Nanotechnology, of course, isn’t a single thing at all. That’s why debates about the subject often descend into mutual incomprehension, as different people use the same word to different things, whether it’s business types talking about fabric treatments, scientists talking about new microscopes, or posthumanists and futurists talking about universal assemblers. I’ve attempted to break the term up a little and separate out the different meanings of the word. I’ll soon put these nanotechology definitions on my website, but I’m going to try out the draft definitions here first. First, the all-important issue of scale.
Nanotechnologies get their name from a unit of length, the nanometer. A nanometer is one billionth of a metre, but let’s try to put this in context. We could call our everyday world the macroscale. This is the world in which we can manipulate things with our bare hands, and in rough terms it covers about a factor of a thousand. The biggest things I can move about are about half a meter big (if they’re not too dense), and my clumsy fingers can’t do very much with things smaller than half a millimeter.
We’ve long had the tools to extend the range of human abilities to manipulate matter on smaller scales than this. Most important is the light microscope, which has opened up a new realm of matter – the microscale. Like the macroscale, this also embraces roughly another factor of a thousand in length scales. At the upper end, objects half a millimeter or so in size provide the link with the macroscale; still visible to the naked eye, handling them becomes much more convenient with the help of a simple microscope or even a magnifying glass. At the lower end, the wavelength of light itself, around half a micrometer, gives a lower limit on the size of objects which can be discriminated even with the most sophisticated laboratory light microscope.
Below the microscale is the nanoscale. If we take as the upper limit of the nanoscale the half-micron or so that represents the smallest object that can be resolved in a light microscope, then another factor of one thousand takes us to half a nanometer. This is a very natural lower limit for the nanoscale, because it is a typical size for a small molecule. The nanoscale domain, then, in which nanotechnology operates, is one in which individual molecules are the building blocks of useful structures and devices.
These definitions are by the nature arbitrary, and it’s not worth spending a lot of time debating precise limits on length scales. Some definitions – the US National Nanotechnology Initiative provides one example – uses a smaller upper limit of 100 nm. There isn’t really any fundamental reason for choosing this number over any other one, except that this definition carries the authority of President Clinton, who of course is famous for the precision of his use of language. Some other definitions attempt to attach some more precise physical significance to this upper length limit on nanotechnology, by appealing to some length at which finite size effects, usually of quantum origin, become important. This is superficially appealing but unattractive on closer examination, because the relevant length-scale on which these finite size effects become important differs substantially according to the phenomenon being looked at. And this line of reasoning leads to an absurd, but commonly held view, that the nanoscale is simply the length-scale on which quantum effects become important. This is a very unhelpful definition when one thinks about it for longer than a second or two; there are plenty of macroscopic phenomena that you can’t understand without invoking quantum mechanics. Magnetism and the electronic behaviour of semiconductors are two everyday examples. And equally, many interesting nanoscale phenomena, notably virtually all of cell biology, don’t really involve quantum mechanical effects in any direct way.
So I’m going to stick to these twin definitions – it’s the nanoscale if it’s too small to resolve in an ordinary light microscope, and if it’s bigger than your typical small molecule.