Technique Creates Patterns in Unique Crystals Formed from Hydrogel Nanoparticles

The development could make possible the fabrication of waveguides, three-dimensional microlenses and other photonic structures from the unusual crystals.

In related work, the Georgia Institute of Technology researchers have also learned to use weak attractive forces between the soft spheres to produce uniform crystalline structures with particle concentrations much lower than possible with hard spheres. The developments were described September 10th at the 226th national meeting of the American Chemical Society in New York.

In April 2002, a research team led by Andrew Lyon, a professor in Georgia Tech's School of Chemistry and Biochemistry, announced it had developed a family of hydrogel-based nanoparticles that could be used to create photonic crystals whose optical properties could be tuned by thermally adjusting the water content of the particles.

The soft, conformable spherical particles provided a unique system for producing self-assembled periodic structures that could be tuned to transmit specific wavelengths of light. Applications were expected in optical switching and optical limiting.

The work discussed at the ACS meeting moves the nanoparticles closer to practical application by providing a way to form complex patterns in the crystalline structures. The patterns could be useful as optical waveguides or lenses.

"This represents a fundamentally new method for patterning self-assembled photonic materials," Lyon said. "By combining a photo-patterning method with a self-assembly technique, we can rapidly make large volumes of very nice optical materials. This provides the best of both worlds - a good optical material that is easy to prepare, combined with a process that allows us to tell the material what kind of overall structure it should have."

Lyon's group creates the pattern with a frequency-doubled Nd:YAG laser whose beam applies specific amounts of heat to the poly-N-isopropylacrylamide nanoparticles, which average 224 nanometer in diameter. To produce the smallest possible features, the researchers include tiny gold nanoparticles with the hydrogels; the gold converts the laser light to heat, allowing precise thermal control.

The heat prompts phase transitions, causing the particles to shrink or swell depending on the temperature. That changes the crystalline structure.

"The gold particles allow us to use a very narrowly-focused laser beam to locally heat the material," Lyon said. "We can have a very sharp temperature gradient between the center of the laser spot and the surroundings. Everything outside of the laser spot experiences mostly ambient conditions and stays crystallized. Everything inside the laser spot goes through a melting phase. Then, the effective cooling rate is very rapid as the laser moves away, trapping the material as an optically transparent, non-diffractive glassy material."

The patterning could be used to create optical waveguides or microlenses.