Second-harmonic generation and photoluminescence properties of colloidal WS₂ monolayers deposited from solution

Monolayered WS2 exhibits a direct band gap, strongly bound excitons, trions and biexcitons as well as no inversion symmetry in the 2D crystal structure (D3h), allowing frequency doubling by second-harmonic generation. The recent progress in colloidal synthesis of transition metal dichalcogenides enables the scalable production of phase-pure monolayer-dominated WS2. In this work, we use parabolic mirror-assisted microscopy to investigate the linear and nonlinear optical properties of deposited colloidal WS2. Spatial and spectral resolution of one- and two-photon photoluminescence and second-harmonic generation correlates structural motifs of the WS2 layers including orientation and agglomeration to the local optical response. We find exciton and trion emission at 613 nm and 639 nm respectively, with significantly varying contribution to the overall intensity between one- and two-photon excitation. This is attributed to the increased carrier density under high fluences needed for two-photon excitation. By evaluating the second-harmonic generation intensity, the nonlinear susceptibility of colloidally synthesized WS2 nanosheets is calculated to range from 4.28-7.55 nm V-1, averaging higher than a commercial WS2 monolayer reference sample. Our results show that colloidal WS2 is suitable for large area coverage creating an isotropic WS2 film, while retaining the highly sought after linear and nonlinear optical properties of WS2 monolayers, even under surface ligand separated aggregation.