Enhancement of Young's modulus in freestanding SrTiO₃ thin films

Recent developments in thin fil growth provide a powerful route to free-standing single-crystal films of perovskite oxides by water etching of a sacrificial underlayer (Di Lu et al., Nature Materials 15, 1255 (2016)). We have used atomic force microscopy to systematically probe the elastic properties of suspended SrTiO₃ films, in a thickness series from 4-98 nm, which has been previously inaccessible and unexplored. We observe the thickness dependence of elastic modulus of SrTiO₃ to be non-monotonic, with a strong enhancement of stiffness for thicknesses below 30 nm. We provide evidence that the nature of this elastic stiffening lies in an effective flexoelectric coupling. This study provides the basis for studying strain manipulation and strain control of oxides at the nanoscale. In particular, oxide membranes can be designed to mimic stiffnesses of 2d materials like graphene and transition metal dichalcogenides.