Tunability of MoO<sub>3</sub> Thin-Film Properties Due to Annealing in Situ Monitored by Hard X‑ray Photoemission LiaoXiaxia JeongAh Reum WilksRegan G. WiesnerSven RusuMarin FélixRoberto XiaoTing HartmannClaudia BärMarcus 2019 The chemical and electronic structure of MoO<sub>3</sub> thin films is monitored by synchrotron-based hard X-ray photoelectron spectroscopy while annealing from room temperature to 310 °C. Color-coded 2D intensity maps of the Mo 3d and O 1s and valence band maximum (VBM) spectra show the evolution of the annealing-induced changes. Broadening of the Mo 3d and O 1s spectra indicate the reduction of MoO<sub>3</sub>. At moderate temperatures (120–200 °C), we find spectral evidence for the formation of Mo<sup>5+</sup> and at higher temperatures (>165 °C) also of Mo<sup>4+</sup> states. These states can be related to the spectral intensity above the VBM attributed to O vacancy induced gap states caused by partial filling of initially unoccupied Mo 4d-derived states. A clear relation between annealing temperature and the induced changes in the chemical and electronic structure suggests this approach as a route for deliberate tuning of MoO<sub>3</sub> thin-film properties.