First-principles Study Of Graphene Quantum Systems

Using first-principles calculations, we have studied structures and electronic properties of metal atoms adsorption on or doping in graphene systems and graphene-related quantum structures.For the functionalization of graphene by the addition of transition metal atoms of Mn, Fe and Co to its surface, we found that the center of the hexagonal ring formed by carbon from graphene is the most stable site for Mn, Fe, Co after structural optimization. The calculated spin-polarized band structures of the graphene with the adsorption of Mn adatom indicate that the conduction bands are modified and move down due to the coupling between the Mn atom and graphene. For Fe adsorbed on the graphene surface, the system is semi-half-metallic and the spin polarization P is found to be 100%. The system with Co adatom on graphene exhibits metallic electronic structure due to the density of states (DOS) peak at the band center with both majority and minority spins. Local densities of states indicate a larger promotion of 4s electrons into the 3d state in Fe and Co, resulting in lower local moments compared to an Mn adatom on graphene.We have studied the doping effects of bilayer graphene, focusing on Au substitute doping in the upper layer of bilayer graphene to form order-disorder separated quantum film. We found that Au doping in the upper layer of bilayer graphene keeps well the structural geometry of the lower graphene layer. Importantly, there is a high energy barrier between the two graphene planes to prevent the diffusion of Au dopant to the lower graphene layer. Moreover, a portion of electrons of Au transfers to the carbon atoms in the lower graphene layer, leading to increase of the carrier density of large mobility.First-principles spin-polarized calculations have been conducted to investigate the structural, electronic and magnetic properties of 3d transition metal Mn doping into two typical sites in the upper layer of bilayer graphene with the AB Bernal structure. One of the doping sites is above the center of a carbon hexagon of the lower graphene layer (called the H site) and the other is directly on top of a carbon atom of the lower graphene layer (called the T site). We found that Mn doping enlarges the interlayer distance in bilayer graphene. Charge density distribution indicates that the region between the upper and lower graphene layer has apparent covalent-bonding characters due to the Mn doping. In the spin-polarized band structure of H site doping, theπandπ* bands separate from each other at the Dirac point both in majority spin and minority spin. In the band structure of T site doping, the Fermi level is located above the Dirac point and moves to the conduction bands in majority spin and minority spin, making the bilayer graphene n doped. A high spin polarization of 95% is achieved due to the H site doping. The local moment of Mn for H and T site doping is reduced to 1.76μB and 1.88μB, respectively, which are smaller than the value (5μB) in its free state.In addition, using first-principles calculations, we have studied several graphene-related quantum systems, such as carbon nanotubes doped with transition metal atoms, carbon chains inserted into carbon nantoubes, and ZnO nanotubes.