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 monolayer1,2,3,4. Transition-metal oxide perovskites host a variety of correlated electronic phases5,6,7,8,9,10,11,12, 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 layer13,14 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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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
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This paper is dedicated to the memory of N. B. Ming, who supported the Center for Microstructure of Quantum Materials at Nanjing University. This work was supported by the National Basic Research Program of China (grant 2015CB654901), the National Natural Science Foundation of China (grants 11574135, 51772143, 11474147, 51672125, 11774153 and 11874199), the Fundamental Research Funds for the Central Universities (grant 0213-14380058), and the National Natural Science Foundation of China/The Research Grants Council of Hong Kong (NSFC/RGC, grant 11861161004). Y.N. and P.W. are supported by the National Thousand-Young-Talents Program and the Program for High-Level Entrepreneurial and Innovative Talents Introduction, Jiangsu Province. S.C. is supported by Program A for Outstanding Ph.D. candidate of Nanjing University (grant 201801A013) and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (grant KYCX18_0045). TEM work at the University of California, Irvine used the facilities of the Irvine Materials Research Institute (IMRI) and was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under grant DE-SC0014430. The research at the University of Nebraska–Lincoln is supported by the National Science Foundation (NSF) under the Nebraska MRSEC programme (grant DMR-1420645). Y.N. acknowledges discussions with H. Hwang, D. Li and S. S. Hong.
Nature thanks Gertjan Koster and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a–d, The growth and surface morphology of ultrathin STO films of thickness four unit cells (a), three unit cells (b), two unit cells (c) and one unit cell (u.c.) (d) on six-unit-cell-thick SAO-buffered STO substrates. The RHEED intensity oscillations (red curves) exhibit four intensity oscillation periods in the growth of one-unit-cell-thick SAO (especially near the SAO surface). Insets, the RHEED diffraction patterns show clear fourfold reconstructed diffraction patterns. The atomic force microscopy (AFM) images (right panels) show clear steps and terraces, indicating the smooth film surfaces. e, High-resolution XRD 2θ–ω scans of a three-layered heterostructure consisting of an STO film (60 unit cells thick) on an SAO (four unit cells thick) on an STO substrate heterostructure and the corresponding freestanding STO film and STO substrate after releasing the sample. The asterisk denotes the background diffraction peak from the PDMS tape. f, Optical image of the freestanding 60-unit-cell-thick STO transferred onto PDMS with the support of a glass slide. g, Magnification of the 2θ–ω scans around the STO(002) substrate peak show clear thickness fringes, indicating the smooth surfaces of both the as-grown and freestanding films.
a–c, Cross-sectional HAADF images (a), SAED patterns (b), and plan-view HAADF images (c) of ultrathin freestanding BFO films of various unit-cell thicknesses (four, three, two and one) below the reported 2D limit. Scale bars, 2 nm (a); 2 nm−1 (b); and 5 nm (c). All BFO films were suspended on holey carbon grids. As the one-unit-cell-thick freestanding film is extremely sensitive to the electron beam, it can survive only at low-dose SAED measurements.
a, b, SAED measurements on a one-unit-cell-thick freestanding BFO film taken at the same region in a sequence showing clear broadening of the diffraction spots. Scale bars, 2 nm−1. c, The fitted full-width at half-maximum (FWHM) of the diffraction spots (red and green dashed boxes in a and b) show a FWHM of 0.2 nm−1 and 0.39 nm−1, indicating that the real-space crystalline lattice domain size decreases by almost half between these two measurements.
a, Topography, b, out-of-plane PFM phase images, c, in-plane PFM phase images, and d, in-plane PFM amplitude images of freestanding BFO films. Scale bars, 500 nm. All freestanding films exhibit a single out-of-plane domain. Only the freestanding films thicker than four unit cells show in-plane polarizations, which is consistent with the R-phase to T-phase transition observed in the atomically resolved HAADF images, as discussed in the main text. e, The corresponding local out-of-plane PFM hysteresis loops show the switchable out-of-plane polarizations in all ultrathin films. The asymmetry of the coercive field is probably due to asymmetric electrostatic boundary conditions and a self-poling effect, which arises at the interface between BFO and the conductive silicon substrate employed in the measurements.
The initial state contains a single out-of-plane domain and multiple in-plane domains. The initial out-of-plane domain with polarization pointing downwards can be switched and this domain-writing process also affects the in-plane domain orientation owing to the trailing field effect. The voltage was applied at the tip during the domain reading and writing. Scale bar, 500 nm.
Extended Data Fig. 6 Lattice distortion as a function of position for an eight-unit-cell-thick BFO slab.
Clear lattice distortions c driven by the surface electric field are found near the polar surface. l is the thickness of the slab and z is the distance from the surface. The dotted line is a guide to the eye.
Extended Data Fig. 7 Schematic of the workflow of the TEM cross-sectional sample preparation using focused ion beam.
a, Deposition of a Pt protection layer on the film surface by electron beam evaporation. b, Etching by gallium ion beam to form a sample lamella. c, Cutting off the lamella by gallium ion beam and pulling it out by in situ micro-manipulator. d, Adjusting the lamella position. e, Transfer of the lamella to a sample grid and detaching of the micro-manipulator. f, Thinning of the lamella to be electron-transparent and removing the superficial amorphous layers by ion-beam fine milling and polishing.
a–e, STEM-HAADF cross-sectional images of BFO freestanding films showing flat (a–c) and buckled shape (d, e) in thicker (ten unit cells) and thinner (four unit cells) freestanding films, respectively. For thicker freestanding films, all atomic columns can be sharply resolved in a single image since their zone axes were well aligned with the beam direction (b) and a mistilt of the lattice of just 1° leads to a blurred image (c). For thinner freestanding films, higher-magnification images (e) taken with different tilting angles (1° per step) indicate that the film is a single crystal but twisted owing to the ripples. The scale bars in the low-magnification images (a, d) are 20 nm and the scale bars in the enlarged images (b, c, e) are 5 nm.
To quantitatively estimate the effect of the degree of mistilt on the analysis of c/a, we calculated the simulated HAADF-STEM images of a hypothetical cubic BiFeO3 crystal as a function of the tilting angle around the  axis (a), and  axis (b), respectively, using a multislice simulation code called QSTEM. The thickness of the BFO model was about 20 nm (50 unit cells). As shown in a and b, within a few degrees of the tilt, the atomic columns in STEM images are noticeably elongated along the direction perpendicular to the rotation axis. Using the same two-dimensional Gaussian fitting procedure as that one used for analysing the experimental data in Fig. 3, the deviations of the fitted lattice constants from the nominal ones are negligible (c). The maximum deviation of the calculated c/a ratio is less than 0.002, which is two orders of magnitude smaller than the increment of the c/a ratio (1.22) observed in the ultrathin freestanding BFO films. The electron energy was 300 kV, the probe convergence angle was 25 mrad, and the angular range of the HAADF detector was 79.5 mrad to 200 mrad in the simulation, values consistent with the experiment. The error bars represent the fitting error of the lattice constants.