Influence of strain on electronic and transport properties of defects in 2D materials

Abstract

Theoretical as well as experimental studies have shown potential applications of two-dimensional (2D) materials in various fields of science and technology, for example field-effect transistors (FET), spin- and valleytronics, optoelectronic, topological insulators, and flexible devices. These devices, fabricated from monolayers (MLs) of one or two elements, are inexpensive, inherently flexible, and amenable to industrial scale processing because of emergent growth techniques. Among all 2D materials, transition metal dichalcogenides (TMDs) monolayers are under intense investigations since they offer unprecedented opportunities in tuning electronic, optical, and transport properties through strain, dielectric screening, stacking confinement, photoluminescence, and crystal defects. Monolayers of Group-6 TMDs form two stable structural configurations, namely a semiconducting phase (H-phase) with a direct bandgap and a semimetallic phase (T-phase). These monolayers can undergo strong elastic deformations, up to about 10%, without any bond breaking. Although, MLs TMDs are highly robust to external mechanical fields, their electronic structure is sensitive to compressive and tensile strain. Besides, intrinsic point defects are always present in their synthetic samples. Hence, it is important to understand both effects on the electronic and optical properties of such monolayers. Moreover, the coexistence of 2H- and 1T-phase of MLs MoS2 have further pushed their strong potential for applications in the next generation of electronic devices based on the 2D lateral heterojunctions. Here, the interfaces of two phases are often imperfect and may contain numerous vacancies, which also have considerable effects on their properties. In this work, we investigate the electronic structure, energetic, and optical properties of defective MLs TMDs, subject to various strain situations, using density functional theory (DFT) simulations. Our results indicate that strain leads to strong modifications of the defect levels inside the bandgap, e.g. splitting their degeneracy up to an amount of 450 meV. We show that a type of shear strain lowers the formation energy of all the point defects. According to the outcomes, presence of vacancy complexes leads to absorption with larger dipole matrix elements in comparison to the case of simple transition metal vacancies. The other objective of this thesis is to explore the charge transport properties of the 1T/2H-MoS2 heterojunctions in the existence of point defects, by means of non-equilibrium Green’s function (NEGF) approach. While vacancies in semiconducting MoS2 act as scattering centers, their presence at the interface improves the flow of the charge carriers. The transmission enhancement was explained by changes in the electronic densities at the T-H interfaces, which open new transport channels for electron conduction.