The existence of stable layered bulk allotropes can be an indication for the existence of corresponding stable tubular structures (BxCyNz, Mo and W chalcogenide tubes, NiCl2 cage and nanotubular structures, phosphorus nanotubes, metal-boron nanotubes, etc.). Such nanotubes may exhibit important physical properties derived from the properties of the corresponding layered structure and quantum-mechanical constraints related to the topology of tubular structures. It is speculated, for example, that hole doping of layered lithium borocarbide (Li:BC), which is isovalent with, and structurally similar to, the superconductor MgB2, may yield an even higher critical temperature for superconductivity than MgB2 .
Using the Density-Functional Tight-Binding method (DFTB) , we have shown that charged borocarbide nanotubes derived from layered lithium borocarbide, are stable and energetically viable . The charged tubes are semiconducting with an energy gap decreasing with diameter. The size of the gap also depends on the helicity of the tubes. Due to characteristic van Hove singularities in the Density of States (DOS) of tubular structures, shifting of the Fermi level into the valence band may result in a large value for the number of electronic states at the Fermi energy. This could have a direct effect on the electron-phonon coupling, similar to that in medium temperature superconductors. We have already shown using the rigid band model, for example, that hole doping of charged BC nanotubes causes them to become metallic by shifting the Fermi energy into the valence band .
In this paper, results will be presented on metal doped neutral BC tubes. Using the ab initio pseudopotential plane wave method, we have shown that doping of BC tubes by Li and Cu atoms does not significantly change the topologies of the valence and conduction bands. The Fermi energy simply moves towards the edge of the valence band showing a rigid band behaviour similar to that exhibited by the charged BC tubes.
These results may have some interesting and important consequences in designing novel superconducting nanodevices and tubular assembled materials.
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 P.Ponomarenko, M.W.Radny, P.V.Smith and G.Seifert, Phys.Rev. B67 (2003) 125401
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