Anisotropic temperature-driven strain dynamics in VO2 solid-state microactuators

VO2 is a particularly appealing material for the development of solidstate micro- and nanoactuators due to its phase transition characterized by a large lattice change associated with a high energy density. Its martensitic transformation is strongly anisotropic: upon heating the c axis contracts by almost 1%, while the a and b axes expand by about 0.5 and 0.4%, respectively. This is a crucial aspect to consider when developing VO2-based actuating schemes, as the employment of polycrystalline VO2 and averaging mechanisms may reduce device performances. This work provides a quantitative analysis of the built-in strain in VO2 across its phase transition by characterizing thin films having different crystalline microstructures, namely singlecrystal and "tessellated" ones. A general method to quantitatively measure the built-in strain along different lattice directions in epitaxial thin films is presented. This method is based on optical profilometry of double-clamped microbridges aligned along different lattice directions. Our results show that the strain dynamics and anisotropy of VO2 devices can be controlled by the VO2 crystalline microstructure. Moreover, we demonstrate that the mechanical degrees of freedom affect the transport properties in VO2 micromechanical systems.