Modeling of electronic transport in scanning tunneling microscope tip–carbon nanotube systems

A model is proposed for two observed current–voltage (I–V) patterns in a recent experiment with a scanning tunneling microscope tip and a carbon nanotube [Collins et al., Science 278, 100 (1997)]. We claim that there are two mechanical contact modes for a tip (metal)–nanotube (semiconductor) junction (1) with or (2) without a tiny vacuum gap (0.1–0.2 nm). With the tip grounded, the tunneling case in (1) would produce large dI/dV with V>0, small dI/dV with V<0, and I=0 near V=0 for an either n or p nanotube; the Schottky mechanism in (2) would result in I≠0 only with V<0 for an n nanotube, and the bias polarities would be reversed for a p nanotube. The two observed I–V patterns are thus entirely explained by a tip–nanotube contact of the two types, where the nanotube must be n-type.