Carbon-based Materials:Preparation, Characterization And Theoretical Studies Of Electronic Structure

Carbon is one of the most fundamental elements in the earth. The abundance of carbon in the earth’s crust is only0.027%, which is far less than that of Fe, Al and O. However, carbon element is everywhere on earth. Carbon can possess almost all the material properties on the planet, even the antithetical properties. For example, carbon-based materials can be the hardest or softest, transparent or not, insulator, conductor or semiconductor, heat insulation or good thermal conductivity. In a sense, the human world is the world of carbon, and the history of human civilization is the history of using of carbon. So far, Carbon-based materials have been employed in many areas, such as using as a neutron moderator in nuclear reactor, fabricating spintronics and electronic devices, etc.For the application of using as a neutron moderator, graphite has been successfully used in conventional nuclear reactor. However, the characterization of nuclear graphite in molten salt reactor is scarcely studied. For all the six promising future reactors proposed by the Generation IV International Forum, molten salt reactor has incomparable advantages:a high negative temperature coefficient of reactivity, a very low vapor pressure, thermodynamic stability, and so on. Notably, the main advantages of molten salt reactor are as a result of the prerequisite that the graphite can adapt the molten-salt and irradiation environment. In order to improve the barrier property of the porous graphite to molten salt, it is necessary to coat the graphite with pyrolytic carbon and investigate the irradiation response of the coating. Subsequently, the mechanism of ferromagnetism in carbon-based materials, which contain only s and p electrons in contrast to traditional ferromagnets based on3d or4f electrons, is important but unclear. Although the magnetic carbon-based materials have been fabricated by doping transition metal, the origin of magnetism and the control of the intensity of magnetization have not been solved. Therefore, it has been one of the important task in the area of spin polarization. Additionally, carbon has4valence electrons and can bond together via single bonds, double bonds or triple bonds in the form of sp3-, sp2-and sp-hybridized carbon atoms, respectively. Moreover, carbon atoms can form covalent bonds with some other atoms to compose materials with diverse structures and properties. In1985, Inagaki published a book entitled Materials Engineering of Carbons in Japanese. In the book, he began to emphasize the importance of nanotexture in carbon materials belonged to the graphite family and discussed carbonization and graphitization of carbon materials on the basis of their nanotextures, planar, axial, point and random orientation schema. The discovery of various fullerenes and later carbon nanotubes opened a new era for carbon materials. Nonetheless, many science researchers have been engaged in seeking for the novel carbon allotropes. Due to the fabrication of graphdiyne in2010, great interest has been aroused in the research on the layer composed of sp2-and sp-hybridized carbon atoms.Based on the above application areas, the carbon-based materials involved in the present work include polycrystalline pyrolytic carbon coating, polycrystalline SiC coating, SiC crystal, SiC monolayer and graphyne (2D or3D). Chemical vapor deposition, i.e., one of the effective methods to obtain the pyrolytic carbon-based coating, was used to prepare the samples. First-principles calculations based on density functional theory was employed to investigate the geometry, energetic stability, electronic structure, etc.With the combination of experimental and the theoretical analyses, in this paper we systematically introduce the preparation, characterization and theoretical studies of electronic structures of carbon-based materials. Chapter I introduces a brief introduction of research background. Chapter II describes the experimental and computational details used in our paper. Chaper III to VI introduce in detail and summarize the work completed during the period of study for a doctorate.The main results are summarized as follows:1. IPyC prepared at1300℃by chemical vapor deposition was implanted with129Xe26+ions to obtain a wide range of information as well as to understand the coating materials in nuclear energy field. Microstructure of the pristine and ion-implanted IPyC on nuclear graphite substrate was investigated using polarized light microscopy, scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, nanoindentation, and X-ray photoemission spectroscopy. It was demonstrated that the Xe ion irradiation resulted in concurrent changes in both physical and chemical structures of our standard polycrystalline sample. Influences of the thermal annealing temperature on the properties of the implanted IPyC at500and1000℃were also studied. Ion-irradiation gave rise to the formation of structural deterioration along a and c axis, accompanying with the appearance of widespread clastic morphology among the irradiated zone of IPyC. There was a dose window that could be used to tune the mechanical properties of IPyC:the nanohardness and Young’s modulus increased after an irradiation, but decreased as the amorphization was reached. Infiltration studies were performed on uncoated nuclear graphite and PyC coated graphite in molten FLiNaK salt at650℃under argon atmosphere at1,3and5atm. Uncoated graphite shows weight gain more obviously than that of PyC coated graphite. Nuclear graphite with PyC coating exhibits excellent infiltration resistance in molten salt due to the small open porosity as conformed from scanning electron microscopy and mercury injection experiments. SiC coating is produced on a nuclear graphite substrate using chemical vapor deposition at1150℃to protect it from molten salt diffusion. Infiltration studies, performed in molten FLiNaK salt under an argon atmosphere at5atm, show that uncoated nuclear graphite exhibits significantly higher weight gain than SiC-coated nuclear graphite. The continuous and compact SiC coating exhibits excellent infiltration resistance in liquid fluoride salt as confirmed by synchrotron radiation X-ray microbeam fluorescence.2. Magnetism of6H-SiC single crystals implanted with3MeV protons is studied both experimentally and theoretically. We found that proton irradiation can induce stable ferromagnetism in6H-SiC with a Curie temperature above300K. There is a dose window available for tuning the magnetization of the samples. The maximum saturation magnetizations (0.17emu/g) are three orders of magnitude larger than that reported in neutron-irradiated SiC crystals (1×10-4emu/g). First-principles calculations indicate that the ferromagnetism is related to the divacancy-related defects (VsiVc+nH,(n=1-3)) generated under proton irradiation. Room temperature, macroscopic magnetization was also induced and could be tuned in6H-SiC using 14N+ion implantation. First-principles density functional theory computation results confirmed that14N+ion implantation can enhance the ferromagnetic ordering of the local magnetic moments caused by vacancy and substitution defects. The calculated magnetization values in the energetically favored ferromagnetic ordering are qualitatively in agreement with the experimental data. We also performed first-principles calculations to investigate the spin-polarization of vacancy defects in SiC monolayer. We show that Si and C vacancy defects play different roles in the magnetism of SiC monolayer. Local magnetic moments can be induced by the presence of Si vacancy (Vsi) whereas no spin-polarization occurs in C vacancy (Vc) defects. The induced states are due to the unpaired electrons on carbon atoms surrounding the silicon vacancy. The spatial distribution of spin density displays the features of ferrimagnetic alignments for the most stable configuration.3. We explored the electronic structures and optical properties of graphyne consisting of sp-and sp2-hybridized carbon atoms using first-principles calculations and tight-binding method. In contrast to zero-band gap graphene, the small band gap in graphyne is related to the inhomogeneous π-π bindings between differently-hybridized carbon atoms. The optical properties of single-layered and bulk graphyne are also calculated and analyzed on the basis of electron density of states. The interlayer interactions of bulk graphyne narrow the band gap to0.16eV and result in redshift of the optical spectral peaks as compared to single-layered graphyne. We also performed first-principles calculations to explore the structural, energetic, and electronic properties of graphyne nanoribbons (GYNRs), the graphyne with a nanometer-size width. We found that structural relaxation mainly takes place at the edges of GYNRs and the armchair-shaped edge is energetically preferable. All the GYNRs are semiconducting independent of edge structures. Due to the lateral quantum-confinement effects, the band gap decreases gradually with the increase of ribbon width. For the GYNRs with bared zigzag edges, the edge states are spin-polarized and coupled in an antiferromagnetic-like way along each side. With the semiconducting properties and tunable band gaps, these novel carbon nanoribbons may find applications in building nanoscaled devices.