Bromine-adsorption-induced change in the electronic and magnetic properties of nanographite network systems

Abstract

Nanographite having open edges has a nonbonding $\ensuremath{\pi}$-electron state of edge origin (edge state), which causes unconventional electronic and magnetic features. Bromine-adsorption effect on the electronic and magnetic properties of nanoporus carbon consisting of a flexible three-dimensional random nanographite network system is investigated by using activated carbon fibers (ACFs) as host material. The irreversible adsorption of bromine into ACFs at room temperature gives compositions up to $\mathrm{Br}∕\mathrm{C}=0.43$. The interaction between nanographite and the adsorbed bromine is classified into three groups; charge transfer, covalent bonding, and physisorption interactions, where the latter two are the majorities. Although the charge transfer rate from carbon to bromine is considerably small (0.0004 per C atom at maximum), in comparison with that in bulk graphite-bromine intercalation compounds, the downshift of the Fermi energy results in the large reduction of the localized spin concentration of edge state and the orbital diamagnetism. This proves the important role of the edge state in the charge transfer process. The electron spin resonance results demonstrate the contribution of the orbital character of bromine to the edge state in nanographite through orbital mixing. The physisorbed bromine accommodated into the nanopores induces the dielectric and structural effects on the nanographite. The former causes the modification of the carrier conduction process due to the charging effect, while the latter results in the magnetic switching phenomenon induced by the effective pressure of physisorbed bromine species to the edge-state spins.