Density-functional study of LixMoS2 intercalates (0<=x<=1)

The stability of Lithium intercalated 2H- and 1T allotropes of Molybdenum disulfide (LixMoS2) is studied within a density-functional theory framework as function of the Li content (x) and the intercalation sites. Octahedral coordination of Li interstitials in the van der Waals gap is found as the most favorite for both allotropes. The critical content of Lithium, required for the initialization of a 2H->1T phase transition is estimated to x ~ 0.4. For smaller Li contents the hexagonal 2H crystal structure is not changed, while 1T-LixMoS2 compounds adopt a monoclinic lattice. All allotropic forms of LixMoS2 - excluding the monoclinic Li1.0MoS2 structure - show metallic-like character. The monoclinic Li1.0MoS2 is a semiconductor with a band gap of 1.1 eV. Finally, the influence of Li intercalation on the stability of multiwalled MoS2 nanotubes is discussed within a phenomenological model.Comment: submitted to Comput.Mater.Sc

Electronic, optical and sodium K edge XANES in disodium helide: a DFT study

Abstract The ground-state properties of the disodium helide (Na2He) in the cubic structure was calculated using the WIEN2k package within GGA, LDA, and mBJ potentials. From our results, the GGA and LDA predict the material to be semiconductor, while mBJ predicts the material to be insulator. The calculated results from the electronic structure show that Na2He is a direct bandgap semiconductor. Excitonic properties were studied and the results provide Mott-wannier type excitonic behavior of the material. The optical properties for Na2He were studied and its application towards optoelectronic devices has been identified. Also, Na K edge x-ray absorption near edge structure (XANES) for Na2He were computed and discussed. To verify the possibility of formation 2D structure (monolayer) of this compound, phonon calculations were performed. The result indicates that the 2D phase for this compound is dynamically unstable

Suitability of amorphous TiO2 nanoparticles as a photoelectrode in dye sensitized solar cells: A DFT-TDDFT study

Ab-initio calculations of nano-sized (∼1 nm) amorphous-TiO2 cluster and dye molecules (N3 and N719) have been carried out. Optimized structures of amorphous cluster and dye molecules have been obtained via molecular dynamics (MD) and density functional theory (DFT) calculations, respectively. The lowest excited state energies of the TiO2 cluster and the dye molecules have been obtained using time-dependent DFT. The calculations show that HOMO-LUMO gap and singlet-singlet excited state energies for amorphous cluster are very similar to that of a crystalline TiO2 cluster. Our calculations also show that the energy levels of the molecular dyes are well aligned with those of the amorphous cluster. © 2013 Elsevier B.V. All rights reserved.K.R.G acknowledges the following support (a) CSIC under JAE-DOC program ‘Junta para la Ampliación de Estudios’ cofinanciada por el FSE, (b) the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (Grant No. IT-366-07) and (c) the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-02).Peer Reviewe

Structural, Mechanical and Thermodynamic Properties under Pressure Effect of Rubidium Telluride: First Principle Calculations

First-principles density functional theory calculations have been performed to investigate the structural, elastic and thermodynamic properties of rubidium telluride in cubic anti-fluorite (anti-CaF2-type) structure. The calculated ground-state properties of Rb2Te compound such as equilibrium lattice parameter and bulk moduli are investigated by generalized gradient approximation (GGA-PBE) that are based on the optimization of total energy. The elastic constants, Young’s and shear modulus, Poisson ratio, have also been calculated. Our results are in reasonable agreement with the available theoretical and experimental data. The pressure dependence of elastic constant and thermodynamic quantities under high pressure are also calculated and discussed

Electronic and optical properties under pressure effect of alkali metal oxides

71.15.Ap Basis sets, 78.40.Fy Semiconductors, 78.20.Ci Optical constants,