Temperature-dependent Raman and ultraviolet photoelectron spectroscopy studies on phase transition behavior of VO2 films with M1 and M2 phases

Structural and electronic phase transitions behavior of two polycrystalline VO2 films, one with pure M1 phase and the other with pure M2 phase at room temperature, were investigated by temperature-controlled Raman spectroscopy and ultraviolet photoelectron spectroscopy (UPS). We observed characteristic transient dynamics in which the Raman modes at 195 cm−1 (V-V vibration) and 616 cm−1 (V-O vibration) showed remarkable hardening along the temperature in M1 phase film, indicating the rearrangements of V-V pairs and VO6 octahedra. It was also shown that the M1 Raman mode frequency approached those of invariant M2 peaks before entering rutile phase. In UPS spectra with high energy resolution of 0.03 eV for the M2 phase film, narrower V3d band was observed together with smaller gap compared to those of M1 phase film, supporting the nature of Mott insulator of M2 phase even in the polycrystalline film. Cooperative behavior of lattice rearrangements and electronic phase transition was suggested for M1 phase film.