Structure and properties of graphene nano disks (GND) with and without edge-dopants

Graphene is one of the most important materials. In this research, the structures and properties of graphene nano disks (GND) with a concentric shape were investigated by Density Functional Theory (DFT) calculations, in which the most effective DFT methods - B3lyp and Pw91pw91 were employed. It was found that there are two types of edges - Zigzag and Armchair in concentric graphene nano disks (GND). The bond length between armchair-edge carbons is much shorter than that between zigzag-edge carbons. For C24 GND that consists of 24 carbon atoms, only armchair edge with 12 atoms is formed. For a GND larger than the C24 GND, both armchair and zigzag edges co-exist. Furthermore, when the number of carbon atoms in armchair-edge are always 12, the number of zigzag-edge atoms increases with increasing the size of a GND. In addition, the stability of a GND is enhanced with increasing its size, because the ratio of edge-atoms to non-edge-atoms decreases. The size effect of a graphene nano disk on its HOMO-LUMO energy gap was evaluated. C6 and C24 GNDs possess HOMO-LUMO gaps of 1.7 and 2.1eV, respectively, indicating that they are semi-conductors. In contrast, C54 and C96 GNDs are organic metals, because their HOMO-LUMO gaps are as low as 0.3 eV. The effect of doping foreign atoms to the edges of GNDs on their structures, stabilities, and HOMO-LUMO energy gaps were also examined. When foreign atoms are attached to the edge of a GND, the original unsaturated carbon atoms become saturated. As a result, both of the C-C bonds lengths and the stability of a GND increase. Furthermore, the doping effect on the HOMO-LUMO energy gap is dependent on the type of doped atoms. The doping H, F, or OH into the edge of a GND increases its HOMO-LUMO energy gap. In contrast, a Li-doped GND has a lower HOMO-LUMO energy gap than that without doping. Therefore, Li-doping can increase the electrical conductance of a GND, whereas H, F, or OH-doping decreases its conductance.

SEARCH FOR LONG-LIVED WEAKLY INTERACTING PARTICLES USING THE PIERRE AUGER OBSERVATORY

Data collected with the surface detector array of the Pierre Auger Observatory during the period from January 1, 2004 to March 17, 2015 was examined for evidence of production of long-lived weakly interacting particles in interactions of ultra high energy cosmic rays in the atmosphere. The search was performed using extensive air showers with primary energies more than 10 EeV and arrival directions in the range of 57.5◦ to 77.5◦. There was no evidence of significant production of such particles. An upper limit on the fraction of extensive air showers in which such particles are produced was set.