Figure 6.

(a) Wavelength-dependent refractive index (both real and complex part) of franckeite, determined from the fit of thickness-dependent optical contrast traces to a model based on the Fresnel law. The shadowed region is the uncertainty of the refractive index extracted from the analysis of different datasets. (b) Calculated optical contrast for a single-layer franckeite flake as a function of the illumination wavelength and the SiO₂ thickness to determine the optimal SiO₂ capping layer to facilitate the identification of franckeite thin layers.