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From the Labs: Materials

New publications, experiments and breakthroughs in materials science–and what they mean.
December 22, 2008

Self-Assembling Optics
Nanoparticles form sophisticated devices

Silver crystals: These octahedral silver nanoparticles suspended in ethanol are assembling into a large crystal. The density of the particles, which changes from top to bottom, determines what colors pass through.

Source: “Self-Organized Silver Nanoparticles for Three-Dimensional Plasmonic Crystals”
Peidong Yang et al.
Nano Letters
8: 4033-4038

Results: Researchers at the University of California, Berkeley, led by Peidong Yang, have shown that silver nanoparticles with very regular octahedral shapes pack together under the influence of gravity to form large crystals. The crystals’ optical properties can be varied by changing the amount of time the nanoparticles have to pack together, which affects the spacing between them.

Why it matters: Light striking the crystals causes the formation of what’s called a plasmon, a wave passing through the electrons at the crystals’ surfaces. Plasmonic crystals could be used to guide light in optical computers or to increase the sensitivity of chemical sensors. They could also serve as lenses for superhigh-resolution microscopy. Using conventional lithography to etch patterns in materials can achieve a similar effect but is more expensive.

Method: Using methods that Yang developed previously, the researchers grew silver nanoparticles in solution, then suspended them in ethanol inside a test tube. By allowing the nanoparticles to pack together for longer or shorter periods of time before evaporating the ethanol, the researchers produced densely and loosely packed crystals, whose optical properties they studied. The crystals transmitted particular bands of radiation while blocking others, and the frequency varied according to how tightly packed the crystals were.

Next steps: The Berkeley group plans to build a large plasmonic crystal on the surface of a six-inch wafer to establish that the crystals can be formed on a scale large enough for many of their potential applications.

Practical Plastic Solar Cells
New dyes and electrolytes improve efficiency of Grätzel cells

Source: “New Efficiency Records for Stable Dye-­Sensitized Solar Cells with Low-Volatility and Ionic Liquid Electrolytes”
Michael Grätzel et al.
Journal of Physical Chemistry C
112: 17046-17050

Results: Scientists at the Swiss Federal Institute of Tech­nology and the Chinese ­Academy of Sciences have used nonvolatile electrolytes and a new dye to improve the sta­bility of flexible dye-­sensitized solar cells (also called Grätzel cells) while achieving efficiencies of up to 10 percent.

Why it matters: Dye-­sensitized solar cells could be cheaper than conventional solar cells, because they’re made of inexpensive materials and can be printed rapidly. They can also be made flexible. But they have been difficult to manufacture and unreliable to operate, because the electrolytes that carry current within them are volatile and must be carefully encapsulated. By using nonvolatile electrolytes, the researchers have made Grätzel cells that are more reliable and potentially cheaper to manufacture. What’s more, the new dye allows the researchers to use the non­volatile electrolytes while maintaining efficiencies of near 10 percent, a level necessary to compete with conventional solar cells.

Method: The researchers coupled two previously synthesized nonvolatile electrolytes with a new dye that absorbs more light. That reduced both the amount of dye required and the thickness of the solar cells, making it easier for electrical charges to move out of the cell.

Next steps: The cells remain stable when exposed to light and high temperatures for 1,000 hours. The researchers are now testing them at higher temperatures, studying their long-term performance in a large solar panel, and working with corporate partners to commercialize the technology.

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