28 June 2012
Despite researchers’ excitement over graphene’s potential in electronics, it doesn’t absorb light in the visible or near-infrared range very well, which limits its use as a photodetector. Now, Naomi Halas and colleagues at Rice University have found a way to significantly boost graphene’s light-harvesting potential (Nano Lett., DOI: 10.1021/nl301774e).
Photodetectors are ubiquitous in devices such as cameras, night vision goggles, and telescopes. But conventional photodetectors made from semiconductors either require expensive materials or only detect photons with high energies. Making photodetectors that work for light with lower energies remains a difficult task.
Halas and her colleagues came up with the idea to build a photodetector that could harvest lower-energy light using antennas that grab light’s energy and transfer it to a semiconductor. Last year, they built a photodetector with gold nanorods as antennas on the semiconductor silicon (Science, DOI:10.1126/science.1203056). The detector absorbs photons and generates waves called plasmons, which transfer their energy to electrons that cross over into the silicon.
Researchers would also like to make photodetectors from graphene because of its flexibility and light weight, so Halas and her team decided to combine the antennas with graphene to improve its photodetection properties. “Graphene is this wonderful atomic-scale thin conductor,” she says. “People are making big sheets of this stuff. Now, we’re bringing a new function onto what people are already excited about.”
In the device, the researchers put a monolayer of graphene on a silicon chip and used electron beam lithography to deposit clusters of gold nanoparticles onto the graphene. The clusters were seven disks arranged in a flower shape, which Halas calls “a very special arrangement,” because it gives a strong response to light in the visible and near-infrared range.
By adjusting the size and spacing of the nanoparticles, the researchers could control the frequency of the light harvested. “The graphene collects and delivers the photocurrent,” Halas explains, while the nanoparticle antennas tune the optical properties.
The researchers drape a second layer of graphene on top of the first. It conforms to the gold clusters like a veil, making the transfer of electrons to the graphene very efficient. The result was an electric current of more than 80 nanoamperes when illuminated with 785-nm laser light — a current eight times greater than for graphene alone. Compared to nanoparticle antennae sitting on a single layer of graphene, the sandwich structure doubled the current, says Halas.
Halas says the photodetector’s practicality is its strength: scientists could construct it on any flexible material.
“The enhancement of the performance is most impressive,” says chemical engineer Nicholas Kotov of the University of Michigan, Ann Arbor. The direct transfer of energy into the graphene, he says, “has potential applications in many areas besides photodetectors.” They include use in solar cells, photoelectrocatalytic materials, and thermoelectrics to convert heat into electricity.
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