Visibility of dichalcogenide nanolayers

Dichalcogenides with the common formula MX₂ are layered materials with electrical properties that range from semiconducting to superconducting. Here, we describe optimal imaging conditions for the optical detection of ultrathin, two-dimensional dichalcogenide nanocrystals containing single, double and triple layers of MoS₂, WSe₂ and NbSe₂. A simple optical model is used to calculate the contrast for nanolayers deposited on wafers with varying thicknesses of SiO₂. The model is extended for imaging using the green channel of a video camera. Using AFM and optical imaging we confirm that single layers of MoS₂ and WSe₂ can be detected on 90 and 270 nm SiO₂ using optical means. By measuring contrast under broadband green illumination we are also able to distinguish between nanostructures containing single, double and triple layers of MoS₂ and WSe_2. We observe and discuss discrepancies in the case of NbSe₂.