SCANNING-TUNNELING-MICROSCOPY (STM) IMAGING OF CARBON NANOTUBES

Carbon nanotubes prepared by thermal decomposition of hydrocarbons on supported Co catalysts were investigated by STM in air. An interpretat ion of the STM images is proposed which accounts for specific distorti ons taking place while scanning three-dimensional objects whose dimens ions are of the order of the curvature radius of the Lip. These distor tions have both geometric and electronic origins, and cannot be neglec ted. The distortion mechanism was found to be different for nanotube d iameters in the ranges of 1 nm and 10 nm. The 1 nm tubes are more stro ngly affected by their apparent broadening, reflecting the finite size of the tip apex. Here the distortion can reach up to 300% of the geom etric diameter, whereas for 10 nm tubes the distortions are in the ran ge of 50% of the geometric diameter. An apparent flattening of the nan otubes in the vertical direction was also found, which is attributed t o differences in electronic densities of states between the substrate and the nanotube, and to an additional tunnelling barrier between the nanotube and the substrate. STM images with atomic resolution and line cut topographic profiles show similar structures as For the case of H OPG. However, the atomic corrugation was found to be five times smalle r on the 1 nm diameter tubules than for the 10 nm family, the latter b eing close to the value obtained with HOPG. Coiled nanotubes have been imaged by STM for the first time. Here both the electrical resistance of the coiled nanotube and its elastic deformation play a significant role in the image formation process, these effects being more importa nt than for straight nanotubes. (C) 1998 Elsevier Science Ltd. All rig hts reserved.