Photoelectric Properties Of Multilayer Graphene System And Hollow Nanospheres Structure

In recent years, theoretical predictions of the optical conductivity of both monolayer and Bernal-stacked bilayer graphene have largely been in agreement with experimental observations. In the beginning of this thesis, we introduce the current research topics at the application of graphene and reviews the electronic structure, optoelectronic properties for both monolayer and bilayer graphene systems. In order to have a better understand-ing of the optical properties of multilayer graphene film, we present a theoretical study on optoelectronic properties of ABA-stacked and ABC-stacked trilayer graphene (TLG). The optical conductance and light transmittance are evaluated through using the energy-balance equation derived from the Boltzmann equation for an air/TLG/dielectric-wafer system in the presence of linearly polarized radiation field. For short wavelength radiation, the universal optical conductivity σ0t=3e2/(4h) can be obtained. Importantly, there also exists an optical absorption window in trilayer graphene in the radiation wavelength range3-400μm. which is induced by different transition energies required for inter-and intra-band optical absorption channels. This feature in trilayer graphene is similar to those of mono-and bi-layer graphene. As a result, we find that the position and width of this window depend sensitively on temperature and carrier density of the system, especially at the lower frequency edge. There is a small characteristic absorption peak at about82μm where the largest interband transition states exist in the ABC-stacked TLG model, in contrast to the relatively smooth curves in a simplified model. At the same time, we also investigate the optical conductivity of ABA-stacked graphene through semi-classic Boltzmann equation approach and Kubo formula. After the comparison of mono-, bi-and tri-layer graphene, some interesting features can be obtained. At short wavelength radia-tion, the universal optical conductance σ0m=e2/(4h), σ0b=2e2/(4h), and σ0t=3e2/(4h) for mono, bi and tri-layer graphene can be obtained. Importantly, there exist optical absorption windows in the radiation wavelength range3-500μm, which are induced by different transition energies required for inter-and intra-band optical absorption chan-nels. As a result, we find that the position and width of the optical windows depend sensitively on temperature, carrier density and number of graphene layers of the system, especially at the lower frequency edge. These theoretical results indicate that a few layers graphene have some interesting and important physical properties which can be utilized to realize bandwith tunable infrared or THz optoelectronic devices. Moreover, we also theoretically study the electronic subband structure and collective electronic excitation associated with plasmon and surface plasmon modes in metal based hollow nanosphere in this thesis. The dependence of the electronic subband energy on the sample parameters of the hollow nanosphere is examined. We find that the subband states with different quantum numbers I degenerate roughly when the outer radius of the sphere r2≥100nm. In this case, the energy spectrum of a sphere is mainly determined by quantum number n. Moreover, the plasmon and surface plasmon excitations can be achieved mainly via inter-subband transitions from occupied subbands to unoccupied subbands. We examine the dependence of the plasmon and surface-plasmon frequencies on the shell thickness d and the outer radius r2of the sphere using the standard random-phase approximation. We find that when a four-state model is employed for calculations, four branches of the plasmon and surface plasmon oscillations with teraherzt frequencies can be observed, respectively.