Analysis of the Gd 4d-XY Auger spectrum using synchrotron radiation

We report the 4d-XY (X, Y = 5p, 4f, and the conduction band) Auger spectra of clean Gd using a monochromatic photon source with energies above and below the 3d threshold. The spectra with higher hv show the existence of intense spectator-hole Auger transitions. Comparison of these spectra with those obtained with a primary electron source allows detailed interpretation of the various features and explains the unusual spin polarization of the electron-induced spectrum reported earlier.

Effect of High Pressure on the Electrical Conductivity of TlInX2 (X = Se, Te) Layered Semiconductors

The dc electrical conductivity of TlInX2 (X = Se, Te) single crystals, parallel and perpendicular to the (001) c-axis is studied under high quasi-hydrostatic pressure up to 7.0 GPa, at room temperature. Conductivity measurements parallel to the c-axis are carried out at high pressures and down to liquid nitrogen temperatures. These materials show continuous metallization under pressure. Both compounds have almost the same pressure coefficient of the electrical activation energy parallel to the c-axis, d(ΔE∥)/dP = −2.9 × 10−10 eV/Pa, which results from the narrowing of the band gap under pressure. The results are discussed in the light of the band structure of these compounds.

Influence of deviation from stoichiometry on the photoluminescence in CdTe doped with indium

Low temperature photoluminescence of vacuum and cadmium annealed CdTe:In is reported here. A new peak at similar to 1.14 eV related to transitions from the conduction band to an acceptor involving a tellurium vacancy has been observed.

Origin of the insulating state in NaCuO2

We study the electronic structure of NaCuO2 by analysing experimental core level photoemission and X-ray absorption spectra using a cluster as well as an Anderson impurity Hamiltonian including the band structure of the oxygen sublattice. We show that the X-ray absorption results unambiguously establish a negative value of the charge transfer energy, A. Further, mean-field calculations for the edge-shared one-dimensional CuO2 lattice of NaCuO2 within the multiband Hubbard Hamiltonian show that the origin of the insulating nature lies in the band structure rather than in the correlation effects. LMTO-ASA band structure calculations suggest that NaCuO2 is an insulator with a gap of around 1 eV.

Photocatalysis on Fine Powders of Perovskite Oxides

Fine powders of semiconductor oxides have been widely used as photocatalysts for many reactions. Among the various photocatalytic reactions, water splitting has been given much importance, since it is a promising chemical route for solar energy conversion. Perovskite oxides, in particular SrTiO, have been commonly used as photocatalysts because some of them can decompose H,O into H, and 0, without an external bias potential (1). In turn, this is because the conduction band (CB) edges of some of the perovskite oxides are more negative than the H+/H, energy level. Since the catalytic activity is related to the surface properties of the solids, fine powders rather than single crystals are used. Photocatalysis on fine powers can be conveniently discussed in three parts, viz. preparation, characterization and their catalytic activity. Presently, photo-decomposition of water using SrTiO, fine powders is discussed in greater detail, although other photocatalytic reactions on various perovskite oxides are also briefly dealt with.

Seebeck Coefficient in Bulk and Quantum Wells of Non-Parabolic Kane-Type Materials

We present a simplified and quantitative analysis of the Seebeck coefficient in degenerate bulk and quantum well materials whose conduction band electrons obey Kane's non-parabolic energy dispersion relation. We use k.p formalism to include the effect of the overlap function due to the band non-parabolicity in the Seebeck coefficient. We also address the key issues and the conditions in which the Seebeck coefficient in quantum wells should exhibit oscillatory dependency with the film thickness under the acoustic phonon and ionized impurity scattering. The effect of screening length in degenerate bulk and quantum wells has also been generalized for the determination of ionization scattering. The well-known expressions of the Seebeck coefficient in non-degenerate wide band gap materials for both bulk and quantum wells has been obtained as a special case and this provides an indirect proof of our generalized theoretical analysis.

Photoemission study of pyrite-type transition-metal chalcogenides MS2-xSex (M=Fe, Co, Ni)

The electronic structures of pyrite-type transition-metal chalcogenides MS2-xSex (M = Fe, Co, Ni) has been investigated by photoemission and inverse-photoemission spectroscopy. The valence-band spectrum of ferromagnetic CoS2 does not show exchange splitting of the Co 3d peak, in disagreement with band-structure calculations. High-resolution photoemission spectra of NiS1.55Se0.45 shows spectral weight transfer from low (similar or equal to 50 meV) to high (0.2-0.5 eV) binding energies, in going from the metallic to the insulating phase.

First-principle description of magnonic PdnFem multilayers

Ab-initio calculations are used to determine the parameters that determine magnonic band structure of PdnFem multilayers (n = 2, m <= 8). We obtain the layer-resolved magnetization, the exchange coupling, and the magnetic anisotropy of the Pd-Fe structures. The Fe moment is 3.0 mu(B) close to the Pd layers and 2.2 mu(B) in the middle of the Fe layers. An intriguing but not usually considered aspect is that the elemental Pd is nonmagnetic, similar to Cu spacer layers in other multilayer systems. This leads to a pre-asymptotic ferromagnetic coupling through the Pd (about 40 mJ/m(2)). Furthermore, the Pd acquires a small moment due to spin polarization by neighboring Fe atoms, which translates into magnetic anisotropy. The anisotropies are large, in the range typical for L1(0) structures, which is beneficial for high-frequency applications. (C) 2011 American Institute of Physics. doi:10.1063/1.3556763]

Thiophene adsorption studies on clean and hydrogen-preadsorbed MoS2(100) surface

It is known from temperature-programmed desorption studies that the binding energy of thiophene over Mo/gamma-Al2O3 and Co-Mo/gamma-Al2O3, hydrodesulfurization catalysts, is lower in the presence of hydrogen. The adsorption of thiophene on clean and hydrogen-adsorbed MoS2 was modelled using extended Huckel tight binding band structure calculations. In the eta(1) adsorption configuration the calculations show a lower binding energy for adsorption on the hydrogen-preadsorbed surface similar to that observed experimentally. The lowering is due to an increased occupancy of the Mo density of states in the presence of hydrogen.

Multi-Mode Phonon-Controlled Field Emission from Carbon Nanotubes: Modeling and Experiments

The main idea proposed in this paper is that in a vertically aligned array of short carbon nanotubes (CNTs) grown on a metal substrate, we consider a frequency dependent electric field, so that the mode-specific propagation of phonons, in correspondence with the strained band structure and the dispersion curves, take place. We perform theoretical calculations to validate this idea with a view of optimizing the field emission behavior of the CNT array. This is the first approach of its kind, and is in contrast to the the conventional approach where a DC bias voltage is applied in order to observe field emission. A first set of experimental results presented in this paper gives a clear indication that phonon-assisted control of field emission current in CNT based thin film diode is possible.

Solute solute and solvent solute interactions in solid solutions of Cu+Sn, Au+Sn and Cu+Au+Sn alloys

The chemical potentials of tin in its α-solid solutions with Cu, Au and Cu + Au alloys have been measured using a gas-solid equilibration technique. The variation of the excess chemical potential of tin with its composition in the alloy is related to the solute-solute repulsive interaction, while the excess chemical potential at infinite dilution of the solute is a measure of solvent-solute interaction energies. It is shown that solute-solute interaction is primarily determined by the concentration of (s + p) electrons in the conduction band, although the interaction energies are smaller than those predicted by either the rigid band model or calculation based on Friedel oscillations in the potential function. Finally, the variation of the solvent-solute interaction with solvent composition in the ternary system can be accounted for in terms of a quasi-chemical treatment which takes into account the clustering of the solvent atoms around the solute.

Role of the cesium antimonide layer in the Na2KSb/Cs3Sb photocathode

When radiation of sufficiently high energy is incident on the surface of a semiconductor photocathode, electrons are excited from the valence band to the conduction band and these may contribute to the photocurrent. The photocurrent in a single-layer cathode is found to be small, because of collisions within the cathode material, the electron affinity condition, etc. It is observed that when a thin layer of n-type cesium antimonide (Cs3Sb) is deposited over a p-type layer of sodium potassium antimonide (Na2KSb), there occurs a sharp rise in the photocurrent. The causes for the dramatic increase in the photocurrent obtainable from a sodium potassium antimonide cathode, by depositing a thin layer of cesium antimonide are analyzed in this article. It has been shown that the interface between sodium potassium antimonide and cesium antimonide can result in lowering of the electron affinity to a level below the bottom of the conduction band of sodium potassium antimonide. The drift field that arises at the heterointerface enables the electrons to reach the surface, leading to the emission of almost all the photogenerated electrons within the cathode. The processes involved in photoemission from such a double-layer cathode are examined from a theoretical point of view. The spectral response of the two-layer cathode is also found to be better than that of a single-layer cathode.

Structural, thermal, and electrical properties of CrSi2

Stoichiometric CrSi2 was prepared by arc melting and compacted by uniaxial hot pressing for property measurements. The crystal structure of CrSi2 was investigated using the powder x-ray diffraction method. From the Rietveld refinement, the lattice parameters were found to be a = 4.427 57 (7) and c = 6.368 04 (11) Å, respectively. The thermal expansion measurement revealed an anisotropic expansion in the temperature range from room temperature 800 K with αa = 14.58×10−6/K, αc = 7.51×10−6/K, and αV = 12.05×10−6/K. The volumetric thermal expansion coefficient shows an anomalous decrease in the temperature range of 450–600 K. The measured electrical resistivity ρ and thermoelectric power S have similar trends with a maxima around 550 K. Thermal conductivity measurements show a monotonic decrease with increasing temperature from a room temperature value of 10 W m−1 K−1. The ZT values increase with temperature and have a maximum value of 0.18 in the temperature range studied. An analysis of the electronic band structure is provided.

Simple theoretical analysis of the Fowler-Nordheim field emission from microstructures and quantum wires of optoelectronic materials

We present a simplified theoretical formulation of the Fowler-Nordheim field emission (FNFE) under magnetic quantization and also in quantum wires of optoelectronic materials on the basis of a newly formulated electron dispersion law in the presence of strong electric field within the framework of k.p formalism taking InAs, InSb, GaAs, Hg(1-x)Cd(x)Te and In(1-x)Ga(x) As(y)P(1-y) lattice matched to InP as examples. The FNFE exhibits oscillations with inverse quantizing magnetic field and electron concentration due to SdH effect and increases with increasing electric field. For quantum wires the FNFE increases with increasing film thickness due to the existence van-Hove singularity and the magnitude of the quantum jumps are not of same height indicating the signature of the band structure of the material concerned. The appearance of the humps of the respective curves is due to the redistribution of the electrons among the quantized energy levels when the quantum numbers corresponding to the highest occupied level changes from one fixed value to the others. Although the field current varies in various manners with all the variables in all the limiting cases as evident from all the curves, the rates of variations are totally band-structure dependent. Under certain limiting conditions, all the results as derived in this paper get transformed in to well known Fowler-Nordheim formula. (C) 2011 Elsevier Ltd. All rights reserved.

Environmental remediation by photocatalysis

Photocatalysis refers to the oxidation and reduction reactions on semiconductor surfaces, mediated by the valence band holes and conduction band electrons, which are generated by the absorption of ultraviolet or visible light radiation. Photocatalysis is widely being practiced for the degradation and mineralization of hazardous organic compounds to CO2 and H2O, reduction of toxic metal ions to their non-toxic states, deactivation and destruction of water borne microorganisms, decomposition of air pollutants like volatile organic compounds, NOx, CO and NH3, degradation of waste plastics and green synthesis of industrially important chemicals. This review attempts to showcase the well established mechanism of photocatalysis, the use of photocatalysts for water and air pollution control,visible light responsive modified-TiO2 and non-TiO2 based materials for environmental and energy applications, and the importance of developing reaction kinetics for a comprehensive understanding and design of the processes.