Tunability of MoO₃ Thin-Film Properties Due to Annealing in Situ Monitored by Hard X‑ray Photoemission

The chemical and electronic structure of MoO₃ 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 MoO₃. At moderate temperatures (120–200 °C), we find spectral evidence for the formation of Mo⁵⁺ and at higher temperatures (>165 °C) also of Mo⁴⁺ 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 MoO₃ thin-film properties.