Sorting the semiconducting wheat from the metallic chaff: Carbon nanotubes get closer to industrial purity


A few years ago, carbon nanotubes (CNTs) were touted as the Wave of the Future that would revolutionize materials engineering from semiconductors to space elevators. These days, CNTs have largely been obscured by the mad rush to research graphene (though not entirely), but there’s still some work being done on nature’s version of the ultimate Fruit Roll-up. This week, scientists with Japan’s National Institute of Advanced Industrial Science and Technology held a demonstration of a new sorting technique that could make sorting carbon nanotubes — and thus building things with nanotubes — much simpler

The problem with CNTs is essentially a quality control issue. We can make them — but aligning them in the proper direction has proven to be difficult. The problem is exacerbated when attempting to build semiconductors. Dope a group of CNTs and you wind up with a set of semiconducting CNTs and a set of metal CNTs — and separating them out from each other is extremely difficult. Past methods have included the use of a centrifuge and a gel-based sorting method that works, but tops out at a purity of 88-94%. While even that was notable, 94% purity is far too low for semiconductor applications at the cutting edge, where impurities of just a few atoms can cause problems.

New nanotube sorting method

This new sorting method works by applying a low (4.5-volt) current to the CNT solutions for up to 19 hours (electrophoresis), after which the measured purity of the solution is 95-98%. That’s still not enough for semiconductor applications, which require a purity of 99.96%, but it’s far better than what we’ve had to date. More to the point, it’s also cheap. One of the greatest problems facing the mass commercialization of CNTs is that all of the developed methods for sorting them have been extremely expensive and time consuming. Self-organizing CNTs that neatly arranged themselves into perfect rows would be the Holy Grail for computing, but then, self-directed nanoscale engineering is considered the Holy Grail for the entire semiconductor market, including solar cells and other applications.

According to the team that developed the technique, the voltage can be applied via simple dry cell batteries and works for CNTs with a diameter between 0.9nm and 2.1nm. The metal semiconductors will separate into one layer, while the semiconductor CNTs will separate into a second layer. Whether this technique can be scaled up to at least Mason jars is unclear, but being able to separate small amounts of CNTs into higher purity layers is, at least, a significant step forward over where we’ve been.

Now read: IBM creates 9nm carbon nanotube transistor that outperforms silicon

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