Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMD) have demonstrated how the new electronic phases 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 transition metal 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[1]. Using the recently developed method based on water-soluble Sr3Al2O6 as the sacrificial buffer layer[2,3] 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. Our results demonstrate the absence of critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films. The ability to synthesize and transfer crystalline freestanding perovskite films without thickness limitation onto any desired substrate opens a new field for the 2D correlated electronic phases and interfacial phenomena that technically have not yet been accessible.

[1] D.X. Ji, et al. Nature (in press, 2019)
[2] S. S. Hong, et al. Sci. Adv. 3, eaao5173 (2017).
[3] D. Lu, D. J. Baek, S. S. Hong, L. F. Kourkoutis, Y. Hikita, & Harold Y. Hwang, Nat. Mater. 15, 1255 (2016).

Bio：Yuefeng NIE received his Ph. D. degree in the department of physics at the University of Connecticut and joined Cornell Center for Materials Research at Cornell University as a research associate in 2011. He joined the department of materials science and engineering in Nanjing University in 2015. His research interests mainly focus on the atomically-precise engineering of emergent strongly correlated phases in complex oxide thin films and interfaces by a combination of oxide molecular beam epitaxy (MBE) and in situ angle-resolved photoemission spectroscopy (ARPES).