Effect of surface oxidation on nonlinear optical absorption in WS2 nanosheets

Abstract

Controlling nanosheet size and understanding size-dependent surface oxidation are of quite importance for the nonlinear optical properties of two-dimensional materials. Herein, size-separation of WS2 nanosheets was prepared successfully using liquid phase exfoliation and a gradient centrifugation method. We confirmed the higher surface oxidation with the decrease of nanosheet size by both high-resolution X-ray photoelectron spectroscopy and Raman spectroscopy. To investigate the effect of size-dependent surface oxidation on nonlinear optical properties in WS2, Z-scan technique equipped with 800 nm femtosecond laser was used. We observed the conversion between saturable absorption and reverse saturable absorption in WS2 dispersions as well as the decrease of saturable absorption in WS2 films due to the increase of reverse saturable absorption contribution of WO3. Energy-level model based on type II WS2/WO3 heterostructure was put forth to understand nonlinear optical absorption of the oxidized WS2. The results show that charge transportation from WS2 to WO3 increases the reverse saturable absorption contribution in WO3 while decreases the saturable absorption in WS2. The results pave the way to design desirable nonlinear optical devices by controlling size with different level of surface oxidation.