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Nanotech: The shape of things to come
27 February 2008
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Mason Inman
IN THE late 1980s, the book Engines of Creation by Eric Drexler caused quite a stir. It described a coming revolution in
nanotechnology - the design of machines and structures on the scale of billionths of a metre. Drexler predicted that within a few
decades we would have swarms of molecular devices that could build virtually anything. He also warned that self-replicating machines
might get out of control, overrunning the world with an all-consuming "grey goo".
It was a compelling vision. Now, 20 years on, are we any closer to it? The clear answer is yes. Well, maybe not the grey goo bit,
and you can safely ignore the scores of products - from wound dressings to anti-graffiti paint - that claim to contain
nanoparticles. Yet away from the hype, researchers have made serious progress in the physics, chemistry and engineering of nanoscale
devices that could eventually power our gadgets, do our computing and monitor the air we breathe, all in super-fast and efficient
ways.
For decades, researchers have known that in the nano-world it is not just size that matters but, crucially, shape. The shapes of
nanostructures dictate their basic properties: how they conduct electricity and heat, how they absorb light, how durable they are,
and how they behave with other structures.
The recent advance has been in getting various shapes of nanostructures to interact properly and reliably. "People have learned how
to make nanodevices with pretty remarkable reproducibility and control," says James Heath, a chemist and nanotech specialist at the
California Institute of Technology in Pasadena. "You can really begin contemplating things that are arbitrarily complex."
So although nanotech has yet to reach its stunning potential, it is time to take stock of what is emerging from the nano-world...
NanotubesIt was the early 1990s when nanotubes started to attract widespread attention. These rigatoni-shaped structures are harder
to bend or break than steel and just a fraction of its weight; they buckle when folded, but can then snap back to their original
shape. What's more, they can be made to conduct electricity or stymie it like a semiconductor: it all depends on the bonds between
their carbon atoms. This versatility makes them ideal for computers and other electronics.
Nanotubes could be used, for instance, as the basis for ultra-dense and robust storage systems that could replace the flash memory
in cameras and the hard drives in PCs and mobile devices. In the last year, Jeong Won Kang and Qing Jiang of the University of
California, Riverside, have designed one such system in simulations (Nanotechnology, vol 18, p 095705). By packing one nanotube
inside another larger one and then slapping electrodes on both ends, they propose to store digital information more simply than
other nanotech methods. Each memory element (see above) would work by sliding the inner nanotube into different configurations to
represent a 1 or 0.
NanowiresWhen shrunk to just a few nanometres in width, solid rods of silicon and other semiconductors take on astounding
properties. Because of their ultra-skinny shape, nanowires are in effect 1-dimensional. The result is that electrons can only flow
along the wire rather than across it, making them easy to control. A nanowire behaves a bit like a water hose: step on it, and you
block the flow. Current flowing through an ordinary wire is more like water in a river: try to block it with your foot, and the
water simply flows around it.
Nanowires make extremely fine sensors, able to detect noxious chemicals down to a few parts per billion; a single molecule binding
to a nanowire will disturb the flow of electrons inside (see above). Nanowires are also highly efficient at converting waste heat to
electricity, or acting as mini-coolers (Nature, vol 451, p 163). Researchers have recently grown nanowires and transferred them onto
flexible plastic chips (Nature Materials, vol 6, p 379). This might enable them to work as cheap, implantable biosensors or energy
sources.
BuckyballsNanotech is improving our chances of harnessing alternative forms of energy. Take organic polymer-based photovolaic cells,
which researchers have pursued for more than 15 years. These promise to be a much cheaper way of turning solar energy into
electricity than traditional solar cells made from crystalline silicon. The problem is that polymers do so much less efficiently.
A well-known nanostructure has come to the rescue: the round, football-like carbon molecule known as a buckyball, or fullerene. When
photons hit an organic solar cell, they knock electrons loose from the polymer, but these electrons tend to quickly recombine into
the mix. If you add buckyballs to the polymer, however, they grab the free electrons and relay them in the form of usable
electricity (see below).
Chemist Alan Heeger and colleagues at the University of California, Santa Barbara - the group that pioneered this kind of solar cell
- have taken a big step by successfully layering one type of composite material on top of another, so as to absorb a wider range of
the light spectrum. Their device converts 6.5 per cent of the absorbed light energy into electricity, which they claim is a new
record for polymer cells (Science, vol 317, p 222).
NanoparticlesThe future of nanotech lies in coaxing molecules to assemble themselves into arbitrary shapes on demand. This could
lead to all manner of molecular-scale devices being built automatically. Researchers from Northwestern University in Illinois and
Brookhaven National Laboratory in New York have developed one of the most advanced approaches to date by controlling how
nanoparticles bind to one another.
By equipping gold nanoparticles with tentacle-like strands of programmed DNA, they have shown for the first time that nanoparticles
can link themselves up with their neighbours to form intricate and ordered 3D structures (Nature, vol 451, p 549 and p 553). Such a
nano-assembler can create fundamental arrangements of molecules such as crystal lattices (left) that could be used for catalysing
chemical reactions, manipulating light or fighting disease within the body, for example. The researchers are already working on
assembling more complex structures.
Nanotechnology - Follow the emergence of a new technology in our continuously updated special report.
Mason Inman is a writer based in Massachusetts

From issue 2645 of New Scientist magazine, 27 February 2008, page 42
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