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Epitaxial growth of a 100-square-centimetre single-crystal hexagonal boron nitride monolayer on copper

Abstract

The development of two-dimensional (2D) materials has opened up possibilities for their application in electronics, optoelectronics and photovoltaics, because they can provide devices with smaller size, higher speed and additional functionalities compared with conventional silicon-based devices1. The ability to grow large, high-quality single crystals for 2D components—that is, conductors, semiconductors and insulators—is essential for the industrial application of 2D devices2,3,4. Atom-layered hexagonal boron nitride (hBN), with its excellent stability, flat surface and large bandgap, has been reported to be the best 2D insulator5,6,7,8,9,10,11,12. However, the size of 2D hBN single crystals is typically limited to less than one millimetre13,14,15,16,17,18, mainly because of difficulties in the growth of such crystals; these include excessive nucleation, which precludes growth from a single nucleus to large single crystals, and the threefold symmetry of the hBN lattice, which leads to antiparallel domains and twin boundaries on most substrates19. Here we report the epitaxial growth of a 100-square-centimetre single-crystal hBN monolayer on a low-symmetry Cu (110) vicinal surface, obtained by annealing an industrial copper foil. Structural characterizations and theoretical calculations indicate that epitaxial growth was achieved by the coupling of Cu <211> step edges with hBN zigzag edges, which breaks the equivalence of antiparallel hBN domains, enabling unidirectional domain alignment better than 99 per cent. The growth kinetics, unidirectional alignment and seamless stitching of the hBN domains are unambiguously demonstrated using centimetre- to atomic-scale characterization techniques. Our findings are expected to facilitate the wide application of 2D devices and lead to the epitaxial growth of broad non-centrosymmetric 2D materials, such as various transition-metal dichalcogenides20,21,22,23, to produce large single crystals.

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Acknowledgements

This work was supported by the National Key R&D Program of China (2016YFA0300903, 2016YFA0300804 and 2018YFA0306800), the Natural Science Foundation of China (11888101, 51522201, 11474006, 11714154, 11634001 and 21725302), the National Equipment Program of China (ZDYZ2015-1), the Beijing Municipal Science & Technology Commission (Z181100004218006), the Beijing Graphene Innovation Program (Z181100004818003), the Science, Technology and Innovation Commission of Shenzhen Municipality (ZDSYS20170303165926217 and JCYJ20170412152620376), the Guangdong Innovative and Entrepreneurial Research Team Program (2016ZT06D348), the Bureau of Industry and Information Technology of Shenzhen (graphene platform contract number 201901161512), the National Postdoctoral Program for Innovative Talents (BX201700014), the Strategic Priority Research Program of CAS (XDB28000000), the National Program for Thousand Young Talents of China and the Institute for Basic Science of South Korea (IBS-R019-D1).

Reviewer information

Nature thanks Kian Ping Loh and Boris I. Yakobson and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

K.L., E.W., D.Y. and L.W. conceived the project. K.L. supervised the project. L.W., X. Xu, M. Wu, Zhihong Zhang, Zhibin Zhang and W.W. conducted the annealing of the single-crystal Cu foil and ex situ growth of single-crystal hBN. F.D. and L.Z. performed the theoretical calculations. Z.-J.W. and M. Willinger performed the in situ growth experiments. X.B., R.Q., L.W., Y.G. and P.G. performed the transfer of hBN and the TEM experiments. L.W. and X. Xu performed the EBSD, UV-Vis and LEED measurements, as well as the etching and oxidization experiments. Y.J. and Z.W. performed the STM experiments. Y.Z., W.C., X. Xie and J.Z. performed the ARPES measurements. S.W., J.L. and Y.S. performed the polarized SHG mapping measurements. Q.L., Zhihong Zhang and S.Z. performed the AFM measurement. All of the authors discussed the results and wrote the paper.

Competing interests

The authors declare no competing interests.

Correspondence to Xuedong Bai or Zhu-Jun Wang or Feng Ding or Kaihui Liu.

Extended data figures and tables

Extended Data Fig. 1 Annealing protocol for the single-crystal Cu (110) foil and growth of single-crystal hBN.

a, Diagram of the annealing procedure used for the single-crystal Cu (110) foil (see Methods for details). b, A small piece of single-crystal Cu (110) with <211> steps was placed on a polycrystalline Cu foil of size 10 × 10 cm2 to guide the annealing of the Cu foil. After annealing at 1,040 °C for 5 min, the nucleus has started to assimilate the polycrystalline Cu foil. c, After annealing for 2 h, about three-fifths of the Cu foil was annealed to give a single-crystal Cu foil. d, After annealing for 5 h, the entire foil is a single-crystal Cu foil. e, f, XRD patterns corresponding to a 2θ scan (e) and a ϕ scan along the Cu (100) direction (f) of as-annealed Cu foil, confirming the single-crystal nature of the Cu (110) foil without in-plane rotation. g, SEM image of unidirectionally aligned hBN grown on as-annealed Cu (110). h, Diagram of the growth procedure of single-crystal hBN film.

Extended Data Fig. 2 Characterizations of as-annealed Cu (110) foil at multiple positions.

a, Markers 1–9 on the as-annealed Cu foil indicate the positions used for the following characterizations. b, c, Representative EBSD maps along the [001] (b) and [010] (c) direction measured at positions 2 (b1, c1), 3 (b2, c2), 4 (b3, c3), 5 (b4, c4), 6 (b5, c5), 7 (b6, c6), 8 (b7, c7) and 9 (b8, c8). d, Representative LEED patterns measured at positions 2 (d1) to 9 (d8). The purple solid and dashed circles correspond to the visible and invisible diffraction points (due to the extinction rule), respectively.

Extended Data Fig. 3 Characterizations of as-grown hBN samples.

a, XPS spectra of the hBN film. The characteristic peaks corresponding to N and B confirm the chemical composition of hBN. b, Representative Raman spectrum. The characteristic peak at 1,367 cm−1 corresponds to the E2g vibration mode of 2D hBN. c, UV-Vis absorption spectrum, showing a bandgap of about 6.1 eV. d, AFM image of the edge of a hBN domain transferred onto SiO2/Si, showing monolayer thickness. eg, Low-magnification STEM image of a hBN film transferred onto a holey-carbon-film TEM grid (e). The SAED pattern (f) and atomically resolved HAADF-STEM image (g) were collected at the suspended area and show monolayer features.

Extended Data Fig. 4 Unidirectionally aligned growth of hBN domains on Cu (110).

a, SEM images of hBN domains covering a width of about 1 cm; the overlap regions are marked by same-colour dashed boxes. Among a total of about 700 domains only 3 are not aligned, which indicates that that the unidirectional alignment probability is greater than 99% (the arrow on the upper-right corner points to the downward growth direction of domains). b, LEED characterizations at positions 2–9 in Extended Data Fig. 2a.

Extended Data Fig. 5 Boundaries of hBN domains.

a, Grain boundary between unaligned hBN domains, shown as a dark line in a polarized SHG map. b, SEM image of hBN domains grown on Cu (110) after H2 etching, where no etched lines can be observed. c, SEM image of hBN domains grown on Cu (111) with a 60° twist angle after H2 etching. A clear etched line can be observed. d, Optical image of hBN domains grown on Cu (110) after UV oxidation for 30 min, where no boundary line can be observed. e, Optical image of hBN domains grown on Cu (111) with a 60° twist angle after UV oxidation for 30 min. Clear oxidation lines can be observed because of the existing boundaries.

Extended Data Fig. 6 TEM characterization of the seamless stitching of unidirectionally aligned hBN domains.

ac, TEM images of increasing magnification obtained at the boundary area between two unidirectionally aligned hBN domains on graphene TEM grids. dl, HRTEM images captured at the regions marked by numbers 0–9 in c, showing the bare graphene TEM grid at 0 (d) and the consistent moiré patterns of monolayer hBN on graphene TEM grids (el), confirming the seamless stitching of unidirectionally aligned hBN domains without the formation of grain boundaries. The insets show the corresponding FFT patterns. The image sizes of dl are identical. mo, TEM images of increasing magnification at the boundary area between unaligned hBN domains on graphene TEM grids, showing distinct moiré patterns and grain boundary (GB). p, FFT pattern of o, indicating a twist angle of 4.9° for the two hBN domains.

Extended Data Fig. 7 ARPES spectra of as-grown single-crystal hBN.

a, Constant-energy mapping at a binding energy of −4.0 eV. The red hexagon indicates the Brillouin zone of hBN (with the high-symmetry points Γ, M and K labelled), in which the top of the σ band located at the Γ point and a section of the π band can be clearly seen. bd, ARPES spectra along the K–Γ (b), M–Γ (c) and K–M (d) momentum directions. The white and red dashed curves depict the dispersions of the σ and π band, respectively. The σ band has a strong signal and a very sharp top, whereas the signal from the π band is relatively weak but can still be clearly distinguished. The top of the π band is blended with the bands of the Cu substrate, so it is hard to distinguish. e, Magnified ARPES spectrum crossing the Γ point along the ky direction, as shown in a. Here, kx and ky denote the momentum components along the horizontal and longitudinal axes of a, respectively; k|| in b, c and d is oriented along the K–Γ, M–Γ and K–M directions, respectively; EEF is the binding energy of band structures; E is the kinetic energy of photoelectrons received by the analyser; and EF is the Fermi level energy.

Extended Data Fig. 8 Evidence of step-edge-guided growth of hBN on Cu (110).

a, AFM images of parallel bunched steps on a bare as-annealed Cu surface. The average surface roughness is about 3.0 nm (including the bunched steps) and the average roughness of the flat plateaus is about 0.3 nm. bd, In situ observation of the hBN growth on Cu (110) at 950 °C. eg, Magnified SEM images of the hBN growth on Cu (110) at the area marked by the box in d, showing unidirectionally aligned growth of trapezoid hBN domains (the arrow points to the downward growth direction).

Extended Data Fig. 9 Edge-coupling-guided hBN growth on Cu (110).

ac, Schematic of edge-coupling-guided hBN growth on a Cu (110) vicinal surface with atomic step edges along the <211> direction. b shows the top view and c shows the side view of a.

Extended Data Fig. 10 Growth of hBN domains on different Cu facets.

a, SEM images of unidirectionally aligned hBN domains on Cu (410). b, SEM images of twisted hBN domains on Cu (100). c, SEM images of hBN domains with antiparallel alignment on Cu (111).

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Fig. 1: Characterization of single-crystal Cu (110) obtained by annealing an industrial Cu foil.
Fig. 2: Unidirectional alignment and seamless stitching of hBN domains on Cu (110).
Fig. 3: In situ observation of the unidirectional growth of hBN domains.
Fig. 4: Mechanism of edge-coupling-guided epitaxial growth of hBN domains on Cu (110).
Extended Data Fig. 1: Annealing protocol for the single-crystal Cu (110) foil and growth of single-crystal hBN.
Extended Data Fig. 2: Characterizations of as-annealed Cu (110) foil at multiple positions.
Extended Data Fig. 3: Characterizations of as-grown hBN samples.
Extended Data Fig. 4: Unidirectionally aligned growth of hBN domains on Cu (110).
Extended Data Fig. 5: Boundaries of hBN domains.
Extended Data Fig. 6: TEM characterization of the seamless stitching of unidirectionally aligned hBN domains.
Extended Data Fig. 7: ARPES spectra of as-grown single-crystal hBN.
Extended Data Fig. 8: Evidence of step-edge-guided growth of hBN on Cu (110).
Extended Data Fig. 9: Edge-coupling-guided hBN growth on Cu (110).
Extended Data Fig. 10: Growth of hBN domains on different Cu facets.

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