In one of the first examples of molecules building themselves into useful synthetic microstructures with little human intervention, chemical engineers have created plastic materials that assemble themselves into sophisticated optical devices known as photonic crystals. Samson Jenekhe and Linda Chen of the University of Rochester describe their work on a process known as "hierarchical self-assembly" in the Jan. 15 issue of Science.
Just last year the same team created the largest synthetic structures ever made by self-assembly, where molecules organize themselves into discrete microscopic objects, such as hollow spheres. In the current work they've gone one step further, developing methods to make billions of those objects come together to form even larger, highly ordered structures visible to the naked eye. The materials are an intricate 3-D composite of air and plastic that manipulate light in the same way that allows opals to produce such striking colors. That's because the materials exhibit "spatial periodicity," a sought-after characteristic that refers to their high degree of order and their optical properties.
The structures themselves don't appear extraordinary; they're simply thin films that coat glass slides. But they do hint at their optical qualities, reflecting the colors of the rainbow like a hologram on a credit card. Under a microscope, the detail and order of the materials is striking: Beneath the sheen, hollow plastic spheres pack together to form a three-dimensional structure that looks like a honeycomb. (Jenekhe points out that there are other optical devices actually called honeycombs, but those 2-D devices are quite different from these 3-D structures).
"This is the next logical step in self-assembly," says Jenekhe, the lead investigator and professor of chemical engineering, chemistry, and materials science. "We start with polymer molecules in solution. They self-organize into hollow spheres, and billions and billions of them come together in a precise, ordered way to form a larger periodic structure."
In the Science paper the scientists describe how hollow spheres assembled together to form photonic crystals - structures that can create and manipulate light signals precisely, transmitting certain wavelengths while blocking others. Photonic crystals were first envisioned by Eli Yablonovitch, now at the University of California at Los Angeles, more than a decade ago. In these crystals, molecules are ordered so that light traveling through them is modulated in a highly controlled fashion. Groups at Sandia National Laboratory, at Allied Signal, and in the Netherlands have built photonic crystals, but most current efforts involve a great deal of technological hand-holding: either laborious and expensive fabrication like drilling tiny holes into a material, or providing a template to begin the assembly process, then somehow removing the starting material.
The Rochester team believes its is the first photonic crystal that literally grows itself. In the process, known as hierarchical self-assembly, hollow spheres stack themselves into larger structures, like bricks forming their own wall. The device, a porous mix of a plastic framework and air pockets, doesn't require any sort of template and isn't so much fabricated as grown. Like opals, gems in which air and silica pack together to produce dazzling sharp colors as light travels through them, the alternating air spheres and plastic framework built into the new materials manipulate the light in predictable, precise ways as it passes through.
The team's devices are typically about a square centimeter and 30 millionths of a meter thick, less than the thickness of a human hair. By using chemical techniques to vary the size of the spheres, the width of the framework, and the structure of the shapes they create, the team is able to precisely manipulate how light travels through the crystals - some of the same features that determine whether opals are red, blue, or another color. In current designs of the new materials the air pockets comprise more than half of the structure.
"The work is extremely creative; it bodes a future world in which we'll be able to make 3-D nanostructures that will assemble themselves," says Yablonovitch, professor of electrical engineering at UCLA.
Applications are widespread for a device that selectively filters out certain wavelengths, or colors, of light. Optical data storage and telecommunications rely on transmission and detection of specific wavelengths, and holographic memory systems are expected to do the same one day. The plastics might make possible better light-emitting diodes (LEDs), materials that are increasingly being used to produce more efficient lighting systems. Also possible are special paints that change colors under different light conditions - perhaps lighter in the harsh glare of sunlight and darker under incandescent light. Another potential application: a super-efficient, plastic laser that could produce intense light with a fraction of the energy now required.
Currently, though, many scientists are simply trying to build components that bend and steer light as handily as current computer chips manipulate electronic signals. Today's computers rely on semiconductors, or "electronic crystals," that give scientists the power to control millions of electronic signals gliding around a computer chip simultaneously. Such control remains a dream when it comes to light: It's impossible to fit millions of tiny conventional lenses and mirrors on a chip the size of one's fingernail. So scientists are still searching for materials for optical systems that would allow them to channel, switch and manipulate optical signals with ease. The payoff would be enormous: Light can carry thousands of times more information than electrons and, if used instead of electronic signals, could boost the speed of telecommunications equipment, modems and other devices dramatically.
"What we're able to do now with electrons led to the microelectronics revolution. We'd like to do the same with photons. For that you need materials like these photonic crystals that allow you to trap light and control the way it propagates," says Jenekhe.
The key to the work, says Jenekhe, is encoding into the polymers information so they will organize themselves into large-scale objects with specific characteristics. Jenekhe and graduate student Chen worked with a polymer known as poly(phenylquinoline)-block-polystyrene, a molecular cousin to materials used in Styrofoam cups and in paint. By taking advantage of the chemical properties of the material, Chen and Jenekhe made the particles assemble into specified shapes and sizes. Once the polymers are prepared using standard chemical techniques, it takes them just minutes or hours to organize into photonic crystals. Besides such crystals, Jenekhe expects self-assembly to be useful in a variety of areas, perhaps including biomaterials, separation media, and sensors.
"Much of nature is a product of hierarchical self-assembly, and humans are the example par excellence. Each of us starts as a single cell encoded with the information to guide our growth into a larger structure - a complete human being. Making materials that are on their own smart, intelligent and able to orchestrate their own growth marks the chemistry and polymer science of the future."
Funding for the project came from the Office of Naval Research and the National Science Foundation. The work was also supported by an Elon Huntington Hooker graduate research fellowship to Chen, who now works at Bell Laboratories of Lucent Technologies.