  Carbon nanotubes are the focus of considerable research efforts due to their
fascinating physical properties. They provide an excellent model system for the
study of one dimensional materials and molecular electronics. The chirality of
nanotubes can lead to very different electronic behaviours, either metallic or
semiconducting. Their electronic spectrum consists of a series of Van Hove
singularities defining a bandgap for semiconducting tubes and molecular
orbitals at the corresponding energies. A promising way to tune the nanotubes
electronic properties for future applications is to use doping by heteroatoms.
Here we report on experimental investigation of the role of many-body
interactions in nanotube bandgaps, the visualization in direct space of the
molecular orbitals of nanotubes and the properties of nitrogen doped nanotubes
using scanning tunneling microscopy and transmission electron microscopy as
well as electron energy loss spectroscopy.
