Creating nanostructures with shadows

Hank Hogan, hank@hankhogan.com

From out of the shadows, researchers at the University of California, Berkeley, have built nanostructures that might lead to more efficient solar power. Another potential application is improved nanoplasmonic-based and label-free chemical and biological sensors.

The idea for the technique came to research leader Luke P. Lee, a bioengineering professor, after he saw very narrow features arising from two shadows. That sparked a realization, he said. “If you combine two shadows, you can create a completely unexpected image.”

Inspired by the interaction of light and shadows in the ruins at Pompeii, researchers used directional deposition and milling to create unique shapes from nanoshadows. Courtesy of Luke P. Lee, University of California, Berkeley.

The nanoscale implementation of this idea is dubbed “shadow overlap ion-beam lithography,” and its description appeared in Nano Letters (2009, Vol. 9, No. 11, pp. 3726-3731). It involves the use of microscopic prepatterned structures, such as beads or pillars.

In a demonstration, the researchers used polystyrene beads measuring 477 nm across. They deposited these on a glass substrate and etched them back to 405 nm, opening space between them. To create the final nanostructure, they deposited a 20-nm layer of gold over the beads, using a directional method. The effect was that the beads created areas of varying thickness, or shadows, in the metal.

The investigators then used ion milling to remove the gold. Once again there was directionality, and once again the beads created shadows. The milling was at an angle, both to the beads and to the deposition. In the final step, they removed the beads. The result was an array of shapes, with the outcome depending on the angle of the deposition and milling. By adjusting either or both, the researchers could create a combination of nanoscale formations.

They characterized the structures using both near- and far-field optical techniques. They found that they could redshift the resonance peak from 580 to 680 nm by increasing the deposition angle, a result that agreed with calculations.

Lee noted several benefits of the new technique. The smallest features are in the nanometer range, far smaller and sharper than can be achieved with other methods. Using this approach, it also is possible to create shapes that cannot be fabricated with conventional mask-based lithography. Finally, unlike other high-resolution techniques such as electron beam lithography, shadow overlap ion-beam lithography offers high throughput and could be very cost effective.

One drawback is the need for prepatterned structures. However, it may be possible to overcome this constraint by adjusting the deposition or milling angle.

Applications outside of a lab are still some time off, but the researchers intend to investigate two in the near term. One is label-free chemical or biological sensors, using nanostructures to create sites for surface-enhanced Raman spectroscopy or nanoscale plasmon resonance electron transfer absorption spectroscopy, a method developed in Lee’s lab.

Another application of interest is the creation of nanoarrays tuned to specific wavelengths. These could be assembled into one device, opening up the possibility of harvesting light from the ultraviolet to the infrared. That would allow solar cells to work even when the sun isn’t shining, Lee said.

“You can collect infrared energy at night. Even when you have cloudy conditions, you can still pick up some other wavelengths,” he added.