Shape-Dependent Defect Structures of Monolayer MoS₂ Crystals Grown by Chemical Vapor Deposition

Monolayer MoS₂ crystals with tailored morphologies have been shown to exhibit shape-dependent properties and thus have potential applications in building nanodevices. However, a deep understanding of the relationship between the shape and defect structures in monolayer MoS₂ is yet elusive. Monolayer MoS₂ crystals in polygonal shapes, including triangle, tetragon, pentagon, and hexagon, are grown using the chemical vapor deposition technique. Compared with other shapes, the hexagon MoS₂ crystal contains more electron-donor defects that are mainly due to sulfur vacancies. In the triangular shapes, the defects are mainly distributed at the vertices of the shapes while they are located at the center of hexagonal shapes. On the basis of the Coulomb interaction of exciton and trion, quantitative calculations demonstrate a high electron density (∼10¹²/cm²) and high Fermi level (E_C - E_F = 15 meV) for hexagonal shape at room temperature, compared to triangular shapes (∼10¹¹/cm², E_C - E_F ≈ 30 meV). These findings verify that a much higher number of donor-like sulfur vacancies are formed in hexagonal MoS₂ shapes. This property allows more electrons or trions to localize in such sites through the physical/chemical adsorption of O₂/H₂O, which results in a strong enhancement of the light emission efficiency in the hexagonal crystal. The findings provide a better understanding of the formation of shape-dependent defect structures of monolayer MoS₂ crystals and are inspiring for applications in fabricating nanoelectronic and optoelectronic devices through defect engineering.