Abstract
Within the family of two-dimensional dielectrics, rhombohedral boron nitride (rBN) is considerably promising owing to having not only the superior properties of hexagonal boron nitride1,2,3,4—including low permittivity and dissipation, strong electrical insulation, good chemical stability, high thermal conductivity and atomic flatness without dangling bonds—but also useful optical nonlinearity and interfacial ferroelectricity originating from the broken in-plane and out-of-plane centrosymmetry5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23. However, the preparation of large-sized single-crystal rBN layers remains a challenge24,25,26, owing to the requisite unprecedented growth controls to coordinate the lattice orientation of each layer and the sliding vector of every interface. Here we report a facile methodology using bevel-edge epitaxy to prepare centimetre-sized single-crystal rBN layers with exact interlayer ABC stacking on a vicinal nickel surface. We realized successful accurate fabrication over a single-crystal nickel substrate with bunched step edges of the terrace facet (100) at the bevel facet (110), which simultaneously guided the consistent boron–nitrogen bond orientation in each BN layer and the rhombohedral stacking of BN layers via nucleation near each bevel facet. The pure rhombohedral phase of the as-grown BN layers was verified, and consequently showed robust, homogeneous and switchable ferroelectricity with a high Curie temperature. Our work provides an effective route for accurate stacking-controlled growth of single-crystal two-dimensional layers and presents a foundation for applicable multifunctional devices based on stacked two-dimensional materials.
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The data that support the findings of this study are available within the paper. Additional data are available from the corresponding authors upon request. Source data are provided with this paper.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (52025023, 51991344, 12334001, 51991342, 52021006, 52272173, 92163206, 11888101, T2188101, 12104018, 12104493, 52172035, 92163206 and 22333005), National Key R&D Program of China (2023YFB4603603, 2022YFA1403500 and 2021YFA1400502), Guangdong Major Project of Basic and Applied Basic Research (2021B0301030002), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB33000000), the National Postdoctoral Program for Innovative Talents (BX20220117 and BX20230022), China Postdoctoral Science Foundation (2022M721224 and 2023M730103), and the New Cornerstone Science Foundation through the XPLORER PRIZE. L.W. thanks support from the Youth Innovation Promotion Association of CAS. W.W. thanks the National Supercomputer Centre in Tianjin for computing support.
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X.B., K.L. and L.W. supervised the project. L.W., K.L., X.B. and X.Z. conceived the project. L.W., K.L., F.D., X.Z. and E.W. developed the growth and ferroelectricity mechanism. X.B. and L.W. organized the structural characterization. L.W., J.Q., Z.Z., Muhong Wu, W. Wang and X.X. conducted the preparation of single-crystal Ni(520) foils and rBN multilayer films. W. Wei, C.Z., Y.S., Menghao Wu and F.D. performed the theoretical calculations. Mengqi Wu, J.Q. and X.Z. performed the scanning probe-based characterizations. X.L., H.S., Q.G., M.C., Z.L., L.W., P.G. and X.B. performed the STEM and TEM measurements. J.Q., Z.Z., Q.W., Z.X., C.L., H.H., Z.-J.W. and L.W. performed the SEM, EBSD, LEED, XRD, SHG, Raman, ultraviolet–visible and XPS experiments. All authors discussed the results and wrote the paper.
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Extended data figures and tables
Extended Data Fig. 1 Designing the most suitable terrace and bevel facets of bunched steps.
a, Schematic illustrations of the interlayer spacing mismatch between the rBN and three low-index facets of Ni substrates, where the spacing distances of the (100), (110) and (111) facets are 1.05, 1.12 and 1.22 times the interlayer spacing of the rBN layers (c), respectively. b, Schematic illustrations of the interlayer sliding mismatch, where the horizontal distance of adjacent edges at the bevel facets of (100), (110) and (111) is 0, 2.43 and 1.72 times the single B-N bond length (a). Considering that the interlayer sliding in rBN layers is constant integer times of the half B-N bond length along the armchair direction, the (110) facet is chosen as the best bevel facet. c, Binding energy of unstable ABA-stacked hBN attached to the bevel facet of bunched steps, suggesting the spontaneous relaxation to ABC stacking. The orange, green, grey and white balls represent the B, N, Ni and H atoms, respectively.
Extended Data Fig. 2 EBSD pole figures of the single-crystal Ni(520) foil measured at different positions.
The multiples of uniform density (MUD) results present the unidirectional orientation without in-plane rotation.
Extended Data Fig. 3 Atomic configuration of rBN.
a, Typical planar STEM images of rBN layers. All images are the same size. b, Typical cross-sectional STEM images of rBN layers with consistent ABC stacking. The zone axes are the zigzag direction of rBN layers here. All images are the same size. c, Representative cross-sectional HRTEM image of as-grown rBN layer on Ni(520) substrate. The bunched steps with angle of 135° on the Ni(520) surface are denoted with yellow dotted lines. Zone axis is zigzag direction of rBN layers here. d-g, Cross-sectional STEM image of rBN layers grown on Ni substrate (d), electron energy-loss spectroscopy (EELS) mappings of Ni (e), B (f) and N (g), implying the absence of Ni-B alloys at interface.
Extended Data Fig. 4 Comparative results for growing AA’A-stacked hBN multilayers.
a, Schematic illustration of the growth of inherently stable hBN multilayer domains on the Ni surface without guidance from bevel edges. Inset, top view of the AA’A-stacked hBN lattice. The orange, green and blue balls represent the B, N and overlapped B and N atoms, respectively. b, Typical SEM image of multilayer hBN domains with the typically opposite orientation in the adjacent layers grown on the Ni foil without bevel edges. Inset, scheme of the anti-parallelly stacked BN layers. c, LEED pattern of as-grown hBN layers. d,e, Planar (d) and cross-sectional (e) HAADF-STEM images of hBN layers. The zone axis of (e) is zigzag direction of hBN layers.
Extended Data Fig. 5 SEM images of unidirectionally aligned rBN multilayer domains on Ni(520) substrates.
All images are the same size.
Extended Data Fig. 6 More seamlessly stitching results.
a,b, Pristine low-magnification STEM images of stitched region shown in Fig. 3c. c-f, Representative HAADF-STEM images collected at zones 1, 2, 4 and 5 marked in Fig. 3c. All images are the same size. g, Low-magnification TEM image obtained at the concave corner in the joint area of the first two layers of aligned rBN domains. Inset, the first layer BN that had been stitched into a continuous film. h-l, Representative HAADF-STEM images collected at zones II to VI marked in (g), showing the uniform lattice of stitched domains. All images are the same size.
Extended Data Fig. 7 Characterization of as-grown single-crystal rBN films.
a, AFM height mappings of transferred rBN films with typical thicknesses in the range of 2.2–12 nm. b, Statistics of the side length of separated domains at the nucleation stage. The average domain size is calculated to be 3.1 μm. c, Statistics of the average film thickness, exhibiting the nearly-linear dependence on the growth time. The estimated growth rate at the film-formation stage is 1.15 nm/h, which is consistent with the previously reported values (ref. 1: Nature 2022, 88, 606; ref. 2: Nano Letters 2016, 16, 3360). d, LEED patterns of the as-grown rBN film collected at different positions. The threefold symmetric patterns with consistent orientation confirm the single-crystal nature of the rBN lattice. e-h, UV-vis spectrum with an absorption peak at 6.1 eV (e), Raman spectrum with a characteristic E2g peak at 1368.5 cm−1 (f), and XPS spectra with B 1 s peak at 190.2 eV (g) and N 1 s peak at 397.9 eV (h), respectively, exhibiting the high quality of the as-grown rBN layers. i,j, In-situ SHG mappings of the rBN single-crystal films before (i) and after (j) etching, showing none of observed etched defect or thickness reduction. The same number corresponds to the same area, and all images are the same size.
Extended Data Fig. 8 Evolution of PFM response by using different a.c. (Vac) and d.c. (Vdc) voltages.
a,b, PFM signals with a fixed Vdc of 8 V and various Vac of 0.8 V to 2.0 V (a), as well as a fixed Vac of 1 V and Vdc ranging from 2.0 V to 9.0 V (b). The Vdc-dependent PFM hysteresis loops show that the rBN system has a specific coercive voltage (Vc). When Vdc is smaller than Vc in rBN, Vdc is not enough to switch the ferroelectric polarization, so a butterfly shaped curve in amplitude and 180° phase hysteresis are absent. In addition, the PFM loops show increasing amplitude and shrinking hysteresis window with increasing Vac from 0.5 V to 2.0 V. c, PFM signals collected at different nine samples, indicating the reproducibility of ferroelectricity in rBN layers.
Extended Data Fig. 9 Theoretical simulation and experimental measurements on hBN.
a,b, Interlayer differential charge density (a) and the corresponding line profiles (b) of the exfoliated hBN flakes. c,d, AFM height mapping (c) and the corresponding KPFM mapping (d) measured on the exfoliated hBN flakes, showing no potential change of hBN with different thickness. The curve data were collected along the orange dashed lines. e, PFM measurements on exfoliated hBN flakes, showing the absence of ferroelectric responses. Inset, AFM height mapping of hBN with a typical thickness of 3.9 nm.
Extended Data Fig. 10 Theoretical simulation of the switching barrier of the ferroelectric domains in rBN with 2–4 layers.
a-c, Energy evolution of the bilayer (a), trilayer (b) and tetralayer (c) systems along with the sliding steps. d-f, Relative displacement curves between adjacent layers of rBN bilayer (d), trilayer (e) and tetralayer (f) under another sliding path. The dashed lines mark the corresponding domain boundary areas, and the switching barriers are also marked in the bottom of figures. The insets are different sliding paths and the orange and green balls represent the B and N atoms, respectively. g, Energy evolution with the nucleus propagation of rBN trilayer from ABC to CBA stacking sequence, where r represents the radius of nucleus. The CBA-stacked nucleus and fixed domain boundary region is marked by the black dashed lines.
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Wang, L., Qi, J., Wei, W. et al. Bevel-edge epitaxy of ferroelectric rhombohedral boron nitride single crystal. Nature 629, 74–79 (2024). https://doi.org/10.1038/s41586-024-07286-3
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DOI: https://doi.org/10.1038/s41586-024-07286-3
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