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Oxides exhibit a remarkable 22 orders of magnitude variation in their electrical conductivity at room temperature. A most remarkable phenomenon manifested in a still rather sparsely populated set of compounds is a metal—insulator transition wherein the electrical conductivity of a single material can be abruptly switched (often at timescales as fast as femtoseconds) across several orders of magnitude and from insulating to metallic behavior in response to temperature, pressure, application of a voltage, and/or photo-excitation The abruptly discontinuous change in electrical conductivity arises from the transformation of localized valence electrons to itinerant electrons in the material, and is often accompanied by similarly abrupt changes in optical transmittance and magnetic susceptibility. The emergent complexity at the cusp of these transitions is underpinned primarily by electron—electron interactions and as such are greatly relevant to the design of new functionality.
In this talk, I will focus on our recent results on the influence of finite size and doping on the metal-insulator phase transitions of the binary vanadium oxide **VO_2**. We have achieved substantial tunability of the critical transition temperature between -20 and 70°C through control of dimensionality, morphology, and dopant concentration in hydrothermally prepared single-crystalline **VO_2** nanostructures. The tunability of the phase diagram portends applications of these materials as dynamically switchable glazings for energy efficient windows (smart windows!). I will further discuss colossal metal—insulator switching recently discovered in **MxV_2O_5** bronze phases. These structures provide a versatile platform for systematically tuning geometric and electronic structure and thus the extent of electron correlation. I will conclude by discussing the implications of electron correlation for the design of cathodes and photocatalysts.