All-optical quality assessment of 2D TMDs using polarization-resolved SHG

While large-area crystal growth techniques, such as chemical vapor deposition (CVD), are successfully used to produce 2D transition-metal dichalcogenides, the presence of grain boundaries, vacancies and arbitrarily oriented grains, substantially affect their crystal quality. We demonstrate a fast, high-resolution non-linear optical method for the quality control of WS₂ monolayers. Polarization resolved second harmonic generation (PSHG) imaging reveals with high-precision the orientation of the main crystallographic axis (armchair). By performing a pixel-by-pixel mapping of the armchair orientations on a CVD-grown sample area, we can distinguish between different domains, locate their boundaries and reveal their detailed structure. We fit experimental PSHG images of sub-micron resolution into a generalized theoretical model and we acquire the armchair orientation for every pixel. This allows us to measure the mean orientational average of armchair angle distributions from specific regions of interest and consequently to define the standard deviation of these distributions as a crystal quality factor. [1] S. Psilodimitrakopoulos, et al., Light Science & Applications 7, 18005 (2018).