Healing of Planar Defects in 2D Materials via Grain Boundary Sliding

Abstract

carrier traps, and reduce coherence in light emission. The growth of single phase TMDC films is complicated by the intralayer sliding in each monolayer (resulting in polymorphs, as in 1T and 1H) [9,10] and interlayer sliding in multilayer films (creating different stacking polytypes, for example, 2H and 3R). [7,11] The inherent threefold symmetry as well as fluctuations in chemical potential during growth, coupled with the small energy barriers for rotation and translation of the atomic layers, provides many possibilities for freezing-in dislocations and stacking faults.Among the different CVD approaches, van der Waals (vdW) epitaxy has emerged as the most promising approach to grow large-scale highly crystalline TMDC films, although the films are not free of stacking faults. [12] The key to the success of vdW epitaxy lies in the fact that the surface of 2D TMDC has no dangling bonds, thereby relaxing the lattice matching conditions between the two crystals. Nevertheless, our previous studies have shown that the interlayer vdW interaction between two atomic layers is sufficiently strong to align the grains so that stacking configurations with the lowest energy can be attained, [4,7] which has implications for the healing of planar defects.In polycrystalline TMDCs, GBs are intrinsic 1D topological defects which divide the film into a cellular network of Understanding the mechanisms and kinetics of defect annihilations, particularly at the atomic scale, is important for the preparation of highquality crystals for realizing the full potential of 2D transition metal dichalcogenides (TMDCs) in electronics and quantum photonics. Herein, by performing in situ annealing experiments in an atomic resolution scanning transmission electron microscope, it is found that stacking faults and rotational disorders in multilayered 2D crystals can be healed by grain boundary (GB) sliding, which works like a "wiper blade" to correct all metastable phases into thermodynamically stable phases along its trace. The driving force for GB sliding is the gain in interlayer binding energy as the more stable phase grows at the expanse of the metastable ones. Density functional theory calculations show that the correction of 2D stacking faults is triggered by the ejection of Mo atoms in mirror twin boundaries, followed by the collective migrations of 1D GB. The study highlights the role of the often-neglected interlayer interactions for defect repair in 2D materials and shows that exploiting these interactions has significant potential for obtaining large-scale defect-free 2D films. 2D MaterialsLarge-scale growth of single-crystalline transition metal dichalcogenide (TMDC) films with a low density of defects is yet to be realized, posing a bottleneck for implementing 2D electronics in industrial applications. To date, most chemical vapor deposition (CVD) grown films are polycrystalline and contain multiple imperfections, such as point defects, [1,2] line defects, [3] grain boundaries (GBs), [4,5] stacking faults, [6,7] and rotational disorder, ...

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