While refining their novel method for making nanoscale wires,
chemists at the National Institute of Standards and Technology (NIST)
discovered an unexpected bonus—a new way to create nanowires that
produce light similar to that from light-emitting diodes (LEDs). These
“nano-LEDs” may one day have their light-emission abilities put to work
serving miniature devices such as nanogenerators or lab-on-a-chip
systems.
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Graphic illustrates a single row of nanowires (cylinders with red Credit: NIST |
Nanowires typically are “grown” by the controlled deposition of
molecules—zinc oxide, for example—from a gas onto a base material, a
process called chemical vapor deposition (CVD). Most CVD techniques form
nanowires that rise vertically from the surface like brush bristles.
Because the wire only contacts the substrate at one end, it tends not to
share characteristics with the substrate material—a less-than-preferred
trait because the exact composition of the nanowire will then be hard
to define. Vertical growth also produces a dense forest of nanowires,
making it difficult to find and re-position individual wires of superior
quality. To remedy these shortcomings, NIST chemists Babak Nikoobakht
and Andrew Herzing developed a “surface-directed” method for growing
nanowires horizontally across the substrate (see “NIST Demos Industrial-Grade Nanowire Device Fabrication” NIST Tech Beat, Oct. 25, 2007, at http://www.nist.gov/public_affairs/techbeat/tb2007_1025.htm#nanowire).
Like many vertical growth CVD methods, the NIST fabrication technique
uses gold as a catalyst for crystal formation. The difference is that
the gold deposited in the NIST method is heated to 900 degrees Celsius
(1,652 degrees Fahrenheit), converting it to a nanoparticle that serves
as growth site and medium for the crystallization of zinc oxide
molecules. As the zinc oxide nanocrystal grows, it pushes the gold
nanoparticle along the surface of the substrate (in this experiment,
gallium nitride) to form a nanowire that grows horizontally across the
substrate and so exhibits properties strongly influenced by its base
material.
In recent work published in ACS Nano,* Nikoobakht and
Herzing increased the thickness of the gold catalyst nanoparticle from
less than 8 nanometers to approximately 20 nanometers. The change
resulted in nanowires that grew a secondary structure, a shark-like
“dorsal fin” (referred to as a “nanowall”) where the zinc oxide portion
is electron-rich and the gallium nitride portion is electron-poor. The
interface between these two materials—known as a p-n
heterojunction—allows electrons to flow across it when the
nanowire-nanowall combination was charged with electricity. In turn, the
movement of electrons produced light and led the researchers to dub it a
“nano LED.”
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Transmission electron microscope image of “nano LEDs” emitting light. Credit: NIST |
Unlike previous techniques for producing heterojunctions, the NIST
“surface-directed” fabrication method makes it easy to locate individual
heterojunctions on the surface. This feature is especially useful when a
large number of heterojunctions must be grouped in an array so that
they can be electrically charged as a light-emitting unit.
Transmission electron microscope (TEM) examination of the zinc
oxide-gallium nitride nanowires and nanowalls revealed few structural
defects in the nanowires and very distinct p-n heterojunctions in the
nanowalls, both affirmations of the effectiveness of the NIST “surface
directed” fabrication method.
Nikoobakht and Herzing hope to improve the nano LEDs in future
experiments using better geometry and material designs, and then apply
them in the development of light sources and detectors useful in
photonic devices or lab-on-a-chip platforms.
* B. Nikkoobakht and A. Herzing. Formation of planar arrays of
one-dimensional p-n heterojunctions using surface-directed growth of
nanowires and nanowalls. ACS Nano. Published online Sept. 15, 2010.
Media Contact: Michael Newman, mnewman@nist.gov, 301-975-3025
