Effect of surface oxidation on nonlinear optical absorption in WS2 nanosheets.

Surface oxidation was easily formed at the edge of WS 2 , thus leading to charge transfer from WS 2 to WO 3. • Size-separation of WS 2 nanosheets was prepared using liquid phase exfoliation. • Surface oxidation defect of WS 2 was formed easily by smaller size. • Size-dependent nonlinear optical Absorption in WS 2 was observed. • Charge transfer at interface is used to explain the tunable nonlinear absorption. 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 WS 2 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 WS 2 , Z-scan technique equipped with 800 nm femtosecond laser was used. We observed the conversion between saturable absorption and reverse saturable absorption in WS 2 dispersions as well as the decrease of saturable absorption in WS 2 films due to the increase of reverse saturable absorption contribution of WO 3. Energy-level model based on type II WS 2 /WO 3 heterostructure was put forth to understand nonlinear optical absorption of the oxidized WS 2. The results show that charge transportation from WS 2 to WO 3 increases the reverse saturable absorption contribution in WO 3 while decreases the saturable absorption in WS 2. The results pave the way to design desirable nonlinear optical devices by controlling size with different level of surface oxidation. [ABSTRACT FROM AUTHOR]