STM and STS investigation of few wall carbon nanotubes containing non-hexagonal carbon rings

Abstract : We performed scanning tunneling microscopy (STM) measurements on few wall carbon nanotubes that exhibited changing diameter. Such change in the diameter may occur if non-hexagonal carbon ring configurations are introduced in the nanotube walls. A few-walled nanotube knee of 4 degrees, with different diameter values on the two sides of the knee was imaged by STM. Theoretical model structures [J. Han et al., Phys. Rev. B 57 (1998) 14983] of single wall carbon nanotubes show that a bend of 4° may occur when a pentagonal and a heptagonal carbon ring is incorporated side by side in the hexagonal nanotube structure (i.e. "fused defect"). Scanning tunneling spectroscopic (STS) measurements show that additional electronic states are present in the energy gap in the region where the bend occurs. We also performed STS measurements on a single-wall nanotube with a conical tip. In agreement with theory, the results show that the energy gap in the tapered end is larger than in the nanotube