First principles calculations of the electronic structure of lithium intercalated oxygen deficient amorphous tungsten and titanium oxides
Title: First principles calculations of the electronic structure of lithium intercalated oxygen deficient amorphous tungsten and titanium oxides
DNr: SNIC 2014/1-360
Project Type: SNIC Medium Compute
Principal Investigator: Gunnar Niklasson <gunnar.niklasson@angstrom.uu.se>
Affiliation: Uppsala universitet
Duration: 2014-12-01 – 2015-12-01
Classification: 10304 21001 20599
Homepage: http://www.teknik.uu.se/ftf/Research.asp
Keywords:

Abstract

Amorphous tungsten oxide (aWO3), and titanium oxide (aTiO2) are of large interest in the field of electrochromic (EC) coatings for smart window applications [1]. EC thin films of these materials display a large reversible optical transmittance modulation between colored and bleached states upon application of coloration and bleaching voltages of approximately 1.0-1.5 V. Because of its fast switching aTiO2 is also a promising material in nonvolatile, crossbar-type bipolar resistive random access memories [NVM-RRAM] [2]. We have recently carried out experimental studies of the structure, optical properties and electrochemical density of states [3] of amorphous aWOx and aTiOx coatings, produced by sputtering. In order to elucidate the role of disorder on the electronic and optical properties of these materials, we have been carrying out ab initio molecular dynamics calculations by amorphization of the monoclinic (WO3) and anatase (TiO2) crystalline phases. We have also performed density functional calculations using as input parameters experimental results of local structure obtained by X-ray diffraction and EXAFS measurements. These studies have allowed us to obtain preliminary results of the electronic structure and optical properties. So far we have made significant advances in the calculations for some amorphous phases, especially stoichiometric ones, which have shown remarkable agreement with experimental results and with computational techniques like reverse Monte Carlo simulations. However, we need to extend our computational efforts since ab initio molecular dynamics, relaxation processes, and calculations of the electronic density of states (DOS) and complex dielectric function, are time consuming and require a very high computational effort in order to reach a good numerical accuracy. In the present project we will extend our studies to the electronic and optical properties of lithium intercalated oxygen deficient amorphous tungsten (aLiyWOx) and titanium (aLiyTiOx) oxides. We have noted the existence of remarkable similarities between the properties displayed by oxygen deficient, and lithium intercalated films. We will compare how oxygen vacancies and inserted Li affects the DOS. It is important to understand processes like localization of states in the band gap, optical properties and Li+-intercalation, which are of prime importance in technical applications of electrochromic coatings. In this project we aim to characterize and understand the electrochromism and its origin in the electronic structure, by first principles calculations for substoichiometric as well as Li+ intercalated tungsten oxides. The overall objective is to gain a detailed understanding of the density of states of the tungsten and titanium oxide systems and in a wider framework, in amorphous materials in general. 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. References 1. G.A. Niklasson and C.G. Granqvist, J. Mater. Chem. 17 (2007) 127-156 2. H. Y. Jeong, J. Y. Lee, and S-Y Choi, Adv. Funct. Mater. 20 3912 (2010). 3. M. Strömme, R. Ahuja and G.A. Niklasson, Phys. Rev. Lett. 93 (2004) 206403