Strain- and temperature-induced phase transitions in single crystalline VO$_2$

The metal-insulator transition (MIT) of VO$_2$ exhibits a rich phase behavior involving two monoclinic (M1, M2), triclinic, and tetragonal phases that can form a complex domain structure and accompany the electronic transition. The interplay between these structural variants arises from strain due to differing lattice constants, temperature-dependent phase stability, and possible external strain from the substrate; the coupling between these effects renders a systematic study of the phase behavior difficult. We report on phase mapping of the structural changes through independent control of temperature and uniaxial strain in individual single-crystal nanorods, using Raman spectroscopy and near-field imaging. This allows us to investigate the transformation between the various insulating phases, elucidating the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, intermediate in the first-order transformation into the monoclinic M2 phase.