Size‐Dependent Nonlinear Optical Properties of Atomically Thin Transition Metal Dichalcogenide Nanosheets

Size-dependent nonlinear optical properties of modification-free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS2, WS2, and NbSe2. Firstly, a gradient centrifugation method is demonstrated to separate the TMD nanosheets into different sizes. The successful size separation allows the study of size-dependent nonlinear optical properties of nanoscale TMD materials for the first time. Z-scan measurements indicate that the dispersion of MoS2 and WS2 nanosheets that are 50-60 nm thick leads to reverse saturable absorption (RSA), which is in contrast to the saturable absorption (SA) seen in the thicker samples. Moreover, the NbSe2 nanosheets show no size-dependent effects because of their metallic nature. The mechanism behind the size-dependent nonlinear optical properties of the semiconductive TMD nanosheets is revealed by transient transmission spectra measurements.