Optical dielectric function of two-dimensional WS2 on epitaxial graphene

Heterostacks composed of two-dimensional WS2 and graphene exhibit interesting opto-electronic properties, including ultrafast charge transfer and fast, efficient photodetection. The optical properties of WS2 are heavily influenced by the presence of graphene in the heterostack, yet, so far, their characterization in the whole visible range is missing. In this work we report the complex dielectric function of two-dimensional WS2 flakes on epitaxial graphene on silicon carbide, obtained by means of spectroscopic ellipsometry (SE). The so-called A, B and C excitonic features are precisely identified, and significant differences with respect to literature data of WS2 on fused silica are highlighted. Since this investigation is based on SE, the complex dielectric function of WS2 is retrieved without performing Kramers Kronig analysis, which is instead necessary when employing reflectance or transmittance spectroscopy. Furthermore, the approach described in this work can be used in principle to characterize any two-dimensional flakes, both in terms of complex dielectric function and percentage of surface coverage.