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. 

https://pubs.acs.org/doi/full/10.1021/acs.nanolett.8b00483