Optical identification of atomically thin dichalcogenide crystals

We present a systematic study of the optical contrast of diselenide (NbSe2) and molybdenum disulphide (MoS2) flakes deposited onto Si wafers with a thermally grown SiO2 layer. We measure the optical contrast of flakes whose thickness ranges from 200 layers down to a monolayer using different illumination wavelengths in the visible spectrum. The refractive index of these thin crystals has been obtained from the measured optical contrast dependence on the flake thickness by using a simple model based on the Fresnel law. With the refractive index of these NbSe2 and MoS2 crystallites, the optical microscopy data can be quantitatively analyzed to determine the thickness of the flakes in a fast and non-destructive way.