A rigorous and accurate contrast spectroscopy for ultimate thickness determination of micrometre-sized graphene on gold and molecular sensing

The thickness of graphene films can be accurately determined by optical contrast spectroscopy. However, this becomes challenging and complicated when the flake size reduces to the micrometre scale, where the contrast spectrum is sensitively dependent on the polarization and incident angle of light. Here we report accurate measurement of the optical contrast spectra of micrometre-sized few-layer graphene flakes on Au substrate. Us-ing a high-resolution optical microscopy with a 100x magnification objective, we accurately determined the lay-er numbers of flakes as small as one micrometre in lateral size. We developed a rigorous theoretical model to accurately take into account the appropriate contribution of light incident at various angles and polarizations, which matched the experimental results extremely well. Furthermore, we demonstrate that the optical contrast spectroscopy is highly sensitive to detect the adsorption of a sub-monolayer airborne hydrocarbon molecules, which can reveal whether graphene is contaminated. Though the technique was demonstrated on graphene, it can be readily generalized to many other two-dimensional (2D) materials, which opens new avenues for devel-oping miniaturized and ultrasensitive molecular sensors.