Modeling the Temperature Dependence of the Optical Constants of MoS₂ Monolayers and Bilayers

We used spectroscopic ellipsometry to determine the dielectric functions of monolayers and bilayers of MoS₂ at varying temperatures. In-situ data were obtained during MOCVD growth of samples, and subsequently, temperature dependent spectra were obtained between 1050°C (i.e., growth temperature) and 300°C, at 50°C increments. We successfully constructed models for monolayer and bilayer MoS₂ by using four oscillators to represent their dielectric functions, each of which is associated with an electron transition in the band structure. In developing the models, the monolayers and bilayers were represented as an effective medium, accounting for the void concentration present in each sample determined by post-growth AFM measurements. The results show that compared to the monolayer, the absorption of the bilayers of MoS₂ is higher. Additionally, for both monolayer and bilayer, the maximum of the absorption red-shifts as a function of temperature.