Strain and temperature dependence of the insulating phases of VO2 near the metal-insulator transition

In addition to its metal-insulator transition (MIT), VO2 exhibits a rich phase behavior of insulating monoclinic (M1,M2) and triclinic (T) phases. By using micro-Raman spectroscopy and independent control of temperature and uniaxial strain in individual single-crystal microbeams, we map these insulating phases with their associated structural changes as represented by their respective phonon frequencies. The competition between these structural forms is dictated by the internal strain due to differing lattice constants, the experimentally applied external strain, and the temperature-dependent phase stability. We identify the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, while a discontinuous transition into the M2 phase occurs from both the M1 and T phases. The results suggest that understanding the driving forces that determine the interplay between M1, M2, and T phases near the MIT could be critical for the identification of the underlying mechanism behind the MIT itself.