Your most-cited papers are virtually all on the subject of nanotubes. What are nanotubes? And how did you move from fullerenes, which won you a Nobel Prize, to nanotubes?

It goes back to December of 1990. Prior to that, certainly, a group of us involved in the fullerene story, including Harry Kroto, often talked about the question of whether larger fullerenes would be cylindrical or more ball-shaped.

And by 1990, it was pretty clear that if you had n carbons in a fullerene—n being a number bigger than 60—the most stable form of that structure would be as ball-shaped as possible. But we also knew that there was a coalescence of balls to make larger balls that happened all the time. And that coalescence almost certainly would be more active at the ends, where the pentagons were, and this would tend to make it a more elongated object.

In any case, in December 1990, after Wolfgang Krätschmer and Donald Huffman had their major breakthrough making fullerenes in a carbon smoke generator, I attended a workshop organized by the Air Force Office of Scientific Research on the subject of carbon-carbon materials. There was a panel discussion scheduled for the end of the meeting. I was asked to be on the panel, and I knew in advance that I was going to be asked to say something about how bucky balls might be related to carbon-carbon composites. I stayed up late the night before, and it occurred to me that if you make an elongated fullerene—the way C₇₀ is the same as C₆₀, but with a belt of carbons around the equator to make it elongated—by adding more and more belts, you would make a long tube with hemispheres of C₆₀ as end caps. It would be a fullerene but it would be a carbon fiber with the virtue of having no exposed edges. It is clear that it is the exposed edges of carbon fibers that is their Achilles heel. That is where the oxidation primarily occurs and also where fractures occur, which was what was being discussed in much of the meeting.

So I talked about this in my little period during the panel discussion. And Millie Dresselhaus [of MIT] was sitting right next to me. She initially didn't quite understand what I had in mind. I said, "I'm talking about a single fullerene extended." Over the subsequent months, Millie and Gene Dresselhaus really got intrigued with the structure, and they started calculating the infrared spectra and the electronic properties. And in a meeting in Philadelphia the next summer, Millie got up and gave a talk about bucky tubes and presented the results of their calculations. I was very attentive, and I asked her if she had any idea how you could make these things. She looked at me and said, "I think you're making them now." I was about to launch into a deep intellectual discussion about why she was wrong, but decided not to. And as we now know, she wasn't. You don’t have to vary the carbon arc method very much to make nanotubes. That was the origin.

  And you worked on it from that point on?

Yes. Nanotubes increasingly dominated my thoughts. And as of about 1993, nanotubes have been pretty much all we've been doing.

  Your 1996 Science paper is about crystalline ropes of nanotubes ("Crystalline ropes of metallic carbon nanotubes," Science 273[5274]: 483-7; 26 July 1996). What is the importance of this rope structure and why has this paper had such impact?

There are two prime stories in this paper. The first is the laser-oven method for making single-wall carbon nanotubes. That came out of an MRS meeting in Boston. I'm guessing it was late 1994. Someone was showing that cobalt and carbon, when arced, produced single-wall nanotubes. It occurred to me that back in 1990 we had found that laser vaporizing graphite in an oven at 1,200 degrees gives the most efficient production of fullerenes, hands-down. It occurred to me that we should put cobalt or nickel in with the carbon and try vaporizing that in the oven. I called back to my group and suggested it, and within a month Ting Guo, my graduate student, had done just that. We found stunningly vast yields; maybe 40 or 50 percent of the stuff collected was nanotubes. We published it in Chemical Physics Letters in 1995. Ting and I and a few other students continued to work on this method, and it occurred to us sometime in late 1995 or early 1996 that the juxtaposition of two lasers, one hitting soon after the other, might dramatically increase the yield. Once again Ting tried it and found stunning results. At times, when we collected the material, we found hardly anything but single-wall nanotube ropes.

We then sent some of it off to Jack Fisher at Penn, who did X-ray diffraction and other experiments on it. And in the spring of 1996, he sent me a copy of a rough draft he had written about this X-ray diffraction work on these nanotubes. He got an electron diffraction pattern for the packing of nanotubes in a regular hexagonal array, and the spacing between the nanotubes was, if I remember correctly, 13.8 angstroms. Jack was saying in the paper that it appeared that the tubes must be very narrow in distribution of diameters to give such a sharp diffraction peak. I was playing around with this question, and I wondered if that particular diameter has any significance. So I looked it up in the literature and found that it is actually the diameter of a (10,10) tube, and there are other tubes of different helicities in that same range. I started toying with the notion that it might be the (10,10) tube that is actually being preferentially made. So I fired off an e-mail to Jack and we got into it quite intensely and after two weeks we had a new manuscript. I insisted on calling these things ropes. Jack wanted to call them crystals or rods. So we ended up with crystalline ropes. Either way, I knew it was something of really stunning importance, if it were in fact true.

  Why "stunning importance?" Why over 800 citations in six years?

Well, this was the first major publication that told people you could get high-quality, single-wall nanotubes in sufficient abundance to do anything with it. Prior to that time, the only methods for making single-wall nanotubes were carbon arc methods. So to have available large quantities of material was the key event and this was the reference people cited. The fact that it was in Science helped. And a good criterion that this was high-quality material was that it gave off a good electron diffraction pattern. So here was the first such thing. It got it over the threshold of interest, particularly for many physicists. And it has continued to be the gold standard for high-quality nanotubes, even now.

  What is it about carbon nanotubes that make them so special?

Carbon nanotubes are particularly important because of their great strength, and among the carbon nanotubes there is a subset that I believe is transcendentally important: the single-wall carbon nanotubes, which are actually giant fullerenes. The reason they have this transcendental importance, at least at the moment, is that the properties you actually measure for these things are very close and sometimes exactly the properties you expect for a perfect fullerene, where no single carbon is missing from the structure. If you take a perfect single-wall carbon nanotube, a nanometer in diameter and 100 nanometers long, the closest description of it is a bucky tube with the ends cut off. When you have that kind of perfection, then you get these incredible properties. When you don’t have that perfection, or there's even a single carbon atom missing in the network, you don’t get the incredible properties.

  And what are the incredible properties?

Well, when you pull on this thing, it's the stiffest damn object in the universe. Nobody really knows what its true tensile strength is, but calculations show it should be somewhere between 30 and 100 times stronger than steel. As for electrical conductivity, if it's a perfect single-wall nanotube, if all the carbons are in it, electrons will flow down the structure in a coherent quantum wave-guide way that is unparalleled in any other structure we know. And it does this at room temperature. It's incredible. But it only does that if it’s perfect. Perfection is a very big deal. Multi-walled nanotubes or carbon fibers before that, wonderful as they are or were, cannot do the stunning things that single-wall nanotubes can. I harp on this point. I call them fullerene fibers or bucky tubes, trying to make this point that with molecular perfection comes the incredible properties that really make this a transcendent area of science and technology. Of course, I have to admit that part of me is still fighting the bucky ball wars of my earlier life, but it’s still an honest and true point.

  Where do you see nanotube research and technology going in the next five years?

Within five years, I'm confident we will find single-wall nanotubes in commercial products. And most likely those will be products that exploit the electronic conduction properties. So, for example, it may very well be that flat-panel displays with single-wall nanotubes as field emitters will be in the commercial market. It may very well be that lithium ion batteries with single-wall nanotubes as the anode structure will be in commercial production. It may very well be that fuel cells with single-wall nanotubes as support for the electrodes will be in commercial production. It may very well be that there will be commercial production of electromagnetic radiation shields—injectionable polymers blended integrally with single-wall nanotubes to buy the electromagnetic shielding necessary for an electronic device. It would not just be part of the molding of the case, but would actually strengthen the case rather than weaken it. It's quite possible we'll have single-wall nanotubes blended in with polymers in special high-value applications where just the mechanical properties are enhanced in such a way that it makes them competitive. All that seems quite likely within the next five years. All require single-wall nanotubes in commercial production at acceptable prices. Almost certainly less than $1,000 per pound. Perhaps as low as $100 per pound.

  What do you think has been the biggest obstacle to research and technological applications?

We really haven't been hung up on any one thing long enough to ever say we're stuck. Certainly the availability of high-quality material is the single biggest limiting factor of how fast things can go. There are thousands of really good researchers around the world dying for something to do and they would all love to get their hands on really good nanotubes. So finding ways to make really good material has been the limiting step. On the other hand, Mother Nature has provided really good answers, the way she often does. Now with single-wall nanotubes, you condense carbon vapors with nickel and cobalt and, bingo, there they are. It's still true that with the laser-oven and the carbon arc method and others, you always make this considerable burden of garbage that comes with it. But there will be techniques, and our own HiPco method is a good example, where the material will be made with very high purity right off the bat in a commercial way. I wouldn't be surprised if these things are being made by the hundreds or thousands of tons within five to ten years in processes that look very typical for the chemical industry.

Dr. Richard E. Smalley
Center for Nanoscale Technology
Rice University
Houston, TX, USA