September 30, 2005

Imagine a future that includes buses changing color depending on the outside temperature, light bulbs giving off light but little or no heat, and microelectronics where the frustrating problem of heat emission has been greatly reduced.

Such a prospect would likely involve 3-D ceramic composites, photonic crystals, and self-assembly of many of the types of multifunctional materials needed for a future boasting heat and energy efficient light bulbs, buses, and microelectronics, among other applications.

A three-year project by a team of researchers from the Beckman Institute at the University of Illinois at Urbana-Champaign, the University of New Mexico, and Stanford University has been investigating self-assembly, as well as directed assembly, of ceramic and composite structures that manipulate light to control the thermal and color properties of multifunctional materials. The group showed that self-assembled colloidal crystals could create photonic band gaps for forming waveguides useful in nanoscale manufacturing. Paul Braun of Beckman’s Advanced Chemical Systems group is one of the principal investigators of the project, which received a Multidisciplinary University Research Initiative (MURI) grant from the Army Research Office.

“The specific accomplishment that I think really sets this MURI apart is we’ve demonstrated that you can use self-assembly to create very robust three-dimensional structures that have interesting optical properties and interesting chemical properties,” Braun said. “Specifically, we’ve designed a number of new sensors, we’ve demonstrated new approaches for the manipulation of light through three-dimensional waveguides, and we have work going on that demonstrates you can build in a layer-by-layer sequence, very complex three-dimensional structures.”

Braun, along with Beckman faculty Jennifer Lewis and Yi Lu, were part of the first group to report 3-D photonic bandgap waveguide structures, which have intriguing properties like the ability to execute 90-degree turns in the space of a few microns. Now, along with Beckman Institute Director Pierre Wiltzius, Shanhui Fan from Stanford and C. Jeffrey Brinker from New Mexico, their investigations also include rapid fabrication techniques, 3-D holographic patterning, and DNA-based assembly, among other areas.

Many of these research topics will be featured at a workshop set for the Beckman Institute Oct. 3-5. The Three-Dimensional Multifunctional Ceramic Composites Workshop will feature participants from around the world taking part in “a forum to explore the state of the art in self-assembly of ceramic-based and hybrid ceramic/organic composites for the creation of multifunctional materials.”

Braun, Wiltzius, Lewis, and Lu will all give tutorials at the workshop, held in conjunction with the Materials Research Society. Braun said the team members not only bring expert knowledge to the MURI project, but also complement each other well.

“One thing that is truly unique about our team is that we’ve brought together people who are very skilled at theory and simulation (Fan), people who really know the self-assembly world, myself and Jeff Brinker, and we have a biochemist to help us think about biological assembly, and that’s Yi Lu,” Braun said. “Jennifer Lewis does the direct writing approaches, and then Pierre concentrates on new optical techniques and optical applications of self-assembly. He’s looking at a holography technique where he interferes light to create three-dimensional structures. So we really have everything from expertise in theory simulation, synthesis and assembly, all the way to optical devices and structures.”

Assembly techniques for these composites are a key a focus of the research. Self-assembly can involve templating with self-organized colloidal crystals, which is one way to form the periodic structure required to realize a photonic band gap capable of the sharp turns necessary for the on-chip integration of optical devices. The team also investigates directed assembly.

“It’s just like writing with a pen, except that you write with a pen in three dimensions and the ink is only a micron in diameter,” Braun said. “Your hair is 50 microns, so we can write with an ink that is 1/50th of your hair and we can write basically arbitrary three-dimensional structures.

“So we span both pure self-assembly and directed assembly. We build structures that are on order of the wavelength of light, so they very strongly manipulate light.”

That ability to manipulate light is one reason why this research is so exciting. Creating these structures in three dimensions adds to their properties and functionality. Braun said that by using self-assembly to create a 3-D photonic structure on top of a surface, that 3-D structure can control the color and thermal properties of a material. Color is important when it comes to heating or cooling, including, for example, large-scale items like a bus.

“It’s the reason why a black car is hot and a white car is not so hot,” Braun said. “So we can make a surface that looks black at some wavelengths and looks white at other wavelengths, so it will absorb some light and reflect other light.”

These techniques also offer scalability.

“One thing we’ve put a lot of effort in to is how we can create this on large areas,” Braun said. “People have demonstrated a number of these ideas over micron-size areas. But for most real applications for these photonic structures, you want to work maybe as small as a postage stamp, or maybe you want to do things that are as big as an aircraft or an automobile-sized structure. So the nice thing about self-assembly and directed assembly is they scale very well for larger structures.”

Controlling heat is a major goal in microelectronics, such as in reducing the heat emitted from integrated circuits, or in more mundane items like a common light bulb, which has a poor light-to-heat ratio. Braun said self-assembled ceramic composites have the ability to withstand high temperatures, making them excellent for use as insulators or regulators.

“As a team we’ve developed ways to make surfaces where we can control very carefully the color of the surface using self-assembly,” he said. “We can control when you heat it up, the emission of the surface, so we can control the thermal properties of surfaces very carefully, and you can make surfaces that are highly insulating.

“The Holy Grail would be to take an incandescent screw-in light bulb, where the filament is only about five percent efficient and most of the energy comes of as heat and only a little bit is the light you see. If you could make it so it didn’t put off any energy as heat, only put it off as light, you could make the efficiency of a regular light bulb go up dramatically. So both these direct-writing approaches and self-assembly approaches may enable controlling the way that hot surfaces give off their heat and light.”

Another area of research involves using DNA-mediated assembly, which could lead to biosensors useful in a number of ways. This research started with Lu’s group using DNA to detect heavy metal irons.

“Yi Lu realized a number of years ago that DNA could work as an enzyme when triggered by a specific chemical. For example, in the presence of lead, DNA enzymes can be trained to split another DNA chain that it’s hybridized to,” Braun said. He said Lu demonstrated this technique for a number of compounds, allowing for a simple readout based on the changing optical properties of the substance being tested. “Instead of needing a very sophisticated readout, what we are working on now is a readout technique that uses something as simple as a laser pointer and a piece of paper,” Braun said. “You just shine through, if you get one pattern it means lead, if you get another pattern it means no lead.”

Such a technique is general in nature and will work for both organic and inorganic materials.

“The idea for the sensor we’re developing is a very general motif,” Braun said. “You pick the specific DNA that senses the molecule you want, and you put that DNA into the sensor and the sensor will detect compound A. If you want a sensor that detects something else, you don’t have to change the structure of the sensor; you just have to use a different kind of DNA. So the DNA is what’s programmed to do the sensing. The DNA both does the assembly and it does the sensing.”

Lu will lecture on this topic at the workshop, while Wiltzius will discuss 3-D holographic lithography, Lewis direct-write assembly of 3-D structures, and Braun’s talk will be on opal synthesis, assembly and characterization.

For more on the workshop, visit http://www.mrs.org/meetings/workshops/2005/ceramic_composites/index.html