Plasmonics Promising for Optically Induced Electronics
HOUSTON, Oct. 12, 2012 — Nanoscale plasmonic antennas, or nonamers, attached to graphene could theoretically create on-demand electronic circuits by hitting them with light at particular frequencies.
Chemically doping silicon adjusts its semiconducting properties, and now scientists at Rice University are applying this concept to graphene — the ultrastrong, highly conductive, single-atom-thick form of carbon — by doping it with light. The method could instantly create optically induced electronics on graphene patterned with plasmonic antennas that manipulate light and inject electrons into the material to affect its conductivity.
Researchers have investigated many strategies for doping graphene, including attaching organic or metallic molecules to its hexagonal lattice. Making it selectively and reversibly capable of doping would provide scientists with a graphene blackboard upon which circuitry could be written and erased, depending on angles, polarizations and colors of the light hitting it.
Nanoscale plasmonic antennas called nonamers placed on graphene have the potential to create electronic circuits by hitting them with light at particular frequencies, according to researchers at Rice University. The positively and negatively doped graphene can be prompted to form phantom circuits on demand. Images courtesy of Rice University.
“One of the major justifications for graphene research has always been about the electronics,” said Peter Nordlander, professor of physics and astronomy and of electrical computer engineering. “People who know silicon understand that electronics are only possible because it can be p- and n-doped, and we’re learning how this can be done on graphene.”
Graphene’s ability to attach to plasmonic nanoantennas affords scientists this possibility. In previous work, Nordlander and colleague Naomi Halas succeeded in depositing plasmonic nanoparticles that act as photodetectors on graphene. (For more on their work, see: Nanoantennas Hold Promise for IR Photovoltaics)
Rather than reflect light, these metal particles redirect its energy; the plasmons that flow in waves across the surface when excited emit light or create “hot electrons” at particular, controllable wavelengths. Adjacent plasmonic particles can interact with each other in ways that are also tunable.
This effect can be seen in graphs of the material’s Fano resonance, where the nonamers, each a little more than 300 nm across, clearly scatter light from a laser source, except at the specific wavelength to which the antennas are tuned.
In their experiment, the nonamers — eight nanoscale gold discs arrayed around one larger disc — were deposited onto a sheet of graphene using an electron beam lithography technique. They were tuned to scatter light between 500 and 1250 nm, but with destructive interference at about 825 nm.
Nonamers in the drawings at top and in the photos at bottom are arrays of nine gold nanoparticles deposited on graphene and tuned to particular frequencies of light. When illuminated, the plasmonic particles pump electrons into the graphene. The technology may lead to the creation of on-demand circuitry for electronic devices.
Most of the incident light energy was converted into hot electrons at the point of destructive interference. These hot electrons directly transferred to the graphene sheet and changed portions of the sheet from a conductor to an n-doped semiconductor. The plasmonic nanoparticle antennas can also be tuned to respond to any color in the visible spectrum or to different polarization states, Nordlander said.
"That's the magic of plasmonics," he said. "We can tune the plasmon resonance any way we want. In this case, we decided to do it at 825 nanometers because that is in the middle of the spectral range of our available light sources. We wanted to know that we could send light at different colors and see no effect, and at that particular color see a big effect."
The technology could also be used to develop novel security and cryptography devices. Instead of using a key, Nordlander foresees a day when people might wave a flashlight in a particular pattern, inducing the circuitry of a lock to open a door on demand.
"Opening a lock becomes a direct event because we are sending the right lights toward the substrate and creating the integrated circuits. It will only answer to my call," he said.
The findings were reported in ACS Nano (doi: 10.1021/nn304028b).
For more information, visit: www.rice.edu
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