Tunability of bandgap from UV to visible region by transition metal ion implantation in CeO₂ and their electronic structures

Abstract

The present report examines the tunability of optical band gap by 150 keV Fe ion implantation in cerium oxide (CeO2) thin films and their electronic structures. X-ray diffraction (XRD) confirms the stable FCC structure and these films lose their crystalline nature with increasing ion fluences. Atomic force microscopy (AFM) shows the morphology changes. Raman measurement exhibits the presence of defect states. UV-Vis spectroscopy reveals that the optical band gap reduces from 3.14 eV to 2.70 eV. These results are well correlated with the electronic structure studies from the X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The XPS indicates the existence of Ce3+ due to the replacement of Fe3+ which reduces the Ce4+ to Ce3+ ions. XAS at the Ce-M4,5 edge reveals the change in Ce4+→Ce3+ with Fe ion fluence and the O-K edge spectra show that the vacancies are introduced after Fe ion implantation. The edge shifting of the O-K edge confirmed the presence of Ce4+-VO-Ce3+ and Ce3+-VO-Fe3+ networks in Fe-implanted thin films. This reversible ability enables the CeO2-based nanomaterial for energy and environmental-related applications.