Complex Exciton Behavior in Monolayer WS₂ by Laser Irradiation

Transition metal dichalcogenides (TMDs) have attracted tremendous attention due to their unique optical and electronic properties. In particular, chemical vapor deposition (CVD) growth technique has provided an opportunity for TMDs to take a step forward for use in large device applications. In previous studies, an understanding of the exciton dynamics in TMDs has been considered important in fabricating and utilizing them in device structures, so several research groups have studied exciton behavior in TMDs such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) depending on laser irradiation. However, despite of the same compositional TMDs, they observed different aspects in the photoluminesence (PL) results, and the mechanism of this is still under debate. In this work, we investigated the exciton behavior of CVD-grown monolayer WS₂ under laser irradiation. The PL spectra of the as-grown and transferred WS₂ flakes were obtained using a confocal PL system with 532 nm excitation. As the laser irradiation time increased, both the blue-shifted PL peak and the red-shifted PL peak were observed at different positions on the same as-grown WS₂ flake. On the other hand, this complex behavior was not noticeable in the transferred WS₂ flake, and only the blue-shifted PL peak was clearly observed. Based on the laser power-dependent PL data, we will discuss how strain, laser heating, and vacancies affect the PL trend of TMDs before and after the transfer process.