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Nanowire electronics: Optical image shows metal
electrodes attached to zinc oxide nanowires using the NIST
technique. Dark spots near the center are the gold pads that start
nanowire growth; red arrow shows direction of growth. Scale bar is
50 micrometers long.
Nanowire electronics: Scanning electron microscope
image shows electrodes connected to group of nanowires. Scale bar
is five micrometers long.
Photos � by NIST
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Despite their name, nanowires are more than
just electrical connectors. Researchers have used nanowires to create
transistors like those used in memory devices and prototype sensors
for gases or biomolecules. However working with objects only tens of
nanometers wide is challenging. A common approach in the lab is to
grow nanowires like blades of grass on a suitable substrate, mow them
off and mix them in a fluid to transfer them to a test surface, using
some method to give them a preferred orientation. When the carrier
fluid dries, the nanowires are left behind like tumbled jackstraws.
Using scanning probe microscopy or similar tools, researchers hunt
around for a convenient, isolated nanowire to work on, or place
electrical contacts without knowing the exact positions of the
nanowires. It�s not a technique suitable for mass production.
Building on earlier work to grow nanowires horizontally on the surface
of wafers (see �Gold Nano Anchors Put Nanowires in Their Place�), NIST researchers used conventional semiconductor manufacturing techniques
to deposit small amounts of gold in precise locations on a sapphire
wafer. In a high-temperature process, the gold deposits bead up into
nanodroplets that act as nucleation points for crystals of zinc oxide,
a semiconductor. A slight mismatch in the crystal structures of zinc
oxide and sapphire induces the semiconductor to grow as a narrow
nanowire in one particular direction across the wafer. Because the
starting points and the growth direction are both well known, it is
relatively straightforward to add electrical contacts and other
features with additional lithography steps.
As proof of concept, the NIST researchers have used this procedure to
create more than 600 nanowire-based transistors, a circuit element
commonly used in digital memory chips, in a single process. In the
prototype process, they report, the nanowires typical grew in small
bunches of up to eight wires at a time, but finer control over the
size of the initial gold deposits should make it possible to select
the number of wires in each position. The technique, they say, should
allow industrial-scale production of nanowire-based devices.
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