Nucleation-controlled hysteresis in unstrained hydrothermal VO2 particles

While nucleation-limited transformation mechanisms are widely implicated in unstrained, undoped VO2 nanoparticles, a direct link between nucleation barriers and hysteresis widths has not yet been established. Here, we investigate microscopic transformation of structural domains optically in hydrothermally grown VO2 particles similar to.5-46 mu m in length, which are not elastically clamped to the substrate. We observe abrupt and generally complete transformation in individual particles, consistent with a nucleation-limited transformation mechanism. The forward and reverse transformation temperatures are not correlated, suggesting a range of potency of nucleation sites for both forward and reverse transformation in undoped particles, resulting in a hysteresis of 2.9-46.3 degrees C. Thus, the macroscopic hysteresis width in bulk VO2 powders and dispersed particulate films is primarily attributable to a distribution of critical nucleation temperatures between different particles. These findings suggest that as VO2 volume elements are scaled down for microelectronic applications, manipulation of nucleation sites via defect engineering may be required to control the degree of the VO2 element reversibility.