Schematic illustration of electrically biased suspended graphene and light emission from the center of the suspended graphene. Credit: Young Duck Kim, Columbia Engineering
Thomas Edison made his earliest incandescent light bulbs with carbon filaments. More than a century later, an international team of researchers has effectively miniaturized the light bulb by making graphene glow brightly in the visible range (Nature Nanotech., doi: 10.1038/nnano.2015.118).
One crucial step to getting graphene to emit visible light was to suspend a thin slab of the crystalline carbon a couple of hundred nanometers above the silicon dioxide substrate, says James Hone, a mechanical engineering professor at Columbia University (USA). Hone's group worked with scientists at Seoul National University and Korea Research Institute of Standards and Science (South Korea).
At either end of the tiny carbon bridge is an electrode. Just as in an incandescent lamp, electrical current running through the suspended graphene sheet makes the carbon give off visible light.
Thanks to an acoustic process known as Umklapp scattering, graphene's thermal conductivity decreases as its temperature increases, according to Hone. Also, any heated wire is hotter at its center than at its ends. These two tendencies combine to localize the visible light emission at the center of the graphene bridge. Electrons at the center reach a temperature of about 2,800 K, which, translated into color temperature, means the glow from graphene is slightly warmer than the light from incandescent lamps.
When they plotted the spectra of the emitted light, the scientists noticed emission peaks, which initially puzzled the team because graphene has no special band gap. The researchers learned these peaks are from interference, “which is not something you usually think about in an incandescent filament,” Hone says. The graphene emits light in both forward and backward directions, and the latter reflects off the substrate and interferes with the forward emission. Changing the height of the gap between the graphene and the substrate allows scientists to fine-tune the emission spectrum.
“This is still not a terribly efficient light bulb,” Hone says. Nevertheless, possible applications of a graphene “light bulb” include displays, optical communications or even light sources for ultra-miniaturized spectrometers.