Kuc, Agnieszka
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Item-typ:Veröffentlichung, Influence of strain on electronic and transport properties of defects in 2D materials(2023-04-26); ; ; 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.Dissertation304 160 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Accurate quantum mechanical modeling of defects in two-dimensional and three-dimensional materials(2021-06-23); ; ; Layered materials beyond graphite such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS2) are presently under intense study, and most applications require knowledge about their defects. In this work, first, density functional theory (DFT) has been employed to study the defects properties in hBN. According to earlier studies, the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), with parameters tuned to reproduce the relative position of the band edges and to satisfy the generalized Koopmans’ theorem (gKT) , is capable of providing defect properties very accurately in traditional bulk semiconductors. This success is concluded to be connected to the proper description of electronic screening. I investigate whether such a functional can be optimized for layer compund such as hBN. I find that while the optimization is possible for bulk hBN, the optimized parameters can only be met approximately for the atomatically thin monolayer (ML) hBN. So, generally, the quantitative accuracy of defect calculations in 2D layers is limited. Using an optimized hybrid functional in bulk hBN, which reproduces the gap and satisfies the generalized Koopmans condition, an Ni configuration is found to be lower in energy than the ones reported so far. The (0/-) charge transition level is also much deeper, so Ni acts as a very efficient compensating center in n-type samples. Its calculated photoluminescence (PL) at 3.0 eV agrees well with the position of an N-sensitive band measured at 3.1 eV. Hyperfine interactions have been also performed to investigate the three boron electron (TBC) electron paramagnetic resonance (EPR) center in bulk hBN. I find that the carbon substitutional on the nitrogen site (CN) is the source of carbon-associated TBC in hBN in agreement with a previous study. I also show that the nitrogen vacancy (VN) cannot be the origin of the electron-associated produced TBC in hBN and in thermal equilibrium it cannot exist in ntype samples. Similarly, defects in van der Waals layered MoS2, play an important role. I present our first principle investigation of a water splitting for a defective monolayer molybdenum disulfide (ML MoS2). Defects such as molybdenum vacancy (VMo) and a vacancy complex of Mo and three S (VMoS3 ) in two configurations are considered. The complex defect VMoS3, where a pair of sulfur vacancies is placed on the top plane and the third vacancy on the bottom plane, is found to be sufficiently active to split the water molecule into OH and H. The water dissociation not occur for the complex defect VMoS3 if all sulfur vacancies are located at the surface. The presented results are critical for designing chemical sensing and hydrogen generation devices from MoS2.Dissertation282 181