What IS a carbon nanotube?

It's not a simple question, and there are lots of top scientists asking the question right now.

Most molecules have a fixed formula - ethanol for example has 2 carbon atoms, 6 hydrogens and an oxygen, arrange them in a certain way and hey presto - ethanol! Arrange them any other way, or any other combination of atoms, and one thing's for sure - it's not ethanol. But there's no such thing as a 'standard' carbon nanotube; they come in all shapes and sizes. And the trouble is that the pen-and-paper model of a carbon nanotube - a rolled up sheet of graphene joined along the edges to form a tube - is a long way from the reality.

Real carbon nanotubes normally have multiple tubes within tubes, but depending on how they were made, and how they were then purified, there can be vast differences.

Some still have metal catalyst particles (nanoparticles) either at the tips of the tubes or in the sample. It's fair to say that no sample of nanotubes is ever totally 'clean', but some have only tiny amounts of amorphous carbon (soot-like carbon) stuck to their surfaces, other samples can be maybe down to only 60% nanotubes with the rest made up of lumps of soot, large 'carbon onions', graphite flakes, etc.

Then there's the question of whether the tubes are really separated out, or are stuck together in bundles. And are they pure? Often there's many impurities, like nitrogen or oxygen, either embedded in the tubes or stuck to the tube surfaces, or solvents stuck to their surfaces. And how long are they? That can change radically from sample to sample.

Finally - those nice walls of hexagons? Depending on how the tubes are grown, the walls can vary from something close to this, to a mish-mash of wobbly irregular walls filled with defects, kinks, bends and junctions; a sort of crazy Picasso rendering of a nanotube that's almost not a tube at all.


The trouble is, that all these variations are important, as each and every one can radically change the properties of the material. You want to know if a sample of nanotubes is toxic? Well, what sort of tubes, and how are they treated? If there are metal nanoparticles present, this could strongly affect their toxicity, and so could many of the other variations described above. Equally if you want to use them in a plastic composite to strengthen it, in a thin film to make it electrically conductive - or in a gas sensor - all of these variations in tube type can dominate the response you get.

That's why the scientific literature on nanotubes is such a tangled and confusing web of results. Different experiments can often contradict each other, a result found by one group can prove frustratingly difficult to reproduce by another. Often the reason lies in exactly what nanotubes were used, and how they were treated before the experiment began.

As a result there's a increasing drive within the nanotube community to come up with a set of standards; an attempt to define a 'standard nanotube sample', whatever that will be, so scientists can be sure they are comparing like with like. As you can imagine, this is a formidable question, as it touches not just on standards for nanotube production and cleaning, but also on the techniques used by the scientists to then measure what they have.

We had a good (short) debate at NanoteC08 about this, and hope to have a longer programmed discussion next year. In the US, NIST (the National Institute for Standards and Technology) are organising a meeting to work through the issues, and have produced a draft report on measurement standards that's useful reading for anyone in the field. In the near future the International Standards Organisation (ISO) is working with various international bodies such as the American National Standards Institute (ANSI) to begin setting standards. So hopefully in the next year or two we may finally get an official standardised answer to the question - what IS a carbon nanotube?