Nano-sized graphene (mono-layer graphite) is an interesting molecule-based nanoscopic electronic system, where a variety of unconventional magnetic features appear in relation to its unique electronic structure. Recently, theoretical studies claim novel magnetic properties of nano-graphene due to the presence of edge-inherited non-bonding π-state, which depends crucially on the shape of nano-graphene. We investigated electronic and magnetic properties of nano-graphene and networked nano-graphites using nano-graphite prepared by heat-treatment of nano-diamond particles and activated carbon fibers (ACF). A single isolated nano-graphene sheet with the mean in-plane size of 10nm is prepared by heat-treatment of nano-diamond. A superlattice pattern appears in the STM image due to the electron confinement effect in large graphene sheets among nano- to submicro-graphene sheets, where a potential gradient produced by inclination of the sheets gives spatial variation of the superlattice periodicity. STM observations of the edges of graphene sheets suggest the presence of the edge-state. The electronic density of states in nano-graphite, which is found to be two orders of magnitudes larger than that of bulk graphite, proves that the electronic structure around the Fermi energy is governed by the π-edge states. ACFs are featured with a 3D network of nano-graphite domains, where each domain is formed with a stacking of 3-4 graphene sheets with the mean size of 2-3nm. The electronic features of ACFs are characterized as Anderson insulator in the Coulomb-gap variable hopping regime. In the 3D nano-graphite network, each nano-graphite domain, in which edge-state spins of constituent graphene sheets are strongly coupled antiferromagnetically to each other through inter-nano-graphene transfer interaction, weak antiferromagnetically interacts with the adjacent domains. The physisorption of guest species such as water into the nano-space (micropore) surrounding nano-graphite domains brings about a switching effect of magnetism between low- and high-magnetic-moment states upon the change in the amount of guest molecules in the reversible adsorption/desorption process. Judging from the fact that guests accommodated in micropores squeeze nano-graphites, which make the inter-graphene layer distance shrunk, this can be explained with the internal-pressure-induced Mott transition in nano-graphite. Heat-treatment induces an insulator-metal transition around 1300°C. Here, the development of an infinite percolation path network between nano-graphite domains is responsible for the metallic state. In the vicinity of the insulator-metal transition, a novel disorder magnetism similar to spin-glass state appears below ca.7K due to the randomness in the strengths of exchange interactions between non-bonding edge-state spins, which are mediated by the conduction π-electrons. Interestingly, the π-conduction-electron-mediated exchange interaction, which is the π-electron analogue of s-d interaction in ordinary metal magnets, has a considerably long ranged feature extending over ca.2nm. The experimental findings presented above reveal novel nanoscopic magnetism of nano-graphite.
Department of Chemistry, Tokyo Institute of Technology
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