Metastability in the Insulator–Metal Transition for Individual Vanadium Dioxide Nanoparticles

While bulk crystals of vanadium dioxide (VO₂) demonstrate a first-order insulator–metal phase transition around 65 °C, the phase transition of VO₂ nanoparticles tends to be superheated and supercooled with a large hysteresis often exceeding 20 °C. The phase transition of the nanoparticles occurs over a uniquely broad temperature range, yet with underlying microscopic mechanisms remaining elusive. In this work, we resolve the phase transition of individual VO₂ nanoparticles based on temperature-dependent infrared nanoimaging. We fabricate isolated VO₂ nanoparticles with an average diameter of 200 nm and a low defect density. Infrared nanoimaging reveals that the individual nanoparticles transition sharply but at different temperatures, accounting for broad insulator-to-metal transitions between 80 and 90 °C observed in ensemble-averaged X-ray diffraction and Raman spectroscopy. Particles with smaller volumes tend to transition to metal at higher temperatures, attributed to smaller numbers of nucleation sites within each particle. A stochastic behavior, where an identical particle exhibits different transition temperatures in repeated heating cycles, also underscores the nucleation-limited transitions. The nanoscale probing of the phase transitions of individual nanoparticles is not only critical to facilitate further control of VO₂ nanodevices but may also serve as a model system to study the physics of metastability at the nanoscale.