Probing the origin of lateral heterogeneities in synthetic monolayer molybdenum disulfide

Synthetic two-dimensional (2D) materials provide an opportunity to realize large-scale applications in next generation electronic and optoelectronic devices. One of the biggest challenges of synthetic 2D materials is the lateral heterogeneity such as non-uniform strain, composition and defect density. The electronic and optical properties are found to be not uniform in many cases, even within a single crystalline domain, potentially limiting synthetic 2D materials in advanced devices. In this work, we probe the origin of the widely observed lateral heterogeneities in synthetic monolayer MoS₂. Epitaxial single crystalline domains (∼10 μm) are optically homogeneous and uniform with 0.3%-0.4% tensile strain, while misoriented domains (>20 μm) exhibit distinct photoluminescence (PL) emissions from the center to the edge, along with released strain at the center. Temperature-dependent Raman and PL mapping reveals that the center of non-epitaxial domains exhibits an enhanced PL due to increased defect density. Density function theory (DFT) calculations suggest that oxygen defects can readily lead the loss of epitaxy, consistent with our observation of a MoO_x core-shell structure that only exists in misoriented domains. Combining experiment and DFT, we hypothesize that two growth mechanisms, solid-solid and vapor-solid growth, may be responsible for the lateral heterogeneities.