Nanotube structures
(Top) Multiwall carbon nanotubes are composed of concentric sets of single-wall tubes and have typical outer diameters of 10--50 nm. This structure can be seen clearly in this high-resolution transmission electron microscope image of a multiwall nanotube grown in an arc discharge (left), whereas other growth techniques produce less ordered structures (right). Images by Jean-Marc Bonard and co-workers at EPFL. (Middle) A scanning electron microscope image of a "nanobrush" of vertically aligned nanotubes obtained by printing a catalyst on a substrate: the nanotubes only grow at the catalyst sites (by the thermal decomposition of a reaction gas). This structure was produced by the groups of Hannes Kind and Laszlo Forró at EPFL, and Louis Schlapbach at the University of Fribourg. (Bottom) By adjusting the growth parameters, different forms of nanotubes, such as these spirals grown by Schlapbach's group, can be produced.