The stylized “Love” symbol on the University of Pennsylvania campus (Credit: Flickr @ InSapphoWeTrust http://www.flickr.com/photos/skinnylawyer/)Researchers from the University of Pennsylvania have demonstrated a new mechanism for extracting energy from light based on the application of the plasmonic nanostructures. This finding could improve technologies for generating electricity from solar energy and lead to more efficient optoelectronic devices used in communications.
Dawn Bonnell, Penn’s vice provost for research and Trustee Professor of Materials Science and Engineering in the School of Engineering and Applied Science, led the work, along with David Conklin, a doctoral student. The study involved a collaboration among additional Penn researchers, through the Nano/Bio Interface Center, as well as a partnership with the lab of Michael J. Therien of Duke University.
“We’re excited to have found a process that is much more efficient than conventional photoconduction,” Bonnell said. “Using such an approach could make solar energy harvesting and optoelectronic devices much better.”
The new work centers on plasmonic nanostructures, specifically, materials fabricated from gold particles and light-sensitive molecules of porphyin, of precise sizes and arranged in specific patterns. Plasmons, or a collective oscillation of electrons, can be excited in these systems by optical radiation and induce an electrical current that can move in a pattern determined by the size and layout of the gold particles, as well as the electrical properties of the surrounding environment.
In physics, plasmons are collective oscillations of the free electron gas density, for example, at optical frequencies. They can be described in the classical picture as an oscillation of free electron density with respect to the fixed positive ions in a metal.Because these plasmonic nanostructures can enhance the scattering of light, they have the potential to be used to advantage in a range of technological applications, such as increasing absorption in solar cells.
Dawn Bonnell. (Credit: University of Pennsylvania)
In 2010, Bonnell and colleagues published a paper in ACS Nano reporting the fabrication of a plasmonic nanostructure, which induced and projected an electrical current across molecules. In some cases they designed the material, an array of gold nanoparticles, using a technique Bonnell’s group invented, known as ferroelectric nanolithography.
The discovery was potentially powerful, but the scientists couldn’t prove that the improved transduction of optical radiation to an electrical current was due to the “hot electrons” produced by the excited plasmons. Other possibilities included that the porphyin molecule itself was excited or that the electric field could focus the incoming light.
“We hypothesized that, when plasmons are excited to a high energy state, we should be able to harvest the electrons out of the material,” Bonnell said. “If we could do that, we could use them for molecular electronics device applications, such as circuit components or solar energy extraction.”
To examine the mechanism of the plasmon-induced current, the researchers systematically varied the different components of the plasmonic nanostructure, changing the size of the gold nanoparticles, the size of the porphyin molecules and the spacing of those components. They designed specific structures that ruled out the other possibilities so that the only contribution to enhanced photocurrent could be from the hot electrons harvested from the plasmons.
“In our measurements, compared to conventional photoexcitation, we saw increases of three to 10 times in the efficiency of our process,” Bonnell said. “And we didn’t even optimize the system. In principle you can envision huge increases in efficiency.”
Devices incorporating this process of harvesting plasmon-induced hot electrons could be customized for different applications by changing the size and spacing of nanoparticles, which would alter the wavelength of light to which the plasmon responds.
“You could imagine having a paint on your laptop that acted like a solar cell to power it using only sunlight,” Bonnell said. “These materials could also improve communications devices, becoming part of efficient molecular circuits.”
Conklin D, Nanayakkara S, Park TH, Lagadec MF, Stecher JT, Chen X, Therien MJ, & Bonnell DA (2013). Exploiting plasmon-induced hot electrons in molecular electronic devices. ACS nano, 7 (5), 4479-86 PMID: 23550717
