Theoretical Study Of The Electronic And Optical Properties Of Material Similar To Graphene

Recently, silicene, germanene, phosphorene and monolayer transition metal dichalcogenides have attracted extensive attention. These ultrathin two-dimensional materials exhibit many outstanding physical properties and hold great potential applications in future electronics and optoelectronics. Among them, similar to graphene, silicene and germanene are semi-metals. Opening bandgap in silicene and germanene to realize their applications in optoelectronics, is still an important scientific issue. Additionally, the physical properties of these low dimensional nano-materials will be greatly influenced by many factors during fabricating and device synthesis, such as strain, defect, surrounding and interaction between layers. To make clear how these factors to influence the electronic and optical properties in these low dimensional nano-materials, we systematically study their electronic and optical absorption properties by employing density functional theory (DFT) combined many-body Green’s function(GW) and Bethe-Salpeter equation (BSE) and explore the mechanism, which will provide a viable theoretical guide for experiments.1) Tunable electronic and optical properties of monolayer silicane under tensile strain. Similar to graphene, silicene is a semi-metal. Full hydrogenated silicene can open a moderate indirect bandgap of 2.1 eV, which impedes its application in optoelectronic device. Thus, some methods are used to change its indirect bandgap nature. Strain is regard as an effective means of tuning material properties and can achieve controllable bandgaps in various 2D layered materials. The electronic structure and optical response of silicane to strain are investigated by employing first-principles calculations based on many-body perturbation theory. The band gap can be efficiently engineered in a broad range and an indirect to direct band gap transition is observed under a strain of 2.74%; the semiconducting silicane can even be turned into a metal under a very large strain. The transitions derive from the persistent downward shift of the lowest conduction band at the f-point upon an increasing strain. The quasi-particle bandgap of silicane is sizable due to the weak dielectric screening; they are rapidly reduced as strain increases while the exciton bound energy is not sensitive to strain. Moreover, the optical absorption edge of the strained silicane significantly shifts towards a low photon energy region and falls into the visible light range, which might serve as a promising candidate for optoelectronic devices.2) Tunable electronic and optical properties of hydrogenated and fluorinated germanene under different configuration and thickness dependent hydrogenated germanene. Germanene is a semi-metal. Opening a bandgap in germanene is an important scientific issue. Using density functional theory, GqWq method and Bethe-Salpeter equation calculations, we systematically explore the structural, electronic and optical properties of hydrogenated and fluorinated germanene. The hydrogenated/fluorinated germanene tend to form chair and zigzag-line configurations and their electronic and optical properties show close geometry dependence. The chair hydrogenated/fluorinated and zigzag-line fluorinated germanene are direct band-gap semiconductors, while the zigzag-line hydrogenated germanene owns an indirect band-gap. Moreover, the quasi-particle corrections are significant and strong excitonic effects with large exciton binding energies are observed. Moreover, the zigzag-line hydrogenated/fluorinated germanene show highly anisotropic optical responses, which may be used as good optical linear polarizers. Furthermore, we investigate the influence of thickness on the electronic and optical properties. Our calculation shows that few-layer and bulk germanane are all direct band-gap semiconductors and the band gaps are tunable in a broad range. The exciton binding energy in monolayer germanane can be as large as 750 meV that is fifteen times larger than bulk germanane. More importantly, the quasi-particle band gaps, optical gaps and exciton binding energies rapidly decrease as the increase of the layer and follow a power law of A+B/Np (0<P<2) with the stacking layer. Also, we make a good understood on the thickness dependent effects, which is similar to the size effects.3) Stacking dependent electronic structure and optical properties of bilayer black phosphorus. Recently, a new member of the 2D materials family, few-layer black phosphorus (BP) has been successfully exfoliated. It has received extensive attention because of its novel physical properties, such as direct bandgap, high hole mobility (1000 cm2·V-1·s-1) and high anisotropy. However, in bilayer black phosphorus, the relationship between the different interlayer interaction induced by different stacking patterns and properties has rarely been discussed and is still unclear, especially on how the stacking structures or faults affect the electronic and optical properties. Bilayer black phosphorus, the thinnest multilayer system, can act as the platform for the observation of the influence of interlayer interaction. We explore quasi-particle energy bands, optical responses and excitons of bilayer black phosphorus (BBP) with four different stacking patterns. All the structures are direct bandgap semiconductors and the bandgap is highly dependent on the stacking pattern. Such dependence can be well understood by the tight-binding model in terms of the interlayer hopping. More interestingly, stacking sensitive optical absorption and exciton binding energy are observed in BBPs, which is attributed to the different interlayer interaction induced by different stacking modes.4) Effect of vacancy on the optical absorption of MoS2 and turnable optical absorption of MoS2 through oxygen passivation. In the past few years, structural defects in molybdenum disulphide (MoS2) monolayer are widely reported and greatly degrade the transport and photoluminescence. However, how they influence the optical absorption properties remains unclear. In this work, by employing many-body perturbation theory calculations, we investigate the influence of sulfur vacancies (SVs), the main type of intrinsic defects in MoS2 monolayer, on the optical absorption and exciton effect. Our calculations reveal that the presence of SVs creates localized midgap states in the bandgap, which results in a dramatic red-shift of the absorption peak and exhibits stronger optical absorbance in the visible light and infrared region. Nevertheless, the SVs can be fine repaired by oxygen passivation and are beneficial to the formation of the stable localized excitons, which greatly enhance the optical absorption in the spectral range. The defect mediated/engineered absorption mechanism is well understood, which offers insightful guides for improving the performance of two-dimensional dichalcogenide based optoelectronic devices.