Scientists at the University of Maryland have shown for the first time that two-metal mixtures, or alloys, of gold, silver, and copper can potentially provide better and more tunable optical characteristics than can be achieved for any these metals individually.
The researchers say their findings have a wide range of potential applications ranging from more efficient solar cells to ultrasensitive molecular detectors and perhaps even to improved performance in functional materials that clog light absorption (think coatings to impart near-invisibility). A peer-reviewed paper based on the study recently was published in and featured on the cover of the journal ACS Photonics.
These three noble metals have long been important in electronics and optics, an importance that has continued to grow as researchers have learned how to tune the optical and electronic properties of metal nanoparticles by changing particle characteristics such as size, shape, and surface chemistry. However, until now, no one had shown a way to tune an inherent determinant of each metal’s ability to reflect, transmit or absorb light, known as its dielectric constant.
“Think about sunlight catching the silver of your wristwatch and projecting those little dancing dots on the wall next to your desk,” explained Marina Leite, assistant professor of materials science and engineering at UMD and corresponding author of the paper. “The wavelengths of light needed to produce that effect are always within the same range. This is called a pre-determined optical response, and it has limited researchers’ ability to change how much light is absorbed in a device made of pure metals such as gold, silver, and copper,”.
To overcome this limitation, Leite and Chen Gong, a graduate student at UMD and co-author of the paper, investigated how the alloying processes of these noble metals affect their optical response to identify combinations that enhance or inhibit the absorption of light.
“This work is a perfect example of the power of materials science and engineering: we discovered a way to control and change metals’ optical properties by mixing them. These alloys obtain a unique functionality that is not achievable using their pure counterparts—making them a better, more powerful tool for tunable optical response than gold, silver, or copper alone,” said Leite, who has a joint appointment in UMD’s Institute for Research in Electronics and Applied Physics (IREAP).
“Our results are relevant to my colleagues working on photonic devices—components for creating, manipulating, or detecting light—as these devices are highly dependent on the tunability of the optical response of their building blocks,” Leite added.
Jeremy Munday, assistant professor of electrical and computer engineering at UMD, agrees. “My colleagues and I have been working to increase the efficiency of solar cells, specifically by exploring the use of all-metal energy harvesting devices. The ability to arbitrarily tune their optoelectronic properties would have a significant impact on their performance,” he said.
This work also has broader economic implications due to the possibility of replacing high-cost metals with low-cost and earth-abundant ones. Though gold is immediately recognizable as a precious and expensive metal, copper and aluminum are much more readily available. Leite and her colleagues are now looking into how they can incorporate alloys using these metals into high-performance optical devices.
This work was supported by the National Science Foundation grant no. HRD1008117, the University of Maryland ADVANCE program, the Minta Martin Award at the A. James Clark School of Engineering at the University of Maryland, and the University of Maryland 2015 Graduate School Summer Research Fellowship program.
The research paper, “Noble Metal Alloys for Plasmonics,” Chen Gong and Marina S. Leite, was published online February 24 in the journal ACS Photonics.