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Crystals Promise New Optical Materials

Photonics Spectra
Jun 2002
Kevin Robinson

Colloidal particles interest scientists because, although the particles are much larger than molecules, they often behave similarly. For example, they can be present in a solid, liquid or vapor phase, and like molecules, their phase behavior is determined by the nature of the forces between them. Two European research groups have reported discoveries of states of matter using colloidal particles that may have an impact on the development of new photonic materials.

Researchers have developed a method for creating self-arranging colloidal crystals that may display interesting optical properties. The work has led to the production of a never-before-theorized crystal structure. Courtesy of Krassimir P. Velikov.

Led by Alfons van Blaaderen, a team from Utrecht University and from FOM Institute for Atomic and Molecular Physics in Amsterdam, both in the Netherlands, discovered a new crystal structure while developing a method to fabricate binary colloidal crystals.

The team immerses a clean substrate in a liquid suspension of the particles, and as the liquid evaporates, the particles become evenly distributed on the substrate in a hexagonal pattern. The process employs the capillary forces accompanying a liquid's drying front to draw and arrange the particles.

To create a binary crystal, which incorporates two particles of different sizes, the researchers first deposit a layer of 203-nm silica colloids. Then they deposit a second layer of silica or polystyrene beads with radii of 101 or 97 nm, respectively. The resulting crystal structure depends on the concentration of the small particles in the solution.

At a low concentration, the drying front places only three particles in the depressions where the larger particles in the first layer meet, forming an LS (or large/small stoichiometry) open hexagonal pattern. At higher concentrations, the small particles fill in all six depressions, forming an LS2 pattern.

At still higher concentrations, six small particles surround the large spheres in a ring, but the rings are rotated so that three small particles arranged in a planar triangle fill the depressions. According to Krassimir P. Velikov, the lead author of a paper that describes the research work, because the new structure has a higher packing fraction than well-known binary colloidal crystals, this last pattern, LS3, holds promise for thermodynamic stability.

Both the new technique for creating binary crystals and the new crystal itself look promising for photonics -- for example, by easing the production of structures with a full photonic bandgap, Velikov said. "So far, there is little research on binary colloidal crystals for photonic applications because it is difficult to make them. With our method, we can make several types of binary crystals that are large enough and suitable for optical characterization."

Melting glass with glue

Researchers from Edinburgh University in the UK, Almería University in Spain and Göteborg University in Sweden, led by Wilson C.K. Poon of Edinburgh, have developed a colloidal suspension of polymethylmethacrylate (PMMA) that can switch between two glassy states. In one state, the colloidal suspension is confined by the repulsion of the particles. In the other, paradoxically, the colloidal suspension is confined by their attraction.

As the concentration of PMMA particles increases, the repulsion of the particles spaces them evenly, but the particles are so numerous that they cannot move very far, explained Poon. Each particle has a "cage" of neighbors that hinders its movement, known as a glassy state because the silicon and oxygen molecules in glass are similarly stuck. The researchers discovered, however, that using a sticky polymer to glue the PMMA particles together melts the first glassy state, and adding more polymer glue forms another.

Poon's research group measured the movement of the particles in solution by watching their effect on laser speckle. Mobile particles cause the speckle pattern to fluctuate, but particles in a glassy state do not exhibit as large an effect.

The experiments illuminate the physics behind glassy states, which may contribute to better optical materials. "One of the main problems with making zirconium-based, rare-earth-doped, high-gain optical materials is that these glasses are prone to crystallization," Poon explained. A better understanding of glasses, he said, could minimize this crystallization.

Both groups plan to continue the work. Poon's group will focus on how colloidal particles move in these systems, and van Blaaderen's will explore potential applications for the crystals in photonics.

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