August 20, 2004  


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. 

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The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Wednesday August 18, 2004 by Emily Ball