Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001476.pdf

Why nitrogen cannot lead to p-type conductivity in ZnO

Model metal/ceria and ceria/metal catalysts have been shown to be excellent systems for studying fundamental phenomena linked to the operation of technical catalysts. In the last fifteen years, many combinations of well-defined systems involving different kinds of metals and ceria have been prepared and characterized using the modern techniques of surface science. So far most of the catalytic studies have been centered on a few reactions: CO oxidation, the hydrogenation of CO2, and the production of hydrogen through the water–gas shift reaction and the reforming of methane or alcohols. Using model catalysts it has been possible to examine in detail correlations between the structural, electronic and catalytic properties of ceria–metal interfaces. In situ techniques (X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, infrared spectroscopy, scanning tunneling microscopy) have been combined to study the morphological changes under reaction conditions and investigate the evolution of active phases involved in the cleavage of C–O, C–H and C–C bonds. Several studies with model ceria catalysts have shown the importance of strong metal–support interactions. In general, a substantial body of knowledge has been acquired and concepts have been developed for a more rational approach to the design of novel technical catalysts containing ceria.

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Ceria-based model catalysts: fundamental studies on the importance of the metal–ceria interface in CO oxidation, the water–gas shift, CO2 hydrogenation, and methane and alcohol reforming

Nanoporous graphene as a reverse osmosis membrane: Recent insights from theory and simulation

The amorphous titanium dioxide (a-TiO2) has drawn attention recently due to the finding that it holds promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The mechanism of hole conductivity at a level much higher than the edge of the valence band is still a mystery. In this work, an amorphous TiO2 model is obtained from molecular dynamics employing the "melt-and-quench" technique. The electronic properties, polaronic states and the hole conduction mechanism in amorphous structure were investigated by means of density functional theory with Hubbard's energy correction (DFT + U) and compared to those in crystalline (rutile) TiO2. The formation energy of the oxygen vacancy was found to reduce significantly (by a few eV) upon amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels, which are comparable to experimental observations and could be responsible for hole conduction in the "leaky" TiO2 recently discovered for the photochemical water-splitting applications.

Oxygen vacancy and hole conduction in amorphous TiO2

Nanocomposites of selenium (Se) and ordered mesoporous silicon carbide-derived carbon (OM-SiC-CDC) are prepared for the first time and studied as cathodes for lithium-selenium (Li-Se) batteries. The higher concentration of Li salt in the electrolytes greatly improves Se utilization and cell cycle stability. Se-CDC shows significantly better performance characteristics than Se-activated carbon nanocomposites with similar physical properties. Se-CDC also exhibits better rate performance and cycle stability compared to similarly produced sulfur (S)–CDC for Li/S batteries.

Micro- and Mesoporous Carbide-Derived Carbon–Selenium Cathodes for High-Performance Lithium Selenium Batteries

Exploring bonding nature, electronic, optical and thermoelectric properties of ternary compound Na2ZnCl4 via first-principles calculations

Lithium oxide (Li2O) has been suggested as a suitable breeder blanket material for fusion reactors. Li+ vacancies are created by neutron irradiation, forming bulk defect complex whose extra character is experimentally unclear. We present a theoretical study of Li2O using density functional theory (DFT) with a plane-wave basis set. The generalized gradient approximation (GGA) and local-density approximation (LDA) were used for exchange and correlation. Here we address the total energy for defect free, cation defect, cation vacancy and vacancy formation energy in Li2O crystal in anti-fluorite structure.Lithium oxide (Li2O) has been suggested as a suitable breeder blanket material for fusion reactors. Li+ vacancies are created by neutron irradiation, forming bulk defect complex whose extra character is experimentally unclear. We present a theoretical study of Li2O using density functional theory (DFT) with a plane-wave basis set. The generalized gradient approximation (GGA) and local-density approximation (LDA) were used for exchange and correlation. Here we address the total energy for defect free, cation defect, cation vacancy and vacancy formation energy in Li2O crystal in anti-fluorite structure.

R. D. Eithiraj

Papers

Exploring bonding nature, electronic, optical and thermoelectric properties of ternary compound Na2ZnCl4 via first-principles calculations

Ab-initio calculation for cation vacancy formation energy in anti-fluorite structure

Vacancy formation energy in K2O