Revealing the role of oxygen vacancies on the phase transition of VO 2 film from the optical-constant measurements.

Vanadium dioxide (VO 2 ) material shows a distinct metal-insulator transition (MIT) at the critical temperature of ∼340 K. Similar to other correlated oxides, the MIT properties of VO 2 is always sensitive to those crystal defects such as oxygen vacancies. In this study, we investigated the oxygen vacancies related phase transition behavior of VO 2 crystal film and systematically examined the effect of oxygen vacancies from the optical constant measurements. The results indicated that the oxygen vacancies changed not only the electron occupancy on V 3d-O 2p hybrid-orbitals, but also the electron-electron correlation energy and the related band gap, which modulated the MIT behavior and decreased the critical temperature resultantly. Our work not only provided a facile way to modulate the MIT behavior of VO 2 crystal film, but also revealed the effects of the oxygen vacancies on the electronic inter-band transitions as well as the electronic correlations in driving this MIT process.