Avoiding photooxidation of monolayer WS₂ with nonlinear optical detection method

Kuang-I Lin^1*, Yao-Pang Chang², Wei-Bang Li¹, Yueh-Chiang Yang², Hsueh-Lung Lu², Po-Wen Chiu²

¹Core Facility Center, National Cheng Kung University, Tainan, Taiwan
²Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Kuang-I Lin, email:kilin@mail.ncku.edu.tw

The preservation of two-dimensional (2D) WS₂ in the environment is a concern for researchers [1]. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption [2]. Most of the current optical detection methods for 2D materials use photoluminescence and Raman spectroscopy. It is difficult to avoid laser damage to the sample. With other methods, such as fluorescence microscopy and laser scanning confocal microscopy, the morphology of WS₂ can be rapidly obtained [2,3], but the energy characteristics of quasiparticles formed by the 2D system cannot be obtained. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics [4-6]. With this detection method, we can explore the oxidation and degradation mechanisms of WS₂ grown by chemical vapor deposition in its original state. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. We also demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow WS₂ can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of WS₂ under environmental conditions.
1. Li, Q. et al., J. Mater. Chem. A 7, 4291 (2019).
2. Kotsakidis, J. C. et al., Nano Lett. 19, 5205 (2019).
3. Liu, H. et al., Nano Lett. 16, 5559 (2016).
4. Lin, K.-I. et al., Nano Lett. 18, 793 (2018).
5. Lin, K.-I. et al., J. Phys. Chem. C 124, 7979 (2020).
6. Tsai, T.-H. et al., ACS Nano 14, 4559 (2020).

Keywords: WS2, second-harmonic generation, photooxidation, chemical vapor deposition, optical detection