Broadband optical properties of redox exfoliated transition metal dichalcogenide films

Abstract

Semiconducting transition metal dichalcogenides (TMDs) continue to attract attention as components of optical devices due to remarkable refractive indices (e.g., n ≥ 4 for MoS2) from ultraviolet to near-infrared wavelengths. In recent years, TMD synthetic processes have advanced to provide sonication- and surfactant-free exfoliation methods. Such methods provide better access to high-yields of oxidatively-resistant TMDs as stable colloidal dispersions. However, inconsistent optical constants (i.e., refractive indices, n, and extinction coefficients, k) have been reported throughout the literature without clear attribution to TMD origin, exfoliation technique, or film processing procedures. Here, we offer insight toward understanding the nature of these reported discrepancies. As such, we derive broadband optical constants of redox exfoliated TMD films from 250nm – 20µm using variable angle spectroscopic ellipsometry. These data illustrate continuation of high n and low k values into the long-wavelength infrared regime. However, all the optical features (250nm – 20µm) are heavily dependent on both the selected raw material source (synthetic or natural) and accompanying post-processing conditions. These experimental optical features show significant changes in n ranging from 2.4 – 3.3. While the intrinsic lattice defect density is most likely to dominate TMD optical properties, residual species critical for exfoliation (i.e., polyoxometalates) are also suspected to contribute to variations in reported optical constants (e.g., extrinsic chemical dopant effects). Understanding such intrinsic and extrinsic optical property dependencies further expands the utility of redox exfoliated TMDs to expedite the development of next-generation optical devices.