Temperature Dependence of Optical Properties of MoS₂ and WS₂ Heterostructures Assessed by Spectroscopic Ellipsometry

We report the complex dielectric function ε = ε₁ + iε₂ of MoS₂/WS₂ and WS₂/MoS₂ heterostructures and their constituent monolayers MoS₂ and WS₂ for an energy range from 1.5 to 6.0 eV and temperatures from 39 to 300 K. Comparisons between the optical properties of the heterostructures and their monolayers were conducted. Critical-point (CP) energies of the heterostructures were traced back to their origins in the monolayers. Low-temperature measurements confirmed the existence of only three excitonic CPs from 1.5 to 2.5 eV due to the overlap of trion B^- of the MoS₂ monolayer and exciton A⁰ of the WS₂ monolayer. Due to the dielectric screening effect, most CPs exhibit red shifts in the heterostructures compared to their monolayer counterparts.