A team of scientists at the U.S.
Department of Energy's (DOE) Argonne National Laboratory and the Carnegie
Institution of Washington has succeeded in "watching" nanoparticles
grow in real time.
The revolutionary technique allows researchers to learn about the early stages
of nanoparticle generation, long a mystery due to inadequate probing methods,
and could lead to improved performance of the nanomaterials in applications
including solar cells, sensing and more.
These silver nanoplates are decorated with silver oxy salt nanoparticles along the edges. These nanostructures were grown under irradiation of high-energy x-rays, which allowed scientists to "watch" them grow in real time. The image is from a scanning electron microscope.
"Nanocrystal growth is the foundation of nanotechnology," said lead
researcher Yugang Sun, an Argonne chemist. "Understanding it will allow
scientists to more precisely tailor new and fascinating nanoparticle properties."
The way that nanoparticles look and behave depends on their architecture: size,
shape, texture and surface chemistry. This, in turn, depends very much on the
conditions under which they are grown.
"Accurately controlling nanoparticles is very difficult," Sun explained.
"It's even harder to reproduce the same nanoparticles from batch to batch,
because we still don't know all the conditions for the recipe. Temperature,
pressure, humidity, impurities—they all affect growth, and we keep discovering
more factors."
In order to understand how nanoparticles grow, the scientists needed to actually
watch them in the act. The problem was that electron microscopy, the usual method
for seeing down into the atomic level of nanoparticles, requires a vacuum. But
many kinds of nanocrystals have to grow in a liquid medium—and the vacuum
in an electron microscope makes this impossible. A special thin cell allows
a tiny amount of liquid to be analyzed in an electron microscope, but it still
limited the researchers to a liquid layer just 100 nanometers thick, which is
significantly different from the real conditions for nanoparticle synthesis.
To solve this conundrum, Sun found he needed to use the very high-energy X-rays
provided at Sector 1 of Argonne’s Advanced Photon Source (APS), which
adjoins the laboratory’s Center for Nanoscale Materials, where he works.
The pattern of X-rays scattered by the sample allowed the researchers to reconstruct
the earliest stages of nanocrystals second-by-second.
"This technique yields a treasure trove of information, especially on
the nucleation and growth steps of the crystals, that we had never been able
to get before," said Sun.
The intensity of the X-rays does affect the growth of the nanocrystals, Sun
said, but the effects only became significant after an especially long reaction
time. "Getting a clear image of the growth process will allow us to control
samples to get better results, and eventually, new nanomaterials that will have
a wide range of applications,” Sun explained.
The nanomaterials could be used in photovoltaic solar cells, chemical and biological
sensors and even imaging. For example, noble metal nanoplates can absorb near-infrared
light, so they can be used to enhance contrast in images. In one possible case,
an injection of specially tailored nanoparticles near a cancer patient's tumor
site could increase the imaging contrast between normal and cancerous cells
so that doctors can accurately map the tumor.
"The key to this breakthrough was the unique ability for us to work with
scientists from the Advanced Photon Source, the Center for Nanoscale Materials
and the Electron Microscopy Center—all in one place," Sun said.
Funding for the research was provided by the U.S. Department of Energy's Office
of Science. The article, “Nanophase Evolution at Semiconductor/Electrolyte
Interface in Situ Probed by Time-Resolved High-Energy Synchrotron X-ray Diffraction”,
was published in NanoLetters.
The Center for Nanoscale Materialsat Argonne National Laboratory is one of
the five DOE Nanoscale Science Research Centers (NSRCs), premier national user
facilities for interdisciplinary research at the nanoscale, supported by the
DOE Office of Science. Together the NSRCs comprise a suite of complementary
facilities that provide researchers with state-of-the-art capabilities to fabricate,
process, characterize and model nanoscale materials, and constitute the largest
infrastructure investment of the National Nanotechnology Initiative. The NSRCs
are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge
and Sandia and Los Alamos national laboratories. For more information about
the DOE NSRCs, please visit http://nano.energy.gov.
Argonne National Laboratory seeks solutions to pressing national problems in
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leading-edge basic and applied scientific research in virtually every scientific
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of Energy's Office of Science.