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Ultrathin Films Retain Bulk Properties

Photonics Spectra
Oct 1999
Robert C. Pini

Polymer films less than 100 nm thick have molecular properties nearly identical to the same material in bulk, according to recent findings of a study published in the July 8 issue of Nature. The findings confirm that thin films for optical coatings retain their physical properties even at only 10 nm thick.

That's great news for the coating industry, which had predicted the results but never achieved lab confirmation. "There's a whole field of thin polymer films based on this assumption that we verified," said Sanat Kumar, professor of materials science and engineering at Pennsylvania State University. The findings can help usher in applications for high precision telecommunications filters and new multilayer films applied to lubricate and protect moving parts.

"The thinner the coating, the greater the precision for any application," said Chris H. Stoessel, a research and development engineer at Optical Coating Laboratory Inc. in Santa Rosa, Calif. "Now you can synthesize properties by making them up out of a stack of ultrathin coatings."

If you can cut the thickness, you can cut the price of optical coatings significantly, said Russ Stiles, an engineer at Optical Coating Laboratory.

What's the limit?

With the help of other materials scientists, Kumar used small-angle neutron scattering to characterize the chain structure and conformation of ultrathin polystyrene films from 10 to 60 nm thick. The researchers found that even in the thinnest films -- thinner than the unperturbed radius of gyration of the polystyrene chains -- the polymer material exhibited chain structure and conformity similar to the bulk form.

Diffusion, glass transition temperature and the dynamic properties of optics are dramatically altered by thinner films, Kumar said, making it easier to know how coatings will react under working conditions.

Since their work appeared in Nature, Kumar said the research team has recorded thin-film characteristics for layers only 5 nm thick, which comes close to the practical limits of measurement using neutron scattering. Pointing to the 0.5-nm-thick lubricant layer on a hard disk drive, Kumar said the research still has room to explore the limits. The next generation of measurement technology will help improve the signal by one order of magnitude.

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