Grain boundaries (GBs) frequently emerge in a CVD-grown large-scale transition metal dichalcogenides monolayer thin film, which affect the electronic and optical properties of the material. Photoluminescence (PL) can be easily quenched/enhanced at GBs, which are, however, merely investigated in relatively large tilt angles (θ>14°) in previous research. Here, we experimentally examine the PL properties of monolayer WS2 GBs with tilt angles as small as a few degrees. Contrary to conventional wisdom, we find that PL intensity remains intact by the GBs when their tilt angles θ8°. The abnormal PL behavior is elucidated by a detailed structure analysis on the dislocation cores. For a small tilt angle, the strain fields introduced by the defective cores are sparsely distributed without mutual coupling, and the chemical stoichiometry along the GBs preserves very well. These two key structural features of the small-tilt-angle GBs allow excitons to diffuse transparently across the GB, leading to a neglectable influence on the optical and electronic properties, as verified by our first-principle simulations. The PL invariant of the small-tilt-angle GBs sheds light on the future development of CVD-grown wafer-scale techniques and their optical applications.

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