With a background in condensed-matter physics and mesoscopic physics, I came in from the field of molecular scale electronics, where by 1993 we had decided to work on conducting polymers and, in particular, measuring the electrical transport properties of a single molecule of a conducting polymer. For that we had to make tiny nanoelectrodes and do scanning probe imaging and this precursor work led us to work on carbon nanotubes, which were discovered in 1991 by Iijima, and then it all took off.

  What was it about carbon nanotubes that sparked your interest?

We were exploring what were the best molecules for successfully measuring current through a single molecule. After our first experiments on conducting polymers, we concluded that they were bad conducting wires at a single-molecule level. In fact, all these systems are semiconductors. We knew about carbon nanotubes, but they had problems of their own. In particular, the technology was such that when you tried to fabricate carbon nanotubes, you came up with this crummy soot that had a few nanotubes in it. They were all lost in the dirt. Then Rick Smalley at Rice developed a technique to get really clean carbon nanotubes in high yields. I contacted Smalley and we started doing experiments on that material. That was quite successful because we did indeed manage to measure single molecules, and to measure all the properties, whether semiconducting or metallic and so on.

  Was this the work that led to your 1997 Nature article "Individual single-wall carbon nanotubes as quantum wires," (Nature 386[6624]: 474-7, 3 April 1997)?

Exactly. It was a direct result of our collaboration with Smalley and his group. They provided the material. We had all the tools to make very tiny contacts and measure the electrical properties. A Ph.D. student in my group, Sander Tans, was leading that effort. He managed to get single molecules between the electrodes and measure the transport through that. What we did was to take tiny metallic electrodes on a chip, and put a nanotube molecule across it. And then we measured the electrical transport through such a nanotube molecule at low temperatures. When we applied a voltage, we could see a current running. The current didn't increase like it would under Ohm's law, but did so in steps. This we could attribute to quantum effects in the electrical transport. This was the start of a whole series of experiments we did on carbon nanotubes.

  What was your guiding philosophy for these experiments?

We were just exploring, in effect; although we were specifically searching for these types of quantum signatures. Indeed, it's much more established now, five years later. It's now clear that carbon nanotubes are ballistic wires, which is an extraordinary finding. It means that if an electron enters the nanotube, it can then travel without any resistance through the nanotube molecule. The nanotube is so defect-free that it doesn’t scatter back, which is where normal resistance comes from. Here the molecule is so pure and nice, the electron just keeps running without any scattering. It's a beautiful phenomenon.

  What is the state of the work now?

Well, we've done a lot of different things with nanotubes. We've done some beautiful experiments from a scientific perspective but also some applied work. For instance, we made the first transistor at room temperature from a semiconducting nanotube. That was a milestone: the first single-molecule transistor working at room temperature. Recently we even made digital logic circuits with nanotubes on a chip. Now we're setting up projects in biophysics. This is a completely different story: we looked at the electrical properties of DNA and we found that DNA is quite a good insulator, which is contrary to some earlier very controversial reports. We also became interested in the assembly properties of DNA and from that we moved even more into biophysics. Now we're interested in molecular motors and, in particular, the molecular motors that do all the work in living cells. We're looking at applying nanotechnology tools to biological systems and vice versa, using bio-assemblies to make hybrid bio-inorganic structures.

  What would your goal be?

We are exploring how far we can push the limits of what can be done. We can make, for example, applied devices like biosensors by taking nanotube structures and putting them in a liquid with some enzymes. We can build molecular electronics devices, assembling them with the double helix of DNA. And maybe we can take advantage of the mechanics of molecular motors for doing some work in totally different environments. We can also use nanotechnology to study biomolecules in a confined area. So we can go into both areas: apply nanotechnology to biology but also apply phenomena from biology to the inorganic world of molecular electronics.

  What is the future of molecular electronics and what role are carbon nanotubes likely to play in it?

If molecular electronics is going to go anywhere, and that's still an "if," then nanotubes are definitely going to be a big part of it. They have truly unique properties. No other material can be metallic at the scale of a single molecule. Typically metals will go semiconducting at the scale of a single molecule. But the special structure of nanotubes implies that they don't have that property and therefore they can be truly metallic. So they are unique in being able to be metallic wires and for that reason are going to be a main component in the future of molecular electronics. The big question is whether molecular electronics will have any real applications. That will depend critically on whether we can find good architectures and ways to build devices and assemble structures. What we, and others as well, have done is to demonstrate prototypes of single elements. We've made a new transistor, a new diode, etc. We have even recently combined a few transistors on a chip. But no one has come up with a strategy for building and assembling a molecular computer. That's the state of the field at the moment and we need real progress before anything substantial appears.

  In your biophysical work, are you aiming to make devices first or use the tools of nanotechnology to study biological mechanisms?

At the moment we are working on studying DNA repair mechanisms. This is more a fundamental scientific rather than an applied study. We're looking at how enzymes do repair work when the double strand breaks in DNA. When you walk in the sun, for instance, UV light damages some of your DNA, and then your body has machinery to repair that damage. Now we are using scanning probes and molecular tweezers and tools like that to study this repair mechanism at a single-molecule level; to see how enzymes walk along the DNA, find the two broken ends, and do the work to repair the breaks. We've also started working recently on using DNA in nanostructures and nanofluids and studying some of the statistical physics of DNA. In the far future, we might then be able to use these techniques to build detection devices or do some sequencing. We're also playing with assembly, using the DNA duplex; organizing C60, for instance, into a linear array along the DNA or binding nanotubes, C60, and DNA into a device. We have various projects in the works in which we use the complementary sequences of DNA to bind molecules or nanotubes and make devices.

  Where would you like to be five years from now in your research?

Ideally, I'd like to be at a point where I have learned a lot more. That's a little vague so let me add two more detailed remarks to it. In molecular electronics, I think the new efforts should really be directed toward assembling circuits. This should be the major direction of research and, hopefully, we'll make some progress in the next five years. In hybrid bio-inorganic structures, I would like to show that we can really combine molecular electronics and biological molecules into working devices. If we could do that, if we could use this combination to create something useful or do some new science, I would be very happy.

Professor Cees Dekker, Ph.D.
Department of Applied Physics and DIMES
Delft University of Technology
Delft, the Netherlands