Theoretical Investigation Of Structure Design And Spin Control On Graphene And Graphene-like Quantum Dots

Due to the large surface-volume ratio and remarkable size confinement effect, low dimension nanomaterials own diverse electronic, optics, machanics, and magnetic properties differ significantly from bulk materials, which have become a topic of increasing interest. Discovery of carbon nanotubes in the1991has inspired an abundant interest in the synthesis of nanomaterials based on group â…¢-â…¤ and â…¡-â…£ semiconductors with remarkable mechanical, thermal and electronic properties.First-principles calculations based on density-functional theory (DFT) combined with molecular dynamics simulation (MDSs) have been proved to be useful theoretical methods in revealing the structures and properties of nanomaterials. The morphologies, energetic stability, electronic structures, magnetism, dynamics of chemical reaction etc. can be reasonably predicted using the theoretical schemes. First-principles calculations have also been successfully employed to handle material design and performance prediction, which can greatly improve efficiency and reducel the development cycle.In this dissertation, we performed first-principles calculations within DFT to reveal the graphene and graphene-like quaumtum dots (QDs). The research mainly considered the stability, electronic and magnetic properties, meanwhile, we focused on the effect of size and goemetrical configuration on those properties. The low dimensional materials considered in this work include boron nitride quantum dots, silicon carbide quatum dots, and graphene quantum dots. The theoretical results revealed in this work are expected to be useful for understanding and interpreting the stability and electronic properties of these nanostructures at atomic scale, which is quite crucial for their potential applications.The thesis is organized as follows:Chapter â…  gives a brief introduction of research background and motivation. Chapter â…¡ introduces the principles of theoretical strategies used in our research work. Chapters â…¢ to V describe in details the works finished during my Ph.D degree studies. The main contents and results in this dissertation are summarized as follows:1. Graphene quantum dots (QDs) hold great promises in spintronics. Here, we report our predictions of honeycomb-patterned QDs beyond graphene, on the basis of firstprinciples calculations and an extended Hubbard model. Our calculations showed that the electronic structures and spin-polarization of boron nitride (BN) and silicon carbide (SiC) QDs can be well tuned by controlling the shape and size of the QDs. Edge hydrogenation can not only greatly improve the stability but also diminish the spin-polarization of BNQDs. Triangular SiC-QDs have spin-polarized ground states, and the magnetic moments increase with increasing QD size. Hexagonal SiC-QDs, however, possess spinunpolarized ground states whose energy gaps decrease with the increase of QD size. To understand the origins of the composition-and shape-dependent spin-polarization of these honeycomb-patterned QDs, we extended the single-orbital Hubbard model of graphene QDs by taking into account the onsite energy differences of the two sublattices. Our extended Hubbard model reproduces well the results of first-principles calculations and offers a simple model to predict the electronic structures of honeycomb-patterned QDs.2. The energetic stability, electronic and magnetic properties of carbon-doped triangular boron nitride quantum dots (BNQDs) were investigated using first-principles calculations within density functional theory (DFT). Different edge structures, doping positions, and carbon concentrations are considered. We find that the substitutional C atom energetically prefers to reside in the minority sublattice of the BNQDs, and the planner structures of the BNQDs are well preserved. When the carbon dopant moves from the inner to the outer region of the BNQDs, the HOMO-LUMO gap decreases in an oscillating way, which is even smaller than that of graphene quantum dots. After carbon doping, BNQDs have non-zero magnetic moment ground states. There is an impurity state above or below the Fermi level of C-doped BNQDs, depending on substituting majority or minority sublattice. This offers a promising way of tuning the electronic and magnetic properties of BNQDs3. The interface bwteen graphene and underlying BN substrate plays an important role in divice applications of the relevant heterostructures. We performed first-principles calculations on the the geometry and electronic properties of graphene QDs on BN substrate. Our calculations showed that graphene QDs prefer to adsorbe on BN substrate forming an AB stacking mode, i.e., boron atoms locate right beneath carbon atoms and nitrogen atoms are below the centers of carbon hexagons. The energy gap can be reduced by the interaction between graphene QD and BN substrate. Changing the adsorption density can be another effective way to tune graphene QD electrionic properties. The energy gap of graphene QDs and the electron spinpolarization can be tuned by applying vertical electric field, the graphene QD energy gap and magnetisim can be decreased rapidly. The change is sensitive to the electronic strength. This offers a promising approach to tune the electronic properties of graphene QDs to achieve specific applications in high-performace nanoscaled devices.