Journal
NATURE
Volume 570, Issue 7759, Pages 87-+Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/s41586-019-1255-7
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Funding
- National Basic Research Program of China [2015CB654901]
- National Natural Science Foundation of China [11574135, 51772143, 11474147, 51672125, 11774153, 11874199]
- Fundamental Research Funds for the Central Universities [0213-14380058]
- National Natural Science Foundation of China/The Research Grants Council of Hong Kong (NSFC/RGC) [11861161004]
- National Thousand-Young-Talents Program
- Program for High-Level Entrepreneurial and Innovative Talents Introduction, Jiangsu Province
- Program A for Outstanding Ph.D. candidate of Nanjing University [201801A013]
- Postgraduate Research and Practice Innovation Program of Jiangsu Province [KYCX18_0045]
- US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering [DE-SC0014430]
- National Science Foundation (NSF) under the Nebraska MRSEC programme [DMR-1420645]
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Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides reveal the electronic phases that emerge when a bulk crystal is reduced to a monolayer(1-4). Transition-metal oxide perovskites host a variety of correlated electronic phases(5-12), so similar behaviour in monolayer materials based on transition-metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report the fabrication of freestanding perovskite films with high crystalline quality almost down to a single unit cell. Using a recently developed method based on water-soluble Sr3Al2O6 as the sacrificial buffer layer(13,14) we synthesize freestanding SrTiO3 and BiFeO3 ultrathin films by reactive molecular beam epitaxy and transfer them to diverse substrates, in particular crystalline silicon wafers and holey carbon films. We find that freestanding BiFeO3 films exhibit unexpected and giant tetragonality and polarization when approaching the 2D limit. Our results demonstrate the absence of a critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films. The ability to synthesize and transfer crystalline freestanding perovskite films without any thickness limitation onto any desired substrate creates opportunities for research into 2D correlated phases and interfacial phenomena that have not previously been technically possible.
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