Nanostructures of Various Shapes and their Structural Phase Transition*

Vanadium dioxide is a strongly correlated oxide material exhibiting first-order metal−insulator transition (MIT) which is accompanied by a structural phase transition from a low T monoclinic to a high T rutile phase at ~68°C in bulk. For potential applications such as catalytic, sensor and thermoelectric devices based on this MIT, controlling the nanoscale morphology and understanding the dependence of MIT on the morphology are important issue. We report a systematic synthesis of VO₂ nanostructures with controlled morphology. Twin boundaries played a key role in determining the shape of nanostructures as revealed in SEM and TEM microscopy images. Comprehensive in-situ x-ray scattering experiments were performed and data illustrate that MIT Temperature depends sensitively on the morphology of nanostructures. We tuned the MIT from 64°C to as low as 54°C by changing the shape and size of VO₂ nanostructures. We found that MIT temperature decreases as the dimension of nanostructure decreases, which can be attributed to the increased number of heterogeneous nucleation centers such as O₂ vacancies. We also present optical microscope images revealing dynamics of the metallic domain growth and reciprocal space map of XRD profile on a single VO₂ Nanowire