Monolayer transition-metal dichalcogenides with polyethyleneimine adsorption

Monolayer transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) have attracted much attention for use in electronic and optical applications. Due to their sensitivity to gases, the surface of these materials has been protected by coating with polymers such as polyethyleneimine (PEI). However, the effects of such coating layers on the electronic and optical properties of MX2 are not well understood. In this work, density functional theory calculations are used to study the effects of adsorption of PEI on monolayer MX2 substrates based on the electronic and optical properties before and after PEI adsorption. The results show that the bandgap of the substrate decreases significantly upon PEI adsorption, while the optical spectra show only slight modification with a decrease of intensity at short wavelengths around 300 nm. The results also indicate that PEI adsorption results in n-type doping states in MX2 but does not deteriorate the optical properties in the visible range.