Carbon nanotubes are often imagined to be structures of great perfection and regularity, but the reality is that, like virtually all materials we encounter, they will have defects – places where there’s a mistake in the crystal structure, like a missing atom or a wrongly connected bond. Defects are tremendously important in materials science, because they’re what stop materials from being anything like as strong as you would estimate they ought to be from a simple calculation. A recent paper in Nature Materials (abstract here, subscription required for full paper) provides what is, I think, the first accurate measurement of defect densities in single walled carbon nanotubes. For typical nanotubes, produced by chemical vapour deposition, one finds one defect every four microns of nanotube length.
It’s these atomic-level flaws that will, in practise, limit both the electronic and the mechanical properties of carbon nanotubes. The study, by Philip Collins and coworkers, at UC Irvine, uses a new technique for decorating the defects electrochemically. It’s not able to distinguish between different types of defects, which could include a substitutional dopant, a broken bond passivated by further chemical group or a mechanical strain or kink, as well as what is perhaps the theoretically best studied nanotube defect – the Stone–Wales defect. The latter occurs if, in a group of four hexagons of carbon, one bond is rotated, leading to two hexagons, a pentagon and a heptagon.
The figure of one defect per 4 microns of tube is, in one way, rather impressive – it translates into there being only one defect for every 10 thousand billion atoms. This is a similar level to the best quality silicon, which is pretty much the most perfect crystalline material available. But, on the other hand, given the essentially one-dimensional nature of a nanotube, it’s pretty significant, since a single defect in a length of nanotube being used in an electronic device would dramatically change its characteristics. And the presence of all these weak spots are likely to mean that it’s going to be difficult to make a macroscale nanotube cable whose strength approaches the theoretical estimates people have been making, for example in connection with the proposed space elevator.