Bundles of ${\mathrm{CaSi}}_2$ tubes were studied by density-functional band structure calculations. The most stable tubular structures contain Ca ions both in the intratubular region and on the outer surface of the tube. The formation energies indicate that the ${\mathrm{CaSi}}_2$ tubes are in the experimentally accessible range. The currently studied ${\mathrm{CaSi}}_2\mathrm{}(6,6)\mathrm{}$ nanotube exhibits ideal properties for a nanosized contact: Metallic conductivity along the tube interior is coupled with the strongly ionic character of the outer tube shell. The regular arrangement of positive and negative charges on the outer surface leads to a preferential aggregation of neighboring tubes within a bundle of tubes. Thus, there exists an additional driving force for the formation of regular patterns that may be exploited in the self-assembly route to nanostructures by template techniques.

• Received 17 March 2003

DOI:https://doi.org/10.1103/PhysRevB.68.075416