Credit: Bonnell Group
Last fall we wrote about Penn’s Nano/Bio Center getting a $11.7 million NSF grant. According to a new release from the university, some of the work is paying off in the form of gold-on-glass nanogrid that can convert light directly into electrical current.
A group of material scientists at the center say that when light strikes gold nanoparticles, it creates surface plasmons (localized, collective oscillations of electrons) on the nanoparticle surface. They noticed that when interparticle distances are small, the surface plasmons in the same region can couple, creating intense electric fields. If the particles are coupled (with something like highly conjugated multiporphyrin chromophoric wire), they can then act as optical antennae capturing and refocusing light between them.
The group likens the effect to that of photovoltaic cells. A paper on their work is published in the current edition of ACS Nano.
One of the authors is ACerS member Dawn Bonnell, a professor of materials science and the director of the center at Penn. She and colleagues at Penn, Duke and the University of Maryland experimented with varying the space between the nanoparticles and eventually found an optimal distance for getting light to excite plasmons. According to the release, Bonnell and the others say:
[When] the nanoparticles are optimally coupled, a large electromagnetic field is established between the particles and captured by gold nanoparticles. The particles then couple to one another, forming a percolative path across opposing [gold] electrodes. The size, shape and separation can be tailored to engineer the region of focused light. When the size, shape and separation of the particles are optimized to produce a “resonant” optical antennae, enhancement factors of thousands might result.
They also say that their work shows that magnitude of the photoconductivity of the plasmon-coupled nanoparticles can be tuned independently of the optical characteristics of the molecule.
“If the efficiency of the system could be scaled up without any additional, unforeseen limitations, we could conceivably manufacture a one-amp, one-volt sample the diameter of a human hair and an inch long,” Bonnell says.
They say there a several immediate applications where this effect may find use. One is for higher efficiency solar energy-harvesting units. Recently, surface plasmons have been engineered into a variety of light-activated devices such as biosensors. A more exotic use might be found non-binary computer data storage that would uses a range of wavelengths of light for data instead of being limited to 1s and 0s.
Because molecular compounds exhibit a wide range of optical and electrical properties, the strategies for fabrication, testing and analysis elucidated in this study can form the basis of a new set of devices in which plasmon-controlled electrical properties of single molecules could be designed with wide implications to plasmonic circuits and optoelectronic and energy-harvesting devices.