Two-probe Systems Research Articles

Substitutional effects of Ni on thermoelectric properties of cobalt spinels to yield more than 10-fold increase in the thermoelectric figure of merit

The low thermoelectric figure of merit, ZT, is severely limiting the wide practical applications of thermoelectric materials. In this research theoretical calculations were performed on Ni substituted cobalt oxide (Co3O4) semiconductor materials to identify the qualitative trend of thermoelectric properties due to substitutions. Ni substitutions were made in the tetrahedral sites of 56 atom Co3O4 super cell. With the increasing Ni substitution, the band gap near the Fermi region started to close, from 0.337 eV for pure Co3O4 to 0 eV with Ni+2 substitutions in the tetrahedral sites of Co3O4, thereby showing an increasing metallic behavior which is also evident from the electrical conductance calculations. To calculate band structure, Seebeck coefficient, electrical conductance and thermal conductance due to electrons, first principles based density functional theory, DFT, combined with non-equilibrium Green's function method for two-probe systems were used. To calculate the lattice thermal conductance, reverse non-equilibrium molecular dynamics was used. The calculated Seebeck coefficients were 720 μV/K for pure Co3O4 and decreased exponentially to 35 μV/K for Co2NiO4, where the qualitative trend aligned very well with the available experimental data for Ni substituted cobalt oxide materials. These calculated properties assisted in calculating the theoretical thermoelectric figure of merit, and to qualitatively assess the effect of doping on the overall performance. Improved thermoelectric properties such as low thermal conductance and high Seebeck coefficient for low Nickel substitutions (12.5% of tetrahedral sites represented with Ni0·125Co2·875O4), was observed which yielded a 10-fold increase in thermoelectric figure of merit, ZT, in comparison to pure Co3O4 at 300 K. Additionally, the effect of Nickel substitutions on the p-type behavior was presented and discussed in this research article.

Designed Graphane-based spin filters by tuning the sp2/sp3 configuration

The electronic transport properties of H-vacancy-defected graphane are investigated by using the density functional theory (DFT) and Non-equilibrium Green’s Function (NEGF) methods. Results show that the band gap of graphane depends on the relative positions of the defect bands introduced by H vacancies and the valence or conduction bands, suggesting the band gap controllability. The defect bands are obviously spin polarized and the ratio is almost 100%. And the magnetization of the system can be tuned by defect concentration and location. Moreover, in the two-probe systems, the presence of H-vacancy-defect bands near the Ef plays an important role in controlling spin-dependent transport of graphane. A perfect spin-filtering effect with 100% spin polarization could be realized by H-vacancy defects. The spin properties indicate the potential of graphane as an option for future graphene-based spin filters and spintronic devices.

A Fast and Efficient Calibration Method for Phased Array Antennas Using Fourier-Structured Excitation Matrix

This article presents a novel phased array antenna calibration method. Unlike conventional phased array calibration methods equipped with only one probe antenna in the far-field setup, multiple measurement probes are introduced in the proposed algorithm to significantly improve the measurement efficiency. Specifically, a novel construction method for an excitation matrix (which contains excitation coefficients for phased array elements) is proposed, where phase differences introduced by the spatial locations of the multiple probes are perfectly incorporated into the Fourier-structured excitation matrix, thereby presenting the best measurement efficiency without sacrificing the array calibration accuracy. The proposed method can offer robust array calibration results (guaranteed by the low condition number of the constructed excitation matrix with Fourier structure) and high measurement efficiency (offered by multiprobe setups). To experimentally validate the proposed algorithm, a multichannel (i.e., multiprobe) far-field measurement setup was utilized. A four-element open-ended waveguide array operating at 2.6 GHz and two-probe far-field systems were employed as the antenna under test (AUT) and multiple probes, respectively, to validate the effectiveness and robustness of the proposed phased array calibration algorithm. The proposed algorithm will be highly valuable for efficient phased array calibration, especially for future antenna systems equipped with large-scale antenna configurations.

Role of asymmetric magnetic electrodes in tuning spin selective rectification action of borazine [B3N3H6

Herein, we examine the role of magnetic and non-magnetic electrodes and molecule-electrode anchor group in inducing simultaneous exhibition of spin filtration and rectification action in borazine. Magnetic electrodes preferred are iron and nickel; where as non-magnetic electrode used is gold; with anchor group varying from thiol ended molecule on either end to that at only one end. Two-probe systems developed, manifests large equilibrium spin filter efficiency (85%) with gold-nickel combinations, however dual nature of spin filtration and rectification is profusely evident with gold-iron combinations with thiol ended electrode-molecule contact on either end. Results obtained, have been explained through the transmission spectra obtained at equilibrium and ±1V. A molecular level explanation has been provided through the analysis of molecular projected Hamiltonian states obtained at equilibrium and ±1V. These results along with projected device density states analysis establish the role of d orbital of nickel or iron in inducing the observed feature.

The spin-dependent transport of transition metal encapsulated B40fullerene

Two-probe systems of transition metal atom (X)-encapsulated B40fullerene contacted with Au electrodes, where X = Fe, Mn, Ni, and Co.

Spin-dependent ballistic transport properties and electronic structures of pristine and edge-doped zigzag silicene nanoribbons: large magnetoresistance

The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magnetic configuration of the two electrodes, are discussed. By switching on and off the external magnetic field, we may convert the metallic ferromagnetic ZSiNRs into insulating antiferromagnetic ZSiNRs. In the ferromagnetic state, even- or odd-width ZSiNRs exhibit a drastically different magnetoresistance. In an odd-width edge-doped ZSiNR a large magnetoresistance occurs compared to that in a pristine ZSiNR. The situation is reversed in even-width ZSiNRs. These phenomena result from the drastic change in the conductance in the antiparallel configuration.

Reliability Analysis of CNT Contacts with Metal Electrodes

Contact resistance has always been an issue for semiconducting carbon nanotubes (CNT). This paper presents a comparative study on open-ended semiconducting carbon nanotube (4, 0) two-probe systems with metal electrodes of platinum, copper and gold. The local density of states and transmission coefficients has been calculated to explain the electronic structure and transmission properties of two-probe systems. Contact resistance for each two-probe system has been derived from their current–voltage characteristics. Calculations are based on first-principle quantum mechanical density functional, matrix Green’s function methods and Landauer–Buttiker formula. Contact resistance is calculated to be highest for the Au electrode while it is least for the Pt electrode.

ELECTRONIC STRUCTURES AND TRANSPORT PROPERTIES OF SINGLE CRYSTALLINE GaN NANOTUBES

The electronic structures and transport properties of single crystalline GaN nanotubes with 0.92 nm inner diameter and different wall thicknesses of 0.08 nm, 0.26 nm and 0.54 nm are studied based on the generalized gradient approximation (GGA) of density functional theory (DFT) and the nonequilibrium green's function (NEGF). The research shows that (1) the three single crystalline GaN nanotubes have direct band gaps, decreasing with the increase of wall thickness; (2) the electronic density of state and electronic transmission spectra of two-probe system have their own pulse-type sharp peaks with almost the same location of electron energy; (3) under different bias-voltages, two-probe systems of the single crystalline GaN nanotubes have the I–V properties which reveal that the single-wall GaN nanotube and the single-layer GaN nanotube are semiconducting and the double-layer GaN nanotube appears nearly metallic.

First-principles calculations of size-dependent giant electroresistance effect in nanoscale asymmetric ferroelectric tunnel junctions

Based on the first principle calculations, we predicted the electronic structures and ferroelectric instability of the asymmetric ferroelectric tunneling junction with the ferroelectric barrier thickness changing, and found two undiscovered and important behaviors, i.e., absence of the critical thickness for the positive polarization state and the larger critical thickness for the negative polarization state. Using nonequilibrium Green function’s approach, the corresponding two-probe systems and their electronic transport properties at different ferroelectric barrier thickness have been constructed. It is found that reorienting the polarization direction in the ferroelectric barrier can dramatically change the internal electric field and macroscopic potential barrier, resulting in several orders of magnitude change in tunneling electroresistance ratio. Results also found that the tunneling electroresistance can be distinctly controlled by adjusting thickness of the ferroelectric barrier, which behavior is defined as the size-dependent giant electroresistance effect. Our results enable architectures of large density and high sensitivity in the next generation of ferroelectric random access memories with nondestructive resistive readout.

First-Principles Study on Transport Properties of Saturated Single Crystalline GaN Nanotubes

Electronic structures and transport properties of the saturated single crystalline GaN nanotubes with 0.92 nm inner diameter and different wall thickness have been investigated by using first-principles and non-equilibrium green's function (NEGF). The result shows, the energy gap of the saturated single crystalline nanotubes changed from 0.2933 to 2.8442 eV. The saturated single crystalline GaN nanotubes have the direct band gap and the band gap decreases with the increase of wall thickness of nanotubes. The electronic density of state and the electronic transmission spectra of two-probe system have pulse-type sharp peak, the change of electronic density of state is in accord with electronic transmission rate. I-V characteristics of two-probe systems reflect that the saturated single-wall GaN nanotube and the saturated single-layer GaN nanotube are semiconducting properties, and the saturated double-layer GaN nanotube appears metallic behavior.

Influence of normal and radial contributions of local current density on local electrochemical impedance spectroscopy

A new tri-electrode probe is presented and applied to local electrochemical impedance spectroscopy (LEIS) measurements. As opposed to two-probe systems, the three-probe one allows measurement not only of normal, but also of radial contributions of local current densities to the local impedance values. The results concerning the cases of the blocking electrode and the electrode with faradaic reaction are discussed from the theoretical point of view for a disk electrode. Numerical simulations and experimental results are compared for the case of the ferri/ferrocyanide electrode reaction at the Pt working electrode disk. At the centre of the disk, the impedance taking into account both normal and radial contributions was in good agreement with the local impedance measured in terms of only the normal contribution. At the periphery of the electrode, the impedance taking into account both normal and radial contributions differed significantly from the local impedance measured in terms of only the normal contribution. The radial impedance results at the periphery of the electrode are in good agreement with the usual explanation that the associated larger current density is attributed to the geometry of the electrode, which exhibits a greater accessibility at the electrode edge.

Ab initio investigations of the charge transport properties of endohedral M@C20 (M = Na and K) metallofullerenes

Using density functional theory and quantum transport calculations based on nonequilibum Green's function formalism, we investigate the charge transport properties of endohedral M@C20(M = Na and K) metallofullerenes. Our results show that the conductance of C20 fullerene can be obviously improved by insertion of alkali atom at its centre. Both linear and nonlinear sections are found on the I—V curves of the Au-M@C20-Au two-probe systems. The novel negative differential resistance behaviour is also observed in Na@C20 molecule but not in K@C20.

Bending of conjugated molecular wires and its effect on electron conduction properties

The electronic structure and electron transport properties of simple conjugated molecularwires like oligophenylene ethynylene (OPE) and oligophenylene vinylene (OPV) are studiedunder compression. If artificially confined to a given shorter length, the oligomers tend tobend and bending causes a loss in the overlap of the conjugated molecular orbitals.Theoretical modeling of electronic transport has been carried out for all undistorted andcompressed OPE/OPV oligomers. OPV exists in step-like or V-like conformations andthey have the same stability with very similar frontier molecular orbitals. Theconductances of these molecular wires are calculated when inserted between twogold probes and the conductances for OPV are found to be comparable to OPEwhen the interfaces are same. The conductance decreases with bending due tothe gradual loss in overlap of the molecular orbitals. It is also found that theconductances of the molecular wires decrease very strongly if the terminal sulfur atom issimultaneously bonded to hydrogen and a gold surface, thus reflecting the importance ofthe interface in determining the conductance in two-probe systems. From theconductance studies it may be concluded that if one or more benzene rings ofOPE are rotated from coplanar conditions, the orthogonal molecular orbitalsmay completely block the electronic transport, rendering the molecule insulating.

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.

Efficient wave-function matching approach for quantum transport calculations

The Wave Function Matching (WFM) technique has recently been developed for the calculation of electronic transport in quantum two-probe systems. In terms of efficiency it is comparable with the widely used Green's function approach. The WFM formalism presented so far requires the evaluation of all the propagating and evanescent bulk modes of the left and right electrodes in order to obtain the correct coupling between device and electrode regions. In this paper we will describe a modified WFM approach that allows for the exclusion of the vast majority of the evanescent modes in all parts of the calculation. This approach makes it feasible to apply iterative techniques to efficiently determine the few required bulk modes, which allows for a significant reduction of the computational expense of the WFM method. We illustrate the efficiency of the method on a carbon nanotube field-effect-transistor (FET) device displaying band-to-band tunneling and modeled within the semi-empirical Extended H\"uckel theory (EHT) framework.

Simulation of STM technique for electron transport through boron-nitride nanotubes

We report first-principles calculations on the electrical transport properties of boron-nitrid nanotubes (BNNTs). We consider a single walled ( 5 , 0 ) boron-nitrid nanotube sandwiched between an Au(1 0 0) substrate and a monatomic Au scanning tunneling microscope (STM) tip. Lateral motion of the tip over the nanotube wall cause it to change from one conformation class to the others and to switch between a strongly and a weakly conducting state. Thus, surprisingly, despite their apparent simplicity these Au/BNNT/Au nanowires are shown to be a convenient switch. Experiments with a conventional STM are proposed to test these predictions. The projection of the density of states (PDOS) and the transmission coefficients T ( E ) of the two-probe systems at zero bias are analyzed, and it suggests that the variation of the coupling between the wire and the electrodes leads to switching behaviour.

Transport Properties of Binary Clusters

We present first-principles studies on the transport properties of small silicon and aluminium clusters: Al2, Si2, Al4 and AlSi sandwiched between two Al (100) electrodes. The variation of the equilibrium conductance as a function of contact distance for these two-probe systems is probed. Our results show that the transport properties are dependent on both the specific nanostructure and the separation distance between the central molecule and the electrodes. For equilibrium transport properties, the clusters with the similar structure show similar transmission spectra at large distances, the small difference can be explained by the electron filling. For current-voltage characteristics, all the clusters show the metallic behaviour at lower bias, however very different non-linear behaviour can be observed at higher bias. For AlSi and Al2, when the distance between the central cluster and the electrodes is 3.5 Å, large negative differential resistance (NDR) can be found in the bias range 0.8 V∼1.4 V.