Electronic Transmission Coefficient Research Articles

Destructive-quantum-interference suppression in crown ether single molecule junction

The electronic transmission coefficient of X-crown ether-Y (X = 3Y ;Y = 4, 5, and 6) have been investigated using density functional theory and Green’s function approximation incorporated with the Huckel method. The results illustrate unexpected role of the oxygen atoms to highly enhance charge transport in the crown ether molecules by moving the destructive quantum interferences (QI) close to the Fermi level. Such slight shifting creates a beneficial peak-valley pattern in the transmission spectra that facilitates the ON/OFF variation. Moreover, the length of the crown ether rings offers an insignificant impact on electronic transmission. Hence, we believe that these findings would deepen our understanding of QI patterns and exploit crown ether molecules more practically and efficiently in molecular devices.

Interplay between effects of barrier tilting and scatterers within a barrier on tunneling transport of Dirac electrons in graphene

Dirac-electronic tunneling and transport properties with both finite and zero energy band gap are investigated for graphene with an in-plane tilted potential barrier embedded with scatters. For a tilted barrier, by using Wentzel-Kramers-Brillouin approximation, an analytical solution is obtained first for transmission coefficient of Dirac electrons in gapped graphene in the absence of any scatters. In the presence of either a single or a continuous distribution of scatters embedded within a tilted barrier, however, a numerical scheme based on finite-difference approach is developed for accurately calculating both transmission coefficient and tunneling resistance of Dirac electrons. Here, the combination of a tilted barrier and a scatter potential can be viewed as an effective barrier-potential profile facilitated by a proper gate structure. Meanwhile, a full analysis and detailed comparisons are presented for the interplay between effects of both distributed scatters in a barrier and barrier tilting on tunneling transport of Dirac electrons in graphene. The barrier-tilting field and scatter position are found to play a key role in controlling a peak of tunneling resistance as well as in its switching to a cusp by a mid-barrier-embedded scatter as the incident energy reaches the Dirac point in a barrier. Different from a single scatter, a continuous distribution within a barrier can enhance the unimpeded incoherent tunneling for head-on collision while greatly suppressing skew ones with increasing barrier-tilting field. All these predicted attractive transport properties are expected to be extremely useful for designing both novel electronic and optical graphene-based devices and electronic lenses in ballistic-electron optics.

Wave vector filtering effect for electrons in magnetically and electrically confined semiconductor heterostructure

Wave vector filtering effect is explored for electrons in magnetically and electrically confined semiconductor heterostructure, which can be realized experimentally by depositing a ferromagnetic stripe and a Schottky metal stripe in parallel configuration on the surface of [Formula: see text] heterostructure. Adopting improved transfer matrix method to solve Schrödinger equation, electronic transmission coefficient is calculated exactly, and then wave vector filtering efficiency is obtained by differentiating transmission probability over longitudinal wave vector. An obvious wave vector filtering effect appears, due to an essentially two-dimensional process for electron transmission through a magnetic nanostructure. Besides, wave vector filtering efficiency is associated closely with width, position and externally applied voltage of Schottky metal stripe, which makes wave vector filtering effect become controllable and results in a manipulable momentum filter for nanoelectronics.

Signatures of driven magnons in electronic conductance of a magnetic nanowire

In this paper, we study the effect of the driven magnons on the electronic magnetoconductance of an anti-/ferromagnetic quantum wire which is connected to two magnetic chains. The proposed model is based on Green’s function technique within the nearest neighbor tight-binding approach and using a semiclassical approximation for the dynamics of magnetic moments. Due to the existence of the external time-periodic magnetic field, the time-average of the electronic transmission coefficient is presented. This quantity is investigated as a function of external field properties and damping force. The numerical results indicate that the conductance of the magnetic quantum wire has the most decrease at frequencies of external field corresponding to magnonic eigenfrequencies of the center wire. The strong damping forces diminish the effect of the external magnetic field specially at the frequencies corresponding to magnonic eigenfrequencies.

Current–voltage characteristics of β-ketoenamines molecular switches induced by intramolecular hydrogen transfer

Using non-equilibrium Green’s function formalism combined with first-principles density functional theory, we analyzed the transport properties of (Z)-4-aminopent-3-en-2-one (which can be transformed into two enol and keto forms) as molecular switches. In addition to H atom, we obtained the transport properties for CH3 and C2H5 groups at R position. The current–voltage curves, ON–OFF ratio, electronic transmission coefficients, HOMO–LUMO gaps, three different adsorption types (hollow, top, and bridge), and the effect of electrode materials, Y (Y = Au, Ag, and Pt) on electronic transport properties of keto and enol forms of the mentioned molecules are discussed in detail. According to our obtained results, the enol and keto forms are ON and OFF states, respectively.

Electronic transport of organic-inorganic hybrid perovskites from first-principles and machine learning

Using the data-mining machine learning technique and the non-equilibrium Green's function method in combination with density functional theory, we studied the electronic transport properties of the organic-inorganic hybrid perovskite MAPbI3. The band structures of MAPbI3 from first-principles show that the ferroelectric and antiferroelectric dipole configurations have very little influence on the energy bandgap. Furthermore, we investigated the tunnel junctions made of MAPbI3 and 48 different metal electrodes, with the same fixed lattice constant as MAPbI3. With the increase in the number of perovskite unit cells, the electron transmission coefficients are found to decrease exponentially in general. For data mining studies, several different methods are employed to develop models for predicting electron transport properties. In particular, the gradient boosting regression tree model was tested and found to be the most effective tool among all these algorithms for fast prediction of the electron transmission coefficients and performance ranking of all studied metal electrodes.

Thermoelectric properties of polycrystalline graphene nanoribbons

Thermoelectric materials, which can convert heat energy into electric energy and also from electric energy into heat energy, have aroused widespread interest of both theoretical and technological researches recently. Graphene is a typical two-dimensional carbon nanomaterial and regarded as a competitive candidate for the next-generation micro/nano-devices. Unfortunately, graphene is an inefficient thermoelectric material due to the extremely high thermal conductivity. To overcome this drawback, exploring an effective way to improve the thermoelectric performance is of critical importance. In this paper, using the nonequilibrium Green’s function approach, we systematically investigate the effects of grain boundary on the thermoelectric properties of graphene nanoribbons. The results show that owing to the existence of grain boundary, the phonons and electrons encounter great scatterings when they transmit through the polycrystalline graphene nanoribbons. These scatterings cause the phononic and electronic transmission coefficient to decrease dramatically, and thus leading the thermal conductance (including both electron and phonon parts) of graphene nanoribbons to be evidently suppressed. Meanwhile, such scatterings induce more intense transmission peaks and pits in the electronic transmission spectrum of polycrystalline graphene nanoribbons. Generally, the Seebeck coefficient depends on the derivative of electronic transmission coefficient. The larger the logarithmic derivative of transmission, the higher the Seebeck coefficient can be obtained. Therefore Seebeck coefficient is improved obviously in the polycrystalline graphene nanoribbons. Based on such two positive effects, the thermoelectric performance of polycrystalline graphene nanoribbons is significantly enhanced. At room temperature, the thermoelectric figure of merit of polycrystalline graphene nanoribbons can approach to 0.3, which is about 6 times larger than that of pristine graphene nanoribbon (figure of merit is about 0.05). It is also found that the quantity of grain boundaries and length of system can further improve the thermoelectric properties of the polycrystalline graphene nanoribbons, while the width of system has a limited influence on it. This is because the quantity of grain boundaries and length of polycrystalline graphene nanoribbons can give rise to more intense phonon and electron scatterings and further decreasing of thermal conductance and enhancement of Seebeck coefficient. The results presented in this paper demonstrate that polycrystalline structure is indeed an effective way to improve the thermoelectric conversion efficiency of graphene nanoribbons, and provide a theoretical guideline for designing and preparing thermoelectric devices based on graphene nanoribbons.

Contact Electrification by Quantum-Mechanical Tunneling.

A simple model of charge transfer by loss-less quantum-mechanical tunneling between two solids is proposed. The model is applicable to electron transport and contact electrification between e.g. a metal and a dielectric solid. Based on a one-dimensional effective-mass Hamiltonian, the tunneling transmission coefficient of electrons through a barrier from one solid to another solid is calculated analytically. The transport rate (current) of electrons is found using the Tsu-Esaki equation and accounting for different Fermi functions of the two solids. We show that the tunneling dynamics is very sensitive to the vacuum potential versus the two solids conduction-band edges and the thickness of the vacuum gap. The relevant time constants for tunneling and contact electrification, relevant for triboelectricity, can vary over several orders of magnitude when the vacuum gap changes by one order of magnitude, say, 1 Å to 10 Å. Coulomb repulsion between electrons on the left and right material surfaces is accounted for in the tunneling dynamics.

A double gate resonant tunneling transistor scheme based on silicene nanotube

In this paper, we propose a scheme of double gate resonant tunneling field effect transistor based on two gates wrapped on a silicene nanotube. By applying a vertical electric field in the side gates, the energy gap in these regions is opened. Due to different bandgap in these regions and other parts of the nanotube, a quantum well is created in the middle which results in confined energy states in the well region. Hence by applying an electric field via lateral gates, we can change the left and right barrier height simultaneously. Moreover, confined states in the well region are tuned by the voltage applied to the central gate. The transmission coefficient of the incident electrons is calculated using the transfer matrix method. Our theoretical results illustrate that the proposed structure has an oscillatory behavior in the transfer characteristics. On the other hand, output characteristic displays a step-like behavior. We also investigate the influence of the barriers and well thicknesses as well as the perpendicular electric field exerted by lateral gates on the current of the proposed device.

Current states in symmetrical SNINS junction for arbitrary concentrations of nonmagnetic impurities

Equilibrium current states in superconducting junctions of SNINS-type (S – superconductor, N – normal metal, I – insulator) for arbitrary concentrations of nonmagnetic impurities in superconducting regions were investigated using the Ginzburg–Landau theory near the critical temperature. We found the current-phase relation valid in a wide range of electron transmission coefficient values and an arbitrary thickness of the normal layer. The influence of depairing effects was taken into account. A new analytical formula for the dependence of the current on the phase difference was obtained and a number of particular cases were considered.

Current–Voltage Characteristics of the Aziridine-Based Nano-Molecular Wires: a Light-Driven Molecular Switch**Supported by the Damghan University, the Ferdowsi University of Mashhad and the Islamic Azad University of Shahrood.

Using nonequilibrium Green’s function formalism combined first-principles density functional theory, we analyze the transport properties of a 4,4-dimethyl-6-(4-nitrophenyl)-2-phenyl-3,5-diaza-bicyclo[3.1.0]hex-2-ene molecular optical switch. The title molecule can convert between closed and open forms by visible or ultraviolet irradiation. The I–V characteristics, differential conductance, on-off ratio, electronic transmission coefficients, spatial distribution of molecular projected self-consistent Hamiltonian orbitals, HOMO-LUMO gaps, effect of electrode materials Y (111) (Y = Au, Ag and Pt) on electronic transport and different molecular geometries corresponding to the closed and open forms through the molecular device are discussed in detail. Based on the results, as soon as possible the open form translates to the closed form, and there is a switch from the ON state to the OFF state (low resistance switches to high resistance). Theoretical results show that the donor/acceptor substituent plays an important role in the electronic transport of molecular devices. The switching performance can be improved to some extent through suitable donor and acceptor substituents.

Strain-induced semiconductor-to-metal transitions in C36-based carbon peapods: Ab initio study

We present a density functional study of the band structure and some elastic properties of semiconductor carbon nanotube endohedrally doped with the C36 fullerenes. We found that the electronic states belonging to C36s lied inside the band gap of the pristine tube. External strain applied along the tube axis results in decreasing of the semiconducting gap from 0.3 to 0 eV. Metallic behavior of the strained system is confirmed via the additional calculations of electronic transmission coefficients. Phonons influence is accounted with the molecular dynamics approach. Strain of about 0.04 corresponds to the linear part of stress-strain curve and is found to be sufficient to observe the semiconductor-to-metal transitions at room temperature.

Study of electron transport in the functionalized nanotubes and their impact on the electron transfer in the active site of horseradish peroxidase

Electrochemical characterization of functionalized carbon nanotubes (f-CNT) including carboxyl (CNT-COOH), amine (CNT-NH2) and hydroxyl (CNT-OH) functional groups were studied using differential pulse voltammetry (DPV). The current-voltage (I-V) curves were obtained from each system and the effect of f-CNT on redox interaction of horseradish peroxidase (HRP) immobilized on the electrode surface was investigated. The non-equilibrium Green's function (NEGF) combined with density functional theory (DFT) were used to study the transport properties of f-CNT. Additionally, the effect of the number of functional groups on transport properties of CNT, I-V characteristics, electronic transmission coefficients and spatial distribution of f-CNTs have been calculated and analyzed. The results showed that the carboxyl derivative has larger transmission coefficients and current value than other f-CNTs. Then, the effect of functional groups on the electron transport in heme group of HRP is discussed. Finally, the effect of a covalent bond between active site amino acids and amine functional group of CNT was investigated and discussed.

High-temperature thermoelectric transport behavior of the Al/γ-Al2O3 interface: impact of electron and phonon scattering at nanoscale metal-ceramic contacts.

The thermoelectric transport properties of a metal-ceramic interface based on Al and γ-Al2O3 are explored by employing the non-equilibrium Green's function formalism (NEGF) coupled with density functional theory (DFT). However, to acquire the phonon thermal conductance, the parameterized ReaxFF potential is utilized for computing the intrinsic force constants of propagating phonons across the interface. Several interfacial electronic properties such as the charge transfer, the potential barrier, and the atomic orbital overlap are critically analyzed based on the DFT derived results of the electrostatic difference potential, the electron density difference, and the spin-polarized density of states in the fully relaxed structure of the interface. Within the NEGF framework, both the electron and phonon transmission coefficients are estimated for the variations of bias voltage and temperature gradient across the interface. The strong orbital overlap and the scattering of electrons and phonons at the nanometer-size interface suppress the lattice thermal conductivity significantly compared to the electron transport, which in turn enhances the thermoelectric performance of the Al/Al2O3 composite, in contrast to the bulk material of Al. Moreover, a steep rise of power factor induced by the increased transmission of charge carriers with temperature improves the energy conversion efficiency of the material. The present findings could pave the way for developing thermoelectric materials based on metal-ceramic composites.

Impact of Mismatch Angle on Electronic Transport Across Grain Boundaries and Interfaces in 2D Materials

We study the impact of grain boundaries (GB) and misorientation angles between grains on electronic transport in 2-dimensional materials. Here we have developed a numerical model based on the first-principles electronic bandstructure calculations in conjunction with a method which computes electron transmission coefficients from simultaneous conservation of energy and momentum at the interface to essentially evaluate GB/interface resistance in a Landauer formalism. We find that the resistance across graphene GBs vary over a wide range depending on misorientation angles and type of GBs, starting from 53 Ω μm for low-mismatch angles in twin (symmetric) GBs to about 1020 Ω μm for 21° mismatch in tilt (asymmetric) GBs. On the other hand, misorientation angles have weak influence on the resistance across MoS2 GBs, ranging from about 130 Ω μm for low mismatch angles to about 6000 Ω μm for 21°. The interface resistance across graphene-MoS2 heterojunctions also exhibits a strong dependence on misorientation angles with resistance values ranging from about 100 Ω μm for low-mismatch angles in Class-I (symmetric) interfaces to 1015 Ω μm for 14° mismatch in Class-II (asymmetric) interfaces. Overall, symmetric homo/heterojunctions exhibit a weak dependence on misorientation angles, while in MoS2 both symmetric and asymmetric GBs show a gradual dependence on mismatch angles.

First-principles study of the electronic transport properties of a 1,3-diazabicyclo[3.1.0]hex-3-ene molecular optical switch

We analyze the transport properties of 4-(6-(4-chlorophenyl)-4-phenyl-1,3-diaza-bicyclo[3.1.0]hex-3-en-2-yl)-2-nitrophenol molecular optical switch using first-principles calculations. Molecule consisting switch can transform among closed and open forms by visible or ultraviolet irradiation. We have studied multiple attributes such as I–V characteristics, electronic transmission coefficients T(E), the effect of electrode materials (Au, Ag, and Pt) on electronic transport properties, on-off ratio and spatial distribution of molecular projected self-consistent Hamiltonian (MPSH) orbitals corresponding to the closed and open forms. The physical origin of switching behavior is interpreted based on the different molecular geometries, location and size of the frontier molecular orbitals and the HOMO–LUMO gap. According to the theoretical results, one can found that when the open form converts to the closed form, there is a switch from low resistance (on state) to high resistance (off state). We hope that the results of this study can help researchers to design new functional molecular devices.

Monte Carlo simulations of heterogeneous electron transfer: New challenges

We report results of MC simulations of electron transfer across a metal electrode/electrolyte solution interface. The model presumes the Landau–Zener theory and a random walk on a two-dimensional lattice formed by crossing parabolic reaction free energy surfaces along the solvent coordinate. Emphasis is put on investigating the activationless discharge regime; the bridge-assisted electron transfer is also partially addressed. We have calculated effective electronic transmission coefficient as a function of the electrode overpotential and temperature in a wide range of orbital overlap. The dependence of the transmission coefficient on the electronic density of states is analyzed as well.

Plasma sheath: An equivalent nonlinear mirror between electron density and transmitted electromagnetic signal

An experiment on the propagation of electromagnetic (EM) signals in continuous time-varying plasma is designed to establish the nonlinear mirror between electron density and transmission coefficient. The nonlinearity is confirmed from the theoretical and experimental results. The amplitude and phase can be considered nonlinear functions of electron density when the complex interaction between plasma and EM waves is ignored. Results show that amplitude and phase distributions are asymmetrical when electron density follows symmetric distribution. The skewness of amplitude is positive, whereas the skewness of phase is negative. The nonlinear degree is closely related to the ratio of plasma frequency to the incident wave frequency and the range of electron density. The conclusions are crucial to the modeling of plasma sheath channels and understanding the blackout problem.

Supercatalysis by Superexchange

In modern transition state theory, the rate constant for an electron transfer reaction is expressed as the product of four factors: an exponential factor, a pre-exponential factor, an electronic transmission coefficient, and a nuclear transmission coefficient. The activation energy of the reaction manifests inside the exponential factor, and on the conventional view, catalysis occurs by decreasing this activation energy below its catalyst-free value. In the present work we report the discovery of an unusual counter-example in which catalysis occurs by increasing the electron transmission coefficient far above its catalyst-free value. The mechanism involves the formation of a superexchange bridge between an electron donor (a graphite cathode) and an electron acceptor (a pentasulfide ion). The bridge consists of a dz2 orbital inside a cobalt phthalocyanine molecule. The dramatic result is the acceleration of the reduction of pentasulfide ions by more than 5 orders of magnitude compared with the catalyst-fre...

Spectral response, dark current, and noise analyses in resonant tunneling quantum dot infrared photodetectors.

Reduction of dark current at high-temperature operation is a great challenge in conventional quantum dot infrared photodetectors, as the rate of thermal excitations resulting in the dark current increases exponentially with temperature. A resonant tunneling barrier is the best candidate for suppression of dark current, enhancement in signal-to-noise ratio, and selective extraction of different wavelength response. In this paper, we use a physical model developed by the authors recently to design a proper resonant tunneling barrier for quantum infrared photodetectors and to study and analyze the spectral response of these devices. The calculated transmission coefficient of electrons by this model and its dependency on bias voltage are in agreement with experimental results. Furthermore, based on the calculated transmission coefficient, the dark current of a quantum dot infrared photodetector with a resonant tunneling barrier is calculated and compared with the experimental data. The validity of our model is proven through this comparison. Theoretical dark current by our model shows better agreement with the experimental data and is more accurate than the previously developed model. Moreover, noise in the device is calculated. Finally, the effect of different parameters, such as temperature, size of quantum dots, and bias voltage, on the performance of the device is simulated and studied.