The Bigger Picture of Nanotechnology
Chris - Nanotechnology. It's a term that gets bandied about in the media a huge amount but people probably don't understand terribly much about what it means. Would you mind making it a little bit easier for us to understand?
David - There are a couple of ways in which you can look at nanotechnology. The first one is what we refer to as the top-down approach. In other words, we take things that are large and try and make them smaller. That would be the sort of work semiconductor fabrication companies are involved in when they try to make transistors smaller and faster. The other way is to use self-assembly and to allow the atoms or molecules or things on a nanoscale to do the work for you. What you do is arrange particular materials and treat them in a particular way. By doing so they self-assemble and form interesting materials.
Chris - When we're talking nanotech, how big is nanotech?
David - Well if you sort of imagine the size of the planet Earth and imagine a football, the ratio between them is about ten to the power of eight. If you then scale down a football by that same ratio, tha'll take you to roughly the size of a carbon60 molecule, or a buckyball. If you go smaller than that, then you'reinto the area of DNA and single walled carbon nanotubes. So it's really at the ultimate scale of length.
Chris - And the reason that it's become a technology that we've embraced only recently is presumable because it is so small and there are constraints when working at this scale.
David - There are lots of constraints but there are also lots of advantages of working at this scale. The constraints are how you can see, manipulate and move these things. But one of the key aspects is how you can take the benefits of this nanoscale. When you get this small, two things really happen. The first is that the surface area to volume ratio changes, so bascially your nanomaterial is all surface. Volume plays much less of a role and the surface is the key. The other thing is that when you're at these very small scales, you're dealing with quantum effects. This rather weird area of physics takes over and you get unusual properties that you don't get with the bulk of the material.
Chris - So they're not a nuisance, they're actually quite useful.
David - They can be very very useful. In fact, there are a number of applications for them in the biotechnology industry. The displaying industry are very interested in using these carbon-based electronic materials to extract electrons and use them in displays for example.
Chris - Are we going to get better television sets out of this?
David - Well that's the ultimate goal. Samsung for example have developed a new type of cathode ray tube.
Chris - You'd better tell us what that is.
David - If you imagine your television and it's got a single source of electrons and the electron gun. If you then want to give your television a wider screen, it also has to get deeper, heavier and more expensive. The other way round it is rather than having a single source, you have billions and billions of sources of electrons. That means that you can have these sources quite lose to where the phosphors are. The phosphors are the things that glow red, green or blue in your TV screen. Getting them very close means that you have a very thin display. A thin display is potentially a cheaper display, but it's also light in weight.
Chris - So just explain the difference between a plasma or LCD screen and a normal television.
David - An LCD or liquid crystal display essentially uses particular molecules that under certain conditions either allow light to go through or they block light. So they tend to be permanently on as a technology, and what you're doing to make something go dark is to put something in the way of it. So that's inherently rather energetically inefficient. In plasma screens, you're setting up and electrical discharge and that tends to use very high voltages and that tends to mean that your display becomes very expensive.
Chris - So how do you actually create that discharge? What's actually happening?
David - What's happening is that you have a gas within a chamber and you're setting up a high voltage discharge which then breaks down and gives off particular colours. By modulating that high voltage you can make it glow red, green or blue.
Chris - So it's a bit like a strip light that's illuminating this studio but obviously in a much more controlled way.
David - Yes that's one way to look at it. But these things tend to be very energetically inefficient and power hungry and you have to use very large areas of silicon as a substrate, which is also expensive.
Chris - So how is nanotech going to make us have much better screens then?
David - The idea is that you can use materials that give off electrons very easily, such as what are called carbon nanotubes. A carbon nanotube is a single sheet of carbon atoms which are arranged in hexagonal rings.
Chris - A bit like graphite then.
David - Exactly like graphite. Graphite is a layered material and the layers are called graphene. If you were to take on eof those graphene layers and roll it up, that would form a single-walled carbon nanotube. You can have single walled ones of these or you can have multiple walls, a bit like a Russian doll structure. These things, because they have a very high aspect ratio, when you put the into an electric field they give off electrons very easily. Those electrons then hit off the phosphorous and therefore your television is red, green or blue.
Chris - How do you make the different colours though? I can understand how you get the electrons out but do you therefore have to connect the nanotube to something that glows a certain colour or gives off a certain colour when it gets excited?
David - What happens is the electrons come off of the carbon nanotube, are accelerated towards towards a phosphor in a straight line and you just decide in your structure that one third of your screen will be red, one third will be blue and one third will be green. It's all to do with the control electronics when the electrons turn on and when the electrons turn off. That's how you get the different colours on a screen.
Chris - Can you just give us the idiot's guide to how these phosphors work? That's the coating on the screen that the excited electrons hit and is then encouraged to change colour.
David - That's right. There are different types of phosphors and phosphorus materials but one type is called the rare earth based phosphors. These are rather interesting materials. Most materials when you shine light on them will glow at different colours, including red, green, blue and a mixture. The rare earths are rather unusual because when light hits them they give off light in very well defined wavelengths. The narrowness of that wavelength is basically the narrowness of your colour. This gives you a very clear and clean image.
Chris - Can we focus on these nanotubes because they really are an exciting bit of science aren't they? Didn't Sir Harry Kroto discover these things and get the Nobel prize for making them or something?
David - Harry Kroto got the Nobel prize for the discovery of C60, which is a molecule of sixty carbon atoms. This was partly when he was at Sussex University but he then went to Rice and did some experiments with the late Richard Smalley. What happened there was that they found C60 molecules by hitting a laser into graphite. Carbon nanotubes came about and were discovered by a Professor Iijima in Japan to some extent by accident. He was trying to do a similar experiment and he looked at the material that was left over expecting to see some C60 molecules because that's what he was interested in. To his surprise, he found some long helical structures made out of just carbon atoms. He then wrote this up and submitted it to Nature and it's been one of the most highly cited papers since 1991 and it's developed the whole field of carbon nanotube electronics.
Chris - Ok, so that's electronics taken care of and I've got a little question for you about nanotubes in a second. But obviously nanotech is a much broader field than that. So set the scene for other things people are doing with it.
David - Nanotechnology and nanomaterials can be used in a number if different areas, particularly in catalysis, enhancing reactions, used in fuel cells, and there's a huge patent portfolio for using nanotechnology for cosmetics.
Chris - Really, how?
David - Well you're dealing with a product that you can rub onto your skin. If you think of things like sun screen, five years ago, they were all coloured pigment. Now if you look at a lot of sunscreens, they're actually clear. This is because the size of the material, the titanium dioxide material has got smaller, and will hopefully improve the response and protection towards the sun. But there are a whole rang eof these nanomaterials that are beginning to emerge in unusual areas such as skin care and things like that.