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

TL;DR

This study uses optical microscopy and micro-reflectance to quickly determine the thickness and refractive index of exfoliated franckeite nanosheets, aiding their characterization.

Contribution

It introduces a rapid optical method to estimate thickness and refractive index of franckeite nanosheets using microscopy and spectral analysis.

Findings

Optical contrast spectra enable refractive index determination.

Transmission and epi-illumination microscopy estimate flake thickness.

Refractive index of franckeite characterized across visible spectrum.

Abstract

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 in a broad range of the visible spectrum through a fit of the acquired spectra to a Fresnel law based model.