Study On Electronic Structure And Optical And Magnetic Properties Of Quantum Dots

Nanostructures made from element carbon which is one of the most widespread elemets in nature, such as fullerence and carbon nanotube, due to their unique and excellent physical properties, they have always been the focus field of nanomaterials and nanotechnology research since their discovery. Especially in2004, the successful preparation of monoatomic layer of graphite, i.e. graphene, stimulates huge excitement in the sciences. This new material has many novel properties and great potential which arise from its strictly two-dimensional structure. The researchers try to explore the properties of graphene thoroughly from every aspect and extend its application field. With the progress of the preparation technoloyg, nanostructures based on graphene, such as graphene ribbon and graphene quantum dot (GQD), have gradually become the hotspot of researching in material science and condensed matter physicals. Compared to monolayer graphene and graphene ribbon, the focus on GQDs arises in recent years and there is a tremendous space for further research. The electronic structure differents from that of graphene and ribbon, in particularly, the existence of energy gap, lead to broader application prospects of GQDs. And it is necessary and important to explore the properties of GQDs in depth.In this paper, numerical calculations are performed to investigate the electronic structure of various kinds of GQDs from different aspect. The corresponding optical and magnetic properties are also investigated. Our findings will provide theoretical guides for the applications of GQDs in nanoelectronic and spintronic devices. Seven chapters are included in this thesis:The opening chapter gives a brief introduction of the basic concepts of graphene. Firstly, we describe the arising and developing of carbon material, the crystal structure, basic physical properties and application prospects of graphene. Then, both the band structure of graphene calculated by the tight-binding method and the behavior of electrons with low energy described by Dirac equation are introduced. Finally, latest progress and research status of graphene nanostructures are presented.In the second chapter, utilized the nearest-neighbor tight-binding model, we calculate the electronic structure of several GQDs with different shape and edge comprehensively and systematically. The band structure near the Fermi level and the highly degenerated states are studied in detail. The calculations show that the shape, edge and size of the dot play a decisive role in its electronic structure. The results are compared with those obtained from the calculations based on the tight-binding model including the next-nearest-neighbor hopping energy. It is shown that the electron-hole symmetry between the states can be destroyed by the next-nearest-neighbor hopping energy. But qualitative changes in the electronic structure are not found. The optical spectra originate from the electronic transitions are investigated, too.Based on the single particle energy spectra obtained in chapter two, the Coulomb and exchange interactions between electrons are taken into account in chapter three. By using the Hubbard model and Hartree-Fock method, the electron arrangements and spin polarization density of the ground state are found. Thus the magnetic properties of GQDs can be gotten. Due to the wide energy gap, armchair-edged GQDs always exhibit a nonmagnetic ground state. And a zigzag-edged dot has a ferromagnetic or antiferromagnetic ground state according to its shape. The effects of electron-hole interaction are also studied in terms of the configuration interaction method. The energy of excitons, the optical gap and the optical response under the excitonic effect are calculated. The funding reveals that the interactions between particles lead to great changes in the dot’s electronic structure.Electric and magnetic field controlled electronic and magnetic properties of GQDs are described in chapter4. Under a weak electric field, the obvious Stark effect is observed. And the total spin of the system begins to depolarize when the electric field strong enough. The magnetic field dependence of the energy spectrum of the GQD displays a clear Hofstadter-butterfly characteristic.Considering the existences of defects are inevitable in actual materials, we discuss the effects of vacancies on the electronic structure of GQDs in the fifth chapter. These vacancies cause the imbalance in the number of atoms between the two sublattices. Thus, localized states with zero energy appear. And the magnetism of the system also changes. Especially, two vacancies when occupying some specific sites of different sublattices still induce doubly degenerate states at the Fermi level. The magnetic moment of the system in its ground state is also in agreement with the Lieb’s theorem. And the variance of the magnetic moment in the presence of an electric field is dependent on the site of the vacancies.In chapter6, the electronic and magnetic properties of bilayer GQDs are also analyzed. The role of interlayer interaction, relative displacement and twist, electric field and vacancy are considered. The electronic structures of GQDs exhibit rich diversity. But the magnetism of the GQDs are less affected. Even under the interlayer electic field, both the ground states of the two types of bilayer dots are still nonmagnetic.Finally, all the work is summarized and the future of our subject is prospected. The aforesaid series of the research achievements show that the electronic structures of GQDs are affected by many factors. And effects are different for different factors. The changes will be reflected in the optical and magnetic properties, which will help to extend the application field of GQDs.