Carbon is Dead. Long Live Carbon?

Last month, the National Nanotechnology Initiative released a report on the state of commercial development of carbon nanotubes. And that state is mostly negative. (Which pains me, because I still love them.) If you’re not familiar with carbon nanotubes, you might know of their close relative, graphene, which has been in the news much more since the Nobel Prize awarded in 2010 for its discovery. Graphene is essentially a single layer of the carbon atoms found in graphite. A carbon nanotube can be thought of as rolling up a sheet of graphene into a cylinder.

CNT as rolled-up graphene.jpg

Visualizing a single-walled (SW) carbon nanotube (CNT) as the result of rolling up a sheet of graphene.

If you want to use carbon nanotubes, there are a lot of properties you need to consider. Nearly 25 years after their discovery, we’re still working on controlling a lot of these properties, which are closely tied to how we make the nanobues.

Carbon nanotubes have six major characteristics to consider when you want to use them:

  • How many “walls” does a nanotube have? We often talk about the single-walled nanotubes you see in the picture above, because their properties are the most impressive. However, it is much easier to make large quantities of nanotubes with multiple walls than single walls.
  • Size. For nanotubes, several things come in play here.
    • The diameter of the nanotubes is often related to chirality, another important aspect of nanotubes, and can affect both mechanical and electrical properties.
    • The length is also very important, especially if you want to incorporate the nanotubes into other materials or if you want to directly use nanotubes as a structural material themselves. For instance, if you want to add nanotubes to another material to make it more conductive, you want them to be long enough to routinely touch each other and carry charge through the entire material. Or if you want that oft-discussed nanotube space elevator, you need really long nanotubes, because stringing a bunch of short nanotubes together results in a weak material.
    • And the aspect ratio of length to width is important for materials when you use them in structures.
  • Chirality, which can basically be thought of as the curviness of how you roll up the graphene to get a nanotube (see the image below). If you think of rolling up a sheet of paper, you can roll it leaving the ends matched up, or you can roll it an angle. Chirality is incredibly important in determing the way electricity behaves in nanotubes, and whether a nanotube behaves like a metal or like a semiconductor (like the silicon in your computer chips). It also turns out that the chirality of nanotubes is related to how they grow when you make them.
  • Defects. Any material is always going to have some deviation from an “ideal” structure. In the case of the carbon nanotubes, it can be missing or have extra carbon atoms that replace a few of the hexagons of the structure with pentagons or heptagons. Or impurity atoms like oxygen may end up incorporated into the nanotube. Defects aren’t necessarily bad for all applications. For instance if you want to stick a nanotube in a plastic, defects can actually help it incorporate better. But electronics typically need nanotubes of the highest purity.

A plane of hexagons is shown in the top left. Overlaid on the plan are arrows representing vectors. On the top right is a nanotube labeled (10, 0) zig-zag. On the bottom left is a larger (10, 10) armchair nanotube. On the bottom right is a larger (10, 7) chiral nanotube. Some of the different ways a nanotube can be rolled up. The numbers in parentheses are the “chiral vector” of the nanotube and determine its diameter and electronic properties.

Currently, the methods we have to make large amounts of CNTs result in a mix of ones with different chiralities, if not also different sizes. (We have gotten much better at controlling diameter over the last several years.) For mechanical applications, the former isn’t much of a problem. But if you have a bunch of CNTs of different conductivities, it’s hard to use them consistently for electronics.

But maybe carbon nanotubes were always doomed once we discovered graphene. Working from the idea of a CNT as a rolled-up graphene sheet, you may realize that means there are  way more factors that can be varied in a CNT than a single flat flake of graphene. When working with graphene, there are just three main factors to consider:

  • Number of layers. This is similar to the number of walls of a nanotube. Scientists and engineers are generally most excited about single-layer graphene (which is technically the “true” graphene). The electronic properties change dramatically with the number of layers, and somewhere between 10 and 100 layers, you’re not that different from graphite. Again, the methods that produce the most graphene produce multi-layer graphene. But all the graphene made in a single batch will generally have consistent electronic properties.
  • Size. This is typically just one parameter, since most methods to make graphene result in roughly circular, square, or other equally shaped patches. Also, graphene’s properties are less affected by size than CNTs.
  • Defects. This tends to be pretty similar to what we see in CNTs, though in graphene there’s a major question of whether you can use an oxidized form or need the pure graphene for your application, because many production methods make the former first.

Single-layer graphene also has the added quirk of its electrical properties being greatly affected by whatever lies beneath it. However, that may be less of an issue for commercial applications, since whatever substrate is chosen for a given application will consistently affect all graphene on it. In a world where can now make graphene in blenders or just fire up any carbon source ranging from Girl Scout cookies to dead bugs and let it deposit on a metal surface, it can be hard for nanotubes to sustain their appeal when growing them requires additional steps of catalyst production and placement.

But perhaps we’re just leaving a devil we know for a more hyped devil we don’t. Near the end of last year, The New Yorker had a great article on the promises we’re making for graphene, the ones we made for nanotubes, and about technical change in general, which points out that we’re still years away from widespread adoption of either material for any purpose. In the meantime, we’re probably going to keep discovering other interesting nanomaterials, and just like people couldn’t believe we got graphene from sticky tape, we’ll probably be surprised by whatever comes next.