Guided Growth of Horizontal Single-Wall Carbon Nanotubes on M-Plane Sapphire

The alignment of carbon nanotubes has been the subject of extensive research. Among the different alignment methods, surface alignment, such as alignment along favorable directions on the anisotropic surface of single-crystal substrates, including sapphire and quartz, has stood out as very effective and robust. Recently, this method was shown to also be highly effective for the guided growth of semiconducting nanowires. Interestingly, while the same directions of alignment were found for the growth of nanotubes and nanowires on R-plane (1102) and A-plane (1120) sapphire (α-Al2O3), nanotubes, unlike nanowires, have been reported not to align on M-plane (1010) sapphire. Here we report the alignment of carbon nanotubes on M-plane sapphire, as well as on the annealed M-plane, which is periodically faceted, in directions that match the directions reported for aligned nanowires. We go on to discuss a possible common alignment mechanism for nanowires and nanotubes on sapphire surfaces.