SCIENCE AT THE SHINE DOME canberra 30 April – 2 May 2003
Symposium: Nanoscience – where physics, chemistry and biology collide
Friday, 2 May 2004
Dr Michael Barber
Executive Director, Science Planning, CSIRO
Michael Barber is Executive Director, Science Planning in CSIRO. Prior to joining CSIRO in December 2002 he was Pro Vice-Chancellor (Research) at the University of Western Australia for nearly nine years. At UWA he was instrumental in the spin-off from the University of one of Australia’s first nanotechnology companies – Advanced Powder Technology Pty Ltd – and in the establishment of the new Motorola Global Software Centre adjacent to the University. Internationally he is recognised for his scientific work in statistical mechanics and theoretical material science. He has also contributed significantly to the development of science policy in Australia. He is a Fellow of the Australian Academy of Science.
Why science at 10-9 metres is fascinating
As convenor of the 2003 Academy of Science AGM symposium I would like to welcome you to what I hope will be an exciting oversight of one of the hot areas of modern science. My primary task in this initial talk is to set the scene for the talks that my colleagues, who are far more expert than I, will present. What I want to try to do is give you a broad sense of nanoscience and in particular why scientists from many different disciplines are finding it so fascinating.
Let me begin with a definition of nanoscience. If you enter 'nanoscience' into a web search engine, as I did the other evening, you come up with quite a range of definitions. Probably the simplest comes from the iNANO Centre at the University of Aarhus in Denmark: 'the study of phenomenon on the scale of 1-100 nanometres'.
(A nanometre is 10-9 of a metre. That's about 1/80,000 of the diameter of a human hair, ten times the radius of a hydrogen atom or about half the diameter of a DNA helix.)
Alternatively, if you want to be more technologically orientated, you might pick up a definition stating that nanoscience is the precursor to nanotechnology, which, to quote from the Scoping Report from the Department of Science, Industry and Resources of a couple of years ago, is 'the creation and use of materials and devices that exploit novel properties arising from the structure and function of matter in the nanometre range'.
Again, notice the explicit mention of a nanometre. But we don't talk about 'metre science' or 'centi-science'. True, we do talk about microelectronics and microbiology, but 'microscience' as a collective to describe the study of phenomena on the scale of microns (10-6m)? Not really. So what is so special about 10-9m that the prefix 'nano' has been applied so broadly to science and technology?
One reason is in the title of this symposium, 'Nanoscience – where physics, chemistry and biology collide' – although I have to admit that converge is probably more accurate than collide! It is illuminating to briefly describe the routes by which these three fundamental scientific disciplines have ended up in what we now call nanoscience.
Let me start first with physics, my own discipline. One tends to think that physics is about the very small – subatomic particles, nuclei, atoms, and so on – the core of twentieth century physics. And that is true. But until very recently the scale on which we observed physical phenomena, and certainly applied them, was much larger. It is the reduction in that scale, depicted in Figure 1 through the transition from electronics involving valves etc., to microelectronics with the increasingly smaller and faster silicon chips that have powered our ICT revolution. This trend, together with developments in material science, have resulted in physicists thinking hard and carefully about material on the nanometre range – still an order of magnitude above the atomic scale but a couple of orders of magnitude below conventional solid state physics for example. As I will illustrate shortly, on that scale both our 'classical intuition' derived from our macroscopic world and our 'quantum intuition' derived from quantum mechanics need to be revised.
Click on image for a larger version of figure 1
At the same time, biology has been moving from whole organism biology through cell biology and molecular biology to the genomics revolution, based on an ability to decode and manipulate individual DNA molecules. As a result, biology is now routinely dealing with molecules and subcellular structures on the scale of nanometres, the range of these phenomena that we are beginning to call, collectively, nanoscience.
Finally, over the last twenty years our chemical colleagues have discovered and begun to explore the chemistry of ever larger and more complex molecules. Developments in organometallic chemistry may lead to a 'molecular electronics' which is built on 'molecular wires' and 'molecular switches'.
Even more significant is another class – called fullerenes – that I will say something more about in a moment. More recently a new branch of chemistry – supramolecular chemistry – has emerged in which the objects of study are complex structures such as micelles and other multi-molecular complexes that have linear dimensions – yes, you guessed it – in the range of say 10-100 nanometres! At the same time, work aimed at improving the efficiency of catalysts has been seeking ways to increase the effective surface area since almost all catalysis occurs at the surface of the catalyst. We will see that one of the characteristics of nanoscale materials is an enhanced surface/ volume ratio.
By whatever route, physicists, chemists and biologists are now talking about science on the same scale. In one sense some of the distinguishing characteristics of the three sciences are washing away in this area called nanoscience. If you take a cluster of 100 atoms, is it a large molecule of primarily chemical interest, or is it a small sample of a condensed matter system, say 100 atoms of helium-4 on a graphite surface – a system that a physicist like me might find interesting, or is it biologically active, even 'potentially alive'? The answer depends on the circumstances and the specific question asked – but the question is also often irrelevant. All these disciplines, and others such as mathematics and information technology, have something to contribute to understanding nature on this fascinating scale, a scale that in the end is the scale on which life operates – the world's greatest nanotechnologist is nature itself.
The potential of this synthesis is the basis of the aspirations – and the hype – of nanotechnology with its expectations that, over the next decades, we will see 'smart' materials, incredibly sensitive biosensors (beyond the AMBRI biosensor about which Vijoleta Braach-Maksvytis may say something in her presentation) and DNA-based computers even smaller and faster than quantum computing that Bob Clark will talk about.
But the emphasis at this symposium is not so much on the potential of nanotechnology but on why science at the nanoscale is so important as science. In one sense it is exciting simply because physicists, chemists and biologists are talking to each other. The cross-inspiration coming from that dialogue is very significant. You will see something of that in Angela Belcher's talk on how ideas derived from the way that viruses work can be applied to the production of high-tech materials. A whole new field of research called biomimetic engineering is emerging and is the topic of Vijoleta Braach-Maksvytis' talk. Ultimately, of course, the holy grail of nanotechnology is to emulate the clean, green, non-polluting, cheap water-based manufacturing that goes on in every cell.
While achievement of that goal in any substantial way I suspect lies well into the future, its pursuit is already showing that science at the nanoscale is fascinating and exciting for scientific reasons alone. In the remainder of my time I would like to dip, almost at random, into this bag called nanoscience and pull out a few little vignettes of some of the those exciting ideas and discoveries that have already been made.
The first comes from chemistry and has given rise to one of the motifs of nanoscience and nanotechnology. When I studied chemistry in first year at university I was taught that pure carbon came in two forms – or allotropes to use the technical term – diamond and graphite. These differed in the way that the underlying carbon atoms are arranged. In diamond they form a tetrahedral array with each carbon atom covalently bonded to four neighbours at the corners of a tetrahedron. The result is a rigid structure that gives rise to the hardness which is one of the characteristics of diamond. In graphite, on the other hand, the carbon atoms are connected in hexagonal rings in two-dimensional sheets. As a result the layers slip easily, giving graphite the softness which is seen in its use as 'lead' in pencils or as a lubricant.
A couple of serendipitous discoveries in 1985 and 1991 changed that simple picture. The first was the discovery in 1985 by Harry Kroto of the University of Sussex and Roger Smalley and Robert Curl at Rice University in Houston of a new form of carbon, in which 60 carbon atoms are covalently bonded together and arranged at the vertices of a truncated icosahedron – rather like a soccer ball. This new form is now called buckminsterfullerene – colloquially referred to as 'bucky balls' – because, as Kroto has recalled, 'the geodesic ideas associated with the constructs of Buckminster Fuller had been instrumental in arriving at a plausible structure'. C60 is only one example of a class of cage-like molecules called fullerenes, with important and interesting properties. Kroto, Smalley and Curl shared the Nobel Prize for Chemistry in 1996 for this discovery.
For most people, C60 is still intuitively a molecule. Admittedly a pretty big molecule with 60 carbon atoms, but there are plenty of organic molecules with as many atoms. However, in 1991, Sumio Iijima from NEC in Japan discovered yet another form of carbon that really begins to stretch the notions of what is a molecule and what is, say, a solid structure. Interestingly Iijima was trying to make C60but instead produced tiny tubes of carbon, now known as nanotubes. In the walls of a nanotube the carbon atoms are arranged as in graphite – indeed the wall of a nanotube is like a rolled up sheet of graphite – whereas the cap bears some resemblance to a bucky ball – notice the presence of pentagons again. Because of their structure, nanotubes have amazing properties; for example, they are possibly the strongest of all synthetic fibres and can be both a metal and a semiconductor – as Max Lu will describe in his presentation.
Click on image for a larger version of figure 2
My next two selections from the nanoscience bag are examples to illustrate how familiar materials can behave rather differently at the nanoscale. I've entitled the first 'White isn't white'! On the left of Figure 3 is a plot (courtesy of Paul McCormick of the University of Western Australia and Advanced Powder Technology Pty Ltd) of the transmissivity of 0.01 wt% aqueous suspensions of particles of zinc oxide, ranging from 250-25 nanometres in diameter, as a function of the wavelength of the incident 'light'. In the bulk, ZnO is a very effective scatterer of light so all the light incident on it is scattered and we see it as white – as in familiar zinc cream sunscreen. However, as the particle size gets smaller and smaller that scattering mechanism becomes less efficient, so that by 25 nanometres zinc oxide is essentially optically transparent. On the other hand, absorption in the UV spectrum occurs by a different process that is hardly impacted by the size of the particles. Consequently, as this rather graphic picture that appeared in the Sunday Times in Perth last year illustrates, nanopowder-based ZnO can form the basis of a cosmetically clear sunscreen with the same UV protection as traditional zinc cream. Already on the market, this example does seem to be fulfilling the prophecy that nanotechnology will change the face of Australia!
Click on image for a larger version of figure 3
My second example is entitled 'Gold is not gold'. In this case, as the size of gold particles decreases classical properties are replaced by quantum properties. Gold appears gold because it absorbs all of the incident light, except the colours of the spectrum necessary for us to see a gold colour. As the size of the gold particles get smaller and smaller the plasmon bands that are responsible for the absorption shift and as a result different wavelengths of light are absorbed resulting in different colours. Paul Mulvaney in his talk will say more about some of the reasons for these effects.
Very briefly, I now want to talk about catalysts. They are important in almost all physical processes and manufacturing processes, so one of the reasons that chemists have got excited about nanoscience is that if we can make more surface we get more catalytic action. A simple dimensional argument would actually say that the surface/volume ratio goes inversely to the cube root of the volume. In physics we argue, 'Well, volume effects aren't very interesting. If you take a big enough volume, the volume dominates the effects.' However, in small systems the surface/volume gets much more important. And our chemists have created some fascinating molecules, starting from a central piece, called dendrimers.
A dendrimer consists of a central molecule with a number of branches – three in the example shown in Figure 4. From the ends of each of these a second molecule is added, and the process continues. (In actuality the most effective process – devised by Jean Frechet of Berkeley – is the reverse: the molecules are grown from the 'surface' inwards.) However, the end result is the same – a very ramified structure bristling with a large number of end points of the chains, marked in Figure 4 by 'A', for active site – such that as the size of the dendrimer grows, generation by generation, the ratio of the number of surface/active sites to say total molecular weight is essentially constant, independent of the size. Since many important chemical reactions occur at surfaces, it is not too surprising that these molecules have staggeringly high reactivity with important applications. Moreover, their size, in the nanometre range, puts that reactivity on the same scale as viruses and other important biologically active molecules. Yet another example of the close connections between one branch of science and another that is a key feature of nanoscience.
Click on image for a larger version of figure 4
For my final vignette I would like to return to physics. In July 2002, if you subscribed to the electronic version of New Scientist, you would have received a dramatic announcement: 'Second law of thermodynamics "Broken"'. The announcement went on to say: 'One of the most fundamental rules of physics, the second law of thermodynamics, has for the first time been shown not to hold for microscopic systems. The demonstration, by chemical physicists in Australia [one of whom, Denis Evans, will address us later]...suggests that micro-scale devices...will not behave like simple scaled-down versions of their larger counterparts – they could sometimes run backwards.'
As anyone who has studied elementary thermodynamics will be aware, the second law is one of the most fundamental laws of physics. It explains in mathematical terms why the world we live in is irreversible; why, for example, if I drop this glass onto the floor and it breaks into a million pieces, we never see the pieces spontaneously rise off the floor and reform into a complete glass in my hand – like a film run backwards. The implication of the experiment conducted by Denis Evans and his colleagues at the Australian National University was that at the nanoscale such an event might be possible. At least the New Scientist in its headline put 'broken' in inverted commas. A few of the other headlines around that time were rather more dramatic. Nature left off the inverted commas and even the venerable Scientific American site screamed 'Second Law of Thermodynamics violated!' I will leave it to Denis to explain precisely the implications of the experiment and simply note that this is a wonderful example of the surprises that seem to lie in wait for us as we explore the nanoworld.
I could go on. I haven't, for example, mentioned that it is possible to 'see' this world and, even with more sophisticated new instruments such as scanning tunnelling or the atomic force microscopes, to not only image individual atoms but to move and manipulate them. You will see something of that in Bob Clark's talk.
Thus, to conclude, the importance of nanoscience and nanotechnology is manifest. As Neil Lane, a former Assistant to the President (of the United States) for Science and Technology, said in testimony before Congress in 1998: 'If I were asked for an area of science and engineering that will most likely produce the breakthroughs of tomorrow, I would point to nanoscale science and engineering.' To most scientists exploring this new world, however, the answer to the question I posed in my title is very simple. Why is science at 10-9m so fascinating? Because it's fun, exciting and surprising!
