Re-reading Feynman – Part 3

As I discussed in part 1 of this series, Richard Feynman’s lecture “There’s plenty of room at the bottom” is universally regarded as a foundational document for nanotechnology. As people argue about what nanotechnology is and might become, and different groups claim Feynman’s posthumous support for their particular vision, it’s worth looking closely at what the lecture actually said. In part 2 of this series, I looked at the first half of Feynman’s lecture, dealing with writing information on a very small scale, microscopy with better than atomic resolution, and the miniaturisation of computers. In the second part of the lecture, Feynman moved on to discuss the possibilities, first, of making ultra-small machines and ultimately of arranging matter on an atomic level.

Feynman enters this subject by speculating about how one might make miniaturised computers. Why, he asks, can’t we simply make them in the same way as we make big ones? (Recall that at the time he was writing, computers filled rooms). Why can’t we just shrink a machine shop: “Why can’t we drill holes, cut things, solder things, stamp things out, mold different shapes all at an infinitesimal level?”

The first problem Feynman identifies is the issue of tolerance – a piece of mechanical engineering, like a car, only works because its parts can be machined to a certain tolerance, which he guesses to be around 0.4 thousandths of an inch (this seems plausible for a 50’s American gas guzzler but I suspect that crucial components in modern cars do better than this). He argues that the ultimate limit on tolerance must derive from the inevitable graininess of atoms, and from this deduces that one can shrink mechanical engineering by a factor of about 4000. This implies that a one-centimeter component can be shrunk to about 2.5 microns. Other problems that come with scale include the fact Van der Waals forces become important, so everything sticks to everything else, and that we can’t use heat engines, because heat diffuses away too quickly. On the other hand, lubrication might get easier for the same reason. So we’ll need to do some things differently on small scales: “There will be several problems of this nature that we will have to be ready to design for”

How are we going to make these devices? Feynman leaves the question open, but he makes one suggestion, recalling the remote handling devices people build to handle radioactive materials, levers that remotely operate mechanical hands: “Now, I want to build much the same device—a master-slave system which operates electrically. But I want the slaves to be made especially carefully by modern large-scale machinists so that they are one-fourth the scale of the “hands” that you ordinarily maneuver. So you have a scheme by which you can do things at one- quarter scale anyway—the little servo motors with little hands play with little nuts and bolts; they drill little holes; they are four times smaller.” And then you use the littler hands to make hands that are even smaller, and so on, until you have a set of machine tools at 1/4000th scale. The need to refine the accuracy of your machines at each stage of miniaturisation makes this, as Feynman concedes, “a very long and very difficult program. Perhaps you can figure a better way than that to get down to small scale more rapidly.”

Reading this with the unfair benefit of hindsight, two things strike me. We do now have mechanical devices that operate on the length scales Feynman is envisioning here, upwards of a few microns. These micro-electromechanical systems (MEMS) are commercialised for example, in the accelerometers that activate car airbags. For an example of a company active in this field, take a look at Crossbow Technology. But the methods by which these MEMS devices are made very different to the scheme Feynman had in mind; just as in the case of computer miniaturisation it’s the planar processes of photolithography and etching that allow one to get down to this level of miniaturisation in a single step.

Returning to Feynman’s idea of the master-slave system in which you input a large motion, and output a much smaller one, we do now have available such a device which can effectively get us not just to the microscale, but to the nanoscale, in a single step. The principle this depends on – the use of piezoelectricity to convert a voltage into a tiny change in dimensions of a particular type of crystal – was well known in 1960, and the material that proves to do the job best – the ceramic lead zirconium titanate (PZT) – had been on the market since 1952. I don’t know when or where the idea of using this material to make controlled, nanoscale motions was first developed, but between 1969 and 1972 David Tabor, at the Cavendish Laboratory in Cambridge, was using PZT for sub-nanometer positional control in the surface forces apparatus which he developed with his students Winterton and Israelachvili. Most famously, PZT nano-actuators were the basis for the scanning tunneling microscope, invented in 1981 by the Nobel laureates Binnig and Rohrer, and the atomic force microscope invented a few years later. As we’ll see, it’s this technology that has allowed the realisation of Feynman’s vision of atom-by-atom control.

Why would you want to make all these tiny machines? Characteristically, the dominant motive for Feynman seems to be for fun, but he throws out one momentous suggestion, attributed to a friend: “it would be interesting in surgery if you could swallow the surgeon. You put the mechanical surgeon inside the blood vessel and it goes into the heart and “looks” around.” Thus the idea of the medical nanobot is launched, only a few years before achieving wide-screen fame in Fantastic Voyage.

Here Feynman asks the ultimate question “What would happen if we could arrange the atoms one by one the way we want them?” The motivation for this is that we would be able to get materials with entirely new properties: “What would the properties of materials be if we could really arrange the atoms the way we want them? They would be very interesting to investigate theoretically. I can’t see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do.”

We do now have some idea of the possibilities that such control would offer. The first, easiest problem that Feynman poses is: “What could we do with layered structures with just the right layers?” The development of molecular beam epitaxy and chemical vapour deposition has made this possible, and just as Feynman anticipated the results have been spectacular. In effect, controlling the structure of compound semiconductors on the nanoscale – making semiconductor heterostructures allows one to create new materials with exactly the electronic properties you want, to make, for example, light emitting diodes and lasers with characteristics that would be unavailable from simple materials. Alferov and Kroemer won the Nobel Prize in Physics in 2000 (with Jack Kilby) for their work on heterostructure lasers. This work is gaining even more commercial importance with the discovery of a way of making blue heterostructure LEDs and lasers by Nakamura, opening the way for using light emitting diodes as a highly energy efficient light-source. Meanwhile new generations of quantum dot and quantum well lasers find uses in the optical communication systems that underly the workings of the internet. You can see an example of the kind of thing that’s been done in a number of labs around the world in this post about work done at Sheffield by my colleague Maurice Skolnick.

This kind of semiconductor nanotechnology still doesn’t quite achieve atomic precision, though. This is Feynman’s ultimate goal: “The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom. “. On this scale, Feynman forsees entirely new possibilities; “We can use, not just circuits, but some system involving the quantized energy levels, or the interactions of quantized spins, etc. We can use, not just circuits, but some system involving the quantized energy levels, or the interactions of quantized spins, etc. “ Some of these ideas are already being realised; quantum dots (even though they are made with slightly less than atomic precision) display quantised energy levels deriving from their size, and the manipulation of spins in such quantised systems is at the heart of the ideas of spintronics and may provide a way of realising quantum computing (another field which Feynman was the first to anticipate). Feynman points out another advantage of making this with atomic precision: the ability to make exact reproductions of the things we make: “But if your machine is only 100 atoms high, you only have to get it correct to one-half of one percent to make sure the other machine is exactly the same size—namely, 100 atoms high! “

Don Eigler, of IBM, demonstrated the possibility of single atom manipulation in 1990 with this famous image of the letters IBM picked out in xenon atoms. Given this capability, what can one usefully do with it? Feynman suggests that it might prove a different route to doing chemistry: “But it is interesting that it would be, in principle, possible (I think) for a physicist to synthesize any chemical substance that the chemist writes down. Give the orders and the physicist synthesizes it. How? Put the atoms down where the chemist says, and so you make the substance. “ Progress towards this goal has been very slow, emphasising just how hard the Eigler experiments were. Philip Moriarty provided an excellent summary of what has been achieved in his correspondence with Chris Phoenix, available as a PDF here. Feynman himself anticipated that this wouldn’t be easy: “By the time I get my devices working, so that we can do it by physics, he will have figured out how to synthesize absolutely anything, so that this will really be useless.” Nonetheless, Feynman stresses the value of these developments for science: “The problems of chemistry and biology can be greatly helped if our ability to see what we are doing, and to do things on an atomic level, is ultimately developed—a development which I think cannot be avoided. “

Now we’ve gone back to the original source to see what Feynman actually said, in my final installment, I’ll assess what validity there is to the various competing claims to the endorsement of Feynman for particular visions of nanotechnology.