nano-Raman

Since a vanadium dioxide nanoparticle (VO₂ NP) contains relatively few potent defect sites that can trigger the phase transition via heterogeneous nucleation, it requires substantial overheating and undercooling (i.e., excess ‘driving forces’) beyond the nominal critical temperature (67 °C) in order to switch between the monoclinic (semiconducting) and tetragonal (metallic) phases, thereby exhibiting a wide thermal hysteresis of the Raman-scattered light (e.g., >50 °C for VO₂ NP “A”). On the other hand, a contiguous VO₂ film contains many nucleation sites due to the larger accessible volume and grain boundaries, so that temperature excursions of just a few degrees can initiate the phase transition, resulting in a narrower hysteresis loop (here, <5 °C). Moreover, defect-harboring grain boundaries can also cause individual nanoparticles of similar volumes to exhibit very different hysteresis widths (e.g., <20 °C for VO₂ NP “B”).