Showing papers on "Lattice constant published in 2008"

Al-doped ZnO thin films as methanol sensors

Abstract: Al-doped zinc oxide (ZnO) thin films were prepared by chemical spray pyrolysis technique. The dopant concentration (Al/Zn at%) varied from 0 to 1.5 at%. Structural analysis of the films reveals that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure with (0 0 2) preferred orientation. The lattice constants calculated from the most prominent peaks are found to be in good agreement with the ICDD reference pattern: zinc oxide, 01-080-0074 (a = 3.2535 A and c = 5.2151 A). The sensing properties of the films towards methanol vapour are investigated for various concentrations of methanol in air at different operating temperatures in the range 200–350 °C. It is observed that compared to the undoped ZnO film, Al-doped films show higher sensitivity to methanol vapour. Among all the Al-doped films studied, the 0.5 at% Al-doped ZnO film shows the maximum response (∼44%) at 275 °C to 500 ppm of methanol vapour in air. Further, the films show fast response and recovery to methanol vapour at higher operating temperatures. The methanol-sensing mechanism of the film has been explained.

Construction of a generalized gradient approximation by restoring the density-gradient expansion and enforcing a tight Lieb–Oxford bound

Abstract: Recently, a generalized gradient approximation (GGA) to the density functional, called PBEsol, was optimized (one parameter) against the jellium-surface exchange-correlation energies, and this, in conjunction with changing another parameter to restore the first-principles gradient expansion for exchange, was sufficient to yield accurate lattice constants of solids. Here, we construct a new GGA that has no empirical parameters, that satisfies one more exact constraint than PBEsol, and that performs 20% better for the lattice constants of 18 previously studied solids, although it does not improve on PBEsol for molecular atomization energies (a property that neither functional was designed for). The new GGA is exact through second order, and it is called the second-order generalized gradient approximation (SOGGA). The SOGGA functional also differs from other GGAs in that it enforces a tighter Lieb-Oxford bound. SOGGA and other functionals are compared to a diverse set of lattice constants, bond distances, and energetic quantities for solids and molecules (this includes the first test of the M06-L meta-GGA for solid-state properties). We find that classifying density functionals in terms of the magnitude mu of the second-order coefficient of the density gradient expansion of the exchange functional not only correlates their behavior for predicting lattice constants of solids versus their behavior for predicting small-molecule atomization energies, as pointed out by Perdew and co-workers [Phys. Rev. Lett. 100, 134606 (2008); Perdew ibid. 80, 891 (1998)], but also correlates their behavior for cohesive energies of solids, reaction barriers heights, and nonhydrogenic bond distances in small molecules.

Plane-wave theory of three-dimensional magnonic crystals

Abstract: We use the plane-wave method to determine spin-wave spectra of three-dimensional magnonic crystals (the magnetic counterpart of photonic crystals) composed of two different ferromagnetic materials. The scattering centers in the magnonic crystal considered are ferromagnetic spheroids (spheres being a special case) distributed in sites of a cubic (sc, fcc, or bcc) lattice embedded in a matrix of a different ferromagnetic material. We demonstrate that magnonic gaps in such structures occur at spontaneous magnetization contrast and/or exchange contrast values above a certain critical level, which depends on the lattice type. Optimum conditions for magnonic gaps to open are offered by the structure in which the scattering centers are the most densely packed (the fcc lattice). We show that in all three lattice types considered the reduced width of the gap (i.e., the width referred to the gap center) is, in good approximation, a linear function of both the exchange contrast and the magnetization contrast. Also, the gap width proves sensitive to scattering center deformation, and its maximum value to correspond to a scattering center shape close to a sphere. Moreover, our numerical results seem to indicate that dipolar interactions in general result in an effective reduction of the gap width, but their impact only becomes of importance when the lattice constant of the cubic magnonic structure is greater than the ferromagnetic exchange length of the matrix material.

InP-Based Transistor Fabrication

Abstract: Methods of forming structures that include InP-based materials, such as a transistor operating as an inversion-type, enhancement-mode device. A dielectric layer may be deposited by ALD over a semiconductor layer including In and P. A channel layer may be formed above a buffer layer having a lattice constant similar to a lattice constant of InP, the buffer layer being formed over a substrate having a lattice constant different from a lattice constant of InP.

Theoretical prediction of perfect spin filtering at interfaces between close-packed surfaces of Ni or Co and graphite or graphene

Abstract: The in-plane lattice constants of close-packed planes of fcc and hcp Ni and Co match that of graphite almost perfectly so that they share a common two-dimensional reciprocal space. Their electronic structures are such that they overlap in this reciprocal space for one spin direction only allowing us to predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. First-principles calculations of the scattering matrix show that the spin filtering is quite insensitive to amounts of interface roughness and disorder which drastically influence the spin-filtering properties of conventional magnetic tunnel junctions or interfaces between transition metals and semiconductors. When a single graphene sheet is adsorbed on these open d-shell transition-metal surfaces, its characteristic electronic structure, with topological singularities at the K points in the two-dimensional Brillouin zone, is destroyed by the chemical bonding. Because graphene bonds only weakly to Cu which has no states at the Fermi energy at the K point for either spin, the electronic structure of graphene can be restored by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property.

Origin of hardness in WB4 and its implications for ReB4, TaB4, MoB4, TcB4, and OsB4

Abstract: First-principles calculations were performed on the superhard material, WB4 (Vicker hardness exceeding 46GPa), to reveal the origin of its high hardness. Our simulated lattice parameters, bulk modulus, and hardness are in excellent agreement with the experimental data. A three-dimensional B network with a peculiar B2 dimer along the z-axis and a xy planar honeycomb B sublattice is uncovered to be mainly responsible for the high hardness. We further predicted that five other transition metal B compounds (TMB4, TM=Re, Mo, Ta, Os, and Tc) within the WB4 structure are potential superhard materials.

Influence of compositional ratio K/Na on physical properties in (KxNa1−x)NbO3 ceramics

Abstract: Lead-free KxNa1−xNbO3 (KNN) ceramics with x=010–080 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on their physical properties It has been found that the KNN ceramics with x=045–060 exhibit quite different microstructures from those with other alkali compositional ratios The lattice parameter discontinuity previously reported at x=0475 has not been confirmed The existence of a change in the line slopes of lattice parameter variation with x is verified in the vicinity of x=035 The permittivity e′, piezoelectric constant d33, and planar electromechanical coupling coefficient kp show broad peaks in the compositional range of x=040–060, where d33 and kp are nearly independent of the K/Na ratio and all the ceramics have the high piezoelectric values of d33∼125 pC/N and kp∼041, respectively From the analyses, we suggest that the high piezoelectric properties for the KNN ceramics near x=050 are closely related to their microstructures of

Influence of substrate bias, deposition temperature and post-deposition annealing on the structure and properties of multi-principal-component (AlCrMoSiTi)N coatings

Abstract: Nitride films are deposited from a single equiatomic AlCrMoSiTi target by reactive DC magnetron sputtering. The influence of the substrate bias and deposition temperature on the coating structure and properties are investigated. The bias is varied from 0 to − 200 V while maintaining a substrate temperature of 573 K. And the temperature is changed from 300 to 773 K whilst maintaining a substrate bias of − 100 V. From X-ray diffraction analysis, it is found that all the as-deposited coatings are of a single phase with NaCl-type FCC structure. This is attributed to the high mixing entropy of AlN, CrN, MoN, SiN, and TiN, and the limited diffusion kinetics during coating growth. Specific aspects of the coating, namely the grain size, lattice constant and compressive stress, are seen to be influenced more by substrate bias than deposition temperature. In fact, it is possible to classify the deposited films as large grained (~ 15 nm) with a reduced lattice constant (~ 4.15 A) and low compressive residual stresses for lower applied substrate biases, and as small grained (~ 4 nm) with an increased lattice constant (~ 4.25 A) and high compressive residual stresses for applied biases of − 100 V or more. A good correlation between the residual stress and lattice constant under various deposition conditions is found. For the coatings deposited at − 100 V, and at temperatures above 573 K, the hardness could attain to the range of 32 to 35 GPa. Even after annealing in vacuum at 1173 K for 5 h, there is no notable change in the as-deposited phase, grain size or lattice constant of the coatings but an increase in hardness. The thermal stability of microstructure is considered to be a result of the high mixing entropy and sluggish diffusion of these multi-component coatings. For the anneal hardening it is proposed that the overall bonding between target elements and nitrogen is enhanced by thermal energy during annealing.

Polymorphic Tin Sulfide Thin Films of Zinc Blende and Orthorhombic Structures by Chemical Deposition

Abstract: Polycrystalline thin films (100-450 nm in thickness) of SnS formed from chemical baths of Sn(II) in acetic acid/HCl solution, triethanolamine, NH 3 (aq), and thioacetamide are polymorphic consisting of zinc blende (ZB) and orthorhombic (OR) structures The ZB structure for the SnS film, reported in this work for the first time, has a lattice constant a = 0579 nm and a direct (forbidden) bandgap of 17 eV, which is distinct from that of SnS(ZB), about 1 eV The electrical conductivity of SnS(ZB) is 6 X 10 -6 (Ω cm) -1 p-type, with activation energies for the conductivity of 05 eV at room temperature and 16 meV near 10 K When a SnS(ZB) film is heated in air at 500°C for 30 min, part of it transforms to SnO 2 and to SnS(OR); after 2 h 30 min at 550°C in air the film converts to transparent SnO 2 Such a film has a bandgap of 37 eV and electrical conductivity, ∼ 1 (Ω cm) -1 Photovoltaic effect in different structures involving these films is presented

Phase transformation induced by lattice distortion in multiprincipal component CoCrFeNiCuxAl1−x solid-solution alloys

Abstract: Multicomponent solid-solution alloys actually form with most of lattice atoms in original elemental crystal being substituted by other component atoms Thus, the regular lattice is destroyed and lattice distortion occurs The classical homogeneous nucleation theory and the concept of fluctuating topology of atomic bonds described in terms of the atomic-level stresses could well explain why face-centered cubic (fcc) solid solution is superior to body-centered cubic (bcc) in forming in CrFeCoNiCu alloy and in the phase transformation from fcc of CrFeNiAl05Cu05 alloy to bcc of CrFeNiAl075Cu025 alloy

Intrinsic relation between ground-state fidelity and the characterization of a quantum phase transition

Abstract: The electronic structure of the highly ordered alloy Cr3Co with the DO3 structure has been studied by FLAPW calculations. It is found that the ferrimagnetic state is stable and that the equilibrium lattice constant of Cr3Co equals 5.77 angstrom. A large peak in majority spin density of states (DOS) and an energy gap in minority spin DOS are observed at the Fermi level, which results in a high spin polarization of 90% in the ordered alloy Cr3Co. The total magnetic moment of Cr3Co is 3.12 mu(B), which is close to the ideal value of 3 mu(B) derived from the Slater-Pauling curve. An antiparallel alignment between the moments on the Cr (A, C) sites and the Cr (B) sites is observed. Finally, the effect of lattice distortion on the electronic structure and on magnetic properties of Cr3Co compound is studied. A spin polarization higher than 80% can be obtained between 5.55 and 5.90 angstrom. With increasing lattice constant, the magnetic moments on the (A, C) sites increase and the moments on the (B, D) sites decrease. They compensate each other and make the total magnetic moment change only slightly. (c) 2007 Elsevier B.V. All rights reserved.

Structural, elastic, and electronic properties of Fe3C from first principles

Abstract: Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010], and [001] directions. The three methods agree well with each other; the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

Ab-initio design of half-metallic fully-compensated ferrimagnets: the case of Cr2MnZ (Z= P, As, Sb, Bi) compounds

Abstract: Electronic structure calculations from first-principles are employed to design some new half-metallic fully-compensated ferrimagnets (or as they are widely known half-metallic antiferromagnets) susceptible of finding applications in spintronics. Cr2MnZ (Z= P, As, Sb, Bi) compounds have 24 valence electrons per unit cell and calculations show that their total spin moment is approximately zero for a wide range of lattice constants in agreement with the Slater-Pauling behavior for ideal half-metals. Simultaneously, the spin magnetic moments of Cr and Mn atoms are antiparallel and the compounds are ferrimagnets. Mean-field approximation is employed to estimate their Curie temperature, which exceeds room temperature for the alloy with Sb. Our findings suggest that Cr2MnSb is the compound of choice for further experimental investigations. Contrary to the alloys mentioned above half-metallic antiferromagnetism is unstable in the case of the Cr2FeZ (Z= Si, Ge, Sn) alloys.

In situ high temperature XRD studies of ZnO nanopowder prepared via cost effective ultrasonic mist chemical vapour deposition

Abstract: Ultrasonic mist chemical vapour deposition (UM-CVD) system has been developed to prepare ZnO nanopowder. This is a promising method for large area deposition at low temperature inspite of being simple, inexpensive and safe. The particle size, lattice parameters and crystal structure of ZnO nanopowder are characterized by in situ high temperature X-ray diffraction (XRD). Surface morphology of powder was studied using transmission electron microscopy (TEM) and field emission electron microscope (FESEM). The optical properties are observed using UV-visible spectrophotometer. The influence of high temperature vacuum annealing on XRD pattern is systematically studied. Results of high temperature XRD showed prominent 100, 002 and 101 reflections among which 101 is of highest intensity. With increase in temperature, a systematic shift in peak positions towards lower 2θ values has been observed, which may be due to change in lattice parameters. Temperature dependence of lattice constants under vacuum shows linear increase in their values. Diffraction patterns obtained from TEM are also in agreement with the XRD data. The synthesized powder exhibited the estimated direct bandgap (E g) of 3.43 eV. The optical bandgap calculated from Tauc’s relation and the bandgap calculated from the particle size inferred from XRD were in agreement with each other.

Growth of highly tensile-strained Ge on relaxed InxGa1−xAs by metal-organic chemical vapor deposition

Abstract: Highly tensile-strained Ge thin films and quantum dots have the potential to be implemented for high mobility metal-oxide-semiconductor field-effect transistor channels and long-wavelength optoelectronic devices. To obtain large tensile strain, Ge has to be epitaxially grown on a material with a larger lattice constant. We report on the growth of tensile-strained Ge on relaxed InxGa1−xAs epitaxial templates by metal-organic chemical vapor deposition. To investigate the methods to achieve high quality Ge epitaxy on III–V semiconductor surfaces, we studied Ge growth on GaAs with variable surface stoichiometry by employing different surface preparation processes. Surfaces with high Ga-to-As ratio are found to be necessary to initiate defect-free Ge epitaxy on GaAs. With proper surface preparation, tensile-strained Ge was grown on relaxed InxGa1−xAs with a range of In content. Low growth temperatures between 350 and 500 °C suppress misfit dislocation formation and strain relaxation. Planar Ge thin films with ...

Optoelectronic and electrochemical properties of nickel oxide (NiO) films deposited by DC reactive magnetron sputtering

Abstract: Nickel oxide (NiO) thin films were deposited onto glass substrates by the DC reactive magnetron sputtering technique. The as-deposited films were post-annealed in air at 450–500 °C for 5 h. The effect of annealing on the structural, microstructural, electrical and optical properties were studied by X-ray diffraction (XRD), atomic force microscope (AFM), four-probe resistivity measurement and UV-vis spectrophotometer. XRD studies indicated cubic structure with a lattice parameter of 0.4193 nm. The band gap of the films was found to be 3.58 eV. Fourier transform infrared (FTIR) studies indicated a broad spectrum centered at 451.6 cm −1 . Photoluminescence studies exhibited room temperature emission at 440 nm. Cyclic voltammetry studies in 1 M KOH solution revealed the electrochromic nature of the NiO films prepared in the present study.

Fast electron-correlation methods for molecular crystals: An application to the α, β1, and β2 modifications of solid formic acid

Abstract: A method for the routine first-principles determination of energies, structures, and phonons of molecular crystals by high-accuracy electron-correlation theories has been proposed It approximates the energy per unit cell of a crystal by a sum of monomer and dimer energies in an embedding field of self-consistent (and, therefore, polarizable) atomic charges and dipole moments First and second energy derivatives with respect to atom positions and lattice constants (useful for characterizing structures and phonons) have also been computed efficiently with a long-range electrostatic correction The method has been applied to solid formic acid modeled as infinite one-dimensional hydrogen-bonded chains Accurate energies (with corrections for basis-set superposition errors), structural parameters, and frequencies have been obtained for three polymorphic structures (β1, β2, and α) with second-order perturbation theory or higher On this basis, reliable assignments of their infrared, Raman, and inelastic neutro

Effect of Co addition on crystal structure and mechanical properties of Ti0.5CrFeNiAlCo high entropy alloy

Abstract: A high entropy alloy of Ti0.5CrFeNiAlCo can form two BCC crystal structures, one is with larger lattice parameter, and another with smaller one. In this paper, more Co was added in the Ti0.5CrFeNiAlCo alloy to detect the crystal structure and the properties changes. It is found that, with the increase of Co addition, the smaller BCC prefers to transit to FCC firstly, and the compressive strength deceased slightly.