Title:	Theoretical descriptions of the electronic mechanisms for polaron formation/migration and hole-electron charge transport in oxygen deficient and lithium intercalated amorphous tungsten and titanium oxides
DNr:	SNIC 2016/1-488
Project Type:	SNIC Medium Compute
Principal Investigator:	Gunnar Niklasson <gunnar.niklasson@angstrom.uu.se>
Affiliation:	Uppsala universitet
Duration:	2016-12-01 – 2017-12-01
Classification:	10304 21001 20599
Homepage:	http://www.teknik.uu.se/fasta-tillstandets-fysik/
Keywords:

Abstract

Amorphous tungsten (aWO3) and titanium (aTiO2) oxide are key components for a number of technological applications, for example sensors, electrochromic devices, photo-electrochemical conversion and water-splitting1. Non-stoichiometric aTiOx has potential applications in memristors2. Although the functionalities of these amorphous oxides are known for applications, realistic descriptions of the structural, electronic, optical and transport processes remain elusive3. This is because methods to characterize amorphous solids are limited by their accuracy in describing the fundamental physical properties as a function of disorder. We are studying aWO3 and aTiO2 through an integrated approach involving experimental data and computational simulations, where we have used ab-initio molecular-dynamics simulation to generate amorphous structures and to verify the local-range structural order by direct comparison with X-ray-absorption data. A more precise understanding of the the local-structural order and its relation to the electronic properties of aWO3 and aTiO2 are now on the way, where for example we recently published how the short-range order induces new electronic effects in aTiO2.4 We are now extending our calculations to assess the electronic properties of oxygen deficient aWO3-x; aTiO2-x and lithium inserted aLiyWO3-x; aLiyTiO2-x systems. Due to disorder very demanding computational schemes have to be used for these materials. Density functional theory [DFT] with a very large unit cell will used to describe the electronic and optical properties in these amorphous oxides; specifically hybrid functional and Hubbard corrected [DFT+U] schemes have to be used, which requires large computational efforts in order to reach a good numerical accuracy. In this project we will expand our study of aWO3-x; aTiO2-x and aLiyWO3-x; aLiyTiO2-x systems to assess polaron formation/migration and hole-electron charge transport. The introduction of oxygen deficiency and doping make it necessary to average over several structures in order to accurately describe the relation between the short-range order and the corresponding electronic states. Thus, it is crucial to carry out simultaneous calculation over several super-cells using different schemes in order to properly reflect the physics of the systems. This requires increased computational resources, although we have optimized the simulation and calculation times by using as input pre-optimized structures and single gamma-point calculations. Knowledge of the representative structure and electronic states makes it viable to extend our analysis to vibrational processes. We have access to experimental vibrational spectroscopy measurements, where linear response DFT in this project would provide the theoretical picture. This project is valuable to improve technological applications of amorphous transition metal oxides. The aim is to provide a unified description of the relationship between structure, electronic and polaron states, and their role for vibrational spectroscopy, electronic transport and optical absorption. The access to SNIC has been very valuable for our studies. However we need to extend our computational capacity in order to obtain accurate results for comparison with experimentals. 1. J. Wu et al., Functional Metal Oxide Nanostructures (Springer, New York, 2012). 2. J.J. Yang et al., Nat. Nanotech. 8, 13 (2013) 3. X. Chen, and S.S. Mao, Chem. Rev. 107, 2891 (2007) 4. C.A. Triana, et al., Phys. Rev. B 94, 165129 (2016)