The glamorous applications for carbon nanotube in electronics focus on the use of individual nanotubes for nanoscale electronics – for example, this single nanotube integrated circuit reported by IBM a couple of years ago. But more immediate applications may come from using thin layers of nanotubes on flexible substrates as conductors or semiconductors – these could be used for thin film transistor arrays in applications like electronic paper. A couple of recent papers report progress in this direction.
From the group of John Rogers, at the University of Illinois, comes a Nature paper reporting integrated circuits on flexible substrates based on nanotubes. The paper (Editors summary in Nature, subscription required for full article) , whose first author is Qing Cao, describes the manufacture of an array of 100 transistors on a 50 µm plastic substrate. The transistors aren’t that small – their dimensions are in the micron range – so this is the sort of electronics that would be used to drive a display rather than as CPU or memory. But the performance of the transistors looks like it could be competitive with rival technologies for flexible displays, such as semiconducting polymers.
The difficulty with using carbon nanotubes for electronics this way is that the usual syntheses produce a mixture of different types of nanotubes, some conducting and some semiconducting. Since about a third of the nanotubes have metallic conductivity, a simple mat of nanotubes won’t behave like a semiconductor, because the metallic nanotubes will provide a short-circuit. Rogers’s group get this round this problem in an effective, if not terribly elegant, way. They cut the film with grooves, and for an appropriate combination of groove width and nanotube length they reduce the probability of finding a continuous metallic path between the electrodes to a very low level.
Another paper, published earlier this month in Science, offers what is potentially a much neater solution to this problem. The paper, “Self-Sorted, Aligned Nanotube Networks for Thin-Film Transistors” (abstract, subscription required for full article), has as its first author Melburne LeMieux, a postdoc in the group of Zhenan Bao at Stanford. They make their nanotube networks by spin-coating from solution. Spin-coating is a simple and very widely used technique for making thin films, which involves depositing a solution on a substrate spinning at a few thousand revolutions per minute. Most of the solution is flung off by the spinning disk, leaving a very thin uniform film, from which the solvent evaporates to leave the network of nanotubes. This simple procedure produces two very useful side-effects. Firstly, the flow in the solvent film has the effect of aligning the nanotubes, with obvious potential benefits for their electronic properties. Even more strikingly, the spin-coating process seems to provide an easy solution to the problem of sorting the metallic and semiconducting nanotubes. It seems that one can prepare the surface so that it is selectively sticky for one or other types of nanotubes; a surface presenting a monolayer of phenyl groups preferentially attracts the metallic nanotubes, while an amine coated surface yields nanotube networks with very good semiconducting behaviour, from which high performance transistors can be made.
6 thoughts on “Nanotubes for flexible electronics”
I must apologise, but I could not help myself…
Is not Zhenan Bao part of the infamous Henrik Schon Scandal?
In particular, how are thing going on the molecular transitor front?
Finally, what is the main lesson Science should learn from the Schon affair (I ask this as some time has passed which may have brought up some deep analysis!)
Thanks in advance
Zhenan Bao was indeed a co-author on some of the discredited Schön papers – she was a postdoc at Bell Labs at the same time as him. However, the universal consensus was that she was in no way complicit in Schön’s misbehaviour, nor, as a fellow postdoc, did she have any supervisory responsibility for him. Since then she’s established an independent career at Stanford, where her group is producing (as we see here) some very nice work.
The wider lessons of the Schon affair and the recent progress of molecular electronics are two good topics for future posts!
So this solves the problem of sorting CNTs for 2D mats? Isn’t this a major innovation?
I apologise in advance for being off topic.
I am writing this post regarding Prof Moriarty of Nottingham University.
He seems to have got a grant to do experiments on diamond mechanosynthesis!
I was wondering what is your take on the matter.
Thanks in advance Zelah
I’m very pleased about it. He’s been funded through the Leadership Fellowship scheme of EPSRC, for which, in this year’s call, nanotechnology was a priority area (I should state here that I wasn’t involved in the peer review process by which his grant was awarded, as in general my role is providing strategic advice rather than intervening in individual granting decisions). I know Philip well and respect his scientific abilities a great deal, so I’m sure he’ll do some exciting work.
Thanks for the very kind words. I should point out that the proposal involves close collaborations with not only Rob Freitas’ group but also Lev Kantorovich at King’s College, Malcolm Heggie at Sussex, and Chris Pakes at La Trobe University in Australia. I’m currently at a SPIE conference in San Diego (which your colleague from Sheffield, Ash Cadby, is also attending). Given that I was in California for the conference, I met with Rob Freitas and Ralph Merkle last week in Palo Alto for some discussions to “kick-start” the project.
The project, which starts in October, will use Qplus (tuning fork) atomic force microscopy in UHV and at temperatures ranging from 4K up to 300 K to explore the viability of diamond mechanosynthesis. The central goal of the project is the fabrication of a row (or particular pattern) of dimers on an appropriately depassivated H:C(100) surface.
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