Photoluminescence studies of WO$_{3}$ and WO$_{3-x }$ single crystals

WO$_{3}$ is an important material to study, not only due to its interesting electronic properties, but also because it has other applications in both electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades [1]. We have studied two WO$_{3}$ single crystals, a transparent and doped WO$_{3-x}$, in an attempt to understand this effect. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not appear to be correlated with temperature. However, polarization studies reveal at least two polarization dependent components of the center. \\[4pt] [1] Satyen K. Deb, Solar energy materials and solar cells \textbf{92}, 245 (2008), and the references therein