with branches containing arbitrary elastic scattering regions connected to nodal sites.

A Time-Dependent Approach to Electronic Transmission in Model Molecular Junctions

We present a simple method to compute the transmission coefficient of a quantum system embedded between two conducting electrodes. Starting from the solution of the time-dependent Schrodinger equation, we demonstrate the relationship between the temporal evolution of the state vector, |ψ(t)>, initially localized on one electrode and the electronic transmission coefficient, T(E). We particularly emphasize the role of the oscillation frequency and the decay rate of |ψ(t)> in the line shape of T(E). This method is applied to the well-known problems of the single impurity, two-site systems and the benzene ring, where it agrees with well-accepted time-independent methods and gives new physical insight to the resonance and interference patterns widely observed in molecular junctions.

Bond breaking electron transfer across a conducting nanowire(nanotube)/electrolyte solution interface: The role of electrical double layer effects

The bond breaking reduction of a peroxodisulphate anion at charged conducting cylinders of nanosize (which mimic metal wires or carbon tubes) in contact with an electrolyte solution is explored in the framework of a quantum mechanical theory and DFT calculations. As example, the electronic transmission coefficients are calculated for Ag(111) surface and silver nanowires of different size. The electron transfer is considered in adiabatic limit; current–voltage curves are computed for both conducting nanocylinders and a plain metal electrode, which is chosen as a reference system. It is argued that the electrical double layer effects play a crucial role and entail a noticeable rise of current at nanowires, as compared with a plain metal electrode. A “pit” on the model current–voltage dependencies was observed to disappear starting from a certain value of the radius of a nanocylinder.

Photoemission mechanism of GaN vacuum surface electron source

GaN photocathode is fully activated by employing a continuous Cs source and an alternate O source. The quantum efficiency curve of transmission-mode photocathode is tested in situ. The quantum efficiency reaches up to 13% in transmission-mode. According to the one-dimensional Schrdinger equation, the electron transmission coefficient formula of GaN vacuum electron source material is deduced. For a certain profile of photocathode surface potential barrier, the electron transmission coefficient relates to the incident electron energy, the height and the width of the surface potential. The energy band of transmission-mode negative electron affinity (NEA) GaN photocathode and the change of surface barrier in the deposit course of Cs,O are given. Using the double dipole layer surface model [CaN(Mg):Cs]:O-Cs, the NEA property formation of GaN vacuum electron source material is analyzed. The results show that the double dipole layer formed in the activation course of Cs,O is conducible to the escape of electrons, and it is the formation of double dipole layer that causes the drop of vacuum energy level of the material surface.

Transmission coefficient through a saddle-point electrostatic potential for graphene in the quantum Hall regime

From the scattering of semicoherent-state wavepackets at high magnetic field, we derive analytically the transmission coefficient of electrons in graphene in the quantum Hall regime through a smooth constriction described by a quadratic saddle-point electrostatic potential. We find anomalous half-quantized conductance steps that are rounded by a backscattering amplitude related to the curvature of the potential. Furthermore, the conductance in graphene breaks particle-hole symmetry in cases where the saddle-point potential is itself asymmetric in space. These results have implications both for the interpretation of split-gate transport experiments, and for the derivation of quantum percolation models for graphene.

Electron conductance in curved quantum structures

A differential-geometry analysis is employed to investigate the transmission of electrons through a curved quantum-wire structure. Although the problem is a three-dimensional spatial problem, the Schrödinger equation can be separated into three general coordinates. Hence, the proposed method is computationally fast and provides direct (geometrical) parameter insight as regards the determination of the electron transmission coefficient. We present, as a case study, calculations of the electron conductivity of a helically shaped quantum-wire structure and discuss the influence of the quantum-wire centerline radius of curvature and pitch length for the conductivity versus the chemical potential.

Determination of the coefficient of electron transmission using temperature measurements

The problem of determining the coefficient of electron transmission through the potential barrier at the metal-vacuum interface is considered. The temperature dependence of the thermofield’s current density is studied using methods of mathematical modeling. The problem is reduced to the solution of a system of algebraic equations. It is shown that regularization of the solution of the problem is required. The results demonstrate that the coefficient of electron transmission can be determined in principle.

Microscopic mechanism of leakage currents in silica junctions

Combining the nonequilibrium Green’s functions with the density-functional theory, we investigated the structural and electronic properties of silica junctions sandwiched between Al electrodes. The results show that the oxygen vacancies and tensile strain field play an important role in the electron transport properties of these two-probe systems. Sizable changes in leakage current across the barrier are found for the oxygen deficient system. It is found that Si dangling bonds formed by the introduction of oxygen vacancies are the main building blocks of the conduction channel in silica thin film. The midband gap states generated by the Si dangling bonds contribute to the leakage current. Detail analysis shows that four conduction channels are generated in silica junction after the presence of oxygen vacancies, resulting in a large enhancement of the electron transmission coefficient at the Fermi level. This leakage current mechanism provides useful information in the microelectronic designs.

Toward the Reactivity Prediction: Outersphere Electroreduction of Transition-Metal Ammine Complexes

The electrochemical reactivity of Co(III), Cr(III), and Ru(III) ammine complexes is discussed within the framework of a quantum chemical approach. The electronic structure of oxidized and reduced complex forms is addressed at the density functional theory level; intramolecular and solvent contributions to the Franck−Condon barrier, as well as electronic transmission coefficients are computed. Standard electrode potentials are estimated for certain redox pairs. The charge distribution in the reactants is employed to calculate electrostatic interactions with a mercury electrode surface and to make double layer corrections more realistic as compared to point/spherical reactant models. The model predictions are employed to explain the rate constant and transfer coefficient. Also discussed are trends in the available experimental data, including new original results on a comparative study of Co(III) and Cr(III) hexaammine complexes.

Properties of spin transport of double quantum rings with structures of ferromagnet/ semiconductor/ ferromagnet

Spin-dependent transport through ferromagnetic/semiconductor/ferromagnetic double quantum rings is studied in this paper. It is found that the average value of the spin-dependent electron transmission coefficient of the double quantum ring is larger than that of the single quantum ring under the condition of zero magnetic flux and antiparallel configration of the ferromagnetic electrodes. When the magnetization directions of the ferromagnetic electrodes are parallel, the average tunneling coefficient of the spin-down electrons in double quantum rings increases distinctly. When the Rashba spin-orbit coupling is considered, the average tunneling coefficient of the spin electrons in double quantum rings is bigger than that in single quantum ring. The applied magnetic field enhences the tunneling coefficient. The δ barrier of the double quantum rings suspresses the tunneling of the electron. The tunneling coefficient decreases monotonically and nonlinearly with the δ barrier strength Z increasing.

Charge-exchange processes involving C60 fullerenes and their ions

We report the results of calculations of charge-exchange processes in collisions involving fullerenes and their ions. For charge-exchange processes in collisions between fullerenes and multiply charged ions, we develop a semiclassical approach based on the decay model combined with the impact-parameter representation for the heavy-particle relative motion. In this model, the charge-transfer process is considered as a transition of the active electron over and under the quasistatic potential barrier formed by the electric fields of the target and projectile. In all our calculations, we represent a fullerene as a perfectly conducting hard sphere, and the energies of the active electrons are assumed to be equal to the corresponding ionization potentials, including the Stark-shift effect. Using the phase-shift approach we evaluate the electron transmission coefficient through the asymmetric potential barrier. It is shown that our theory is in fairly good agreement with the available experimental data on single-electron charge-exchange processes. The advantage of the approach developed consists in the fact that it allows us to provide an accurate description of multielectron-transfer processes. Special attention in our work is also paid to the consideration of charge-exchange processes in fullerene–fullerene collisions. Here we show that the cross sections of charge-transfer processes in collision systems of C60 + + C60 2+ and C60 + C60 + turn out to be close to each other.

3D real-space quantum transport simulation of nanowire MOS transistors: Influence of the ionized doping impurity

We report a numerical study of both donor- and acceptor doping impurity effects in the quantum transport of silicon nanowire metal-oxide-semiconductor (MOS) transistors. The code is based on a full three-dimensional (3D) real-space non-equilibrium green function (NEGF) formalism self-consistently coupled with the 3D Poisson equation. The general results show that the influence of an impurity strongly depends on its type. Indeed, an acceptor or a donor will create a repulsive or attractive potential, giving rise to tunneling effect or resonances, respectively. Our calculations analyze the impact on electron density, transmission coefficient and drain current (ID) which undergoes variations up to 50%. This pinpoints the importance of intrinsic fluctuations due to doping in ultimate nano-transistors whose magnitude cannot be neglected in the next generations of integrated circuits.

A critical look at electric field emission of electrons from metallic surfaces

A critical look at the commonly used Fowler's expression for the transmission coefficient of electrons through the potential barrier leads to the conclusion that this expressions is physically untenable. An expression for the field emission current density, corresponding to the expression for the transmission coefficient obtained from the exact solution of Schrödinger’s equation, has been derived. Numerical results have been expressed in the form of tables and graphs and discussed. It is seen that in the range of 10−4<kT/EF<10−3, the effect of temperature on the field emission current density is negligible.

Charge pumping and noise in a one-dimensional wire with weak electron-electron interactions

We consider the adiabatic pumping of charge through a mesoscopic one-dimensional wire in the presence of electron-electron interactions. A model of static potential in-between two-delta potentials is used to obtain exactly the scattering matrix elements, which are renormalized by the interactions. Two periodic drives, shifted one from another, are applied at two locations of the wire in order to drive a current through it in the absence of bias. Analytical expressions are obtained for the pumped charge, current noise, and Fano factor in different regimes. This allows us to explore pumping for the whole parameter range of pumping strengths. We show that working close to a resonance is necessary to have a comfortable window of pumping amplitudes where charge quantization is close to the optimum value; a single electron charge is transferred in one cycle. Interactions can improve the situation, the charge $Q$ is closer to one-electron charge, and noise is reduced, following a $Q(e\ensuremath{-}Q)$ behavior, reminiscent of the reduction in noise in quantum wires by $T(1\ensuremath{-}T)$, where $T$ is the electron transmission coefficient. For large pumping amplitudes, this charge vanishes, and noise also decreases but slower than the charge.

On the theory of one-dimensional spin-magnetic scattering of electrons

We consider a heterostructure, the potential profile of which consists of two magnetic barriers with noncollinear magnetizations separated by a nonmagnetic quantum well. It is shown that the transmission coefficient of a nonspin-polarized electron is modulated and the angle between the magnetization vectors of barriers is the modulation parameter.

Medium and Interfacial Effects in the Multistep Reduction of Binuclear Complexes with Robson-Type Ligand

We present a combined experimental and computational approach to the modeling and prediction of reactivity in multistep processes of heterogeneous electron transfer. The approach is illustrated by the study of a Robson-type binuclear complex (-Cu(II)-Cu(II)-) undergoing four-electron reduction in aqueous media and water-acetonitrile mixtures. The observed effects of solvent, pH, buffer capacity, and supporting electrolyte are discussed in the framework of a general reaction scheme involving two main routes; one of them includes protonation of intermediate species. The main three problems are addressed on the basis of modern charge transfer theory: (1) the effect of the nature of reactant and intermediate species (protonated/deprotonated, bare or associated with supporting anion/solvent molecule) on the standard redox potential, the electronic transmission coefficient, and the intramolecular reorganization; (2) possible effect of protonation on the shape of the reaction free energy surfaces which are built using the Anderson Hamiltonian; (3) electron transfer across an adsorbed chloride anion. Quantum chemical calculations were performed at the density functional theory level.

Charge transfer between fullerenes and highly charged noble gas ions

A semiclassical model for the description of charge-exchange processes in collisions between fullerenes and multiply charged ions is developed. It is based on the decay model combined with the impact-parameter representation for the heavy particles' relative motion. The charge-transfer process in our model is treated as a transition of the active electron over and under the quasistatic potential barrier formed by the electric fields of the target and projectile. Due to the high electron delocalization on the surface of fullerene we represent it as a perfectly conducting hard sphere, whose radius is determined by the dipole polarizability of C60. The energies of the active electrons are assumed to be equal to the corresponding ionization potentials including the Stark-shift effect. We have developed an efficient technique for the evaluation of the electron transmission coefficient through the asymmetric potential barrier. It is shown that our model provides a good agreement with the available experimental data on single-electron charge-exchange processes. Moreover, it allows us to get an adequate description of multi-electron transfer processes. The first theoretical results on charge exchange between the fullerene ions and highly charged ions have been obtained.