Direct in situ optical and photoelectron emission microscopy observations of the nucleation and growth of VO(2) meso- and nanostructures using thermal transport of V(2)O(5) precursor in a vacuum or in an inert-gas environment were conducted. During nanostructure reductive growth, the formation, coexistence, and transformation of the intermediate oxide phases and morphologies were observed and characterized structurally and compositionally. The composition, structure, and morphology of the resultant nanostructures appeared to be a product of the interplay between kinetic and thermodynamic factors during multiple phase transformations. By rationally "navigating" the growth parameters using knowledge of the vanadium-oxygen temperature-composition phase diagram, wetting behavior, and epitaxial relationships of the intermediate phases with the substrate, control over growth direction, faceting, shape, and elastic strain of the nanostructures can be achieved. Such versatile control over the properties of single-crystal VO(2) nano- and mesostructures will facilitate their application in MEMS, sensors, and optoelectronics.
Controlled and convenient synthetic approaches for more complex-shaped nanotubes and nanowires of different materials are needed to further advance the field of nanoscience and nanotechnology. In this paper, we report the synthesis and characterization of nonlinear nanopores (such as curved and dendritic nanopores) containing alumina films. We show that control over the orientation of nanopores can be accomplished by controlling the geometric shape of aluminum substrates on which nanoporous alumina is grown. The anodically grown alumina on square and rectangular aluminum substrates showed cracks at the sharp edges of the substrates but no cracks were observed in nanoporous alumina films grown on cylindrical substrates. Our electrostatic calculations suggest that the electric field intensity at sharp edges of the rectangular/square substrates is significantly larger than that at the flat faces of the substrates. This leads to a faster alumina growth rate and hence increased stresses at these edges. We also propose a simple model that predicts the shape and orientation of nonlinear nanopores grown on different geometric shaped substrates. Through the use of proposed nanoporous films as template, one can easily synthesize highly complex-shaped nanotubes and nanowires of different materials.
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