Nanotech: Art of the Possible
In George M. Whitesides’ line of work things are measured in nanometers. A nanometer is one-billionth of a meter, and to get a sense of how small that is, forget about analogies to the width of a human hair or the head of a pin. The “nanoscale” has nothing to do with familiar items. You need to think about a place where objects-including the devices being worked on by Whitesides and others-are only slightly larger than atoms.
Building things on that scale is called nanotechnology. It’s a rapidly developing field with immense potential; tiny devices could revolutionize computing, information storage, communications and any number of unforeseen areas. But it’s also an area prone to overblown promises, with speculation about nanomachines that are more likely to be found in Star Trek than in a laboratory.
A distinguished chemist and materials scientist, Whitesides has been exploring this very small world for years. After nearly 20 years at MIT, Whitesides joined Harvard University’s chemistry department in 1982. The Harvard researcher has provided micro- and nanofabrication with some of its most useful construction techniques. But Whitesides also keeps a well-trained reality check on the nano world. Despite his obvious enthusiasm for the field, he’s intent on defining what is, and what isn’t, going to be possible.
TR Senior Editor David Rotman recently visited Whitesides, Mallinckrodt Professor of Chemistry at Harvard, at his Cambridge office to sort out fact from science fiction in nanotechnology.
TR: Let’s start with a basic question. Just how small qualifies for nanotechnology?
WHITESIDES: The standard definition is functional structures that have feature sizes less than 100 nanometers, but I think the number probably should be 50 nanometers or less.
TR: Why is nanotechnology so intriguing?
WHITESIDES: It’s an extension of microtechnology. And microtechnology is the basis of making computer components, and that’s a very big deal. Microtechnology has gotten along for years on the idea that making things smaller brings benefits-they’re less expensive, you get more portability and more performance per dollar. The idea is that since “smaller” has worked with microelectronics, you can continue that trend beyond the current sizes in microelectronics, and this shrinking gets you into the world called “nano.”
TR: What specific technologies could come out of such work?
WHITESIDES: A good example is information storage. Right now the size of a spot on a CD disk is on the order of 10 square micrometers. People, particularly the IBM folks, have made CD equivalents that use pits on a spinning disk but the pits are now 50 nanometers in size. You could get, in something the size of a wristwatch, the equivalent of maybe 1,000 CDs. That starts approaching a fraction of the reference library that you need for your life. That raises interesting questions: What happens when you’re able to put all of the information you need for some major fraction of your life on your wristwatch, rather than actually having to learn it? It’s one of those ideas that shifts a little bit the notion of how a life should be led. You can take those ideas and extrapolate them. You put a micro or global positioning system on your wristwatch, so you know where you are. You could have the capability to locate yourself, to do computations, to use information, to communicate.
TR: Where are we in terms of actually realizing some of these things?
WHITESIDES: At the stage of laboratory prototypes. People have made little CDs, but they are not something that you can use outside the laboratory. It’s going to be very hard to extend the fabrication methods that people now use for making these small structures to anything that’s really manufacturable, but it will happen.
TR: How big a challenge is this?
WHITESIDES: We have a demonstration in principle from microelectronics that if you can make things smaller, people will find lots of exciting things to do with them. Right at the moment, we can make small things in laboratories, but we don’t know how to make really small things en masse in a manufacturing environment. And we don’t know how to use them once we make them. Until that manufacturing problem is solved, we won’t know what things can be made, and we will not know what the technology is going to be like.
TR: Photolithography that uses ultraviolet light to etch out patterns on silicon chips is the dominant technology in making microelectronics on the micrometer scale. But, I take it, you don’t see it having much of a future in terms of nanotechnology?
WHITESIDES: Photolithography has been very successful. We’ve had a wonderful run with this technology, and there’s every reason to think this will continue for a while. But now you begin to step back a little bit and say, well, it’s hard to go below 100 nanometers, and you can’t build 3-D things. It really doesn’t work for too many materials other than silicon and stuff like that. We would like to make things small so they can be fast and cheap and portable and not power consuming. How do we do that?
TR: What are some of the alternatives?
WHITESIDES: One is electron beams, an embodiment of which is Scalpel. [Scalpel is a system developed at Lucent Technologies’ Bell Labs that uses electron beams to pattern silicon wafers]. Another contender is X-ray lithography [this process uses X-rays to pattern wafers]. Both of them have a lot of very difficult technical issues, which we presume will be worked out more or less, but whether they’re really cheap enough is another question. And then there are newer technologies coming along which are, I think, legitimately long shots, such as lithography using neutral atoms or ion beams. It is, at the moment, a real horse race as to which technologies will be used in nanotechnology. But this is just the beginning.
TR: Are the limitations in photolithography fundamental enough to drive a whole new area of technology for making microelectronics?
WHITESIDES: They could be. Photolithography is getting very complicated and very expensive. For the new generation fabs [fabrication facilities for making semiconductor chips] that are being planned now for the years past 2000, the capital cost per fab is estimated to be $3 billion to $10 billion. If you want 20 percent return on investment, and you put in $10 billion, how many microwidgets do you have to sell every year for the few years that that fab is the state of the art? The answer is, a lot. And people who have to put up the money don’t like that.
TR: So it still remains a question how very small things will be made. What are some of the other unknowns about the future of nanotechnology?
WHITESIDES: There’s the problem that the devices, when you get down to true nanoscale size, no longer work as expected based on extrapolations of existing devices. You get somewhat different opinions as to how far existing technology can be extrapolated. My guess is that one can take existing systems and extrapolate them to somewhere in the 50 to 100 nanometers region. As you start getting wires, transistors and other components closer together, they start to talk to one another, and this crosstalk becomes a very serious problem. The properties of the basic materials used-doped silicon-also become hard to control.
TR: Beyond shrinking microelectronics smaller and smaller, there’s been a lot of talk about using nanotechnology for other, mechanical, types of applications.
WHITESIDES: There are a lot of things that range from being potentially real to things that are science fiction. There’s the idea of very small autonomous machines that swim around in the bloodstream or something like that. I can see no way of realizing those. The reason is that, aside from the problems in building them, there are horrendous problems with power in anything that’s an autonomous system. There’ll have to be some truly deep invention before anyone figures out how to power small autonomous systems. We have examples of powered systems: for example, living cells, or organelles in the cell. But the cell is not actually a small object. Mammalian cells are about 25 micrometers across and even bacterial cells are 1 to 3 micrometers. Viruses, which are much smaller, are not powered. So power is one fundamental question. Friction in small moving systems is a second. Manufacturing is a third.
TR: Do you think some of these applications have been overhyped?
WHITESIDES: What Eric Drexler [K. Eric Drexler is a research fellow at the Institute for Molecular Manufacturing in Palo Alto, Calif.; his book Engines of Creation helped popularize nanotech] and others do is to construct a series of ideas based on making existing things smaller. They say if you have a big Rotorooter, why not have a tiny Rotorooter?
TR: But it’s clearly the case where just because they’re smaller…
WHITESIDES: They’re not necessarily better. Smaller is not necessarily always better.
TR: And they don’t always work as just a smaller copy.
WHITESIDES: Right. Not only are they not necessarily better, particularly if they’re more expensive, but also they may not work using the same principles. Which means that for really small structures, we’d probably have to invent new architectures and new ways of thinking about the problem, so that we can deal with the peculiarities of these small machines. And of course one of the interesting questions is, where is it going to be worth the effort to make machines that are really very small?
TR: If we had this conversation five or 10 years from now, any guesses what we’d be talking about?
WHITESIDES: I think we might be having a slightly different conversation. One that is less about how nanotechnology has changed the world and more about how inexpensive microtechnology has changed it. Right now, we reserve the world of microfabrication-making structures between several hundred nanometers and a couple of microns [a micron is a micrometer, one-millionth of a meter]- for electronic microprocessors and computer systems. It’s a very legitimate question to ask what happens when you extend many things that are now made at centimeter and millimeter scales to the micrometer scale, and what new functions do you get?
TR: What do you have in mind?
WHITESIDES: A phrase that I use is “micron-scale technology with the economics of newsprint.” For example, instead of buying a newspaper, you might buy a sheet of paper; the back side of it would be a battery, the front side of it would be a display. You read it, scroll to find reference works on it, see animated illustrations, and when you’re done, you throw it away. One of the things that we might be talking about in 10 years is how micron-scale electronics using new technologies has crept into all kinds of things. My belief is that almost everything-shoes, windows, children’s toys, grocery labels, shipping labels, credit cards-will have electronics in a few years.
TR: You often mention biology and natural systems. What does biology tell you about nanotechnology?
WHITESIDES: Biology makes all kinds of very functional small structures. Drexler talks about small motors; we’ve got a terrific example of a small motor in biology, which is the flagellar motor in bacteria. This motor really works very well, and it actually looks a lot like a motor. Can we either learn how to use these biological things in some appropriate way in our devices, or understand the principles of biology better and then learn how to embed these principles in non-biological systems? Another example is sensors. A lot of what’s done in any biological system is sensing. The retina, the nose, all of these rely on molecules that are nanoscale sensors. How can we use these ideas to build artificial eyes and noses?
TR: Does biology tell you anything about the challenges ahead?
WHITESIDES: We’re made up of a hierarchical set of structures and components. We have molecules at the nanoscale level collected into organelles, which are 10 nanometers to maybe 100 nanometers, collected and working collectively in cells, which then aggregate into tissues that become us. One of the issues in electronics is that we work only in two scales. Transistors and collections of transistors-and that’s the device. But to take full advantage of nano, we’re going to have to think about that full hierarchy of levels of structure.
TR: What are some of the larger lessons that your research in nanotechnology has taught you?
WHITESIDES: One is the notion that function is often hierarchical and prioritized. Molecules do certain kinds of things, objects that are 10 nanometers do certain different kinds of things, objects that are 100 nanometers do yet other different kinds of things. For complex functionality, one has to learn how to build from small pieces into large objects taking advantage of the unique capabilities of each. The second is that there are phenomena that are size-specific. One of the things that one does on any scale is to look for commensurability between the phenomenon that you’re looking at and the object. Whenever you see that the phenomenon and the structures have similar sizes, there are interesting things you can do. The third thing is that for the nanometer scale in particular there is no richer storehouse of interesting ideas and strategies than biology.
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