The current paper continues the presentation of results of a quantum-chemical study of the interface formed by titania nanoparticles and a set of carboxylated moieties, namely, benzoic and bi-isonicotinic acids and tris-(2,2'-debipyridine) Fe(H) complex placed on the surface of either rutile or anatase polymorphs. The calculations were performed in the spd-basis using semi-empirical quantum-chemical codes, both sequential and parallel. The results are mainly addressed to the geometry optimization of the adsorbed molecules on the surface as well as to the adsorption mechanism and the energy of adsorption.
We present an extension of a new Gaussian halving method for the calculation of path integrals introduced in Refs. [1-3]. The original method leads to recursion relations that are nonlinear and could not be solved analytically. In this paper we present a general analytical solution (including the continuum limit) of linearized recursions. We show that the error associated with this linearization is of the order O(1/N-2), where N is the number of discretized steps, i.e. it is of the same order of magnitude as the error inherent in the derivation of the original method. Using this result we derive an improved algorithm for calculation of path integrals.
The electronic structure of low-index (001) and (110) surfaces of ZrO2 in cubic and tetragonal phases has been calculated with the full-potential linearized augmented plane wave method (FLAPW) in the local density approximation for the exchange-correlation potential. We focused on the influence of surface on the electronic structure in ZrO2. The obtained results allow to conclude that the band gap states can be related to oxygen vacancies.
Adsorption sensitivity of semiconductor field emission structures has been studied. Measurements of the current-voltage characteristics of the field emission current show the great sensitivity of the emission properties to adatoms on the emitting surface. It is found that the increase in the emission current from a semiconductor emitter can exceed that for a metal emitter by several orders of magnitude, for the same adsorbate atoms under the same adsorption conditions. A physical mechanism of the observed influence is discussed. The feasibility of using field emission structures as efficient gas sensors is demonstrated for a number of semiconductors.
Green light waveguide behavior of micron-sized single-crystalline Zinc Oxide (ZnO) rods was for the first time investigated by using far-field and near-field optical systems. Micron-sized hexagonal single-crystalline ZnO rods were synthesized with a simple self-organization growth process by annealing ZnO powders in an electric muffle furnace at 1100 degrees C for 1 hour. The green fluorescence excited by ultraviolet laser may result from the impurities on the surface of ZnO rods. The optical waveguide behavior of single ZnO rods shows its potential applications to future micro-optoelectronic devices.
A series c f polycrystalline hexagonal ferrites with the SrCu2-x/2TixFe16-xO27 (X = 0.0, 0.8, 1.2, 1.6 and 2.0) composition were prepared by using the conventional ceramic technique to investigate their AC conductivity and dielectric properties as a function of frequency (f) and temperature (T). The results of AC conductivity measurements reveal three distinct regions in sigma'(f) curves. The first region covers the range of f <= 10 Hz where dispersion of sigma' was attributed to the interfacial electrode polarization and the grain boundaries effect. The second region in the frequency range of 10 Hz <= f <= 10(4) Hz is characterized by approximately frequency-independent sigma'. In the third region, f >= 10(4) Hz, the AC conductivity obeys the universal power law. The frequency exponent S in the power law was found to decrease with increasing temperature. This behavior suggested that the classical barrier hopping model is the most probable mechanism in these samples. The dielectric loss tangent measurements show dielectric relaxation peaks which were explained on the basis of the assumption that a strong correlation between the conduction mechanism and the dielectric polarization exists in ferrites.
Mossbauer spectra of the MnAlxFe2-xO4 spinel ferrites, 0 <= x <= 1, were measured at room temperature. The spectra have been analysed into two magnetic spectra due to Fe ions at the tetrahedral A-sites and octahedral B-sites and two quadrupole doublets; C-B for x >= 0.2 and C-A for x = 1. However, the broad B-site spectra have been further decomposed into their multicomponents "subspectra". The hyperfine interaction parameters have been studied and discussed as functions of the Al content x, and the cation distribution has been estimated. Dependence of the quadrupole doublet splitting QS on the positional oxygen parameter u and molar ratio x has been studied. The X-ray diffraction parameters: the average lattice parametera, the mean ionic radius of the A- and B-sites, the A-site bond length d(AL) and hopping length L-A and those of the B-sites d(BL) and L-B, and the oxygen parameter u have been calculated and discussed depending on the Al content x. The hyperfine magnetic field of the A-sites H-A and B-sites H-B have been found to increase with d(AL) and d(BL), respectively, whereas H-B and the deduced bulk magnetization have been found to increase with L-B.
Cadmium indate films were deposited on glass substrate by sputtering of cadmium-indium alloy target in an oxygen partial pressure of 5x10(-4) mbar under various substrate bias voltages in the range of 0 to -100 V employing dc reactive magnetron sputtering technique. The effect of substrate bias voltage on the structural, electrical, and optical properties was systematically studied. The films formed at the substrate temperature of 523 K and bias voltage of -80 V were polycrystalline with cubic spinel structure. These films exhibited low electrical resistivity of 6x10-(4) Omega cm, Hall mobility of 34 cm(2)/V.sec, optical transmittance of 88%, optical band gap of 3.26 eV and figure of merit of 1.5x10(-2) Omega(-1).
The current paper presents new aspects of a quantum-chemical study of the interface formed by the titania nanoparticles and a set of carboxylated moieties considered as a network of size-limited building blocks. The building blocks are viewed as molecular binary systems whose constituents are subjected to strong donor-acceptor interaction. The instantaneous delivery of electron to the titania conduction band due to photoexcitation is explained by a peculiar partner-space composition of the systems molecular orbitals.
For a se niconductor quantum dot (QD), the contributions made to the exciton energy spectrum by the electron and hole kinetic energies, the energy of Coulomb interaction between them, and the energy of their polarization interaction with the spherical interface between the QD and the dielectric medium have been analyzed.
The boric oxide (B2O3) decomposition process on the Si(100) surface and the surface structure after B2O3 exposure were studied by the reflection high-energy electron diffraction (RHEED) method. B2O3 was deposited on silicon surface at substrate temperatures ranging from 650 to 800 degrees C. From the analysis of diffraction patterns and specular beam intensity upon B2O3 deposition it was proposed that the decomposition rate of B2O3 molecules on the Si(100) surface depends on the silicon atoms supply to the B2O3 molecules adsorption places. For structural analysis, the rocking curves (RC) for specularly reflected beam were measured before and after B2O3 deposition on the Si(100) surface. Theoretical RC's were calculated within the dynamical theory. Comparison of theoretical RC's calculated for various models of the (4x4) structure with the experimental ones shows that two structures can coexist on the surface, namely, the beta-c(4x4) and gamma-(4x4) structures.
The switching properties of amorphous CuInSeTe thin films have been investigated. The amorphous quaternary semiconductor CuInSeTe thin films of 217 nm and 332 nm thicknesses have been prepared by thermal evaporation of the bulk compound under vacuum of about 10(-6) Torr and with evaporation rate of about 80 A(0)/sec. The structure of the bulk and thin films were investigated by X-ray diffraction technique. The compositional studies of CuInSeTe in both powder and thin films form were carried out by atomic absorption GBC 980 and Perkin Elmer (model 1100). Annealing of the films at different temperatures (300, 350, 400, 450, and 500 K) improves the switching characteristics and decreases the threshold voltage V-th. The threshold switching voltage and threshold activation energy E-s were found to decrease linearly with increasing annealing temperature. Moreover, the threshold switching voltage decreased exponentially with temperature.
As2S3 and As4S4 nanocrystals were prepared by the method of open solvent thermal reaction using arsenic trioxide and oleic acid as the precursors. The size of As2S3 and As4S4 nanocrystals can be tuned by changing the reaction temperature and the concentration of the precursors and octadecanoic amine. The UV-vis absorption and photoluminescence spectra were used to characterize them well. The SEM and SAED were also used. The synthesis route diagram for mutual conversions among AsO3, As2S3 and As4S4 has also been obtained.
We study the error associated with the recently developed method of Gaussian halving [1] for calculating path integrals in a general theory in d = 1. We show that it is of order O(1/N-2) - an improvement over the standard O(1/N) error of approximating the path integral of a continuum amplitude with its corresponding discretized expression in the mid-point prescription. The obtained generic results are illustrated by the case of an anharmonic oscillator.
A series of Co14xTixFe2(1-x)O4 spinel samples with 0 <= x <= 0.5 were studied at room temperature using the Mossbauer and X-ray diffraction spectra. The Mossbauer spectra showed a sharp six-line pattern and a growing central paramagnetic phase for x >= 0.1. The determined hyperfine interaction parameters of the tetrahedral A-sites and octahedral B-sites and the bulk magnetization were studied and discussed as functions of x, which allowed to estimate the cation distribution. The B-site quadrupole doublet splitting was affected by the trigonal distortion of the B-sites. The obtained true lattice parameter was increasing at x >= 0.3, whereas its decrease at x >= 0.4 indicated a cluster formation in the samples. The theoretical lattice parameter, the deduced site ionic radius, bonds, edges and hopping lengths of the A- and B-sites and the oxygen parameter u were calculated and their dependence on x was discussed The hyperfine interaction parameters and the bulk magnetization were dependent on the hopping length and the Fe-O bond length at the A- and B-sites.
Considerable advancement has been made in the understanding of << single spin logic >> since its first proposal in 1994. In this classical paradigm, classical binary bits 0 and 1 are encoded in anti-parallel spin polarizations of single electrons confined in quantum dots and placed in a magnetic field. Nearest-neighbor spins interact via exchange, resulting in the singlet state being the preferred spin configuration. Different types of Boolean logic circuits can be realized by arranging the dots in appropriate two-dimensional patterns to elicit the desired relationships between input and output bit(s). Logic signal is propagated unidirectionally from input to output ports using a three phase clock, much like in bucket brigade devices. This idea is the progenitor of spin based quantum logic processors. Here, we present the designs for a large number of combinational and sequential logic circuits, showing the universality of single spin logic.
Correlation between the structural, morphological and optical properties of the electrochemically etched porous silicon (PS) and the nanocrystalline silicon films (nc-Si) prepared by plasma.-enhanced chemical vapor deposition (PECVD) has been studied. X-ray diffraction analysis of the nc-Si layers showed growth of a system of nanocrystals oriented along the (004)-plane. The average size of nanocrystals was estimated to be 4.8 nm. Atomic force microscopy imaging demonstrates the developed surface of deposited films which is cone-like. The observed room-temperature photo luminescence of nc-Si has a maximum at 1.55 eV The comparison of chemical bonds in nc-Si and PS was carryied out by means of Fourier transform infrared spectroscopy. The similarity of these properties of nc-Si and PS is discussed.
Heterojunction devices of n-AlSb/p-Si were fabricated by growing n-AlSb films onto p-type Si single crystal wafers using liquid phase epitaxy (LPE). The X-ray diffraction studies indicate that all films were monocrystalline with cubic structure containing a single AlSb phase and showing one significant peak oriented along the (I 11) direction. Current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed to determine the electrical characteristics of these structures. Rectifying properties were obtained, which are definitely of the diode type. Analysis of the dark I-V characteristics of n-AlSb/p-Si at several temperatures aimed at the elucidation of conduction mechanisms and evaluation of the heterojunction parameters is presented. The forward current growing exponentially with the applied voltage in the range of V <= 0.4 V was dominated by the thermionic emission over the n-AlSb/p-Si. In the 0.5 < V <= 1 V range, the current transport is due to the space-charge limited current controlled by a single dominating trap level in the band gap of AlSb. The carrier transport mechanism was considered to be mainly generation and recombination of carriers in the p-Si substrate under the reverse bias. Information on the potential barrier width and the built-in potential can be successfully obtained by studying the dark C-V measurements at 1 MHz. Discussion of the obtained results and their comparison with the previous published data are also given.
Using a professional program from Gaussian Inc. and the SCF method implemented therein, we have obtained the energy for a Hydrogen molecule under confinement in a Single Wall Carbon Nanotube (SWNT). The energy results from these ab initio calculations are analysed with special attention to the molecule oriented perpendicularly to the nanotube axis. The hydrogen molecule was located inside a section of the nanotube previously optimised in geometry. We analyze the strength of the confinement and its effect on the hydrogen's bond length (R) through the molecule's movement along the radius towards the wall. At each new position, the energy minimization was done allowing bond length relaxation. Differences ranging from 6.0% to 8.2% of R in vacuum were observed in SWNT's (9,0) and (6,0), respectively. The energy curves show the existence of a threshold for dissociation as the molecule approaches the nanotube wall; in the zizag (9,0) case it can not pass the 66% of radius limit.
In this work the growth of iron andiron monosilicide nanolayers on Si(111)7x7 was investigated. The films have been grown by using two types of metal (Fe) sources: Fe strip on Ta ribbon and Fe core in the inner part of W spiral, respectively. The investigation was made by Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). It was shown that a layer-by-layer growth of FeSi and Fe took place for the first and second types of sources, respectively, and that nucleation centers play a key role for the Fe layer-by-layer growth.
