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Scanning electron microscope image shows rows
of horizontal zinc-oxide nanowires grown on a sapphire surface. The gold
nanoparticles are visible on the ends of each row.
Illustration shows how crystalline zinc oxide
nanowires (blue) push "seeds" of gold nanoparticles (red) forward as they
grow.
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The method uses nanoparticles of gold arranged in rows on a
sapphire surface as starting points for growing horizontal semiconductor "wires"
only 3 nanometers (nm) in diameter. Other methods produce semiconductor
nanowires more than 10 nm in diameter. NIST chemists' work was highlighted in
the Oct. 11 issue of Applied Physics Letters. Part of the vision of nanotechnology is the possibility of
building powerful, extraordinarily compact sensors and other devices out of
atomic-scale components. So-called "nanowires" - long thin crystals of, e.g., a
semiconductor - could not only link nanoelectronic devices like conventional
wire but also function as devices themselves, tipped with photodetector or
light-emitting elements, for example.
An obvious stumbling block is the problem of working with
components so small that only the most sophisticated measurement instruments can
even track them. To date, the most successful nanowire alignment method involved
growing large numbers of the rod-like crystals on a suitable base like blades of
grass, shearing them off, mixing them in a solvent, and forcing them to align by
either flow or surface confinement on the test substrate to orient most of the
crystals in a specific horizontal direction. Further photolithography steps are
required to ensure that nanowires are positioned correctly.
In contrast, the NIST technique grows arrays of nanowires
made of zinc oxide, a semiconductor widely used in optoelectronics, with precise
alignments. The gold "anchors" are placed with a chemical etching step and the
orientation of the wires - horizontal, vertical or at a 60 degree angle from the
surface - is determined by tweaking the size of the gold particles. |
