  The physical and electronic properties of ultrathin two-dimensional (2D)
layered nanomaterials are highly related to their thickness. Therefore, the
rapid and accurate identification of single- and few- to multi-layer nanosheets
is essential to their fundamental study and practical applications. Here, a
universal optical method has been developed for simple, rapid and reliable
identification of single- to quindecuple-layer (1L-15L) 2D nanosheets,
including graphene, MoS2, WSe2 and TaS2, on Si substrates coated with 90 nm or
300 nm SiO2. The optical contrast differences between the substrates and 2D
nanosheets with different layer numbers were collected and tabulated, serving
as a standard reference, from which the layer number of a given nanosheet can
be readily and reliably determined without using complex calculation nor
expensive instrument. Our general optical identification method will facilitate
the thickness-dependent study of various 2D nanomaterials, and expedite their
research toward practical applications.
