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Posted: Oct 10, 2012

Doping graphene with light

(Nanowerk Spotlight) Controlling the density of electron carriers – which are essential to the operation of electronic devices such as transistors – is achieved by doping conventional three-dimensional semiconductors. But graphene, a semi-metallic layer that is just one atom thick, has properties very different from traditional materials such as silicon. However, the doping of graphene is a key parameter in the development of graphene-based electronics (read more: "Comparing fundamental techniques for doping graphene sheets"). Although there are no commercial graphene-based electronic devices on the market yet, there is no question that manufacturers are putting a lot of effort into this area because of these devices' very high speed potential.
Researchers have investigated numerous strategies for doping graphene, including attaching organic or metallic molecules to its hexagonal lattice. Now, researchers have managed to dope graphene with light in a way that could lead to more efficient design and manufacture of electronics, as well as novel security and cryptography devices.
"Making it selectively – and reversibly – amenable to doping would be like having a graphene blackboard upon which circuitry can be written and erased at will, depending on the colors, angles or polarization of the light hitting it," Zheyu Fang, a research associate in the Halas Nanophotonics Group at Rice University, tells Nanowerk. "The ability to attach plasmonic nano antennas to graphene affords just such a possibility."
In a recent paper in ACS Nano ("Plasmon-Induced Doping of Graphene"), first authored jointly by Fang and Yumin Wang, a graduate student in Peter Nordlander's Nanophotonics Group at Rice, the team demonstrated photoinduced n-doping of graphene by hot electrons generated from plasmonic nano antennas, observed electrically as a shift of the Dirac point.
Plasmonic nonamer antenna-graphene phototransistor
Plasmonic nonamer antenna-graphene phototransistor. (a) Schematic of a nonamer antenna on single-layered graphene with back-gated voltage and source and drain contacts. The device was fabricated on a Si substrate with a 285 nm thick native oxide layer. (b) Illustration of hot electron injection resulting from optically exciting the nonamer at resonance, where the plasmon decay generates hot electrons which transfer into the graphene sheet. (c) SEM image of a nonamer antenna array patterned on graphene by electron beam lithography. The diameters of the center and satellite nanodisks are 190 and 112 nm, respectively. The gap size is 15 nm. (d) SEM top view for an individual nonamer plasmonic antenna on the graphene substrate. A slight wrinkling of the graphene sheet, which occurs upon transfer to the silica substrate, is also shown. (Reprinted with permission from American Chemical Society)
"We have shown that the degree of hot electron doping can be controlled by varying the plasmonic antenna size, incident laser wavelength, and the laser power density," explains Fang. "We achieved a larger doping efficiency for the n-type graphene in comparison with p-type graphene."
In their experiments, the researchers synthesized a monolayer graphene sheet on a copper foil and the transferred it to a silicon wafer with a 285 nm thick oxide layer. They then patterned source and drain electrodes and plasmonic nonamer antennas onto the graphene using electron beam lithography and gold evaporation. Under laser irradiation, the hot electrons generated from plasmon excitation in the gold nonamer antennas are injected into the graphene sheets, resulting in n-type doping.
Having managed to fabricate gold nano antennas on graphene monolayers, the team now need to figure out how to fabricate the metallic structures without destroying the graphene underneath, and how to improve the doping efficiency with an optimized antenna geometry and incident light.
Fang points out that a microsecond doped carrier relaxation time scale could enable the development of a wide variety of active optical and optoelectronic applications, such as graphene switches and photodetectors and optically induced electronics.
A particularly interesting concept derived from these finding of efficient photo induced doping of graphene is the possibility of generating what the researchers refer to as optically induced electronics.
Schematic illustration of optically induced electronics
Schematic illustration of optically induced electronics (OIE) made possible by nanoantenna n-doping and quantum dot p-doping, inducing (top) a diode or (bottom) an npn transistor, respectively. Different regions of the undoped graphene structures are covered by either plasmonic antennas (tuned to red) or quantum dots (tuned to green). When a specific pattern of light at the requisite wavelengths illuminates the structure, HE from the plasmonic antennas induce n-doping and the holes injected from the quantum dots induce p-doping. Since the optical response of the plasmonic antennas and the quantum dots can be tuned, a similar effect could also be generated using light of the same color. (Reprinted with permission from American Chemical Society)
"While the present plasmon induced hot electron injection results in n-doping, our process could be combined with the recently observed p-doping process observed when a graphene sheet is covered by a thin layer of quantum dots ("Hybrid graphene–quantum dot phototransistors with ultrahigh gain")" says Fang. "By patterning distinct regions of an undoped graphene structure with plasmonic antennas or quantum dots tuned to different – or the same – wavelength and then illuminating by light, it might be possible to create pn junctions and realize simple electronic circuitry."
"By using more complicated patterns, it may be possible to create simple electronic circuits that would turn on only when illuminated by an appropriate unique light pattern" he adds.
Combing graphene with plasmonics could open a new interdisciplinary research field for condensed matter physics and optics, especially for photo detectors at the nanometer scale.
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