Thickness-dependent optical bandgap of WS2/glass thin films deposited PLD method and their optical limitations and nonlinear optical properties by using the CW laser beam(532 nm)

Abstract

WS2 thin films were deposited on glass substrates (WS2/glass) using the pulsed laser deposition (PLD) method with thicknesses of 30 and 190 nm. The crystalline structure and elemental composition of the WS2 thin films were analyzed through X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to measure uniform, hexagonally cross-sectioned cones with smooth and even surfaces. Raman spectroscopy was utilized to assess the crystallinity and layer count in the WS2 thin films. Optical absorption, Tauc, and Urbach plots were employed to determine the optical bandgap energy (Eg) and defect energy Urbach (ED) of WS2 thin films with thicknesses of 30 nm and 190 nm. The 190 nm thick film exhibited an indirect bandgap transition with an energy value of 1.41 eV, whereas the 30 nm thin film showed a direct bandgap transition with an energy value of 1.98 eV. The defect energy Urbach was measured at 1.28 eV for the 30 nm WS2 film and 0.83 eV for the 190 nm film. Open aperture (OA) Z-scan analysis was conducted using a continuous-wave (CW) laser at 532 nm. The nonlinear absorption coefficient (β) was calculated for two different intensities, yielding values of 20.62 and 17.4 cmW for the 30 nm WS2 thin film and 13.75 and 13.35 cmW for the 190 nm film. The values of Imχ(3) and the figure of merit increased as the thin film size decreased for different sizes of WS2 films (30 nm and 190 nm). The optical limiting threshold and clamping point for the 30 nm WS2 thin film were found to be between 30–110 mW. It is evident that the higher β value for the WS2 thin film (30 nm) indicates a more effective limiting behavior.