Quantum Size Effects in Carbon Nanotube Intramolecular Junctions

We calculated the effect of varying the length of a metal-semiconductor carbon nanotube junction on its electrical properties. Joining a metallic (5,5) tube to a semiconducting (10,0) tube leads to the creation of new states near the Fermi energy and produces a larger conductance gap (about 2 eV) than the band gap of the semiconducting segment (about 1 eV). The new states reflect the charge transfer from the (5,5) to the (10,0) segment. The larger conductance gap is due to the mismatch in the conducting states of the (5,5) and (10,0) segments. Although the number of states in the vicinity of EF increases significantly with increasing nanotube length, the electrical behavior of the junction does not acquire the characteristics of the semiconducting segment. The calculations suggest that the (5,5)/(10,0) nanotube junction could behave as an intrinsic diode for lengths as small as 4 nm.