Theoretical Study Of The Effect Of Defects On The Growth,Electronic And Optical Properties Of Two-Dimensional Material

Low-dimensional materials, especially graphene and transition metal dichalcogenides, exhibit great promise in the field of electronic and optoelectronic devices, sensors and energy materials because of their rich structure, good stability and many outstanding physical and chemical properties. They have received enormous attention in recent years from the physics, chemistry and material science community. However, the growth of any material can never avoid the introduction of defects, which are also necessary for any practical applications. A deep understanding of the role played by defects from substrate, surface and interface in affecting electronic properties is essential for realizing functionalization and fine-tuning of the properties. In this dissertation, we employed density functional theory (DFT) simulations to investigate the approach to determine the binding energy distribution on large scale defective substrate, the mechanism of vacancy defect affecting the transport and optical properties in MoS2 and the dependence of the electronic structure of graphene/MoS2 and MoS2/MoS2 on their interlayer misorientation. The main conclusions are summarized below:1) A multiscale approach to determine binding energy distribution on a strained and/or defective surface. The quality of graphene sample prepared by chemical vapor deposition is closely related to its nucleation stage. To investigate the effect of substrate dislocation and grain boundary on the nucleation of graphene, it is required to simulate large atom configurations with complex surface morphology, which is beyond the capacity of first-principle calculations. We developed a multiscale approach combining first-principle calculations with classical molecular mechanics simulations to investigate the local reactivity of defective substrate with strain field. The main idea of this method is using atomic strain to bridge different length scales. Sampling calculations and local structure analysis are combined to predict the local reactivity of large scale surfaces with accuracy close to first principle calculations. The method can be further extended to study other complex systems such as curved or rippled surfaces. Based on the approach, it was found that the dislocation or the core of grain boundary can increase the local adsorption energy by about 1 eV. At 1000K, the proportion of the carbon concentration in defect core regions to that in unstrained area can reach 104. These results demonstrate the great trapping ability of substrate defects against surface atoms, and well explain the weakness of polycrystalline substrate for high quality graphene CVD synthesis.2) Influence of vacancy defect on the electronic and optical properties of single layer MoS2. The mobility of monolayer MoS2 transistor is 1-2 orders of magnitude lower than the theoretical limit. Aiming at this problem, through a combination of first-principles calculation and experiment, we propose a hopping transport mechanism mediated by defect-induced localized states. The introduction of sulfur vacancies lead to highly localized defect states in the band gap of MoS2, where electrons are easily confined. The defects result in a charge transport through hopping between the localized states and a great degradation of device mobility. This model successfully illustrates the physical origin of low mobility and intrinsic n-type doping in molybdenum sulfide. We also proposed a facile low-temperature thiol chemistry route to approach intrinsic charge transport by repairing the sulfur vacancies through surface passivation. On the other hand, a theoretical assumption is made to significantly enhance the photoluminescence of MoS2 by defect engineering and chemical bonding. The conclusions will provide new insights and ideas to improve the electronic and optical properties of layered materials.3) Electronic structure of twisted bilayers of graphene/MoS2 and MoS2/MoS2. Vertically stacked van der Waals heterostructures exhibit novel structures and properties. However, the impact of interlayer misorientation on their property remains unclear. We explored the electronic properties of twisted bilayers of graphene/MoS2 and MoS2/MoS2 within the framework of density functional theory. Our calculations showed that the electronic structures of twisted bilayers of Gr/MoS2 and MoS2/MoS2 are different from those of the nontwisted ones. The band structures of twisted Gr/MoS2 with different rotation angles are very different from each other, which are mainly attributed to the misorientation-induced lattice strain and the sensitive band-strain dependence in MoS2. The influence brought by a pure interlayer rotation is nearly negligible. Furthermore, the position of CBM of MoS2 relative to the Dirac cone can be tuned effectively by strain or twist, which is beneficial for doping MoS2. For twisted bilayer MoS2, the band structure presents a distinct behavior from that of the AB-stacked bilayer, which is a result of reduced interlayer coupling and Brillouin zone folding. The results suggest that interlayer twist between stacked layers of MoS2 or other TMDCs can be utilized for their property engineering.