Direct observation of localized defect states in semiconductor nanotube junctions

We report scanning tunneling microscopy of semiconductor-semiconductor carbon nanotube junctions with different band gaps. Characteristic features of the wave functions at different energy levels, such as a localized defect state, are clearly exhibited in the atomically resolved scanning tunneling spectroscopy. The peaks of the Van Hove singularity on each side penetrate and decay into the opposite side across the junction over a distance of similar to2 nm. These experimental features are accounted for, with the help of tight-binding calculation, by the presence of pentagon-heptagon pair defects at the junction.