Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers

Polycrystalline growth of molybdenum disulfide (MoS₂) using chemical vapor deposition (CVD) methods is subject to the formation of grain boundaries (GBs), which have a large effect on the electrical and optical properties of MoS₂-based optoelectronic devices. The identification of grains and GBs of CVD-grown monolayer MoS₂ has traditionally required atomic resolution microscopy or nonlinear optical imaging techniques. Here, we present a simple spectroscopic method for visualizing GBs of polycrystalline monolayer MoS₂ using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities. We were able to distinguish a GB between two MoS₂ grains with tilt angles as small as 6° in their grain orientations and, based on the inspection of several GBs, found a simple empirical rule to predict the location of the GBs. In addition, the large number of twist angle domains traced through our facile spectroscopic mapping technique allowed us to identify a continuous evolution of the coupled structural and optical properties of bilayer MoS₂ in the vicinity of the 0° and 60° commensuration angles which were explained by elastic deformation model of the MoS₂ membranes.