MoS2 is one of the well-known transition metal dichalcogenides. The moderate bandgap of monolayer MoS2 is fascinating for the new generation of optoelectronic devices. Unfortunately, MoS2 is sensitive to gases in the environment causing its original electronic properties to be modified unexpectedly. This problem has been solved by coating MoS2 with polymers such as polyethyleneimine (PEI). Furthermore, the application of pressure is also an effective method to modify the physical properties of MoS2. However, the effects of polyethyleneimine and pressure on the electronic and optical properties of monolayer MoS2 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 MoS2 to 0.55 eV because of the occurrence of the new energy levels in the bandgap region due to the contribution of the N-2pz state of the PEI molecule. Remarkably, the transition from semiconductor to metal of the monolayer MoS2 and the MoS2/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 MoS2 and MoS2/PEI appear in the ultraviolet region under compressive pressures and the infrared region under tensile strains.
