Tuning The Electronic Structures Of Low-Dimensional Carbon Nano-materials

In this thesis, we performed first-principles calculations based on density functional theory (DFT) by utilizing the SIESTA and VASP codes, focusing on functional modification and manufacturing the electronic properties of low-dimensional carbon nano-sized materials (graphite and carbon nanotubes). The systems under study are: point defects produced by carbon ion implantation of graphite, graphite monolayer films (graphene) surface chemical modification, surface chemical modification of carbon nanotubes, silver-containing carbon nanotubes. We have analyzed the various features of the modification of electronic structures of these systems. By performing spin polarized calculations, we discussed the magnetic mechanisms in these systems. Finally, we made conclusions of the possibility and ways of manufacturing magnetic nanomaterials, electron transport devices based on graphite and carbon nanotubes.This thesis covers the following aspects:(1) We have studied the magnetic mechanisms of graphite induced by ion implantation. We analyzed the possible point defects produced in the implantation process and related structure stability. Point defects like interstitials, vacancies, interstitial-vacancy (I-V) pair and Stone-Wales defect have been systematically studied. It is amazing that only the vacancies contribute to the magnetization of the graphite. SRIM03 simulations indicate that heavy ions like C, N can cause more vacancies than light ions like H, and thus give a more significant magnetic signal. We hope this phenomenon is helpful for the researchers to prepare magnetic materials.(2) The possibility of tuning the magnetic properties of graphene layer by surface monovalent- and divalent- modifications was then studied. It was found that the monovalent addations induced magnetism depends on two aspects: (i) high modification concentration; (ii) topological distributions of the addations. As contrast, divalent addations can induce magnetism even at low concentration independent of addation distribution. The advantages of divalent additions make it a potentional candidate for the future magnetic materials and spintronic devices.(3) The atomic and electronic structures of silver-filled (n, 0) single-walled carbon nanotubes (Ag@SWCNTs) for n - 6, 7, 8, and 10 were explored by performing first-principle calculations to search for an efficient way to tune the electronic properties of SWNTs. We found that silver atoms encapsulated in SWCNTs self-aggregate to form ultrathin nanowires, of which the atomic arrangement depends on the diameter of the SWCNTs as well as the silver content. The electronic structures of the Ag@SWCNTs can be changed from semiconducting to metallic by controlling the silver content. The hybridization of electronic states and the charge transfer between the encapsulated silver nanowires (AgNWs) and the SWCNTs were also addressed.(4) The magnetic properties of hydrogenated single walled carbon nanotubes (SWNTs) were studied at different hydrogen concentrations. The hydrogen adsorption on SWNTs generates quasi- localized states near the Fermi level and opens substantial gaps. The magnetic properties of the compounds depend on the hydrogen concentration. At high hydrogen concentration, the localized states split into spin-up and spin-down branches locating above and below the Fermi level, making the systems spontaneous magnetic. However, at low hydrogen concentration, the spin-up and spin-down branches of the localized states are energetically degenerated and the systems are nonmagnetic. This result is understandable from the point of view of the direct interaction between the local states at adjacent adsorption sites.(5) The electronic structures of nitrogen-doped single-walled carbon nanotubes ((5,5) and (8,0) tubes) were calculated. Various forms of nitrogen-doped defects, including divalent nitrogen adatoms, nitrogen substitutions, amido adsorptions and vacancy defects on tube walls were systematically considered. Divalent nitrogen addations can induce magnetism even at low concentration on these two kinds of CNT, independent of addition distribution. The spin polarized (5,5) CNTs with nitrogen adatoms have good electron transport abilities, thus would have potential applications in spin transport devices. Vacancies, from our calculations, can trigger magnetism only in (8, 0) carbon nanotube (semiconductor). If the unpaired p electrons localized on the dangling C/N atoms are saturated by hydrogen atoms, the magnetic moment will vanish.