Coherence of electron waves on suspended carbon nanotubes at room temperature

We present a comparative study on supported and suspended carbon nanotubes, using room temperature scanning tunneling microscopy measurement, in order to assess the effect of substrate interactions on the electronic properties. Our results show two remarkable differences between suspended and supported molecules; first there is about 0.1 eV less shift in the band structure toward higher energies which indicates less doping from the Au(1 1 1) substrate; second there are persistent coherent oscillations that are very sensitive to the bias voltage. The undamped behaviour of these oscillations is a manifestation of long coherence length, suggesting that the electron-electron interactions between nanotube and substrate is the main mechanism for phase relaxation. The origin of these oscillations is the constructive interference of electron waves that are scattered from defects on the nanotube. We present a high-resolution topographic image of a unique type of such defects where a void on the wall of a pristine nanotube is observed for the first time.