Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

New technique creates patterns in photonic crystals formed from hydrogel nanoparticles

11.09.2003


Creating unique structures



Researchers have developed a laser-based technique for creating patterns in self-assembled colloidal crystals produced from hydrogel nanoparticles – soft spheres that respond to heat by changing size. 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 will be 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 to be discussed at the upcoming 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.

"By controlling the beam intensity profile, we can create a wavefront gradient index lens," Lyon explained. "Across the surface of the crystal, you would have a constantly varying refractive index which provides a lensing effect."

The patterning process is reversible, however. Before they are locked into place in the final production step, patterns created in the soft spheres can be erased and redrawn. The technique can also be used as an annealing step to remove defects where they are not wanted.

"We are trying to push the limits of fidelity, trying to make very small line widths so we can get down toward a useful patterning length scale, which is on the order of five microns," Lyon added. "That could be very useful for transmitting optical information through these structures."

Such patterning would not be possible with hard spheres, whose size cannot be changed significantly once they are created.

Beyond developing the patterning technique, Lyon and collaborators Saet Byul Debord, Clinton Jones and Michael Serpe have also studied the fundamental physics governing formation of crystals from the nanoparticles. Because they are soft and conformable, the hydrogels perform in ways that are very different from hard spheres.

"We are changing the phase diagram dramatically," Lyon said. "That opens up new opportunities for different ways to accomplish the self assembly of optical materials because you are no longer limited by simple hard-sphere packing. With these spheres, we can have a material that displays self-assembly properties based on weak attractive forces – and still ends up as a crystal."

For instance, attractive forces between the spheres cause them to deform, moving closer to one another to maximize particle-to-particle contact. That allows formation of crystals from solutions with particle contents as low as 12 percent – well below the 50 percent concentration required to create uniform crystals from hard spheres.

"This was completely unexpected," Lyon added.

Before the hydrogel nanoparticles can be useful in photonic applications, however, the researchers must reduce the size of the spheres smaller and refine the technology to create smaller features.

Lyon and his team have fabricated nearly 100 different types of monodisperse hydrogel nanoparticles, in sizes ranging from 50 nanometers to 10 microns in diameter. The temperature at which the particles transition to a crystalline state can be controlled chemically during the synthesis process in a range from 10 degrees C to 60 degrees C.

After synthesis and precipitation polymerization in aqueous media, the particles are separated from the surrounding water by simple centrifuging. The resulting glassy gelatinous material, which has a faint blue, green or red hue, is more viscous than honey.

To give it desirable optical properties, the material must be annealed by heating it past the volume phase transition temperature of the component hydrogel particles, at which the photonic crystal loses its order and the nanoparticles begin to give up water content. After removing small amounts of water, the material is allowed to cool, re-absorb water and re-crystallize. This thermal cycling process, repeated several times, packs the soft hydrogel particles into an ordered 3-D hexagonal array, which produces the periodic dielectric structure needed for optical applications.

The research has been sponsored by the Office of Naval Research, and by the Arnold and Mabel Beckman Foundation.


###
Technical Contact: Andrew Lyon (404-894-4090); E-mail: (andrew.lyon@chemistry.gatech.edu)


John Toon | EurekAlert!
Further information:
http://gtresearchnews.gatech.edu/

More articles from Materials Sciences:

nachricht Borophene shines alone as 2-D plasmonic material
21.11.2017 | Rice University

nachricht Quantum dots amplify light with electrical pumping
21.11.2017 | DOE/Los Alamos National Laboratory

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Previous evidence of water on mars now identified as grainflows

21.11.2017 | Physics and Astronomy

NASA's James Webb Space Telescope completes final cryogenic testing

21.11.2017 | Physics and Astronomy

New catalyst controls activation of a carbon-hydrogen bond

21.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>