Low-temperature CVD synthesis of patterned core–shell VO2@ZnO nanotetrapods and enhanced temperature-dependent field-emission properties

Abstract

VO₂ nanostructures are attractive materials because of their reversible metal–insulator transition (MIT) and wide applications in devices. When they are used as field emitters, a new type of temperature-controlled field emission device can be fabricated. Vapor transport methods used to synthesize traditional VO₂ nanostructures are energy-intensive, low yield, and produce simple morphology (quasi-1D) that exhibits substrate clamping; thus they are not suitable for field emission applications. To overcome these limitations, ZnO nanotetrapods were used as templates, and patterned core–shell VO₂@ZnO nanotetrapods were successfully grown on an ITO/glass substrate via a low-temperature CVD synthesis. SEM, TEM, EDX, XPS analyses and X-ray diffraction revealed that the cores and shells of these nanotetrapods were single crystal wurtzite-type ZnO and polycrystalline VO₂, respectively. The VO₂@ZnO nanotetrapods show strongly MIT-related FE properties, the emission current density at low temperature is significantly enhanced in comparison with pure VO₂ nanostructures, and the emission current density increased by about 20 times as the ambient temperature increased from 25 to 105 °C at a fixed field of 5 V μm⁻¹. Although the VO₂@ZnO nanotetrapods show a worse FE performance at low temperatures compared with pure ZnO nanotetrapods, the FE performance was substantially improved at high temperatures, which was attributed to the MIT-related band bending near the interface and the abrupt resistance change across the MIT.