Sunday, February 15, 2009

In which I drink the James Tour Koolaid

As I mentioned last week, James Tour of Rice University gave an invited talk at my university last week (unfortunately, it looks like the abstract is no longer available online).

I was really looking forward to it, as the internets have lots to say about his work, particularly regarding his "nanocars" and other buzzwordy "nano" objects, and on a few of his personal beliefs.

He gave a talk in a pretty packed lecture hall (one of the professors in the department brought his class of undergrads to the talk, which I thought was pretty nifty), and I was also able to ask him some questions afterwards when he met with a group of students for an hour or so.

I was really impressed with a lot of his work, no ifs ands or buts. Contrary to my expectations, his group does a lot of awesome research outside the nanocar dealership, particularly concerning work on carbon nanotubes and graphene.

In particular, he highlighted two areas that were particularly impressive.

Area #1: The Tour group has recently performed some work on the chemical synthesis of graphene:

Graphene is an allotrope of carbon that is simply a sheet[1] of graphite, one atomic layer thick. Think of it as the world's biggest planar aromatic molecule. It has lots and lots of predicted applications as it seems to be incredibly conductive along the sheet.

The main issue is that there really aren't any good ways to make it yet. I kid you not, probably the best way to do so is to pay an undergrad $10 an hour to take a piece of graphite and use Scotch tape to peel off layers until you may or may not get graphene. Yikes.

But, have no fear, because chemistry can come to the rescue! A single-walled carbon nanotube is basically like a sheet of graphene rolled up into a straw. By cleverly oxidizing a nanotube with an oxidizing agent like KMnO4, researchers have shown than you can "un-zip" the nanotube into a sheet of graphene oxide.

What Tour has done that is kind of spiffy is used some diazonium chemistry that his group has helped develop along with a hydrazine reduction to reduce those sheets of graphene oxide back down to functionalized graphene. After, the graphene shows high conductivity along the sheet, indicating that any residual oxygen is not too detrimental to the use of this method for electronic applications. Cool!

Reference: Lomeda, J.R., Doyle, C.D., Kosynkin, D.V., Hwang, W.-F., Tour, J.M. "Diazonium Functionalization of Surfactant-Wrapped Chemically Converted Graphene Sheets." JACS, 2008, 130 (48), pp 16201–16206, DOI: 10.1021/ja806499w

Area #2: As I mentioned above, there's a lot of interest in the electronic properties of graphene as a potential replacement for Si in devices. As well, sensible ways to make graphene are currently few and far between. Or is there?

Researchers have been able for a while to make thin layers of carbon using chemical vapour deposition, called graphitic sheets. Whether or not this constitutes graphene depends on how strict you are at defining these somewhat arbitrary and (decidedly not IUPAC, at least for now) terms[2].

Tour et. al grew thin films of graphitic sheets on "nanocables" of SiO2 (read: thin wires) across two Pt electrodes. Since the SiO2 is an insulator, this means they were able to check the electrical properties of the graphitic sheets wrapped around the wires.

(Above: schematic depicting how they made the nanocables. The squiggly lines in image D correspond to the graphitic thin film)

They observed some very peculiar electronic information, as shown below in figure A. Namely, as they ran through the CV curve, they noted at past a set voltage, the current would rapidly drop off. Furthermore, the switch off seemed extremely reversible, simply requiring the application of a voltage below that what was required to switch it off to turn it back on. The difference between the "off" and "on" states was extremely high, on the order of 10^6 or 7 (figure B). Finally, this switching process was not impacted significantly by the introduction of ambient conditions, nor upon the irradiation of the wires by X-rays.

Now, what is a switch with a very high signal-to-noise ratio good for? Memory applications! Tour attributed the switching process to a "nanoelectromechanical" mechanism whereby the application of a high voltage (erasing the memory) made the wire break (shown below), preventing any current from flowing, and the application of a lower voltage (writing the memory) could heal it back again. Crazy!

This sort of memory device has some nifty advantages over the status quo flash memory ones. In particular, it's a two terminal device that doesn't require a gate; the write/erase voltage is controlled by the size and thickness of the graphitic film. As well, since it is based on a pseudomechanical process (almost like a relay switch), it is resistant to charge interference. Flash memories would be wiped out in the case of an EMP or nuclear explosion, but this (in theory) would be okay. Finally, as I mentioned above, making these sorts of materials by CVD is straightforward, and amenable to industrial-type scale up. Similar two terminal memories exist, based on topics such as phase-change materials, but have been unable to match this extremely high signal to noise ratio.

Reference: Li, Y., Sinitskii, A., Tour, J.M. "Electronic two-terminal bistable graphitic memories." Nature Materials, 2008, 7 (12), pp. 966-971,DOI: 10.1038/nmat2331.

So, in summary: James Tour does some really cool work beyond what you might think! He also presented on some of his nanocar work, of which I am slightly less enthused compared to the above work (he has made several different "nanoautomobiles" including a "nanotruck", a "nanoworm" and a motor-driven nanocar. No word on any "nanobailouts" for the nascent nanoChrysler company- I guess self-assembly machines haven't quite figured out how to unionize yet). I'm just nanokidding! Anyways, enough with the nanopuns. Check out some of Tour's other work!

[1] Graphene is currently defined as anything less than 10 atomic layers of graphite, but as you increase the number of layers, you lose the funky conductivity that makes it so interesting. Hence, the search for a chemical method for large-scale production of single sheet graphene.

[2] Will and Beyond made a great list of some really out-there nano terms. The use of "graphene" to describe carbon-based large aromatics may soon suffer a similar fate. While I was really impressed with almost everything Tour presented on, I somewhat disagree with his overuse of the "nano" prefix, particularly for his nanocars projects. It may get a lot of mainstream media, but it really dilutes what the term can mean, and can induce a lot of eye-rolling from established people in the field.


Will said...

I'm imagining a lab full of undergrads lined up like a sweatshop, taking looseleaf paper, scribbling on it with pencils then peeling off layers with scotch tape, then placing them on a conveyor belt where they get sent down the assembly line into an automated TEM to get analyzed.

Nanocar dealership... I just died a little bit inside. Yeah, I should do a followup to that post, damn nanostuff.

Sounds like it was a really interesting talk nonetheless.

Joel Kelly said...

No joke, that's seriously how it was made for some time: