Optical contrast and refractive index of natural van der Waals heterostructure nanosheets of franckeite

We study mechanically exfoliated nanosheets of franckeite by quantitative optical microscopy. The analysis of transmission-mode and epi-illumination-mode optical microscopy images provides a rapid method to estimate the thickness of the exfoliated flakes at first glance. A quantitative analysis of the optical contrast spectra by means of micro-reflectance allows one to determine the refractive index of franckeite over a broad range of the visible spectrum through a fit of the acquired spectra to a model based on the Fresnel law.We acknowledge funding from the European Commission under the Graphene Flagship, contract CNECTICT-604391, from EU Horizon 2020 research and innovation program under grant agreement No. 696656 (GrapheneCore1 – Graphene-based disruptive technologies), from the FP7 ITN MOLESCO (project no. 606728), from the MINECO (FIS2015-67367-C2-1-P, MAT2014-57915-R, MDM-2014-0377 and Ramón y Cajal 2014 program RYC-2014-16626), from the Comunidad de Madrid (MAD2D-CM Program (S2013/MIT-3007)) and NANOFRONTMAG-CM program (S2013/MIT-2850). RF acknowledges support from the Netherlands Organisation for Scientific Research (NWO) through the research program Rubicon with project number 680-50-1515