Reversibly Tailoring Optical Constants of Monolayer Transition Metal Dichalcogenide MoS2 Films: Impact of Dopant-Induced Screening from Chemical Adsorbates and Mild Film Degradation

Abstract

The refractive index for pristine monolayer MoS2 in the near-infrared is predicted to be ≥4. However, experimental literature reports n can decrease by ∼2 at energies below the band edge. In this work, we show variability in the optical response can correspond to changes in oscillator amplitude and dielectric polarizability of all measured excitons—suggesting exemplary charge transfer and local dielectric media effects discussed here are significant contributors in quantum nanophotonic interactions. Monolayer metal organic chemical vapor deposited MoS2 films are evaluated herein using spectroscopic ellipsometry to assess changes in the refractive index (n) and extinction coefficient (k) due to dopant-induced screening effects from chemical adsorbates and mild film degradation. Notably, large reversible changes in the refractive index (Δn ≈ 2.2) are observed by varying n- and p-type chemical adsorbates. The extent of tailorable dopant-induced screening of MoS2 optical constants illustrated in this work is also shown to be highly dependent on film quality. This suggests a dominant role of pre-existing structural defects on the optical properties of reported films to date. The tailoring of semiconducting transition metal dichalcogenide optical constants in a reversible manner is expected to have broad implications in the development of quantum optical and optoelectronic devices (e.g., high-precision engineered excitonic effects, electroabsorption modulators, and high-efficiency semiconductor nanophotonic technologies).