Alignment Of Vacancies Research Articles

Counterintuitive Reconstruction of the Polar O-Terminated ZnO Surface with Zinc Vacancies and Hydrogen.

Understanding the structure of ZnO surface reconstructions and their resultant properties is crucial to the rational design of ZnO-containing devices ranging from optoelectronics to catalysts. Here, we are motivated by recent experimental work that showed a new surface reconstruction containing Zn vacancies ordered in a Zn(3 × 3) pattern in the subsurface of (0001)-O-terminated ZnO. Reconstruction with Zn vacancies on (0001)-O is surprising and counterintuitive because Zn vacancies enhance the surface dipole rather than reduce it. In this work, we show using density functional theory (DFT) that subsurface Zn vacancies can form on (0001)-O when coupled with adsorption of surface H and are in fact stable under a wide range of common conditions. We also show that these vacancies have a significant ordering tendency and that Sb-doping-created subsurface inversion domain boundaries (IDBs) enhance the driving force of Zn vacancy alignment into large domains of the Zn(3 × 3) reconstruction.

Photon induced L3 vacancy alignment at tuned photon energies

Photon induced L3 X-ray measurements for L α /L l cross-section ratios in elements, 66 ⩽ Z ⩽ 83, at tuned photon energies on synchrotron Beamline-16 at Indus-2, India have been used to study the effect of Coster-Kronig (CK) transitions and photon energies on alignment of L3 vacancies. Certainty and reliability of the measurements were checked from comparison of measured L α and L l fluorescence cross-sections at E 1 excitation with available theoretical/empirical/experimental values that required additional measurements for source, geometry and efficiency factor S 0 GeL α/l in the used set-up. Fall/rise trend of the ratios with energy for different Z’s was found to resemble the off/on-set pattern of CK transitions as pointed out by Bambynek et al. and Campbell. Evaluated alignment parameter A 2 values are very much within the limits, 0.05 <A 2< 0.5, except for Lu, Ta and Pb at E 2 excitation and for W, Pt and Hg at E 1 excitation where values are <0.05. At E 3 excitation, the vacancies only in L3 are aligned (A 2 ≦ 0.2) in all the cases. The pattern of A′ 2 (Coster-Kronig corrected A 2) variation with energy for Dy, W, Pt, Hg and Bi resembles our previously reported theoretical patterns that lends mutual support for both current measurements and earlier theoretical results.

Understanding the early stages of growth of Ge on Si(001) from lattice based simulations

A modified lattice model is developed to study the growth of Ge on Si(001). In addition to the usual bond energies, surface reconstruction and elastic strain are explicitly taken into account in this study. We introduce an additional rebonding energy for formation of vacancies in the dimer chains. Kinetic Monte Carlo simulations of growth and annealing successfully reproduce several important features in the range of growth from a small fraction of a monolayer to about one monolayer thick. These include the formation of dimer chains, formation and alignment of vacancies in the dimer chains and the peak in the dimer chain length distribution. We carefully analyze separately the role of mismatch strain and surface reconstruction on this behavior. Through this work, we show a significantly simplified, but nevertheless quantitatively accurate model for the early stages of growth of Ge on Si(001).

Disparity in formulations used for fluorescent X-ray intensity measurements

The paper presents a problem in computations of X-ray fluorescence cross-sections, shell/sub-shell fluorescence yields, Coster–Kronig yields, vacancy alignment, etc. from X-ray fluorescence (XRF) studies. While using barn/atom as a unit for cross-sections if the atomic masses are not considered it causes a discrepancy in the measured cross-section, yield and alignment values. Most of the earlier publications are being quoted where such an oversight has occurred and discrepancy is evident.

Analytic perturbation theory to study the effect of screening on alignment of atomic inner shell vacancies

An analytic perturbation theory has been developed for screened Coulomb radial wave functions by interpolating the screened Coulomb potential of McEnnan for elements lanthanum to uranium and was used to calculate alignment of vacancies in L3 sub shell. To check the authenticity of the present method of calculation, the intermediate steps are compared with our earlier formulation of point Coulomb potential for the calculations. A close agreement has been obtained among the values of intermediate steps involved in both the formulations that proved the reliability of present calculations and prompted us to extend the calculations for the elements lanthanum to uranium in the energy region threshold to 60 keV.

Reconstruction and alignment of vacancies in carbon nanotubes

Vacancies in carbon nanotubes usually aggregate into larger vacancies. Using first-principles and tight-binding calculations, we investigate the alignment of missing atoms and the movement of pentagon-heptagon defects that are formed by reconstructions in large vacancy clusters ${V}_{n}$ $(n\ensuremath{\le}36)$, where $n$ is the number of missing atoms. In nanotubes with small diameters, missing atoms have a tendency to form a serial network rather than a large hole due to the existence of large curvatures. It is generally found that the parallel alignment of missing atoms along the tube axis is energetically more favorable than the spiral alignment. Thus, the removal of atoms leads to the longitudinal movement of a pentagon-heptagon defect on the tube wall, which is in good agreement with the kink motion observed during superplastic deformation of single-wall nanotubes. The preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.

Equilibrium vacancy concentrations inAl-Σ=33(554)[11¯0]by grand canonical Monte Carlo simulations

A Monte Carlo simulation of equilibrium thermal vacancies is extended to grain boundaries in Al. The vacancy localization is explored by intensive molecular-dynamics simulations. It is found that, in the symmetrical tilt boundary $\ensuremath{\Sigma}=33(554)[1\overline{1}0]$, the reference lattice, based on the coincident site lattice geometrical model, is good enough to describe the microstates of the system up to $T=700\text{ }\text{K}$ in the presence of vacancies and vacancy clusters. A microstate is therefore defined by site occupancies and continuous displacements from the lattice nodes. This model, intermediate between an Ising model and the representation of states in classical statistical mechanics, is used to perform grand canonical Monte Carlo simulations. Vacancy concentrations along the grain boundary are computed from the average site occupancies. Vacancy alignment along the tilt axis is reported. The results are analyzed in the framework of a one-dimensional Ising model which incorporates vacancy free formation energies and pair interaction free energies.

Oxygen vacancy configuration of δ‐Bi2O3: an ab initio study

AbstractThe oxygen vacancy configurations and electronic structures of δ‐Bi2O3 have been studied using the ab initio calculation full‐potential linearized augmented plane‐wave (FPLAPW) method and considering the relativistic effect in Bi atoms in terms of spin–orbital coupling. Three vacancy structures, 〈111〉, 〈110〉 and 〈100〉, were analysed. It was found that the layered δ‐Bi2O3 with 〈110〉 vacancy alignment is the most stable. An indirect band gap of 0.46 eV was observed for this structure. Additionally, the role of lone‐pair electrons as well as polarization during the diffusion of oxygen atoms have been investigated. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electronic structure of theαandδphases ofBi2O3: A combinedab initioand x-ray spectroscopy study

$\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ is the thermodynamically stable phase of ${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ at room temperature. We have performed a theoretical and experimental investigation of its electronic structure using a combination of gradient corrected density functional theory (DFT), along with x-ray photoemission and $\mathrm{O}\text{\ensuremath{-}}K$ shell x-ray absorption and emission spectroscopies. We examine the nature of bonding in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ and in particular explore the nature of the stereochemically active Bi electron lone pair. The Bi $6s$ states are found to be concentrated at the bottom of the valence band but the states contributing to the lone pair on Bi are derived from the top of the valence band. Mixing between O $2p$ and Bi $6s$ states is found to be crucial in producing the asymmetric density on Bi. The role of the lone pair in the fast ion conductor $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ is also investigated, through calculation of the electronic structure with ⟨100⟩, ⟨110⟩, and ⟨111⟩ alignment of oxygen vacancies. Alignment of the vacancies along ⟨100⟩ results in the most energetically favorable configuration of the $\ensuremath{\delta}$ phase, contrary to previous force field calculations and electrostatic arguments which favor the ⟨111⟩ alignment.

Study of photon-induced L3 vacancy alignment for elements La to U

Alignment of photon-induced L3 vacancies is studied in rare earth and highZ elements at energies of experimental interest, near thresholds to 60 keV, under nonrelativistic dipole approximation. Numerical calculations of the matrix element are undertaken to produce theoretical data for comparison with the experimental findings. The A2 values being s>0.1 at photoelectron energies <20 keV are certainly higher than 5–8% uncertainties quoted in experimental results. Present findings are from a very basic model, hydrogen-like and can further be treated as reference to observe the impact of screening, relativistic, multipole and retardation corrections to the model

Electric-field and interface effects on the orientation of Jahn-Teller vacancy distortions in semiconductors

Energy variation of various Jahn-Teller vacancy configurations in thin near-surface layers of a semiconductor is considered. The calculations are made within a model in which each dangling bond near a vacancy is described by the wave function of an electron localized at a defect with a zero-range potential. The Jahn-Teller stabilization energy was assumed to be much smaller than the crystal-field splitting but substantially larger than the energy variations introduced by other perturbations. It is shown that vacancy alignment in semiconductor layers with thickness ⩾100 A at liquid-nitrogen temperatures and lower can be most effectively caused by the electric field which can exist in such layers due to a surface-state charge. The direct effect of the interface on both the electronic and nuclear energies of various configurations is weak and can become manifest only in layers less than 10 A thick at liquidhelium temperature.

Theory of alignment of atoms following ion-atom collisions

The theory of inner-shell vacancy alignment in ion-atom collisions is reviewed and comparisons to the experimental data are made. Alignment in total as well as differential inelastic scattering cross sections are considered, and the importance of Coulomb deflection and higher multipolarities is stressed. Whereas first-order theories generally work well for projectiles with low charge number, higher-order (coupled-channel-) effects are important for the less asymmetric systems. Despite much recent work in this field many problems remain open, both on the theoretical and the experimental side.

Integral and differential alignment of atomic inner-shell vacancies in the semiclassical approximation

The semiclassical approximation (SCA) is used to obtain general expressions for the statistical tensors describing the polarisation state of vacancies in inner shells of atoms produced by fast proton or ion impact. The asymptotic behaviour of alignment is examined in the range of high impact parameters. The restrictions imposed on the statistical tensors by the symmetry of the experiment and by the additional symmetry introduced by the various SCA versions are discussed. The integral and differential alignment of vacancies in the L and M shells of atoms is calculated systematically allowing for the Coulomb deflection of ions by the target nucleus. The deflection is allowed for by a tangential approximation of hyperbolic trajectory. The atom is described by the Hartree-Salter self-consistent-field model. The results of the calculation are compared with the experimental and calculated data published elsewhere. The effect of the screening of nuclear charge of an ion by its electrons on the differential alignment is investigated.

Polarization of the characteristic X-radiation induced by channeled particle impact

The polarization of characteristic X-rays emitted by atoms of a single crystal, when their inner shells are excited by fast protons (ions) moving under planar channeling conditions, is studied theoretically. A quantum mechanical treatment based on first order perturbation theory is suggested for calculating the production cross section and alignment of inner-atomic-shell vacancies induced by channeled particle impact. Calculations have shown that the vacancy alignment and polarization of the following X-radiation can differ considerably for the inner shell ionization by an ion moving in a planar channel and in a random direction. The degree of polarization of X-rays turns out to be sensitive to the shape of the average planar potential.

Application of distorted-wave approximation to the study of inner-shell alignment by electron impact

The distorted-wave Born approximation is used to calculate the 2p3/2 vacancy alignment in ionisation of Mg and Ar by electron impact. The distortion of incoming and outgoing electron waves by a static atomic field is taken into account. The distorted-wave calculation allows one to explain the experimentally observed drastic increase of the alignment with decreasing incident electron energy. The threshold behaviour of the alignment in the Born approximation and in the distorted-wave approximation are discussed.