8,9įurthermore, control of the electronic and optical properties of the monolayer MoS 2 by changing the crystal structure, 10 layer thickness, 4 and applying strain 11 was achieved. 7 To solve this problem, the dichalcogenide can be covered with polymers such as polyethyleneimine (PEI). Thus, the bandgap becomes narrower and even disappeared after a few days of exposure to oxygen gas. 5 The main problem of MoS 2 is that it is sensitive to oxygen gas in air, which modifies its electronic properties unexpectedly. 6 Field-effect transistors (FETs) of monolayer MoS 2 demonstrated a high on/off current ratio exceeding 10 8 and the electron mobility of at least 200 cm 2 V −1 s −1. 1.8 eV, 4 which makes this material suitable for transistors, 5 photodetectors, and light-emitting diodes. In particular, the monolayer MoS 2 has a direct bandgap of ca. Therefore, the monolayer of MoS 2 can be fabricated by exfoliation, which has a thickness of about 0.65 nm. Similar to graphene, the bonding between the layers of MoS 2 is the weak van der Waals interaction. Introduction MoS 2 is one of the most often studied transition metal dichalcogenides (TMDs) 1–24 due to the convergence of many advanced properties for optical, 1 electronic, 2 and mechanical 3 applications. Importantly, new peaks in the optical spectrum of the clean MoS 2 and MoS 2/PEI appear in the ultraviolet region under compressive pressures and the infrared region under tensile strains. The bandgap of these systems approaches 0 eV at the corresponding pressures. Remarkably, the transition from semiconductor to metal of the monolayer MoS 2 and the MoS 2/PEI system occurs at the tensile pressure of 24.95 and 21.79 GPa, respectively. The results showed that the adsorption of the PEI molecule significantly reduces the width of the direct bandgap of the monolayer MoS 2 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 z state of the PEI molecule. Therefore, we elucidated this matter by using density functional theory calculations. However, the effects of polyethyleneimine and pressure on the electronic and optical properties of monolayer MoS 2 remain unknown. Furthermore, the application of pressure is also an effective method to modify the physical properties of MoS 2. This problem has been solved by coating MoS 2 with polymers such as polyethyleneimine (PEI). Unfortunately, MoS 2 is sensitive to gases in the environment causing its original electronic properties to be modified unexpectedly. The moderate bandgap of monolayer MoS 2 is fascinating for the new generation of optoelectronic devices. MoS 2 is one of the well-known transition metal dichalcogenides.
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