"Rainbow Metal," Similar To Opal, Suggests Light-Steering Computer Parts And Catalysts

Because it's riddled with regularly spaced holes only slightly wider than the wavelength of light, the UD material acts like a prism, diffracting a spectrum of colors--from gold and blue to red, green and purple.

"It reflects different wavelengths of light, just like opal, only it's much sturdier," explains Research Assistant Prof. Orlin D. Velev, lead author of the Nature paper, with coauthors including graduate student Peter M. Tessier and Prof. Abraham M. Lenhoff.

In fact, tiny holes in the new material are 20,000 times smaller than the pores in an existing metal mesh that can be used to direct radar waves, reports coauthor Eric W. Kaler, the University's Elizabeth Inez Kelley Professor of Chemical Engineering and chairperson of his department.

Consequently, "It should help guide the much shorter light waves, perhaps in photooptic computer components," says Kaler. Such devices will be crucial in next-generation computers, he says, because "fiber optics can't turn sharp corners, and you don't have much room to maneuver in nanoscale devices."

A sister version of the rainbow metal features light-sized pores--20 times smaller than the smallest red blood cell--as well as even smaller pores, all of which are aligned in tightly packed, uniformly spaced rows. This versatile, "meso/macroporous" form of the material may also be photoactive, and could prove useful as a catalyst for, say, cracking hydrocarbon to produce gasoline, or as a filter for rapidly separating molecules of different sizes.

Dots of gold

Creating the material is a low-energy process involving latex beads, much smaller microspheres of gold and "very simple chemistry," Kaler says. The size of the resulting pores can be "tuned" or changed simply by selecting different sizes of latex and gold beads, he adds.

First, the UD researchers pour a watery solution containing the latex particles onto a polycarbonate membrane. Water slips right through the membrane's 50-nanometer pores. But, the 300-nanometer latex beads are trapped on top.

"It's a bit like dumping a bunch of marbles into a bathtub and then pulling the drain plug," Kaler explains. "After the water escapes, you're left with a densely packed layer of these spheres."

After many hundreds of layers of latex are deposited, bits of gold just one-tenth the size of the polymer beads can be slowly filtered onto them, to fill the gaps between neighboring spheres.

As a final step, the researchers either "bake" or "pickle" their sample to remove the latex.

A multicolored metal with pores about 600 nanometers wide--close to the wavelength of light--is created by heating the latex and fusing the gold for 30 minutes at 300 degrees Celsius (572 degrees Fahrenheit). For a sample with both large and small pores, the researchers instead chemically oxidize and dissolve the latex.

"Our system is a very powerful, versatile way to make porous nanostructures in a variety of materials," Kaler says.

In the past, Velev notes, researchers have used ion beams to drill individual holes into metals, one pore at a time. "Obviously," he says, "that's very time-consuming and expensive. We think we've found another way."

And, UD's material manufacturing technique could be applied to any type of material, Velev says, theoretically allowing polymers and carbon to "shine," or reflect light.

The UD research team, working from the University's Center for Molecular and Engineering Thermodynamics, has been at the forefront of the rapidly accelerating effort to develop porous nanostructures by using arrays of particles as templates, field launched by the same group just two years ago. Their earlier work with porous silica--supported by collaborator Raul Lobo, an Assistant Prof. with the UD Center for Catalytic Science and Technology--appeared in the Oct. 2, 1997 issue of Nature (Vol. 389, pp. 447-448). Similar efforts to develop porous metals are under way by teams at Rice University and the University of Minnesota.