Geometric and electronic structure of new carbon-network materials: Nanotube array on graphite sheet

We design a new class of carbon-network materials with a periodically modified graphite sheet. The modified part corresponds to (6,6) carbon-nanotube geometry. Their tube parts form triangular lattice on graphite sheet. On these systems each tube has six heptagons at the bottom, giving rise to a seamless sp(2-) C network with a negative curvature. We consider these nanotube arrays on graphite sheet with three kinds of tube-end geometries and various sizes for both graphite and tube parts. We report their electronic structures obtained by using a realistic tight-binding model, and for selected systems the density-functional theory. Interestingly, results show that most of them are semiconductors although both (6,6) tube and graphite are metallic. The difference in their tube-end geometries and the sizes of graphite and tube parts affect their electronic structures. Some have nearly flat band states around the Fermi level, showing a possibility of ferromagnetic behavior if hole or electron is doped. Some are direct-gap semiconductors whose interband transition is optically allowed. Their typical gap energies are about 1 eV. Therefore they should emit infrared light.