Experimental and theoretical investigation of the pyroelectric effect of the p-n junction in a paraelectric non-polar semiconductor

We describe here the first comprehensive investigation of a pyroelectric response of a p-n junction in a non-polar paraelectric semiconductor. The pyroelectric effect is generated by the, temperature dependent, built-in electrical dipole moment. High quality PbTe p-n junctions have been prepared specifically for this experiment. The pyroelectric effect was excited by a continuous CO₂ laser beam, modulated by a mechanical chopper. The shape and amplitude of the periodic and single-pulse pyroelectric signals were studied as a function of temperature (10-130 K), reverse bias voltage (up to −500 mV) and chopping frequency (4-2000 Hz). The pyroelectric coefficient is ≈10⁻³ μC/cm²K in the temperature region 40-80 K. The developed theoretical model quantitatively describes all the experimental features of the observed pyroelectric effect. The time evolution of the temperature within the p-n junction was reconstructed.     

The comparative study in transport properties of furan, thiophene and selenophene dithiols in nano electronic

The electron transport properties of furan, thiophene and selenophene dithiols based molecular wires through two electrodic systems using non-equilibrium Green’s functions technique (NEGF) are investigated. The electron transport of the above systems is systematically studied by analysis of transmission function, density of states, current–voltage characteristics, and conductance of the systems. The maximum current is occurred at the vicinity of 2.0 V and the values are 90.37, 98.82 and 100.31 μA for furan, thiophene and selenophene dithiols, respectively. These results can be attributed to the molecular projected self consistent Hamiltonian (MPSH) of two electrodic systems with different molecules at different bias voltage and also to quality of resonance of π electrons of heterocyclic ring. We can foresee that the furan, thiophene, and selenophene dithiols can be applied at electronic devices because of switching the high and low current.     

金纳米线接触构型相关的双重负微分电阻与整流效应

运用第一性原理密度泛函理论(DFT)和非平衡格林函数(NEGF)方法, 研究了[111]Au纳米线与1, 4-二硫苯酚(DTB)构成的分子结的电子输运性质. 构建并优化不同的Au-DTB接触构型, 计算发现: 尖端顶位构型最利于电流输运; 非对称构型大多具有很好的整流特性(最大整流比为25.6); 部分结构出现双重负微分电阻(NDR)效应. 分析表明, 整流效应主要源于非对称接触构型两端S-Au键的稳定性差别; 尖端金原子与硫原子的耦合能级中, 近费米面的能级对低压区电子传输起主要作用; 电压增大, 离费米面较远的能级对输运起主导作用, DTB的本征能级也逐渐参与, 这一转变致使电流出现两峰一谷的双重NDR效应.     

Negative differential resistance of TEMPO molecules on Si(1 1 1)

Negative differential resistance (NDR) has been observed for individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules on Si(1 1 1) in ultra high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements at room temperature. NDR effects were observed exclusively at negative bias voltage using an n-type Si(1 1 1) sample. At 77 K no NDR effects were observed, but the I(V) curves were similar in shape to those recorded on bare Si(1 1 1) sites. TEMPO was observed to adsorb preferentially at corner adatom sites of the Si(1 1 1)-7 × 7 structure. Although the Si(1 1 1)-7 × 7 reconstruction was conserved, local defects were frequently observed in the vicinity of the TEMPO adsorbates.     

Negative Differential Resistance in Atomic Carbon Chain-Graphene Junctions

We investigate the electronic transport properties of atomic carbon chain-graphene junctions by using the density-functional theory combining with the non-equilibrium Green's functions. The results show that the transport properties are sensitively dependent on the contact geometry of carbon chain. From the calculated I-V curve we find negative differential resistance (NDR) in the two types of junctions. The NDR can be considered as a result of molecular orbitals moving related to the bias window.     

Effect of UV lamp irradiation during oxidation of Zr/Pt/Si structure on electrical properties of Pt/ZrO2/Pt/Si structure

Metal-insulator-metal (MIM) capacitors were fabricated using ZrO₂ films and the effects of structural and native defects of the ZrO₂ films on the electrical and dielectric properties were investigated. For preparing ZrO₂ films, Zr films were deposited on Pt/Si substrates by ion beam deposition (IBD) system with/without substrate bias voltages and oxidized at 200 °C for 60 min under 0.1 MPa O₂ atmosphere with/without UV light irradiation (λ = 193 nm, Deep UV lamp). The ZrO₂(∼12 nm) films on Pt(∼100 nm)/Si were characterized by X-ray diffraction pattern (XRD), field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM), capacitance-voltage (C-V) and current-voltage (I-V) measurements were carried out on MIM structures. ZrO₂ films, fabricated by oxidizing the Zr film deposited with substrate bias voltage under UV light irradiation, show the highest capacitance (784 pF) and the lowest leakage current density. The active oxygen species formed by UV irradiation are considered to play an important role in the reduction of the leakage current density, because they can reduce the density of oxygen vacancies.     

CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction as a spin current diode predicted by first-principles calculations

We report on first-principles calculations of spin-dependent quantum transport in a CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction and predict a strong diode effect of charge and spin current. The minority spin current is absolutely inhibited when the bias voltage is applied to the terminals of both CrAs and AlAs. The majority spin current is inhibited when the bias voltage is applied to the terminal of CrAs and “relaxed” when the bias voltage is applied to the terminal of AlAs. The charge and spin current diode are promising for reprogrammable logic applications in the field of spintronics.     

Atomic force microscopy study of surface morphology change in spinel LiMn2O4: Possibility of direct observation of Jahn-Teller instability

Surface morphology in 3.5 × 3.5 μm² area of spinel LiMn₂O₄, which is a typical cathode material for Li ion secondary batteries, is studied using an atomic force microscopy (AFM) with a conductive probe. Negative bias voltage is applied to the probe to attract Li⁺ ions toward LiMn₂O₄ surface during the AFM observation. Before applying the voltage (0 V), the whole LiMn₂O₄ surface is covered with scale-shaped grains. Under the negative voltage of 5.5 V, electric current abruptly increases, indicating Li⁺ ionic conduction. Simultaneously, part of the scale-shaped grains expand and flatten. Jahn-Teller phase transition, which is induced by the repulsive interaction between the Mn-e_g and O-2p electrons in Li accumulated layer, is proposed as a possible origin of these results.     

First principles study of the electron transport through cis-polyacetylene based molecular wires

The electron transport properties of cis-polyacetylene and cis-polyacetylene based molecular wires (oligo(cyclopentadiene), oligo(pyrrole), and oligo(furan)) have been studied theoretically using a combination of density-functional theory and non-equilibrium Green′s functions method. The results demonstrate that the introduction of bridging group X (X=CH₂, NH, and O) in cis-polyacetylene has a profound effect on the electron transport behavior of the molecules. The conductance of the four molecular wires decreases in the order of polyacetylene>oligo(cyclopentadiene)>oligo(furan)>oligo(pyrrole). In particular, the conductances of oligo(furan) and oligo(pyrrole) are much lower than those of polyacetylene and oligo(cyclopentadiene). The mechanism of this difference of electron transport properties of these four molecular systems is analyzed in terms of their geometric structures, electronic structures, transmission spectra, and spatial distribution of frontier orbitals. It is found that the energy levels of frontier molecular orbitals and the evolution of spatial distribution of frontier molecular orbitals with the applied bias are the essential reason for generating this difference of electron transport behaviors of the four molecular systems.     

Transport properties of boron-doped single-walled silicon carbide nanotubes

The doped boron (B) atom in silicon carbide nanotube (SiCNT) can substitute carbon or silicon atom, forming two different structures. The transport properties of both B-doped SiCNT structures are investigated by the method combined non-equilibrium Green’s function with density functional theory (DFT). As the bias ranging from 0.8 to 1.0 V, the negative differential resistance (NDR) effect occurs, which is derived from the great difficulty for electrons tunneling from one electrode to another with the increasing of localization of molecular orbital. The high similar transport properties of both B-doped SiCNT indicate that boron is a suitable impurity for fabricating nano-scale SiCNT electronic devices.     

Modulation structure in the STM images of the non-metallic misfit-layer compound (PbS)1.12VS2

This paper presents the scanning tunneling microscopy (STM) results of the misfit-layer compound (PbS)_1.12VS₂, which is constructed of alternately stacking of PbS (Q) and VS₂ (H) layers. Temperature dependent resistivity measurements show a semiconducting behavior with small activation energies. Unlike the metallic 1Q/1H type of compounds we have succeeded to take both the STM images of a Q layer and a H layer, because electron tunneling from the underlying H layer is suppressed when intermediate positive bias voltage (V_b) is applied to a tip. At V_b = 0.15 V the image shows pseudo-tetragonal arrays of bright spots, although it is obscure with decreasing bias voltage and disappears at less than 10 mV. A modulation structure is found on the H layer of a stepped surface on which surface atoms are undulated in a period being twice the V-V interatomic distance in the [1 0]_H or the [1 1]_H direction.     

First-principle study of the electronic transport properties of a new dumbbell-like carbon nanocomposite

Using a first-principle density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of a new dumbbell-like carbon nanocomposite, in which one carbon nanotube segment is capped with two C₆₀ fullerenes. Our results show that the current–voltage curve reveals a highly nonlinear feature. A negative differential resistance (NDR) behavior is obtained at a very low bias, which is expected to be helpful for the development of low bias NDR-based molecular devices. Moreover, the carbon nanotube length and fullerene type can affect the NDR behavior strongly. The electronic transport is analyzed from the transmission spectra and the molecular projected self-consistent Hamiltonian states under different applied biases.     

Effect of end groups on contact resistance of alkanethiol based metal-molecule-metal junctions using current sensing AFM

Tuning the charge transport through a metal-molecule-metal junction by changing the interface properties is widely studied and is of paramount importance for applications in molecular electronic devices. We used current sensing atomic force microscopy (CSAFM) as a tool to study the contact resistance of metal-molecule-metal (MmM) junctions formed by sandwiching self-assembled monolayers (SAMs) of alkanethiols with various end groups (-CH₃, -OH and -NH₂) between Au(1 1 1) substrates and Au coated AFM tips. The effect of interface chemistry on charge transport through such SAMs with varying end groups was studied in an inert, non-polar liquid (hexadecane) environment. We find that the contact resistances of these MmM junctions vary significantly based on the end group chemistry of the molecules.     

Synthesis and characterization of silicon carbonitride films by plasma enhanced chemical vapor deposition (PECVD) using bis(dimethylamino)dimethylsilane (BDMADMS), as membrane for a small molecule gas separation

Silicon carbonitride thin films have been deposited by plasma enhanced chemical vapor deposition (PECVD) from bis(dimethylamino)dimethylsilane (BDMADMS) as a function of X = (BDMADMS/(BDMADMS + NH₃)) between 0.1 and 1, and plasma power P (W) between 100 and 400 W. The microstructure of obtained materials has been studied by SEM, FTIR, EDS, ellipsometrie, and contact angle of water measurements. The structure of the materials is strongly depended on plasma parameters; we can pass from a material rich in carbon to a material rich in nitrogen. Single gas permeation tests have been carried out and we have obtained a helium permeance of about 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and ideal selectivity of helium over nitrogen of about 20.     

Structural and electrical transport properties of Nb-doped TiO2 films deposited on LaAlO3 by rf sputtering

We have investigated the structural and electrical transport properties of Nb-doped TiO₂ films deposited on (1 0 0) LaAlO₃ substrates by rf magnetron sputtering at temperatures ranging from 873 K to 1073 K. Films deposited below 998 K are anatase, and mixed phases between anatase and rutile exist in the film grown at higher temperatures. We find that films deposited at low temperatures exhibit semiconductor behavior, while metallic conductivity is observed in the most conducting film deposited at 998 K. For this sample, compared to electron-phonon scattering mechanism, electron-phonon-impurity interference effect plays an important role in its electron transport process. Moreover, the temperature coefficient of the resistivity for the film deposited at 1073 K is negative from 2 K to 300 K. The temperature dependence of resistivity for the film is described by ∼exp(b/T)^1/2 at temperatures from 80 K down to 30 K, and by the fluctuation induced tunneling model from 80 K to 300 K.     

Materials and ion transport property studies on hot-press casted solid polymer electrolyte membranes: [(1 − x) PEO: x KIO3]

Materials and ion transport property characterization in Solid Polymer Electrolyte (SPE) membranes: (1 − x) PEO: x KIO₃, where x = 0, 10, 20, 30, 40, 50 wt.%, have been studied. SPE films have been prepared following two casting techniques: a novel hot-press (extrusion) and the traditional solution cast. Hot-press technique is a completely dry/solvent free/rapid/inexpensive procedure as compared to solution cast method and has recently been receiving wider acceptability to cast membranes of ion conducting polymeric electrolytes.‘Log σ − x’ study revealed σ-maxima at salt concentration x = 30 wt.% for SPE film prepared by both the methods. However, hot-pressed SPE film: 70 PEO: 30 KIO₃ exhibited relatively higher room temperature conductivity (σ ∼ 4.40 × 10^− 7 S cm^− 1) than that of the solution casted film. This has been referred to as Optimum Conducting Composition (OCC) SPE film. Materials characterization in OCC SPE film has been done by XRD, FTIR and DSC techniques. These studies confirmed the complexation of salt in the polymeric host. Some basic ionic parameters viz. conductivity (σ), ionic mobility (μ), mobile ion concentration (n), ionic transference number (t_ion) have been determined using different experimental procedures to understand the ion transport behaviour in OCC SPE material. The temperature dependent conductivity measurement has also been carried out and the activation energy (E_a) has been computed from the linear least square fitting of ‘log σ − 1 / T’ Arrhenius plot.     

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three (iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene (m/z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl₂ group favours the diradical dechlorination mechanism. The single m/z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.     

Investigation of diode parameters using I-V and C-V characteristics of In/SiO2/p-Si (MIS) Schottky diodes

A study on interface states density distribution and characteristic parameters of the In/SiO₂/p-Si (MIS) capacitor has been made. The thickness of the SiO₂ film obtained from the measurement of the corrected capacitance in the strong accumulation region for MIS Schottky diodes was 220 Å. The diode parameters from the forward bias I-V characteristics such as ideality factor, series resistance and barrier heights were found to be 1.75, 106-112 Ω and 0.592 eV, respectively. The energy distribution of the interface state density D_it was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. The interface state density obtained using the I-V characteristics had an exponential growth, with bias towards the top of the valance band, from 9.44×10¹³ eV⁻¹ cm⁻² in 0.329-E_v eV to 1.11×10¹³ eV⁻¹ cm⁻² in 0.527-E_v eV at room temperature. Furthermore, the values of interface state density D_it obtained by the Hill-Coleman method from the C-V characteristics range from 52.9×10¹³ to 1.11×10¹³ eV⁻¹ cm⁻² at a frequency range of 30kHz-1 MHz. These values of D_it and R_s were responsible for the non-ideal behaviour of I-V and C-V characteristics.     

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the Cl and Cl isotopomers of p-chloroanisole

The geometric structures and vibrations of p-chloroanisole isotopomers in the first electronically excited state were studied by mass-analyzed resonance-enhanced two-photon ionization spectroscopy and by theoretical calculations. The band origins of the S₁ ← S₀ electronic transitions of ³⁵Cl and ³⁷Cl isotopomers were found to be equivalent at 34 859 ± 3 cm⁻¹. Assignments of the observed vibrational bands of the two isotopomers were made mainly based on the CIS/cc-PVDZ calculations and on conformity with the available data in the literature. Although the general spectral features of these two isotopomers are similar, the frequencies of some vibrational modes are different. This frequency shift partially depends on the degree of involvement of the chlorine atom in the molecular vibrations.     

Ab-initio pseudopotential study of electronic structure and chemisorption of oxygen on aluminium surface

Chemisorption of oxygen atom on aluminium (1 1 1), (1 1 0) and (1 0 0) surfaces is studied using ab-initio plane wave pseudopotential method based on density functional theory (DFT). Oxygen atom chemisorbed on three different high symmetry sites; top, short-bridge and hollow sites on the aluminium surfaces are examined. It has been found that the O-adatom adsorbed at the hollow site on aluminium (1 1 1), (1 1 0) and (1 0 0) plane yield energetically most stable structure. Calculation of chemisorption energies of O-adatom on aluminium surfaces shows that oxygen is most strongly bound to aluminium atoms on Al(1 1 1) plane and the calculated value of the chemisorption energy of O-adatom at the hollow site on Al(1 1 1) surface is 4.8 eV. In this work, the chemisorption energies calculated for O-adatom on Al(1 1 0) and Al(1 0 0) surfaces are reported for the first time. The electronic structures and the electronic charge density distributions of the oxygen chemisorbed aluminium surfaces are also investigated. Calculations show that for aluminium, p orbitals also contribute significantly along with the s orbitals during the bond formation with oxygen atom. Therefore, the possibilities of hybridizations lead to the strong bonding configurations.