First principles calculations of the electronic structure of amorphous tungsten oxides
Title: First principles calculations of the electronic structure of amorphous tungsten oxides
SNIC Project: SNIC 2013/1-283
Project Type: SNAC Medium
Principal Investigator: Gunnar Niklasson <gunnar.niklasson@angstrom.uu.se>
Affiliation: Department of Engineering Sciences, Uppsala University
Duration: 2013-11-13 – 2014-12-01
Classification: 10304 21001 20599
Homepage: http://www.teknik.uu.se/ftf/Research.asp
Keywords:

Abstract

Amorphous tungsten oxide is of large interest in the field of electrochromic coatings for smart window applications [1]. Thin films of the material exhibit a large reversible optical transmittance modulation between colored and bleached states upon application of coloration and bleaching voltages of approximately 1.5 V. In the present project our main interest lies in characterizing and understanding the physical properties of tungsten suboxides. Their optical properties, in particular the electrochromism and its origin in the electronic structure will be compared to the stoichiometric tungsten oxide, WO3. It is probable that the presence of localized states in the band gap will affect the properties of the tungsten suboxides. We have recently carried out experimental studies of the structure, optical properties and electrochemical density of states of amorphous WOx coatings, produced by sputtering, with x spanning the range from 2 to 3. The next step is to compare these results to ab initio calculations of the electronic structure and density of states, including structure optimization by computations of total energy and molecular dynamics. One aim is to study the localized states in the band gap as well as in the bands, and their relation to stoichiometry and nanostructure in the system WO2 -WO3. The overall objective is to gain a detailed understanding of the behavior of the density of states of the tungsten oxide system and in a wider framework, in amorphous materials in general. We will attempt to extract structural information from X-ray diffraction measurements and EXAFS and use these as input in the computation. The computational techniques should be applicable to amorphous structures, which require a large computational effort. Previous work on amorphous WO3 used quite small clusters for the computation [2]. We will use state of the art first-principles methods based on quantum and statistical mechanics, viz. density functional theory (DFT), molecular dynamics (MD) simulations and stochastic quenching method [5]. The amorphous structures will be generated by these last two methods using a heating-melting and supercell approaches. The calculations will be carried out using the implementation of DFT in the software VASP (Vienna Ab Initio Simulation Program). This methodology will lead to an improved understanding on the behavior of the band gaps and localization of the states of amorphous materials. However, these calculations, including hundreds of atoms require a massive computational effort. The use of the SNIC resource and the VASP code are fundamental to this research project, and the results obtained by these theoretical models will be compared to our experimental results, obtained by our previously published electrochemical technique [3,4]. References 1. G.A. Niklasson and C.G. Granqvist, J. Mater. Chem. 17 (2007) 127-156 2. G.A. de Wijs and R.A. de Groot, Phys. Rev. B 60 (1999) 16463-16474 3. M. Strömme, R. Ahuja and G.A. Niklasson, Phys. Rev. Lett. 93 (2004) 206403 4. G.A. Niklasson, R. Ahuja and M. Strömme, Mod. Phys. Lett. B 20 (2006) 863-875 5. C. Århammar et al., Proc. Natl. Acad. Sci., 108 (2011) 6355-6360