Scientists claim 'world's thinnest graphene lightbulb'
15 Jun 2015
A team of scientists from the Columbia University School of Engineering, Seoul National University and the Korea Research Institute of Standards and Science has developed an on-chip visible light source using graphene as a filament.
To achieve this, the research team attached small strips of graphene to metal electrodes, suspended the strips above the substrate, and passed a current through the graphene filaments - causing them to heat up.
Research group leader James Hone said: “We’ve created what is essentially the world’s thinnest lightbulb.”
“The visible light from atomically thin graphene is so intense that it is visible even to the naked eye
Co-lead author Young Duck Kim
“This new type of ’broadband’ light emitter can be integrated into chips and will pave the way towards the realisation of atomically thin, flexible, and transparent displays, and graphene-based on-chip optical communications,” Hone said.
By measuring the spectrum of the light emitted from the graphene filaments, the team was able to show that it was reaching temperatures of above 2500°C, which is hot enough to glow brightly.
Co-lead author Young Duck Kim said: “The visible light from atomically thin graphene is so intense that it is visible even to the naked eye, without any additional magnification.”
During the study, the researchers discovered that the spectrum of the emitted light showed peaks at specific wavelengths - a phenomenon that is due to interference between the light emitted directly from the graphene and light reflecting off the silicon substrate and passing back through the graphene.
“This is only possible because graphene is transparent, unlike any conventional filament, and allows us to tune the emission spectrum by changing the distance to the substrate,” Kim said.
According to the researchers, the ability of graphene to achieve such high temperatures without melting the substrate or the metal electrodes is due to another interesting property: as it heats up, graphene becomes a much poorer conductor of heat. This means that the high temperatures stay confined to a small ’hot spot’ in the centre.
“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,” said Myung-Ho Bae, a senior researcher at the Korea Research Institute.
“These unique thermal properties allow us to heat the suspended graphene up to half of temperature of the sun, and improve efficiency 1000 times, as compared to graphene on a solid substrate,” Bae said.
Looking ahead, the research team is working to further characterise the performance of these graphene lightbulb devices - for example, how fast they can be turned on and off to create ’bits’ for optical communications, and to develop techniques for integrating them into flexible substrates.
