In situ electronic structural study of VO2 thin film across the metal—insulator transition

The in situ valence band photoemission spectrum (PES) and X-ray absorption spectrum (XAS) at V L-II-L-III edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal-insulator transition (MIT) temperature (T-MIT = 67 degrees C). The spectra show evidence for changes in the electronic structure depending on temperature. Across the T-MIT, pure V 3d characteristic d(parallel to) and O 2p-V 3d hybridization characteristic pi(pd), sigma(pd) bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V L-III-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator-semiconductor, semiconductor-metal processes, and vice versa. The conventional MIT at around the T-MIT = 67 degrees C is actually a semiconductor-insulator transformation point.