Reconfigurable Vanadium Dioxide Nanomembranes and Microtubes with Controllable Phase Transition Temperatures.

Abstract

Two additional structural forms, free-standing nanomembranes and microtubes, are reported and added to the vanadium dioxide (VO₂) material family. Free-standing VO₂ nanomembranes were fabricated by precisely thinning as-grown VO₂ thin films and etching away the sacrificial layer underneath. VO₂ microtubes with a range of controllable diameters were rolled-up from the VO₂ nanomembranes. When a VO₂ nanomembrane is rolled-up into a microtubular structure, a significant compressive strain is generated and accommodated therein, which decreases the phase transition temperature of the VO₂ material. The magnitude of the compressive strain is determined by the curvature of the VO₂ microtube, which can be rationally and accurately designed by controlling the tube diameter during the rolling-up fabrication process. The VO₂ microtube rolling-up process presents a novel way to controllably tune the phase transition temperature of VO₂ materials over a wide range toward practical applications. Furthermore, the rolling-up process is reversible. A VO₂ microtube can be transformed back into a nanomembrane by introducing an external strain. Because of its tunable phase transition temperature and reversible shape transformation, the VO₂ nanomembrane-microtube structure is promising for device applications. As an example application, a tubular microactuator device with low driving energy but large displacement is demonstrated at various triggering temperatures.