Nitrogen doped carbon nanotubes have been synthesized using pyrolysis and characterized by Scanning Tunneling Spectroscopy and transmission electron microscopy. The doped nanotubes are all metallic and exhibit strong electron donor states near the Fermi level. Using tight-binding and ab initio calculations, we observe that pyridine-like N structures are responsible for the metallic behavior and the prominent features near the Fermi level. These electron rich structures are the first example of n-type nanotubes, which could pave the way to real molecular hetero-junction devices.

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Topology related changes in the local density of states near the ends of closed carbon nanotubes are investigated using spatially resolved scanning tunneling spectroscopy and tight binding calculations. Sharp resonant valence band states are observed in the experiment at the tube ends, dominating the valence band edge and filling the band gap. Calculations show that the strength and position of these states with respect to the Fermi level depend sensitively on the relative positions of pentagons and their degree of confinement at the tube ends