We investigate the optical properties of two-dimensional monolayer films of MoSe2 and WSe2 (each 0.7nm thick) assembled on SiO2/Si substrates (285nm/0.5mm thick). These films are interesting because they are direct band gap semiconductors that have large excitonic responses. However, due to numerous challenges, including the lack of a quick, contactless, and reliable method, obtaining the optical constants and exciton binding energies in-situ remains a difficult endeavor. Here, we report the optical properties based on contactless ellipsometry to retrieve the optical constants (n,k) and excitonic properties of both monolayers (MoSe2 and WSe2). The optical properties of these materials away from the exciton (~700 nm) are generally not well understood. In this work, we will explore the optical response of these films over a broad range that includes the UV/visible and near infrared (200-2000nm) in order to understand if there are other spectral regions with a strong or tunable refractive index. The current samples are intrinsic without doping. The SiO2 on the Si substrate would be used as a gate capacitor which would allow to vary the density by ~1011-1012 cm-2. These transition metal dichalcogenides (TMD) offer new possibilities for designing modern photonic and optoelectronic components.
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