Accessibility
of electron microscopes could make technique standard practice
“The next generation of high-performance
materials that scientists are studying for applications like batteries, fuel
cells, drug delivery, and photovoltaics are highly complex. We are trying to
engineer them at the nanoscale to give them particular properties to improve
their performance. A huge experimental challenge is to characterize
experimentally these heterogenous, nanostructured, complex materials —
including determining where the atoms are located, their dynamics, and how they
interact with outside stimuli such as photons of light. If people want lighter
laptops with more computing power and longer battery life to take with them in
their emission-free cars that go 400 miles without a fill-up/recharge — and
which pull away from the stoplight like a Ferrari — then we scientists are
going to have to solve these problems! Our research is a tiny (but important)
step in that direction.”
— Simon Billinge
UPTON, NY — With dimensions measuring
billionths of a meter, nanoparticles are way too small to see with the naked
eye. Yet it is becoming possible for today’s scientists not only to see them,
but also to look inside at how the atoms are arranged in three dimensions using
a technique called nanocrystallography. Trouble is, the powerful machines that
make this possible, such as x-ray synchrotrons, are only available at a handful
of facilities around the world. The U.S. Department of Energy’s Brookhaven
National Laboratory is one of them — home to the National Synchrotron Light
Source (NSLS) and future NSLS-II, where scientists are using very bright,
intense x-ray beams to explore the small-scale structure of new materials for
energy applications, medicine, and more.
But a Brookhaven/Columbia Engineering
School team of scientists, in collaboration with researchers at DOE’s Argonne
National Laboratory (ANL) and Northwestern University, has also been working to
develop nanocrystallography techniques that can be used in more ordinary
science settings. They have shown how a powerful method called atomic pair
distribution function (PDF) analysis — which normally requires synchrotron
x-rays or neutrons to discern the atomic arrangements in nanoparticles — can be
carried out using a transmission electron microscope (TEM) — an instrument
found in many chemistry and materials science laboratories.
The researchers describe the TEM-based
data-collection technique and computer-modeling analyses used to extract
quantitative nanostructural information in a paper just published in the May
2012 issue of the journal Zeitschrift fur Kristallographie.
“The ability to collect PDF data using
an electron microscope places this powerful nanocrystallographic analysis
method into the hands of scientists who need it most — the people synthesizing
novel nanoparticles and nanostructures,” said Simon Billinge, a researcher at
both Brookhaven and Columbia University’s School of Engineering and Applied
Science and a long-term user of the NSLS, who led the research.
“State-of-the-art
experiments will still be carried out at x-ray synchrotrons and high-tech neutron-scattering
facilities,” said Billinge, a professor of Materials Science and Applied
Physics and Applied Mathematics at Columbia Engineering. “But this new
development removes significant barriers to more widespread use of the method,
potentially making PDF part of the standard toolkit in materials synthesis
labs. It’s rather like moving nanocrystallography from being available only
with a prescription to being available over the counter,” he said.
In both the synchrotron and TEM-based
methods, the essential technique is the same: bombard a sample with a beam —
x-rays, in the case of a synchrotron, or electrons at a TEM — and measure how
the rays/particles interact with and bounce off the atoms in the sample. The
result is a diffraction pattern that can be translated into measurements of the
distribution of distances between pairs of particles within a given volume —
the atomic pair distribution function (PDF). Scientists then use computational
programs to convert the PDFs into 3-D models of atomic structure.
Electron diffraction had been used to
study the structure of molecules in the gas phase and amorphous thin films, but
initially, scientists didn’t think that electrons would be appropriate for
obtaining reliable PDFs from critical nanocrystalline materials because, unlike
x-ray photons, electrons scatter strongly, distorting the diffraction pattern.
This new work demonstrates that, under the right circumstances and with the
correct data processing, quantitatively reliable PDFs of small nanoparticles —
precisely the ones that are difficult to characterize using standard methods —
can be obtained with the TEM.
Another advantage is that the technique
allows analysis of atomic-level structural arrangements using the same tool
already used to obtain low- and high-resolution images and chemical information
for nanostructures — that is, the same TEM can be used to provide complementary
kinds of information.
“The fact that the real-space images and
the diffraction data suitable for structural analysis can be obtained at the
same time from the same region of a material results in more complete
information for the characterization of the sample,” said Milinda Abeykoon, a
postdoctoral researcher at Brookhaven and the first author of the paper.
In the current study, scientists working
with co-author Mercouri Kanatzidis at Northwestern University and ANL
synthesized nanocrystalline thin films and gold and sodium chloride (NaCl)
nanoparticles and used a TEM at Northwestern to acquire PDFs of these samples.
The Brookhaven/Columbia group studied similar samples using synchrotron x-rays
at NSLS, and analyzed all the data before comparing the resulting PDFs and
atomic structures.
The PDFs from the x-ray and electron
data were highly similar.
“In some cases the strong electron
scattering did introduce some distortions in the PDF, as originally feared,”
Billinge said. “However, surprisingly these problems only affected certain less
important structural parameters — and even resulted in an enhancement of the
signal in a way that may be used in the future to yield a higher resolution
measurement. That was an unexpected gift!”
The research team is continuing to look
for ways to remove barriers to data processing to make the method more
straightforward — and move it from proof-of-principle concept into widespread
standard use.
This research was funded by DOE’s Office
of Science and by the National Science Foundation. The National Synchrotron
Light Source at Brookhaven is also supported by the DOE Office of Science.
DOE’s Office of Science is the single
largest supporter of basic research in the physical sciences in the United
States, and is working to address some of the most pressing challenges of our
time. For more information, please visit science.energy.gov.
Columbia Engineering
Columbia University's Fu Foundation
School of Engineering and Applied Science, founded in 1864, offers programs in
nine departments to both undergraduate and graduate students. With facilities
specifically designed and equipped to meet the laboratory and research needs of
faculty and students, Columbia Engineering is home to NSF-NIH funded centers in
genomic science, molecular nanostructures, materials science, and energy, as
well as one of the world’s leading programs in financial engineering. These
interdisciplinary centers are leading the way in their respective fields while
individual groups of engineers and scientists collaborate to solve some of
modern society’s more difficult challenges.
http://www.engineering.columbia.edu/
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