Nano Focus: Optical confinement modifies graphene transistor characteristics
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Subretinal photodiode array with triple-diode pixels arranged in a hexagonal pattern. Pixels of 70 μm and 140 μm in size were made. Left inset: Central electrodes are surrounded by three diodes connected in series, and by the common return electrode. Right inset: The subretinal implant.
photosensitive pixels but they depend on an external power source. Recently, however, researchers from the Palanker group at the Hansen Experimental Physics Laboratory and the Department of Ophthalmology at Stanford University designed a photovoltaic retinal prosthesis where video goggles were used to deliver both power and visual information through pulsed NIR illumination, preserving the natural link between image perception and eye movement without complex electronics and wiring. In an article published in the June issue of Nature Photonics (DOI: 10.1038/ nphoton.2012.104; p. 391), Keith Mathieson, James Loudin, and co-research-
Nano Focus Optical confinement modifies graphene transistor characteristics
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he interaction between light and matter can be greatly enhanced within an optical cavity in which the spacing of two mirrors defines a standing electromagnetic wave. Placing a sheet of graphene in such a cavity can therefore have profound effects on its optoelectronic properties, as shown by
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VOLUME 37 • SEPTEMBER 2012
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ers from Stanford University and the University of California–Santa Cruz, describe their prosthesis design in which video images captured by a head-mounted camera are processed by a portable computer. The video goggles use a liquid-crystal display (LCD) illuminated by pulsed near-infrared light (880–915 nm) to project the images onto a subretinal photodiode array (consisting of 70 μm pixels, each with ~20 μm stimulating electrodes), which converts the light to local currents that stimulate the nearby neurons in the inner nuclear layer of the retina. The researchers fabricated silicon photodiode arrays consisting of pixels with
M. Engel of the Karlsruhe Institute of Technology, M. Steiner of the IBM T.J. Watson Research Center in New York, A. Lombardo of the University of Cambridge, and their colleagues. Their article in the June 19 issue of Nature Communications (DOI: 10.1038/ncomms1911) describes how such optical confinement of a graphene transistor allows spectrally selective generation of photocurrent and even alters the electrical transport properties of the material. The team embedded a sheet of gra-
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single diodes as well as those consisting of pixels with three diodes connected in series. These triple-diode pixels can produce 1.5 V, which triples the charge injection on the sputtered iridium oxide film electrodes (from 0.5 mC cm−2 for a single-diode pixel to 1.5 mC cm−2). The triple-diode pixels require light intensities three times higher than single-diode pixels because the photosensitive pixel area is divided into three subunits. However, the researchers found that their singleand triple-diode devices had very similar thresholds for eliciting retinal res
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