Structure of V-shape twinned VO2 nanocrystals epitaxially grown on sapphire

Abstract

We report on the synthesis and structure of ‘V’ shape twinned vanadium dioxide (VO2) nanocrystals epitaxially grown on c-plane sapphire substrates using a vapor transport method. The (100)M twin plane played a key role in determining the morphology of VO2 nanocrystals. The growth of VO2 nanocrystals begins at the twin plane and proceeds toward two possible monoclinic [100] (aM−axis) direction resulting in ‘V’ shape twinned crystals with the angle between the sides of approximately 115.4°. At a relatively low growth temperature of 900 °C, the growth of the sides of ‘V’ was limited producing ‘coffee-bean’ shape crystals in which flat crystal facet regions are connected with rounded edges. The twinned VO2 nanocrystals were epitaxial to the c-plane sapphire substrate with the monoclinic [010] (bM−axis) normal to the substrate. In the in-plane direction, the (001)M planes of the VO2 twin crystals were aligned to the direction ±2.3° away from the sapphire (112‾0) plane. The sides of ‘V’ exhibit a rectangular cross-section truncated by the substrate. In-situ synchrotron x-ray diffraction measurement across the metal-insulator transition of the twinned nanocrystals implies that the phase transition of the coffee-bean shape nanocrystals occurs at a lower temperature with a smaller hysteresis gap than the fully grown V-shape nanocrystals.