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Disorderly Conduct: The Unusual
Behavior of Nanomaterials
By Heather Rock Woods
Extremely small pieces of a material
aren’t always a chip off a bigger block. How nanomaterials behave is
tremendously important to know when trying to understand the roles of
mineral nanoparticles in the environment, or design devices for
nanotechnology.
Researchers taking data at SSRL and the
Advanced Photon Source (APS) in Illinois recently found that zinc
sulfide at 3.5 nanometers (nm) in size (3.5 billionths of a meter)
behaves quite differently than ‘bulk’ zinc sulfide (several hundred nm
and up. The method they developed should also prove useful for studying
other kinds of nanomaterials.
“Zinc sulfide is one of a class of
materials potentially very useful as a semiconductor; it can also be
found in the environment as nanoparticles,” said Ben Gilbert, a postdoc
working with Jill Banfield at UC Berkeley. The group from Berkeley and
LBNL published their results in the July 1 issue of Science.
Their work shows that structural disorder
may be common in nanomaterials and that this can modify the material’s
properties, which are important for designing nanotechnology.
The study uses two types of synchrotron
x-ray information: small-angle x-ray scattering (SAXS) at SSRL to get
size and shape information, and wide-angle x-ray scattering (WAXS) at
APS for structure information. Combining the two sets of data allowed
researchers to quantify the difference in behavior between 3.5 nm and
bulk zinc sulfide.
“When you play with things at small
sizes, one question is: how small is small? Things start to change
around 5 to 10 nm,” Gilbert said.
Some materials have been shown to have
significant changes in structure at nanosizes. Gilbert said that zinc
sulfide showed some subtle modifications that nevertheless cause a
strong effect on the materials properties.
The structural changes the researchers saw
did not follow a simple pattern. The nanomaterial’s structure did not
change completely, but it did exhibit a good deal of disorder and
distortion. The length of bonds between atoms had contracted a little.
In addition, the average atomic positions, which define a material’s
crystal structure, had shifted, so the atoms were in different locations
than expected.
An important consequence of these changes
is that the atoms were vibrating faster than in the bulk material. Thus,
the zinc sulfide nanoparticles they studied are stiffer than bulk
version.
It’s difficult to understand what drove
the complex change, but it appears to result from surface effects. Atoms
on the surface of nanomaterials aren’t ‘happy’ because they don’t have
enough neighbors to bond to. This surface ‘unhappiness’ occurs at the
surfaces of all materials, but when the material is very small, the
surface forces can be relatively very large and can lead to large
internal distortions.
“To my knowledge, there are not yet any
general rules about what happens to materials at small sizes. The method
we used could be a good way to look at this,” Gilbert said.
To learn more about this work, attend the
31st Annual SSRL User’s Meeting on Oct. 21-22.
For further information, see:
http://www-ssrl.slac.stanford.edu/research/highlights_archive/nano.html.
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