Micro-Photoluminescence spectroscopy of two distinct types of WS2 monolayers using fs-Ti: Sapphire laser under vacuum and ambient conditions and multivariate calibration models

Abstract

We utilized Micro-Photoluminescence (μPL) and time-resolved photoluminescence (TRPL) spectroscopies with a femtosecond laser (440 nm) to assess the optical properties of two types of WS2 monolayers (MLs) obtained through mechanical exfoliation of different crystals. Based on cryogenic μPL spectra (4 K), the monolayers were classified as low-defect-concentration (LDC) and high-defect-concentration (HDC). By subjecting the MLs to laser excitation at various photon fluxes under ambient and vacuum conditions at room temperature, we characterized their optical properties. The spectra exhibited two main regions corresponding to optical transitions of neutral excitons (X) and trions (T). Notably, two distinct regimes were observed for quasi-particle decay: X-X annihilation at low photon fluxes and Mott transition at high photon fluxes. The photoluminescence quantum yield (PLQY) was measured under ambient and vacuum conditions, with the presence of physisorbed O2 on the ML surface influencing the results. Additionally, we employed a novel analytical approach combining PL and TRPL spectra with chemometric methods for the regression analysis of the two types of WS2 MLs. Utilizing partial least squares regression and principal component regression models, we estimated the decay times of X and T quasi-particles based on the spectral datasets.