nano world is getting brighter. Nanowires made
of semiconductor materials are being used to make
prototype lasers and light-emitting diodes with
emission apertures roughly 100 nm in diameter--about
50 times narrower than conventional counterparts.
Nanolight sources may have many applications, including "lab on
a chip" devices for identifying chemicals and biological agents, scanning-probe
microscope tips for imaging objects smaller than is currently possible, or ultra-precise
tools for laser surgery and electronics manufacturing.
Researchers at the National Institute of Standards
and Technology (NIST) are growing nanowires made
of gallium nitride alloys and making prototype devices
and nanometrology tools. The wires are grown under
high vacuum by depositing atoms layer by layer on
a silicon crystal. NIST is one of few laboratories
capable of growing such semiconductor nanowires without
using metal catalysts, an approach believed to enhance
luminescence and flexibility in crystal design. The
wires are generally between 30 and 500 nanometers
(nm) in diameter and up to 12 micrometers long. When
excited with a laser or electric current, the wires
emit an intense glow in the ultraviolet or visible
parts of the spectrum, depending on the alloy composition.
A paper in the May 22 issue of Applied Physics Letters*
reports that individual nanowires grown at NIST produce
sufficiently intense light to enable reliable room-temperature
measurements of their important characteristics.
For example, the peak wavelength of light emitted
with electric field parallel to the long axis of
a nanowire is shifted with respect to the peak wavelength
emitted with electric field perpendicular to the
wire. Such differences in emission are used to characterize
the nanowire materials and also may be exploited
to make sensors and other devices.
has grown a variety of nanowires and extensively
characterized their structural and optical properties,
finding few defects, strains or impurities, which
results in high light output compared to the bulk
material.** The wires also can be transferred from
the silicon crystal to other substrates, such as
sapphire, and arranged using electric fields. The
NIST team has used the nanowires to make a number
of prototype devices, including light-emitting
diodes, field-effect transistors, and nanowire "bridge" structures
that may be useful in sensors and nanoscale mechanical
*J.B. Schlager, N.A. Sanford, K.A. Bertness, J.M.
Barker, A. Roshko and P.T. Blanchard. 2006. Polarization-resolved
photoluminescence study of individual GaN nanowires
grown by catalyst-free MBE. Applied Physics Letters.
** K.A. Bertness, N.A. Sanford, J.M. Barker, J.B. Schlager, A. Roshko, A.V.
Davydov and I. Levin. 2006. Catalyst-Free Growth of GaN Nanowires. Journal
of Electronic Materials 35, 576. April.