First-Principles Study Of Electronic And Spin Properties Of Graphene Nanostructures

In recent years, the magnetic property of graphene has attracted the attention of people around theworld. Unlike some elements containing d or f-shell electrons, the carbon material itself does not presentmagnetism. By modifying and forming defects, however, graphene can be transformed into magneticmaterial. In addition, duing to the negligible spin-orbit coupling of the carbon-based system, graphene isconsidered to be an ideal material for spintronic devices such as spin field effect transistor (SFET).In this paper, using Vienna Ab-initio Simulation Package (VASP) based on density functional theory(DFT), we have investigated the electronic and spin properties of several graphene nanostructures includinggraphene with vacancy clusters, graphene nanoribbon (GNR) and graphene biribbon, and some interestingresults have been obtained.1. Laws of magnetism induced by vacancy clusters in graphene.We carry out an investigation to unveil the magnetic properties of graphene containing diferentvacancy clusters.The impacts of nine different vacancy cluster defects on the electron spin properties ofgraphene have been studied. The results show that dangling bonds in the vacancies induce magnetism andthe unpaired2p electrons of C play a pivotal role in the formation of magnetic moment. Spin densitydistribution calculations show the spin-polarization of unpaired electrons in the C atoms at the vacancydominates the magnetization of the system. These results provide conclusive evidence that the magnetismin graphene due to vacancies depends strongly on the local bonding environment. On the basis of thesecalculation results, we suggest a simple relation between ground state magnetic moment and the size andshape of the vacancy defects and the latter provides a reasonable estimate to the magnetic moment ofvacancy containing graphene systems. 2. Interedge coupling in zigzag GNR (ZGNR) and the effects of edge reconstruction on the coupling.The first-principle plane wave pseudopotential method has been employed to study the efects ofribbon size on interedge magnetic coupling in ZGNRs with and without edge reconstruction. It is found thatsize effect is crucial for determining the relative importance of the edge state. Interedge coupling bringsabout edge state band splitting for narrow ribbon. With the increasing of the ribbon width, the interactionbetween two edges of ribbon is gradually diminishing and the splitting is becoming smaller and smaller.The direct exchange coupling between the spin moments of the two edges play an important role for narrownanoribbon, while RKKY-like (Ruderman-Kittel-Kasuya-Yosida-like) interaction mediated by π electronsdominate the coupling between the edges for slightly wider ribbon. However, the RKKY-like interactionalmost disappears when ribbon width is greater than about26.31(n=13) for ZGNR. Thespin-polarization of unpaired electrons C atoms in the edges of ZGNR dominates magnetization of thesystem. The edge pentagon-heptagon reconstruction can achieve the demagnetization and weaken theinteredge coupling in ZGNR. In perfect and single-edge reconstructed ZGNR, magnetic moment dependson the ribbon width, while the change of magnetic moment is getting smaller with increasing ribbon width.In double edge reconstructed ZGNR, the relative position of pentatomic and heptatomic carbon ringslocated at diferent edges afects the properties of nanoribbon only for narrow graphene ribbon.3. Lateral in-plane coupling between graphene nanoribbonsThe first principles method to study the electronic and magnetic properties of lateral in-plane coupledGNRs. With the increase of inter-ribbon distance, the total energy exhibits a degenerative oscillation for themodeled systems. The underlying physics can be ascribed to Coulomb interaction and spin-spin couplingbetween ZGNRs, while only Coulomb interaction is operative in armchair GNRs (AGNRs). When theinitial interribbon distance is greater than10(6) for ZGNRs (AGNRs), the total energies reach a constant value. Energy band of the GNRs with small inter-ribbon distance shows that the lateral in-planecoupling (LIPC) will result in edge-state band splitting. This indicates that LIPC should be considered inlateral parallel GNRs with small inter-ribbon distances, but the coupling is negligible when the inter-ribbondistance exceeds10(6) for ZGNRs (AGNRs). The LIPC between ZGNRs is stronger than thatbetween AGNRs, and the spin-spin interaction between the edges atoms of the ZGNRs may be one of thefactors. The inter-ribbon displacement along the ribbon direction influences the energy band structure ofGNRs only when the initial inter-ribbon distance is less than5.4. Zigzag graphene biribbon seamed with S atomsThe effects of adsorption of S atoms at the ribbon edges on the structure, electronic and spin propertiesof bilayer zigzag graphene nanoribbons have been investigated in detail. It is found that graphene biribbonseamed with sp2hybridized S atoms can practically be considered as a radially deformed carbon nanotubedopped with S atoms. The S adsorption is a chemical adsorption and the bond between S adatom and Catom is covalent bond with ionic character.The magnetic moment induced by vacancies may change the transport properties of the carriers' spinin graphene, which has practical significance in graphene spintronics. The maximum inter-ribbon andinteredge distance for effective coupling may determine the physical limits of miniaturization ofgraphene-based devices in future. These results may provide predictive theoretical guidance to fabricationof graphene-based nanoelectronics and spintronic devices in the future.