Scanning tunnelling microscopy (STM) imaging of carbon nanotubes Original Research Article

Carbon nanotubes prepared by thermal decomposition of hydrocarbons on supported Co catalysts were investigated by STM in air. An interpretation of the STM images is proposed which accounts for specific distortions taking place while scanning three-dimensional objects whose dimensions are of the order of the curvature radius of the tip. These distortions have both geometric and electronic origins, and cannot be neglected. The distortion mechanism was found to be different for nanotube diameters in the ranges of 1 nm and 10 nm. The 1 nm tubes are more strongly affected by their apparent broadening, reflecting the finite size of the tip apex. Here the distortion can reach up to 300% of the geometric diameter, whereas for 10 nm tubes the distortions are in the range of 50% of the geometric diameter. An apparent flattening of the nanotubes in the vertical direction was also found, which is attributed to 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 HOPG. However, the atomic corrugation was found to be five times smaller on the 1 nm diameter tubules than for the 10 nm family, the latter being 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 important than for straight nanotubes.