Charge doping and large lattice expansion in oxygen-deficient heteroepitaxial WO3

TL;DR

This study demonstrates how oxygen pressure during pulsed laser deposition can control oxygen vacancies in WO3 thin films, leading to significant lattice expansion and an insulator-to-metal transition, with insights from experimental and ab initio calculations.

Contribution

It reveals a method to tune the structural and electronic properties of WO3 thin films through controlled oxygen vacancy formation during growth.

Findings

Lattice volume changes up to 10% with oxygen vacancy tuning

Observation of insulator-to-metal transition in WO3 films

Ab initio calculations elucidate defect states and charge evolution

Abstract

Tungsten trioxide is a versatile material with widespread applications ranging from electrochromic and optoelectronic devices to water splitting and catalysis of chemical reactions. For technological applications, thin films of WO3 are particularly appealing, taking advantage from high surface-to-volume ratio and tunable physical properties. However, the growth of stoichiometric, crystalline thin films is challenging because the deposition conditions are very sensitive to the formation of oxygen vacancies. In this work, we show how background oxygen pressure during pulsed laser deposition can be used to tune the structural and electronic properties of WO3 thin films. By performing X-ray diffraction and low-temperature transport measurements, we find changes in WO3 lattice volume up to 10%, concomitantly with an insulator-to-metal transition as a function of increased level of electron doping. We use advanced ab initio calculations to describe in detail the properties of the oxygen vacancy defect states, and their evolution in terms of excess charge concentration. Our results depict an intriguing scenario where structural, electronic, optical, and transport properties of WO3 single-crystal thin films can all be purposely tuned by a suited control of oxygen vacancies formation during growth.