Spectroscopic imaging ellipsometry of two-dimensional TMDC heterostructures

TL;DR

This study uses spectroscopic imaging ellipsometry to determine the dielectric functions of MoSe2/WSe2 heterobilayers and their individual layers, revealing how stacking alters optical properties and affects optical measurement interpretations.

Contribution

It provides the first detailed dielectric function measurements for MoSe2/WSe2 heterobilayers, showing they behave as a new artificial homobilayer rather than a simple stack.

Findings

Redshift of 18-44 meV in interband transitions

Dielectric function differs from individual layers at higher energies

Implications for interpreting optical measurements like Raman and photoluminescence

Abstract

Semiconducting two-dimensional materials and their heterostructures gained a lot of interest for applications as well as fundamental studies due to their rich optical properties. Assembly in van der Waals heterostacks can significantly alter the intrinsic optical properties as well as the wavelength-dependent absorption and emission efficiencies making a direct comparison of e.g. photoluminescence intensities difficult. Here, we determine the dielectric function for the prototypical MoSe2/WSe2 heterobilayer and their individual layers. Apart from a redshift of 18 meV - 44 meV of the energetically lowest interband transitions, we find that for larger energies the dielectric function can only be described by treating the van der Waals heterobilayer as a new artificial homobilayer crystal rather than a stack of individual layers. The determined dielectric functions are applied to calculate the Michelson contrast of the individual layers and the bilayer in dependence of the oxide thickness of often used Si/SiO2 substrates. Our results highlight the need to consider the altered dielectric functions impacting the Michelson interference in the interpretation of intensities in optical measurements such as Raman scattering or photoluminescence.