The structural phase transition process of free-standing monoclinic vanadium dioxide micron-sized rods: temperature-dependent Raman study

Abstract

We investigated the temperature-driven structural phase transition (SPT) process of free-standing monoclinic vanadium dioxide (VO₂ (M)) micron-sized rods by Raman spectroscopy. With increasing temperature, the phase transition sequence goes monoclinic M₁ → monoclinic M₁/monoclinic M₂/rutile (R) → monoclinic M₂/R → R. The fully metallic R phase is reached at 50 °C, which is 7 °C lower than the value of nanoparticles in our study and 18 °C lower than the value of early reports on VO₂ (M) bulk materials. The intermediate M₂ phase, which plays a critical role in the monoclinic to rutile transition (MRT), was clearly observed in both heating and cooling process of the micron-sized free-standing VO₂ (M) rods. We suggest that the existence of nucleating defects in our samples, which have an important effect on the nucleation of the R phase, are responsible for the reduction of the phase transition temperature and the appearance of the M₂ phase. This supplies a possible way to expand the practical applications of one dimensional VO₂ materials.