X-ray irradiation induced effects on the chemical and electronic properties of MoO3 thin films

The time and photon-flux dependent x-ray irradiation induced effects on the chemical and electronic properties of MoO3 thin films have been systematically investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). After exposure to Mg Kα x-ray irradiation of different photon flux (realized by using different x-ray source powers: 30, 100, and 300 W), the Mo 3d XPS spectra indicate a partial conversion of the Mo6+ states in MoO3 to Mo5+ states. The degree of reduction significantly depends on exposure time and x-ray source power (i.e., photon flux). To minimize x-ray irradiation induced effects on the collected data, a measurement time of 1-2 h at a low photon flux should not be exceeded. Related UPS analysis shows that no effects of UV irradiation can be detected and reveals the x-ray irradiation induced appearance of defect states above the valence band maximum (VBM). The intensity of these above-VBM states increases linearly inverse with the XPS derived Mo6+/(Mo5+ + Mo6+) ratio, suggesting an x-ray induced loss of oxygen as the common explanation for both effects.