Impact of amorphization on the electronic properties of Zn-Ir-O systems.

We analyze the geometry and electronic structure of a series of amorphous Zn-Ir-O systems using classical molecular dynamics followed by density functional theory taking into account two different charge states of Ir (+3 and  +4). The structures obtained consist of a matrix of interconnected metal-oxygen polyhedra, with Zn adopting preferentially a coordination of 4 and Ir a mixture of coordinations between 4 and 6 that depend on the charge state of Ir and its concentration. The amorphous phases display reduced band gaps compared to crystalline ZnIr2O4 and exhibit localized states near the band edges, which harm their transparency and hole mobility. Increasing amounts of Ir in the Ir(4+) phases decrease the band gap further while not altering it significantly in the Ir(3+) phases. The results are consistent with recent transmittance and resistivity measurements.This is the final version of the article. It first appeared from IOP Science via https://doi.org/10.1088/0953-8984/28/34/34550

The effect of defects and disorder on the electronic properties of ZnIr2O4.

We analyze by means of ab initio calculations the role of imperfections on the electronic structure of ZnIr2O4, ranging from point defects in the spinel phase to the fully amorphous phase. We find that interstitial defects and anion vacancies in the spinel have large formation energies, in agreement with the trends observed in other spinels. In contrast, cation vacancies and antisites have lower formation energies. Among them, the zinc antisite and the zinc vacancy are the defects with the lowest formation energy. They are found to act as acceptors, and may be responsible for the spontaneous hole doping in the material. They may also induce optical transitions that would reduce the transparency of the material. Amorphization of ZnIr2O4 leads a large decrease of the band gap and appearance of localized states at the edges of the band gap region, which may act as charge traps and prevent amorphous ZnIr2O4 from being a good hole conductor.Financial support for this work is provided by the European Commission through contract No.NMP3-LA-2010-246334 (ORAMA). We acknowledge computational support from the UK national high performance computing service ARCHER, for which access was obtained via the UKCP consortium and funded by EPSRC grant EP/K014560/1.This is the final version of the article. It was originally published by AIP in The Journal of Chemical Physics here: http://scitation.aip.org/content/aip/journal/jcp/141/8/10.1063/1.4893556

A computational study of the quantum transport properties of a Cu-CNT composite.

The quantum transport properties of a Cu-CNT composite are studied using a non-equilibrium Green's function approach combined with the self-consistent-charge density-functional tight-binding method. The results show that the electrical conductance of the composite depends strongly on CNT density and alignment but more weakly on chirality. Alignment with the applied bias is preferred and the conductance of the composite increases as its mass density increases.The European Research Council provided financial support for this work under the Seventh Framework Program FP7/2007-2013 (ERC grant agreement no. 259061). Computational support from the Cambridge High Performance Computing Cluster is gratefully acknowledged.This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C5CP01470

Synthesis, Crystal Structure and Properties of a Perovskite-Related Bismuth Phase, (NH4)3Bi2I9

Organic-inorganic halide perovskites, especially methylammonium lead halide, have recently led to a remarkable breakthrough in photovoltaic devices. However, due to the environmental and stability concerns of the heavy metal, lead, in these perovskite based solar cells, research in the non-lead perovskite structures have been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, was prepared in solution and the structure was solved by single crystal X-ray diffraction. The results from DFT calculations showed the significant lone pair effect of the bismuth ion and the band gap was measured as around 2.04 eV, which is lower than the band gap of CH3NH3PbBr3. Conductivity measurement was also performed to examine the potential in the applications as an alternative to the lead containing perovskites

Effect of magnetism and temperature on the stability of (Cr-x, V1-x)(2)AlC phases

The stability of (Crx,V1-x)2AlC MAX phases, materials of interest for a variety of magnetic as well as high temperature applications, has been studied using density-functional-theory first-principles calculations. The enthalpy of mixing predicts these alloys to be unstable towards unmixing at 0 K. The calculations also predict, however, that these phases would be thermally stabilised by configurational entropy at temperatures well below the values used for synthesis. The temperature Ts below which they become unstable is found to be quite sensitive to the presence of magnetic moments on Cr ions, as well as to the material’s magnetic order, in addition to chemical order and composition. Allowing for magnetism, the value of Ts for (Cr0.5,V0.5)2AlC with chemically disordered Cr and V atoms, is estimated to be between 516 K and 645 K depending on the level of theory, while if constrained to spin-paired, Ts drops to 142 K. Antiferromagnetic spin arrangements are found to be favored. The combination of antiferromagnetic frustration and configurational disorder should give rise to interesting spin textures at low temperatures

Breaking the electrical barrier between copper and carbon nanotubes.

Improving the interface between copper and carbon nanotubes (CNTs) offers a straightforward strategy for the effective manufacturing and utilisation of Cu-CNT composite material that could be used in various industries including microelectronics, aerospace and transportation. Motivated by a combination of structural and electrical measurements on Cu-M-CNT bimetal systems (M = Ni, Cr) we show, using first principles calculations, that the conductance of this composite can exceed that of a pure Cu-CNT system and that the current density can even reach 1011 A cm-2. The results show that the proper choice of alloying element (M) and type of contact facilitate the fabrication of ultra-conductive Cu-M-CNT systems by creating a favourable interface geometry, increasing the interface electronic density of states and reducing the contact resistance. In particular, a small concentration of Ni between the Cu matrix and the CNT using either an "end contact" and or a "dot contact" can significantly improve the electrical performance of the composite. Furthermore the predicted conductance of Ni-doped Cu-CNT "carpets" exceeds that of an undoped system by ∼200%. Cr is shown to improve CNT integration and composite conductance over a wide temperature range while Al, at low voltages, can enhance the conductance beyond that of Cr

Polymorphism in M(H2PO2)3 (M = V, Al, Ga) compounds with the perovskite-related ReO3 structure.

Trivalent metal hypophosphites with the general formula M(H2PO2)3 (M = V, Al, Ga) adopt the ReO3 structure, with each compound displaying two structural polymorphs. High-pressure synchrotron X-ray studies reveal a pressure-driven phase transition in Ga(H2PO2)3 that can be understood on the basis of ab initio thermodynamics