Synthesis, characterization and phase transitions of single-crystalline vanadium(IV) oxide nanostructures
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The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid-solid phase transitions such as those involved in metal-insulator switching, ferroelectricity, and superconductivity. The peculiarities in the electronic structure of the seemingly simple binary vanadium oxide VO 2 , as manifested in a pronounced metal-insulator phase transition in proximity to room temperature, have made it the subject of extensive theoretical and experimental investigations over the last several decades. VO 2 exhibits a first-order metal-insulator phase transition near room temperature at 68 °C in the bulk. Associated with the phase transition are dramatic changes in the electrical conductivity, optical properties of VO 2 at all wavelengths, and a structural transition from an insulating, low-temperature monoclinic phase to a metallic, high-temperature tetragonal phase. Such properties make VO 2 a suitable material for Mott field-effect transistors, optical switching devices, thermochromic coatings, and electronic devices exhibiting sharp thresholdlike variation of electrical and optical properties in response to external stimuli such as temperature and voltage. Scaling VO 2 to nanoscale dimensions has recently been possible and has allowed well-defined VO 2 nanostructures to serve as model systems for measurements of intrinsic properties without obscuration from grain boundary connectivities and domain dynamics. Geometric confinement, substrate interactions, and varying defect densities of VO 2 nanostructures gives rise to an electronic and structural phase diagram that is substantially altered from the bulk. In my talk, I will outline two distinct hydrothermal approaches for the synthesis of 1D single-crystalline VO 2 nanostructures exhibiting a substantial diminution in the metal-insulator phase transition temperature based on (a) the hydrothermal hydration, exfoliation, and recrystallization of bulk V 2 O 4 and (b) the hydrothermal reduction and doping of V 2 O 5 using small-molecule reducing agents and tungsten precursors. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution W x V 1- x O 2 . Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal → insulator and insulator → metal phase transformations. Notably, the in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W 6+ doping in the W x V 1- x O 2 nanobelts. We postulate that design principles extracted from fundamental understanding of phase transitions in nanostructures will allow the predictive and rational design of systems with tunable charge and spin ordering.