Xiahan Sang1 Xufan Li1 Wen Zhao2 Jichen Dong2 Christopher Rouleau1 David Geohegan1 Feng Ding2 Kai Xiao1 Raymond Unocic1

1, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
2, Ulsan National Institute of Science and Technology (UNIST), Ulsan, , Korea (the Republic of)

The edge structure of a two-dimensional (2D) material plays an important role in its growth and greatly affects electronic properties. To develop controlled edge engineering methods, it is critical to understand the mechanisms and kinetics of how edge structures form and evolve when subjected to different chemical environments. Here, using atomic-scale in situ scanning transmission electron microscopy (STEM), we observe the dynamics of edge structure formation and by combined electron beam irradiation and thermal effects. The most commonly observed edges are Se- or Mo- terminated zigzag edges, and MoSe nanowire (NW) terminated edges. Density functional theory (DFT) calculation on 59 different hypothesized edges confirms that the experimentally observed edges have the lowest formation energy under metal-rich environments. Unique functional properties of NW terminated edges are revealed using DFT calculation. Edge reconstruction from zigzag edges to NW terminated edges was directly observed experimentally and understood using ab initio molecular dynamics (AIMD). The combined atomic scale engineering, theory and simulation presented in this work helps to pave the way to engineering the edge of 2D materials for targeted functional applications via controlled electron beam irradiation.

In situ aberration-corrected STEM imaging was conducted at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy Office of Science User Facility. Synthesis science (XL, DBG, CMR, KX) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. W.Z., J.D. and F.D. acknowledge the support from the Institute for Basic Science (IBS-R019-D1) of Korea.