Researchers have taken an important step toward high-volume production of new
nanometer-scale structures with the first systematic study of growth conditions
that affect production of one-dimensional nanostructures from the optoelectronic
material cadmium selenide (CdSe).
the results from more than 150 different experiments
in which temperature and pressure conditions were
systematically varied, nanotechnology researchers
at the Georgia Institute of Technology created a “road
map” to guide future nanomanufacturing using the
vapor-liquid-solid (VLS) technique.
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. “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 hundred of degrees Celsius in
a simple horizontal tube furnace under the flow of nitrogen gas, using gold as
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
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
• 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
“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 applications in biomedical imaging.
Zinc oxide is a semiconducting, piezoelectric and optical material with potential
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.”
Source: Georgia Institute of Technology