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We demonstrate the growth of highly textured VO 2 nanocrystals via annealing of e-beam deposited amorphous metallic V. Temperature dependent ellipsometry results reveal the pronounced reflection near the IR spectrum above the transition and an almost temperature independent weak reflection in the visible spectrum. The IR reflection displays a strong hysteresis during heating and cooling near the transition temperature at 68°C, indicating a first order transition and a strain-free structure. Our work demonstrates the feasibility to obtain high quality phase change nanostructures that transmit the visible spectrum but reflect IR and is suitable for large scale fabrication.

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In this work, we study the spectral reflectance of VO2 thin films and identify the specific contributions of the morphology and phase transition to optical spectra. The formation of highly [011] textured VO2 thin films on Si was achieved by an oxidation process starting with a metallic V thin film grown on an [001] Si substrate by an evaporation technique. Structural (XRD and Raman) and spectroscopic (XPS) characterization results indicate high purity VO2 formation with different sizes at various annealing temperatures without any change in the composition. Temperature dependent spectral reflectance distributions reveal that the insulator-to-metal transition (IMT) phase transition temperature of the VO2 nanostructures shows a slight size-dependence (∼3 °C), but this feature can be overshadowed by morphology that can lead to the misinterpretation of transition characteristics. The spectral line shape of the reflectance curves in the visible and near-infrared regions show substantially different characteristics for the samples annealed at different temperatures. Using numerical scattering calculations, we conclude that the changes in the optical response can be explained by morphological effects instead of changes in the intrinsic material properties such as a shift in the IMT temperature. Furthermore, the main mechanism leading to different spectral line shapes is the morphological differences leading to diffuse and specular reflectance.

An extensive study of the surface oxidation process on the properties of TiVN/VN multilayer coatings deposited by RF magnetron sputtering on Mg alloys was performed. TiVN/VN multilayer coatings with a total thickness of 660 nm and different layer numbers of 5, 11, and 33 were oxidized by annealing at 450K under partial oxygen pressure. Electronic, mechanical, and tribological properties of these coatings were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscope, nanoindentation measurements, and scratch tester. The experimental results showed that the oxidation of TiVN/VN multilayer coatings leads to better performance in terms of surface roughness, hardness, and wear. The hardness of the coating increased significantly (up to 1400 HV) with the oxidation of the surface due to the release of stresses between each layer after in situ annealing. The decline of the friction coefficient after oxidation can be attributed to the formation of a self-lubricating V 2 O 5 layer on the surface, the presence of which was confirmed by XPS. The findings in this work reveal a new technique for applying a better thin film coating for industrial applications of Mg alloys.