Spatial mapping of exciton transition energy and strain in composition graded WS_{2(1- x)}Se_{2 x} monolayer

We studied the optical properties of a composition graded WS 2 (1 - x)Se 2 x alloy monolayer. A symmetric gradual composition gradient from a Se-rich center to the relatively less Se-rich edges of an equilateral triangle shaped flake is confirmed by Raman mapping. Photoluminescence (PL) mapping shows a large 100 meV variation in the exciton energy, resulting from the composition dependent bandgap variation and carrier localization. The alloying leads to symmetry breaking and large nonlinear optical susceptibility. Second harmonic generation (SHG) mapping was carried out to study the non-linear properties and additionally to determine the lattice strain of the alloy flake. In contrast to PL and Raman mappings, SHG intensity is found to be spatially uniform. However, polarization dependent SHG reveals a unidirectional strain parallel to the (zigzag) edge of the flake, in addition to the sixfold symmetry expected from the transition metal dichalcogenide (TMD) lattice. Our results suggest potential applications of composition graded TMD alloys as ultra-compact color-tunable light sources and miniaturized spectrometers.