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Xiaolong Liu1 Zhuhua Zhang2 Boris Yakobson3 Mark Hersam1

1, Northwestern University, Evanston, Illinois, United States
2, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu, China
3, Rice University, Houston, Texas, United States

Following theoretical predictions [1], two-dimensional (2D) boron (i.e., borophene) was experimentally realized less than two years ago via molecular beam epitaxy on noble metal substrates [2,3], showing intriguing properties including high in-plane anisotropy and unique surface chemistry [4]. As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, implying that the intrinsic defects of borophene remain unexplored. To fill this knowledge gap, we perform here an atomically-resolved study of borophene structures with ultra-high vacuum scanning tunneling microscopy and density functional theory. Borophene phases corresponding to the v1/6 and v1/5 models are found to coexist and intermix under suitable growth conditions. In each phase, we identify parallel line defects with atomic structures that correspond to the constituent units of the other phase. Furthermore, these line defects are observed to preferentially self-assemble into periodic arrays, giving rise to new borophene phases. Therefore, the distinction between borophene crystals and defects is blurred by the fact that line defects become building blocks of new phases with a single tuning parameter being the mixing ratio of the v1/6 and v1/5 rows. This unique phenomenon in borophene is enabled by its high in-plane anisotropy, similarities of the v1/6 and v1/5 lattice constants, and atomic registry with the underlying Ag(111) substrate. Overall, this work reveals the unique behavior of borophene line defects, which will likely inform future fundamental studies and emerging efforts to realize borophene-based applications.

[1] Y. Liu, E. S. Penev, B. I. Yakobson, Angew. Chem. Int. Ed. 52, 3156–3159 (2013).
[2] A. J. Mannix, X.-F. Zhou, B. Kiraly, J. D. Wood, D. Alducin, B. D. Myers, X. Liu, B. L. Fisher, U. Santiago, J. R. Guest, M. J. Yacaman, A. Ponce, A. R. Oganov, M. C. Hersam, N. P. Guisinger, Science 350, 1513–1516 (2015).
[3] B. Feng, J. Zhang, Q. Zhong, W. Li, S. Li, H. Li, P. Cheng, S. Meng, L. Chen, K. Wu, Nat. Chem. 8, 563–568 (2016).
[4] X. Liu, Z. Wei, I. Balla, A. J. Mannix, N. P. Guisinger, E. Luijten, and M. C. Hersam, Sci. Adv. 3, e1602356 (2017).

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