The beauty and elegance of Nanocrystals: How invisibly small particles will colour and shape our future

[ UniNews Vol. 12, No. 13  28 July - 11 August 2003 ]

Materials scientists are fascinated by minute particles measuring between one and 20 nanometres (nm) in diameter and containing as few as 100 atoms. These nanoscale particles exist in a strange realm where the properties of crystals and molecules begin to merge – often with novel and astounding results. Such are the implications from research in this transition zone that in the next 10 or 20 years developments in nanocrystals and nanotechnology are set to make a major impact in many areas of industry and daily life. Dr Paul Mulvaney, a world leader in nanocrystal research working in the University of Melbourne’s School of Chemistry, recently gave a public lecture on nanocrystals at the Australian Academy of Science. He outlines here the story of these invisibly small particles and how they will both colour and shape our future.

By Paul Mulvaney

Something as prosaic as a grain of table salt (sodium chloride) has properties with which we are all familiar. But cut it in half, and half again, and into ever smaller portions and eventually this common crystalline commodity enters the strange realm of the nanoworld – a transition zone where the properties of materials change; in effect, the crystal is all surface with no bulk.

A 1nm nanocrystal of sodium chloride – essentially cube-shaped and containing a mere 10 or 12 sodium and chloride ions – is so small that all but one chloride ion is on the outside. This crystal cannot possibly behave like a piece of table salt any more. Its exposed ions are in a halfway world between the solute and the crystal. Only the single, central chloride ion still thinks it is in a salt crystal. In such an ‘altered state’ it is perhaps not surprising that crystals show unusual properties. In this case the salt nanocrystal has a solubility a hundred times higher than normal and its melting point is several hundred degrees lower than for a normal table salt crystal.

Materials scientists can ask the same questions of any material. On the nanoscale, what will happen to its properties, and over what size regime will these changes take place? Particularly important are optical, electrical, magnetic and catalytic properties. Gold is a good example. How small can you ‘dice’ a piece of gold before it is no longer gold? What will you see then? How will gold nanoparticles behave chemically or physically? Are they as strong as ‘bulk’ gold?

Obviously, ‘dicing’ is very time-consuming and inefficient. In fact it is much quicker to build a nanocrystal from the atom up – either physically or chemically. At the University of Melbourne we have developed a number of ways to grow gold nanocrystals of various sizes from 1nm up to 60 or 70nm. Around 10-20nm in size, the result is a beautiful ruby red colloidal solution of gold nanocrystals. Larger particles are purple or blue, whilst the smallest nanocrystals are bright orange.

The nanocrystals can be dispersed in solution and can be tuned through their size to absorb and reflect many different colours, opening the way for their use in novel optical devices and colour coatings with special properties.

Preparation of nanocrystals goes back a long way. Michael Faraday, in 1847, was the first person to attribute the red colour of colloidal gold to its finely divided state. But Faraday cannot claim to have been the first to have carried out nanocrystal science. The Romans added gold salts to their sand and soda ash mixtures and found that by careful annealing they could produce a red transparent glass – attributable to gold nanoparticles. Later in the Middle Ages, gold and silver nanoparticles were used to produce the bright red and yellow stained glass windows in churches.

Nanoparticles retain these unique optical properties while they remain separate. However, bring them together and they interact, tending to re-form the original bulk material. How to produce stable nanoparticles in macro quantities for practical use was a world-first discovery by a research team in Melbourne’s School of Chemistry in 1996.

Their answer was to coat the nanoparticles with silica (glass) – which can be different thicknesses. When silica-coated, the gold nanoparticles can be packed together to form (say) a film or powder or homogeneously distributed in polymers. The particles can be close together yet controllably separated.

Given very thick silica shells, such gold nanoparticle films are red. Packed closer and closer together, the particles begin to ‘talk’ to each other – offering an unexpected range of colours. Films can be made with tunable optical properties, from red to crimson to purple, turquoise, cyan or blue, by changing the thickness of the silica shells accordingly. A worldwide patent for this process was granted to the University of Melbourne this year.

Shaping the future

The focus now in nanotechnology research extends to growing structures – such as machines, electrical circuits, and bone. This requires making nanocrystals grow in the direction you want and to do this you must ‘drive’ them.

In 2000 we achieved our first successes in steering gold to form rods or wires on a surface. Interestingly, rods of different lengths have different colours. The shortest rods, like spheres, are red; longer ones become purple and those even longer are blue. The shape controls the colour – and the colour tells you about the shape.

If you zoom in on these rods with a very good electron microscope they are seen to be true single crystals of gold, but for some reason they prefer to grow in one direction. We’re currently investigating why that happens and how to drive the rods to change direction – to (say) grow left. The idea of doing that in a homogeneous solution may sound quite strange, but we have found that under certain conditions we can get growth occurring at an angle – a good starting point. If we can build on that, and understand why it works, we are well on the way to attempting the creation of quite complex nanostructures.

Nanomachines

Once we can build nanostructures, then just around the corner (in the next few years) we will confront the reality of nanomechanics and a new wave of questions. Will an incredibly small piece of metal – which has a very high proportion of its atoms on its surface – have the same strength as the bulk material? How do you pick up a nanorod and bend it to see how strong it is? A fundamental limitation on the design of circuitry, machines, sensors or motors or any nanoscale object is that they may lack robustness – thermal energy alone will be quite disruptive on this length scale, and the material may start to stretch, bend, weaken or even break.

It turns out that if you hit nanorods with a short laser pulse, the heat causes the particles to expand and contract. Since this changes their shape slightly, their colours also fluctuate as they bend or twist. Dr John Sader, in the University’s Department of Mathematics and Statistics, has analysed the mechanical modes that will be excited. He confirms the rods stretch and contract like springs when they absorb light – but at around 1000 million times a second!

This startling behaviour of nanomaterials is counter-intuitive to our normal knowledge about life around us. Things do not usually change colour because they change shape or size. In the nanoworld, however, virtually all metals behave this way. Silver becomes yellow at the nanoscale, sodium red and potassium purple – all sorts of colours can appear. Conversely, we can now use these colours to analyse the behaviour of these exotic materials.

Lighting the way

One of the most spectacular materials in nanocrystal form is cadmium selenide. A semiconductor, cadmium selenide is normally a brown-black colour and it is grown as decimetre-sized single crystals for optoelectronics. School of Chemistry PhD student Craig Bullen is looking at what happens to cadmium selenide at a length scale of one to five nanometres. The effects are quite dramatic. Nanoscale cadmium selenide becomes a tunable fluorophore. It can emit any colour across the visible part of the spectrum. At a size of around seven nanometres it emits deep red but is still brown-looking. At smaller nanosizes it can emit photons ranging across the visible part of the spectrum.

These materials are more photo-stable than any conventional dye molecule and in Craig’s experiments the light output accounts for around 80 per cent of the energy emitted, which puts his results at the international cutting edge – his are perhaps the most luminescent nanocrystals made to date. Their photostability makes them candidates for a range of applications, including tunable lasers and LEDs, as well as in biolabelling where current dyes tend to degrade rapidly under illumination. The University has begun exploring the biolabelling potential of these novel materials with the Ludwig Institute for Cancer Research. The Quantum Dot Corporation, a California biotech start-up company, has sponsored much of Craig’s PhD research into quantum dot chemistry.