Scientists from the United States and Switzerland have demonstrated a novel mechanism for extracting energy from light.
“Using such an approach could make solar energy harvesting and optoelectronic devices much better,” said Dr Dawn Bonnell of the University of Pennsylvania, who is a senior author of a study published in the journal ACS Nano.
The study focuses 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.
Because these materials 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.
In 2010, Dr Bonnell and her colleagues reported 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 Dr Bonnell’s team 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. If we could do that, we could use them for molecular electronics device applications, such as circuit components or solar energy extraction,” Dr Bonnell said.
To examine the mechanism of the plasmon-induced current, the team 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. And we didn’t even optimize the system. In principle you can envision huge increases in efficiency,” Dr Bonnell said.
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.
Bibliographic information: David Conklin et al. 2013. Exploiting Plasmon-Induced Hot Electrons in Molecular Electronic Devices. ACS Nano, 7 (5), pp. 4479 – 4486; doi: 10.1021/nn401071d