Electronic and optical properties of monolayer MoS under the influence of polyethyleneimine adsorption and pressure.

MoS is one of the well-known transition metal dichalcogenides. The moderate bandgap of monolayer MoS is fascinating for the new generation of optoelectronic devices. Unfortunately, MoS is sensitive to gases in the environment causing its original electronic properties to be modified unexpectedly. This problem has been solved by coating MoS with polymers such as polyethyleneimine (PEI). Furthermore, the application of pressure is also an effective method to modify the physical properties of MoS. However, the effects of polyethyleneimine and pressure on the electronic and optical properties of monolayer MoS remain unknown. Therefore, we elucidated this matter by using density functional theory calculations. The results showed that the adsorption of the PEI molecule significantly reduces the width of the direct bandgap of the monolayer MoS to 0.55 eV because of the occurrence of the new energy levels in the bandgap region due to the contribution of the N-2p state of the PEI molecule. Remarkably, the transition from semiconductor to metal of the monolayer MoS and the MoS/PEI system occurs at the tensile pressure of 24.95 and 21.79 GPa, respectively. The bandgap of these systems approaches 0 eV at the corresponding pressures. Importantly, new peaks in the optical spectrum of the clean MoS and MoS/PEI appear in the ultraviolet region under compressive pressures and the infrared region under tensile strains.