Valence Structure Research Articles

Effects of heat treatment on diffusion of Cu atoms into CdTe single crystals

Angular dependence of x-ray fluorescence and x-ray absorption fine structure techniques have been used to study the diffusion of Cu atoms into the photovoltaic material CdTe. Depth profile, effective valency, and local structure of Cu atoms in a Cu-doped single crystal of CdTe were investigated before and after a second heat treatment. Enhanced Cu diffusion into the CdTe single crystal was observed as a result of heating at a moderate temperature around 200 °C, resulting in a redistribution of the Cu impurities through a broader depth profile. Some of the Cu atoms are believed either to form small complexes with Te or occupy interstitial sites in the host but accompanied by a large local lattice distortion while others substitute for Cd on the cation sites. The results thus demonstrate that these nondestructive x-ray characterization methods are useful for probing microstructural changes in CdTe photovoltaic materials/devices in which some Cu-containing compounds are used as back contacts.

All acyclic hydrocarbons: formula periodic table and property overlap plots via chemical combinatorics

It is sought to extend the characterization of isomer classes by using combinatoric techniques long used for isomer enumerations. To this end a general family of isomers (which is challenging to deal with even at the level of conventional enumeration) is considered, namely that of all acyclic hydrocarbons CnH2m with classical valence structures. This set of hydrocarbons is partitioned into structural-isomer classes, and these in turn are presented in the form of a ‘‘ formula periodic table’’ with n and m identifying rows and columns, respectively. Then various isomer-class-averaged characteristics (beyond isomer enumerations) are sought to be entered into the associated positions in this periodic table. Such characteristics include heat of formation, magnetic susceptibility and index of refraction, each of which may be estimated via substructural ‘‘cluster expansion’’. These cluster-expansion-estimated property averages as well as associated standard deviations are developed by way of a generating-function chemical combinatorics for isomers of up to n=25 carbons and 2m=52 hydrogens. These results then are incorporated in property ‘‘ overlap plots’’ to reveal overall isomer-class property trends in the periodic table for this set of ≈1013 structural isomers distributed over ≈360 isomer classes.

Quark-hadron duality and nucleon valence structure

A newly-obtained data sample of inclusive electron-nucleon scattering from deuterium and hydrogen targets at Jefferson Lab has been analyzed for precision tests of quark-hadron duality. In all cases, duality appears to be a non-trivial dynamic property of the nucleon structure function. Assuming duality, the proton magnetic form factor is extracted from the inelastic data alone, and found to be in good agreement with the world's data. Higher twist contributions are found to be small on average, even down to Q 2 ∥ 0.5 GeV 2. The investigation yields a scaling curve from duality arguments which resembles deep-inelastic neutrino-nucleus scattering data, indicating a potential sensitivity to valence and valence-like structure.

Electronic band structure of magnesium and magnesium oxide: experiment and theory

Electron momentum spectroscopy (EMS) has been used to measure the valence band electronic structure of thin magnesium and magnesium oxide films. The band structures have also been calculated within the linear muffin-tin orbital (LMTO) approximation. The free-electron-like parabola characteristic of metallic solids was observed for magnesium with a bandwidth of approximately 6 eV, in agreement with previous measurements. The inclusion of energy broadening due to finite hole-lifetime effects and a Monte Carlo simulation of multiple scattering events gives good agreement between calculated and measured band structures. However, we measure a much higher intensity due to plasmon excitation compared with the simulated intensity. Upon oxidation the valence structure splits into two distinct, less dispersive bands typical of an ionic solid. Intensity due to plasmon excitation was almost completely absent in the experimental spectra for magnesium oxide. The LMTO calculation reproduces the overall structure and dispersion range of the oxide. The measured and calculated energy gap between upper and lower valence bands and their relative intensities do not agree quantitatively. This discrepancy may be due to a contribution of magnesium s states to the predominantly oxygen p states in the upper band.

Embedded graph invariants in Chern-Simons theory

Chern-Simons gauge theory, since its inception as a topological quantum field theory, has proved to be a rich source of understanding for knot invariants. In this work the theory is used to explore the definition of the expectation value of a network of Wilson lines — an embedded graph invariant. Using a generalization of the variational method, lowest-order results for invariants for graphs of arbitrary valence and general vertex tangent space structure are derived. Gauge invariant operators are introduced. Higher order results are found. The method used here provides a Vassiliev-type definition of graph invariants which depend on both the embedding of the graph and the group structure of the gauge theory. It is found that one need not frame individual vertices. However, without a global projection of the graph there is an ambiguity in the relation of the decomposition of distinct vertices. It is suggested that framing may be seen as arising from this ambiguity — as a way of relating frames at distinct vertices.

Electronic structure of silicon nitride

The electronic structure of valence and conduction bands of silicon nitride SiN x layer deposited on a silicon substrate was studied using X-ray photoelectron spectroscopy (XPS) and X-ray bremsstrahlung isochromat spectroscopy (BIS), respectively, and compared with the density of states (DOS) taken from the literature. In the isochromat spectrum measured at photon energy 5415 eV, a strong maximum at 9.7 eV and a weak extended structure in the energy range up to 200 eV above the BIS edge were observed. A theoretical calculation of extended X-ray bremsstrahlung isochromat fine structure (EXBIFS) for silicon nitride has been performed using a multiple scattering method and partial probabilities of X-ray bremsstrahlung transitions calculated for continuum states in a dipole approximation. The results have shown that the X-ray bremsstrahlung isochromat fine structure for silicon nitride appears mainly due to electron transitions to the s states localized near Si ions.

X-ray photoemission characterization of La0.67(CaxSr1−x)0.33MnO3 films

The Curie temperature and x-ray photoemission spectra of thin films of La0.67(CaxSr1−x)0.33MnO3 have been studied as a function of the Ca/Sr ratio. The films were grown by off-axis cosputtering from individual targets of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) onto (100) oriented NdGaO3 substrates. The films grow with a (100) orientation, with no other orientations observed by x-ray diffraction. For the alloy mixtures, the Curie temperature, TC, varies slowly as the Ca/Sr is decreased, remaining ≈300 K, while for the LCMO and LSMO films TC is 260 and 330 K, respectively. The Mn-O valence structure is composed of two dominant peaks, whose positions undergo a change as the Ca fraction is decreased. The core lines behave as linear combinations of lines from pure LCMO and LSMO.

Proton and pion structure functions in the improved valon model

The proton structure functions and their moments, at low ${Q}^{2},$ and pion structure functions are derived and analyzed in an improved version of the valon model. Proton structure functions are compared with experiment data from deep inelastic scattering while pion structure functions are compared to Drell-Yan scattering where the invariant mass of the produced muon pairs falls in the range $4.05<{m}_{\ensuremath{\mu}\ensuremath{\mu}}<8.55 \mathrm{GeV}{/c}^{2}$ at a fixed ${Q}^{2}$ of $25 ({\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{c}}^{2}{)}^{2}.$ The first two moments for the pion valence structure function at ${Q}^{2}=49 ({\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{c}}^{2}{)}^{2}$ are also compared with the results from the other models and experimental data. We conclude that this simple, improved physical picture is relevant as far as the presently attainable experimental data are concerned.

Density Functional Study on the Structures and Energies of the Ti2C2Cluster

The structures and energies of the binary dititanium dicarbide, Ti2C2, in the lowest singlet, triplet, and quintet states have been investigated by density functional theory using the hybrid B3LYP functionals. Geometries and frequencies for a number of isomeric structures are presented at the B3LYP level. A rhombic structure with a transannular C−C bond (VI) is found to be the global Ti2C2 minimum. The other four-membered planar ring structures (VII and VIII) derived from the two-fold addition of carbon to the Ti−Ti or the Ti−C bond of the cyclic (C2v) Ti2C are characterized as higher energy local minima. A linear structure (I) with terminal titanium atoms is found to be a higher energy minimum in its singlet, triplet, and quintet potential energy surfaces (PES) and has a bonding characteristic of a cumulene-like valence structure. Besides, a unique nonplanar C2v structure, which has not been obtained in the isovalence electronic Si2C2 and SiC3 systems, has been identified as a minimum. Cyclic structures ...

Formula periodic table for acyclic hydrocarbon isomer classes: combinatorially averaged graph invariants

The overall isomer classes of acyclic hydrocarbons CnH2m with classical valence structures can be presented in the form of a formula “periodic table’' with n and m identifying rows and columns, respectively. Such a table may be used to reflect property trends of isomer classes. A general approach is presented for obtaining isomer-class averages of specified molecular properties, and the approach is extended to deal with mean square-deviations, so as to estimate the property overlap between the different isomer classes. Here the properties considered include graph diameters, molecular branching, counts of bonds of different multiplicities, and counts of sites of different hybridizations. Contour plots in the “periodic table’' illustrate overall property trends for families of up to about 1012 structural isomers (as arise for isomer classes of up to n=25 carbon atoms). Evidently, an overall view of various property trends of acyclic hydrocarbon isomer classes is possible.

Construction of interstellar cumulenes and heterocumulenes: Mass Spectrometric Studies

The last few years have brought an increasing interest in the chemistry of the interstellar and circumstellar environs. Many of the molecular species discovered in remote galactic regions have been dubbed `non-terrestrial' because of their unique structures (Thaddeus et al., 1993). These findings have provided a challenge to chemists in many differing fields to attempt to generate these unusual species in the laboratory. Of particular recent interest have been the unsaturated hydrocarbon families, CnH and CnH2, which have been pursued by a number of diverse methodologies. A wide range of heterocumulenes, including CnO, HCnO, CnN, HCnN, CnS, HCnS, CnSi, and HCnSi have also provided intriguing targets for laboratory experiments. Strictly, the term cumulene refers to a class of compounds that possess a series of adjacent double bonds, with allene representing the simplest example (H2CCCH2). However, for many of the non-terrestrial molecules presented here, the carbon chain cannot be described in terms of a single simple valence structure, and so we use the terms cumulene and heterocumulene in a more general sense: to describe molecular species that contain an unsaturated polycarbon chain. Mass spectrometry has proved an invaluable tool in the quest for interstellar cumulenes and heterocumulenes in the laboratory. It has the ability, in its many forms, to (i) generate charged analogs of these species in the gas phase, (ii) probe their connectivity, ion chemistry, and thermochemistry, and (iii) in some cases, elucidate the neutrals themselves. Here, we will discuss the progress of these studies to this time. © 1999 John Wiley & Sons, Inc., Mass Spec Rev 18: 131–151, 1999

The Mixed Valence Structure of “R-NiF3”

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The Mixed Valence Structure of “R–NiF3”

XAS at the nickel K-edge and UV-visible studies of rhombohedral nickel trifluoride indicate that the compound has a mixed valence composition Ni[NiF6] and is isostructural with Pd[PdF6]. XAS data are reported for NiF2, [NiF6]2–, [NiF6]3–, Pd[PdF6], CoF3 and RhF3 in support of the Ni[NiF6] study. New data from refinements of X-ray powder diffraction of CoF3 and IrF3 are reported in the context of the distortion from hexagonal close packing of rhombohedral trifluorides. Die Struktur von gemischt-valentem „R–NiF3” Rontgen-Absorptionsspektren (XAS) an der Nickelkante und UV-Vis-Untersuchungen an rhomboedrischem Nickeltrifluorid zeigen, das diese Verbindung, Ni[NiF6], eine gemischt-valente Zusammensetzung hat und isostrukturell mit Pd[PdF6] ist. XAS-Daten von NiF2, [NiF6]2–, [NiF6]3–, Pd[PdF6], CoF3 und RhF3 werden zur Bestatigung der Untersuchung an Ni[NiF6] mitgeteilt. Neue Daten der Verfeinerung von Rontgen-Pulverdiffraktogrammen von CoF3 und IrF3 werden im Zusammenhang mit der Verzerrung der hexagonal-dichtesten Packung der rhomboedrischen Trifluoride mitgeteilt.

Inner valence molecular orbitals and structure of the X-ray O4,5(Th,U) emission spectra in thorium and uranium oxides

In the present work, the possibility of an influence of the chemical environment on the structure of O4,5(Th,U) emission spectra of thorium and uranium oxides was studied. It is noted that the fine structure in such spectra is caused by the formation of inner valence molecular orbitals, involving relatively deep Th(U) 6p and O(F) 2s atomic orbitals of the neighbouring atoms. This leads to differences between the O4,5(Th,U) emission spectra of ThO2, ThF4, UO2 and γ-UO3. Also, a comparison between X-ray photoelectron spectroscopy and X-ray emission spectroscopy data was made for the studied compounds. This allowed us to conclude that they are in good agreement.

Functional and Smart Materials: Structural Evolution and Structure Analysis

Structure and Structural Evolution: Structure, Bonding and Properties. Sodium Chloride and Rutile Related Structure Systems. Perovskite and Related Structure Systems. Fluorite-Type and Related Structure Systems. From Structural Units of Materials Engineering via Soft-Chemistry. Structure Characterizations: Electron Crystallography for Structure Analysis. Structure Analysis of Functional Materials. Chemicals and Valence Structure Analysis of Functional Materials. Appendices: Physical Constants, Electron Wavelengths and Wave Numbers. Crystallographic Structure Systems. FORTRAN Program for Calculating Crystallographic Data. Electron Diffraction Patterns for Several Types of Crystal Structures. FORTRAN Program for Calculating Single Valence-Loss EELS Spectra in TEM. Index.

Density Functional Study on the Structures and Energies of the Ti2C3 Cluster

The structures and energies of the binary dititanium tricarbide Ti2C3 in the lowest singlet, triplet, and quintet states have been investigated by density functional theory using the hybrid B3LYP functionals. Geometries and frequencies for a number of isomeric structures are presented at the B3LYP level. A pentagonal structure with a C2 unit at the base and a carbon at the apex, Xs, is found to be the global Ti2C3 minimum. Cyclic structures V−VIII derived from the 2-fold addition of titanium to the most stable “fan” and “kite” isomers of TiC3 are characterized as higher energy local minima. Linear structures I−IV with all probable relative positioning of the two titanium atoms have been identified as higher order saddle points with respect to the degenerate bending vibration, except for the cumulene-like valence structure I. Cyclic structures are energetically favored over the linear structures. A comparison with the penta-atomic group IVA silicon−carbon clusters is given, where appropriate.

Nitrogen in germanium

The known properties of nitrogen as an impurity in, and as an alloy element of, the germanium network are reviewed in this article. Amorphous and crystalline germanium–nitrogen alloys are interesting materials with potential applications for protective coatings and window layers for solar conversion devices. They may also act as effective diffusion masks for III-V electronic devices. The existing data are compared with similar properties of other group IV nitrides, in particular with silicon nitride. To a certain extent, the general picture mirrors the one found in Si–N systems, as expected from the similar valence structure of both elemental semiconductors. However, important differences appear in the deposition methods and alloy composition, the optical properties of as grown films, and the electrical behavior of nitrogen-doped amorphous layers. Structural studies are reviewed, including band structure calculations and the energies of nitrogen-related defects, which are compared with experimental data. Many important aspects of the electronic structure of Ge–N alloys are not yet completely understood and deserve a more careful investigation, in particular the structure of defects associated with N inclusion. The N doping of the a-Ge:H network appears to be very effective, the activation energy of the most effectively doped samples becoming around 120 meV. This is not the case with N-doped a-Si:H, the reasons for the difference remaining an open question. The lack of data on stoichiometric β-Ge3N4 prevents any reasonable assessment on the possible uses of the alloy in electronic and ceramic applications.

Resonant Photoemission Studies of Thulium Monochalcogenides around the Tm 4dThreshold

Electronic structures of thulium monochalcogenides (TmS, TmSe, TmTe) were studied both experimentally and theoretically using a resonant photoemission technique in the photon energy range around Tm 4 d edge (100–190 eV). The experimental results show a large resonant enhancement of 4 f photoelectron emission around the 4 d threshold with sharper multiplet structures than those reported before. The calculated results are in good agreement with the experimental ones both for divalent and trivalent features of Tm ions. We observed different types of valence structures for TmS, TmSe and TmTe compounds, due to change in mean valence. We have estimated the mean valence of each sample from absorption spectra and from off-resonant photoemission spectra. The meaning of these values is discussed as compared with those for the values obtained from the measurements of magnetic susceptibilities.

Clar Polynomials of Large Benzenoid Systems

We discuss the constructions of Clar-like valence structures and the associated Clar polynomials for large benzenoids. We have shown that Clar polynomial satisfy a theorem analogous to the Clarke's theorem for the characteristic polynomial of graphs, which allows one to reconstruct the polynomial of a system from the polynomials associated with qualified substructures. In the case of Clar's polynomial the substructures are obtained by deleting individual benzene rings from the considered benzenoid, one at a time. This property of Clar polynomial allows one to construct the polynomial of larger systems from known Clar's polynomials of smaller benzenoids.

Band Dispersions in Photoluminescent Porous Si

The same band dispersions as those obtained from crystalline Si along the $\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Delta}\ensuremath{-}X$ symmetry line are first observed from photoluminescent porous Si independent of its microstructure and porosity using an angle-resolved photoemission technique. The electronic structures of porous Si observed in the normal photoemission, valence photoemission, and $L\ensuremath{-}\mathrm{edge}$ x-ray absorption spectra are interpreted consistently to result from the overlap of that of bulk crystalline Si and that relating to the receding valence structure, which is suggested experimentally to be linked to the luminescence origin.