The nanotubes are usually quite short, and tend to form clusters, as shown in the image. They are sometimes coated with a layer of amorphous carbon. Other carbonaceous contaminants such as carbon nanoparticles, graphite, and possibly crystals of carbyne can also be found on support films for TEM. Some references are given below.
Carbon nanotubes and related structures as contaminants on evaporated carbon films, P.J.F. Harris J Microscopy 186 88 (1997).
"Carbonaceous contaminants on support films for transmission electron microscopy", P.J.F. Harris, Carbon 39 909 (2001).
"Multi-walled carbon nanotubes on amorphous carbon films", R. F. Klie et al., Carbon 42 1953 (2004).