  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 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
thickness 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, WSe2 and NbSe2 can be detected on 90nm and 270 nm SiO2 using optical
means. By measuring contrast under broad-band green illumination we are also
able to distinguish between nanostructures containing single, mono and triple
layers of MoS2, WSe2 and NbSe2.
