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Scanning electron microscope images show nanosaws, nanobelts and nanowires of
cadmium selenide grown using the vapor-liquid-solid process.
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The results, reported this month in the journal
Advanced Materials (Vol. 17, pp.1-6), join earlier Georgia Tech work
that similarly mapped production conditions for nanostructures made
from zinc oxide – an increasingly important nanotechnology material.
Together, the two studies provide a foundation for large-scale,
controlled synthesis of nanostructures that could play important roles
in future sensors, displays and other nanoelectronic devices.
The research was supported by the National Science
Foundation (NSF), the NASA Vehicle Systems Program, the Department of
Defense Research and Engineering (DDR&E) and the Defense Advanced
Research Projects Agency (DARPA).
"For the future of nanomanufacturing, we needed a
systematic map to show the best conditions for producing these
structures reproducibly with high yield," explained Zhong Lin Wang,
director of Georgia Tech's Center for Nanoscience and Nanotechnology
and a Regent's professor in Georgia Tech's School of Materials Science
and Engineering. "This information will be necessary for scaling up
the production of these interesting structures for the applications
that will be developed."
In work that required more than a year to complete,
Wang and collaborator Christopher Ma collected information on more
than 45 separate combinations of growth conditions governing the
production of cadmium selenide nanostructures. In their experimental
set-up, powdered cadmium selenide was heated to hundreds of degrees
Celsius in a simple horizontal tube furnace under the flow of nitrogen
gas, using gold as a catalyst.
The technique produced three different types of
nanostructures:
"Nanosaws/nanocombs," unusual structures that form
with "teeth" on one side and a smooth surface on the other;
"Nanobelts," which are ribbon-like structures, and
"Nanowires" that resemble grass and grow vertically
from the substrate.
The researchers varied the temperature at the
cadmium selenide source, the temperature of the silicon substrate
where the structures grew, and the gas pressure inside the furnace.
They repeated each experimental condition three times, each time
determining where the structures grew on the substrate and counting
the number of nanosaws/nanocombs, nanobelts and nanowires in samples
that were examined with electron microscopy.
"These three different structures are all produced
using the same general experimental conditions, but somehow you get
different percentages of each," Wang said. "Our goal was to determine
how to control the conditions to learn how to get close to 100 percent
yield of each structure. This required a systematic study of the
experimental conditions."
Each experiment required approximately two days to
produce the structures and analyze the samples.
Based on their experimental work, Wang and Ma
mapped the optimal conditions for producing each of the three
structures – and learned more about the fabrication process. For
instance, they found that growth of the nanostructures is primarily
controlled by the nitrogen gas pressure inside the chamber and the
temperature of the substrate where the structures are deposited. They
also learned where each type of structure was likely to be deposited
on the substrate under each set of conditions.
Cadmium selenide nanosaws and nanocombs are the
most finicky to grow. At the other end of the scale, nanowires can be
produced from cadmium selenide at a broad range of temperature and
pressure conditions. Specifically, the researchers reported:
Lower temperatures at the source material (630
degrees C), higher pressures (600 millibars) and substrate
temperatures of approximately 575 degrees C produce the highest
percentage of nanosaws and nanocombs.
Lower temperatures at the source material (700
degrees C), lower chamber pressures (4 millibars) and substrate
temperatures of approximately 575 degrees C produce the highest
percentage of nanobelts.
Growth of nanowires can be carried out at a broad
range of temperatures and pressures, with higher source temperatures
favoring the growth of nanowires over nanosaws.
"If other groups want to produce these structures,
they can use our plots to determine the pressures that will be
required, the temperatures and the locations within the chamber where
they will grow," Wang said. "Until now, researchers have had to
determine these parameters by trial and error."
Cadmium selenide has been studied for applications
in optoelectronics, luminescent materials, lasing materials and
biomedical imaging. It is perhaps best known as the basis for quantum
dots that have potential applications in biomedical imaging.
Zinc oxide is a semiconducting, piezoelectric and
optical material with applications in sensors, resonators and other
nanoelectronic structures. The systematic study of growth parameters
for these structures involved more than 100 experiments and was
published in the Journal of Physical Chemistry (B, Vol. 109 (2005)
9869-9872).
"Now that we have determined the optimal
requirements for growth, it should be straightforward to scale up the
production of these structures," Wang concluded. "We have a lot of
ideas for potential applications."
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