Two-dimensional (2D) boron (that is, borophene) was recently synthesized following theoretical predictions1,2,3,4,5. Its metallic nature and high in-plane anisotropy combine many of the desirable attributes of graphene6 and monolayer black phosphorus7. As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, which implies that the intrinsic defects of borophene remain unexplored. Here we investigate borophene line defects at the atomic scale with ultrahigh vacuum (UHV) scanning tunnelling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Under suitable growth conditions, borophene phases that correspond to the v1/6 and v1/5 models are found to intermix and accommodate line defects in each other with structures that match the constituent units of the other phase. These line defects energetically favour spatially periodic self-assembly that gives rise to new borophene phases, which ultimately blurs the distinction between borophene crystals and defects. This phenomenon is unique to borophene as a result of its high in-plane anisotropy and energetically and structurally similar polymorphs. Low-temperature measurements further reveal subtle electronic features that are consistent with a charge density wave (CDW), which are modulated by line defects. This atomic-level understanding is likely to inform ongoing efforts to devise and realize applications based on borophene.
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X.L. and M.C.H. acknowledge support from the Office of Naval Research (ONR N00014-17-1-2993) and the National Science Foundation Materials Research Science and Engineering Center (NSF DMR-1720139). The computational work at Rice University was supported by the Army Research Office (W911NF-16-1-0255) and by the Robert Welch Foundation (C-1590); Z.Z. acknowledges NSFC-11772153 support at a later stage. Z.Z., L.W. and B.I.Y. also acknowledge support by the US DOE Office of Science (DOE DE-SC0012547) and the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (no. MCMS-0415K01). X.L. further acknowledges support from a Ryan Fellowship that is administered through the Northwestern University International Institute for Nanotechnology. The authors thank A. J. Mannix, M. Han, I. Balla, S. Li, E. S. Penev and Q. Ruan for valuable discussions.
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Nature Materials (2018)