Sprinkled Silver Nanocubes Make Super Light Absorbers

Microscopic silver cubes, when sprinkled at random on a polymer-coated gold surface, can provide a simple and tunable way to create large-area absorbers that “perfectly” absorb light of a given wavelength.

The ability to absorb light efficiently and over a desired wavelength range is essential for many photonic applications. Ideal, “perfect” absorbers of infrared or visible light have been made using lithography to create patterned structures on metallic surfaces. But such an approach is expensive and difficult to scale up to the large surface areas required for many applications.

Metallic nanocubes, developed at Duke University, provide a simple way to create a material that “perfectly” absorbs light of a given wavelength when sprinkled at random on a polymer-coated gold surface. Courtesy of Cristian Ciracì.

David Smith and colleagues at Duke University have developed a simple chemical synthesis method, using a dusting of silver nanocubes to modify the absorptive properties of a metallic surface. When separated from the underlying metal by a very thin insulating layer, the cubes act as tiny antennas that cancel out the reflectance of the metal surface. The cost-effective absorbers could find use in applications ranging from sensors to energy-harvesting devices.

“Our new approach is more of a bottom-up process,” said Cristian Ciracì, a research scientist at Duke’s Pratt School of Engineering. “It may allow us to create devices — such as efficient solar panels — that cover much larger areas. In our experiments, we demonstrated an extraordinarily simple method to achieve this.”

The results, published in Nature (doi: 10.1038/nature11615), were conducted in Smith’s lab. He is a senior researcher and the William Bevan Professor of electrical and computer engineering.

The key for many applications or devices is the material’s ability to control the absorption of electromagnetic waves. Metals, for example, can be highly reflective on their own, which can benefit some applications, but for something like a solar cell, optimal light absorption is desired.

“Metamaterials based on metallic elements are particularly efficient as absorbers because both the electrical and magnetic properties of the material can be controlled by how we design them,” Ciracì said.

Duke’s metamaterial has three major components — a thin layer of gold film coated with a nanothin layer of an insulator, topped off with a dusting of millions of self-assembled nanocubes. The nanocubes used in the experiments were made of silver.

“The nanocubes are literally scattered on the gold film, and we can control the properties of the material by varying the geometry of the construct,” Ciracì said. “The absorptivity of large surface areas can now be controlled using this method at scales out of reach of lithography.”

Ciracì believes that, by combining different components of the metamaterial elements into a single composite, more complicated reflectance spectra could be engineered, and a “level of control needed in more exotic applications, such as dynamic inks,” could be achieved.

For more information, visit: www.duke.edu