Single wall carbon nanotubes are currently the focus of intense research efforts in particular for their potential in the building of future electronic devices. The atomic structure of these tubes with nanometer-scale diameter can be seen as the rolling up of a graphene sheet, made only of carbon atoms. Depending on the angle chosen for rolling, the resulting nanotube can be metallic or semiconducting. Because of this link between atomic and electronic structure, the nanotubes are fascinating candidates for molecular electronics or spintronics. The main characteristic of the semiconducting tubes is their electronic gap within which the current can not be driven. However there are still conundrums about the value of this gap. For example, optical experiments give a larger value than electronic measurement obtained in scanning tunneling microscopy experiments. The STM team of the Laboratoire Matériaux et Phénomènes Quantiques in collaboration with the LEM laboratory at ONERA and LPQM at Ecole Normale Supérieure de Cachan have evidenced the important role played by the substrate. Using a scanning tunneling microscope they observed bundles of nanotubes that provide a collection of tubes at different distances from the supporting metal. They showed that the substrate acts like a mirror for the electrons, leading to the creation of an image charge in the metal. The interaction between tube’s electrons and this charge leads to a decrease of the measured gap. By taking this effect into account, the intrinsic electronic properties can be deduced and the experimental results obtained by different techniques can be conciliated. From these results it is clear that the substrate has a strong influence on the electronic properties of the nanotubes, an effect that will have to be considered in the future design of nanotubes based electronic devices.
Reference :
“Many-body effects in electronic bandgaps of carbon nanotubes measured by scanning tunnelling spectroscopy”, H. Lin, J. Lagoute, V. Repain, C. Chacon, Y. Girard, J.-S. Lauret, F. Ducastelle, A. Loiseau and S. Rousset, Nature Materials 9, 235 - 238 (2010)