Nanoscience involves working with objects on a very very small scale. How small? Well, have a look at the back of your hand. Using just your eyes you can focus down to a scale of 1 centimetre to 1 millimetre. At this scale the skin looks flat. However, get out a magnifying glass and you can see it's actually wrinkly with cracks and folds. The magnifying glass allows you to study the fine structure of the skin at less than a millimetre (or one-thousandth of a metre). If you were to look closer with a microscope you could examine the cells that make up your skin. Now you're working at the scale of micrometres (one-thousandth of a millimetre), sometimes referred to as the microworld. Cells and bacteria are measured in micrometres, and electronic components on a silicon chip are usually around 1 micrometre in size.
To reach the nanoworld you have to go smaller again. A nanometre (nm) is one-thousandth of a micrometre (or one-billionth of a metre). This is the scale at which we measure atoms and the molecules they make. For example, ten hydrogen atoms laid side by side measure a nanometre across and a pin head is around a million nanometres wide. The 'machines' inside our cells and the molecular constructions they put together are measured in nanometres. Working at the nanoscale The prospect of changing the big world by working in the nanoworld was first proposed by Richard Feynman back in 1959 (Box 1: Room at the bottom). But it's only been in the last decade that science and technology has given scientists sufficient mastery to enable them to start working directly in this strange world. While it might be right under our noses, the world of the ultra small, in practical terms, is a distant place. It exists beyond any of our senses. We can't see or touch it. Light microscopes can't provide images of anything smaller than the wavelength of visible light (ie, nothing smaller than 380 nanometres) so they don't help much. Physical and chemical properties change
Recent developments in technology are only now allowing scientists to understand what is happening down at this level. While we can't literally see the atomic landscapes of molecules, we can model their composition and structure by using a variety of techniques such as X-ray crystallography, nuclear magnetic resonance spectroscopy and scanning probe microscopy. Part of the challenge of operating in the nanoworld is that things behave differently when you go ultra small. Consider a lump of gold, yellowy gold in colour. If you were to break that lump into nanosize chunks, the gold changes colour depending on the size of the chunks. In the 10 to 100 nanometre range it can appear reddish. Indeed, by breaking down a 'bulk' material into nanosized particles you can often change many of its properties. By controlling the manner in which nanometre-scale molecular structures are formed, it is possible to control the fundamental properties of the materials these molecules build: properties such as colour, electrical conductivity, melting temperature, hardness, crack-resistance and strength.
This is quite amazing when you think we are not changing the chemical composition of the substance. We're not adding a red pigment to the gold, just breaking it into smaller pieces. The physical and chemical properties change because we're opening up and exposing more surface area of the material. For example, if you break a cube of ice in half you increase its surface area without adding any extra volume of ice. When particle sizes are reduced to the nanoscale, the ratio of surface area to volume increases dramatically. Since many important chemical reactions – including those involving catalysts – occur at surfaces, it is not too surprising that very small particles are staggeringly reactive. This is one of the reasons that chemists are very excited about nanoscience – if they can make more surface area, they get more catalytic action, giving the potential to speed up almost all physical and manufacturing processes. Nanoscale construction Constructing things on the nanoscale requires new ways of operating – ways that make a complete break from traditional construction techniques. One good example of this is the move away from traditional 'top-down' manufacturing processes in favour of 'bottom-up' approaches. This is best explained by considering work on nano-electronics. Constructing electronic circuits from the top down The miniaturisation of electronic circuits is a testament to the power of science and technology in the twentieth century. The best computer chips around these days can pack in around 40 million transistors, each measuring around a micrometre (one-thousandth of a millimetre) or less. This is electronics on a microscale, and it is done using what is called a top-down approach. It involves taking a large chunk of material, usually silicon, and slicing it up into many wafers. Patterns (circuits consisting of many transistors) are drawn on light-sensitive films on the surface of each wafer, using light, and then unwanted material between the circuits is etched away with acid. This approach is called top down because you start with a large piece of material and whittle, cut and etch it down to the product you want. Most traditional manufacturing processes follow a top-down process. When you start getting down to the nanoscale with electronics, the traditional top-down techniques using light-generated patterns aren't precise enough to draw and cut the circuits (for the same reasons you can't see the nanoworld – visible light is simply too 'big' to work with, with wavelengths ranging upwards from 380 nanometres). Although it is possible to pattern the circuits at the nanoscale without using visible light (eg, using X-rays or electron beams with much smaller wavelengths than visible light), there is a completely new way of building circuits. Constructing electronic circuits from the bottom up A possible way forward for nanocircuits was put forward by IBM scientists more than 25 years ago. They proposed that by tailoring the atomic structure of organic molecules it should be possible to create a molecular-sized transistor. However, it has only been in the last few years that a variety of nanoscale electronic components made from molecules have been designed and created by researchers. They are now able to build transistors and other electronic components from organic molecules, carbon nanotubes and semiconductor nanowires. But building individual molecular components is only part of the problem. The next challenge is how to assemble these nano-components into integrated devices. This is a completely different way of building circuits. Instead of whittling down a big block of silicon, you're building from the ground up: creating molecules on a surface and then allowing them to assemble into larger structures. This approach is referred to as the bottom-up approach and it's one of the characteristics of a lot of the new nanoscience. Manipulating atoms
Scientists are now attempting to manipulate individual atoms and molecules. (To get an idea of how difficult this is, try building something from LEGO blocks while wearing oven mitts.) But techniques for manipulating atoms are being refined and building with individual atoms is becoming easier. Eventually it could be possible to precisely construct any object using individual atoms and molecules (Box 2: Nanomanipulation). Building by modules (just like living cells do)
Researchers already have some capacity to control many aspects of the nanoworld. However, many of the structures being built are in two dimensions and are relatively simple compared to some of nature's handiwork. Living cells, for example, are nanoscale devices that are constantly assembling complex structures one or two molecules at a time (eg, DNA synthesis). Increasingly scientists are looking to see how the machinery found within living cells manufactures natural molecules and structures. An Australian company has combined several of nature's devices to create a biosensor that enables doctors to obtain results from blood samples in less than five minutes and diagnose patients more quickly (Box 3: In nature's footsteps). Applications of nanoscience The biosensor is not the only product to come from Australian nanoscience. An Australian company has used knowledge about the altered reflective ability of nanoparticles to produce a sunscreen that is effective in filtering UV radiation, yet does not leave a coloured film on the skin. Australian scientists have also been involved in the construction of nanosize capsules that could be used as an effective drug delivery system. Dreams or nightmares?
As investment in nanoscience and nanotechnology continues, some people are voicing ethical, environmental and economic concerns. Media headlines have warned of the 'grey goo' (millions of self-replicating nanomachines) that could engulf the world, and Michael Crichton based his 2002 novel, Prey, on this concept. While this science fiction scenario is far-fetched, there are valid concerns, such as the effect of nanoparticles on health and the environment. But most scientists believe that nanoscience will lead to huge advances in medicine, biotechnology, manufacturing, information technology and other equally diverse areas. Boxes |
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Posted September 2003.
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