The electrical resistance of single VO(2) nanobeams was measured while simultaneously mapping the domain structure with Raman spectroscopy to investigate the relationship between structural domain formation and the metal-insulator transition. With increasing temperature, the nanobeams transformed from the insulating monoclinic M(1) phase to a mixture of the Mott-insulating M(2) and metallic rutile phases. Domain fractions were used to extract the temperature dependent resistivity of the M(2) phase, which showed an activated behavior consistent with the expected Mott-Hubbard gap. Metallic monoclinic phases were also produced by direct injection of charge into devices, decoupling the Mott metal-insulator transition from the monoclinic to rutile structural phase transition.
Phase-selective growth of VO2 and V2O5 nanowires was realized via catalyst-free physical vapor deposition from bulk VO2 powder. Single nanowire Raman spectroscopy was used to analyze the distribution of the vanadium oxide phases within the reactor. VO2 (V2O5) nanowires were identified by characteristic peaks at 197, 224, and 620 cm−1 (149, 700, and 994 cm−1). Electron diffraction and polarization-dependent Raman spectra indicated that the growth directions of VO2 and V2O5 nanowires were [100] and [010]. Analysis of Raman spectra in two polarization configurations is sufficient to distinguish between low-index nanowire growth directions for the V2O5 phase. Single nanostructure Raman measurements thus provide a means to rapidly analyze the phase and growth direction of anisotropic nanostructures.
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