Wires one-millionth of a millimetre wide change composition along their
length.
PHILIP BALL
Wires one-millionth of a
millimetre wide that change
chemical composition along
their length, just as fruit
pastilles change flavour along a
packet, have been grown in the
United States. These
multi-flavoured nanowires can
act as miniature bar-codes,
diodes and light sources.
Conventional microelectronics
components are etched into flat
layers of semiconducting
material. Charles Lieber and
colleagues at Harvard
University in Cambridge,
Massachusetts, grow their
wires - smaller than the
thinnest wire on a commercial
silicon chip - from vapours of
the atomic ingredients.
Lieber's group perfected their
method for making semiconductor nanowires two years ago. They use
a tiny blob of a catalyst, which stays at the growing tip of the wire like
the point of a pencil tracing out a line. The size of the catalyst particle
controls the wire's width.
Now the researchers report that, by choosing their catalyst carefully -
they use gold nanocrystals - they can grow sequential lengths of a
single wire from different chemicals1. They use a laser to blast a
semiconductor into a vapour, which then condenses into nanowires.
Exposing the growing wires first to one kind of vapour and then to
another varies the composition along the wire.
Superlattice lines up
The team has made wires about 20 nanometres across that contain
alternating sections of the semiconductors gallium arsenide and gallium
phosphide. Microelectronic engineers often use structures like this,
called superlattices, in electronic devices. They are currently made by
carving up flat sandwiches of layered semiconductors.
Superlattices are used,
for example, as mirrors
in microscopic lasers,
or as waveguides to
capture and confine
light. If electrons are
trapped in a thin layer
of a semiconductor
sandwiched between
barriers of a different
semiconductor, quantum wells are created that emit light. The colour of
the light can be tuned by varying the well thickness.
Nanowire superlattices could be used in all these applications. Their
size means that many more could be packed onto a single chip than
today's microelectronics components. The researchers envisage
making nanowire lasers, for example.
Up the junction
To demonstrate the wires' potential, Lieber's group made structures
called p-n junctions. They grew silicon nanowires in two sections,
each spiced with a different additive to fine-tune the electrical
behaviour of the silicon. These nanowire p-n junctions behave like
diodes - they let current flow in only one direction.
The team also made p-n junctions that act as light-emitting diodes.
Because these glowing devices are so small, the researchers hope to
make them expel light one photon at a time. This could be useful in a
new type of ultra-powerful information processing called quantum
computing.
References
1.Gudiksen, M. S., Lauhon, L. J., Wang, J., Smith, D. C. & Lieber,
C. M. Growth of nanowire superlattice structures for nanoscale
photonics and electronics. Nature, 415, 617 - 620, (2002).
© Nature News Service / Macmillan Magazines Ltd 2002
07 February 2002
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