Defect-induced distinct exciton-exciton interactions in WS 2 monolayers

The optoelectronic properties of atomically thin transition metal dichalcogenides (TMDs) are largely influenced by defect populations (DPs). In this work, we fabricate WS 2 monolayers with different DPs by varying the fabrication methods and further reveal their distinct exciton-exciton interactions. Steady-state photoluminescence (PL) experiments show that the monolayer with the lowest DP shows optimal PL intensity at low excitation power; however, it is overtaken and significantly surpassed by monolayers with higher DPs at high excitation powers. Excitation-power-dependent experiments demonstrate that these monolayers exhibit distinct PL saturation behaviors with the threshold power differing by four orders of magnitude. Combined with in situ PL imaging and time-resolved PL experiments, we attribute such PL evolution discrepancies to the different DPs within these monolayers, which largely influence the exciton diffusion behavior and subsequently bring about distinct nonradiative exciton-exciton annihilations (EEAs). Valley polarization experiments are further employed to re-examine the DPs of these monolayers. This work reveals the distinct PL behaviors and underlying exciton dynamics in TMD monolayers with different DPs, which can largely facilitate the engineering of relevant high-performance devices for practical applications.