Size, composition and alignment of VO2 microrod crystals by the reduction of V2O5 thin films, and their optical properties through insulator-metal transitions

Abstract

VO2 microrods (MRs) have been successfully synthesized by the reduction of V2O5 thin films. The effects of reduction temperature and thickness of the precursor V2O5 thin films on the stochiometry and morphology of the resulting VO2 MRs have been studied in this work. Results show that size of the VO2 MRs increases with increasing precursor thickness due to the enlargement of the droplets during the melting of V2O5 thin films. Further, the growth of VO2 MRs from V2O5 droplets on a sapphire substrate form a unique angle between each other, which is probably due to epitaxial growth. Reduction temperatures between 600 °C and 650 °C are preferred for the growth of VO2 MRs, while temperatures lower or higher than this range produce V2O5 nanostructures or VO2 nanoparticles, respectively. HRTEM shows that during VO2 MR’s growth, the (110) plane remains parallel to the Si substrate, and a 10 nm amorphous interface was formed between the VO2 MR and the Si substrate. Optical properties of the MRs were examined by carrying out light scattering experiments as a function of temperature to identify the temperature range of insulator-to-metal (IM) and metal-to-insulator (MI) transition characteristics. The optical scattering experiments show the large influence of microrod (MR) size and distribution on the characteristics of MI and IM transitions. The transition behavior can range from polycrystalline film type transition to new unique features.