Quantitative functional imaging of VO₂ metal-insulator transition through intermediate M₂ phase

VO₂ exhibits metal-insulator transition (MIT) accompanied by structural phase transformation between rutile R phase and monoclinic M₁ phase, and an intermediate monoclinic phase M₂ may also emerge, stabilized by strain. The evolution of microstructures and properties across phase transition is not only critical for understanding the nature of MIT, but also important for numerous device applications, yet they are quite challenging to characterize. Utilizing advanced atomic force microscopy (AFM) techniques in combination with polarized light microscopy, X-ray diffraction, and Raman spectroscopy, we map the microstructure evolution of VO₂ when it is heated from room temperature M₁ phase to high temperature R phase through intermediate M₂ phase, and acquire functional imaging of VO₂ spanning these three phases as well. These result in quantitative mapping of electric conduction and Young's modulus of VO₂ in one-to-one correspondence to its domain patterns, especially those with austenite-martensite interface between M₂ and R phases. Young's modulus of M₁, M₂, and R phase of VO₂ are determined to be 95 GPa, 65-117 GPa, and 98-100 GPa respectively, and significant anisotropy is observed in M₂ phase. Rigorous continuum analysis has also been carried out to analyze the complicated domain pattern, validating our experimental observations that match theoretical expectation well.