Freestanding Crystalline Monolayers of Oxide Perovskites*

Two-dimensional (2D) materials like graphene have demonstrated new electronic phases can emerge when a bulk crystal is thinned down to a mono-layer. As transition metal oxide perovskites host a variety of correlated phases, realizing the analogs with oxide perovskite materials would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report on the fabrication of freestanding perovskite films with high crystalline quality down to a single unit cell. Using the recently developed method based on water-soluble Sr₃Al₂O₆ as the sacrificial buffer layer we synthesize freestanding SrTiO₃ and BiFeO₃ ultrathin films by reactive molecular beam epitaxy and transfer them to different substrates such as crystalline silicon wafers and holey carbon films. We find that freestanding BiFeO₃ films exhibit an unexpected giant tetragonality and polarization when approaching the ultimate 2D limit. The ability to synthesize crystalline freestanding perovskite films without thickness limitation opens a new field for the 2D correlated electronic phases and interfacial phenomena.