Following the arc of carbon light sources

Joseph Swan who was born in the coal-rich North East of England would hopefully approve of the natural successor to his invention - the incandescent light bulb - being made from strips of the 2D carbon allotrope, graphene, he having used carbonized strips of paper as his experimental filaments.

In work led by Young Duck Kim, researchers from Columbia University, New York, USA, Seoul National University (SNU) and the Korea Research Institute of Standards and Science (KRISS) have demonstrated for the first time an "on-chip" visible light source that uses graphene as its "filament". To construct their light source, the team attached small strips of graphene to metal electrodes, which were suspended above the substrate. When they applied a current, the filaments heat up and emit bright, visible light [Hone et al., Nature Nanotechnol, 2015, DOI: 10.1038/nnano.2015.118]

"We've created what is essentially the world's thinnest light bulb," says Kim's group leader James Hone. "his new type of 'broadband' light emitter can be integrated into chips and will pave the way towards the realization of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications," he suggests.

Photonic microcircuitry will only be possible with the development of on-chip light sources that can be integrated into the technology and do with photons what conventional integrated circuits do with electrons. Researchers have, of course, experimented with light emitting diodes (LEDs) and their ilk, but the oldest and simplest electric light source, the incandescent light bulb, seemed off limits because they have to get very hot, thousands of degrees, to produce a visible glow and these temperatures would simply melt the surrounding circuitry.

However, graphene has some interesting properties that make it the ideal candidate for a viable filament light source. Fundamentally, as it gets hotter graphene becomes a poor conductor of heat, which means that high temperatures are confined to a tiny "hot spot" in the center of a piece of the material. This neatly sidesteps the problem of overheating as the glowing graphene itself protects its surroundings from the heat generated. Team member Myung-Ho Bae of KRISS explains that, "At the highest temperatures, the electron temperature is much higher than that of acoustic vibrational modes of the graphene lattice, so that less energy is needed to attain temperatures needed for visible light emission." He adds that, "These unique thermal properties allow us to heat the suspended graphene up to half of the temperature of the sun, and improve efficiency 1000 times, as compared to graphene on a solid substrate."

An additional interesting characteristic of the graphene filament light source is that light reflected from the silicon substrate interferes with the direct light being generated and passes through the transparent graphene. This allows the team to tune the emission spectrum of the light source simply by changing the distance between filament and substrate.

The team is now investigating how fabrication might be scaled and also characterizing the device's performance for future optical communications applications. In addition to ICT applications, Hone also suggests that arrays of the devices might be used for rapid heating of chemical reactions on a micro-hotplate for fast, high-temperature studies in catalysis and other areas of chemistry.

"We are now developing graphene-based transparent and flexible displays and on-chip photonics circuits," Kim told Materials Today. "We are also studying the unique physical properties and new phenomena of low-dimensional materials under extremely high temperatures (T > 2000 K)."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".