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. The size‐dependent nonlinear optical response of three, atomically thin, transition metal dichalcogenides is studied. Size separation of transition metal dichalcogenides is achieved by gradient centrifugation. Most of the large flakes show saturated absorption but WS2 nanosheets thinner than 50–60 nm exhibit reverse saturated absorption.