Screening Effect of Ultrathin Gold Films on Excitons in Monolayer WS2

Screening plays an important role in determining the energy levels and binding energies of excitons in two-dimensional (2D) semiconductors. However, the photoelectric properties of 2D materials deposited upon metal films are often assumed to remain unaffected by the screening effect. Herein, we derive the reflectance contrast spectra of monolayer WS2 on different thicknesses of ultrathin gold films by using the transfer matrix method, and investigate the screening effect of ultrathin gold films on excitons in monolayer WS2. We show that the exciton binding energy of the ground state gradually decreases as the gold film thickness increases due to the enhanced screening effect. Nevertheless, it may be due to the impact of free-electron scattering on the permittivities of gold films: exciton resonance energies of the ground state and the first excited state gradually increase as the gold film thickness increases, which are different from previously reported variation trend of exciton energy levels with the thickness of screening media of 2D materials. As a consequence, the electronic bandgap of the exciton only presents a small variation range of less than 4 meV with thickening of the gold film from 0.0 to 40 nm. These findings shed light on understanding to what extent the screening effect of gold films affects the excitons in 2D materials.