Direct evidence of M2 phase during the monoclinic-tetragonal (rutile) phase transition of W-doped VO2 nanowires

Identifying different phases of VO2 during the metal−insulator phase transition is critical for device application due to the difference of electrical, mechanical and magnetic properties of phases. However, most studies so far were carried out using microprobe analyses, which lack the spatial resolution needed to identify nanoscale phases and changes. Taking advantage of in situ low temperature aberration-corrected scanning transmission electron microscopy, we observed the existence of M2 phase alongside M1 and R phase in the W-doped nanowires close to transition temperature. The localized stress caused by adding W in the structure results in the stabilization of nanosize grains of M2 phase in structure along with M1 and R phases. The observation of the metastable M2 phase even for unclamped nanowires suggests the possibility of finely modulating the phase diagram of VO2 through a combination of finite size and doping.