The origin of edge-enhanced second harmonic generation in monolayer MoS2 flakes

The sensitiveness of second-harmonic generation (SHG) signal to crystal lattice orientation and crystal symmetry makes it a powerful tool for structural characterization of low dimensional transition metal dichalcogenide (TMD) materials. In this paper, we report the strain-induced SHG enhancement at the edge of the monolayer MoS2 flakes. By performing raster mapping of SHG intensity, the images show an enhanced SHG intensity at the flake edge with cubic-power dependence. The AFM analysis of the MoS2 flake edge with enhanced SHG, indeed, detected a topographic height change as compared to the center region. Micro-Raman spectroscopy analysis shows a blue shift in A1g phonon mode at the flake edge. This edge-enhanced SHG may be attributed to the intrinsic strain effect induced during the chemical vapor deposition growth condition, which modulates the nonlinear susceptibility. The micro-Raman spectroscopy implies that the flake edge has a compressed and stiffened structure in an out-of-plane direction. This work brings out the awareness of the residual strain effect on 2D TMD device performance, and the adequate characterization of the strained structure by SHG intensity mapping.