A team of researchers from Seoul National University (SNU), Columbia, and Korea Research Institute Standards and Science (KRISS) have, for the first time, demonstrated an on-chip visible light source with a graphene filament.
Bright Visible Light Emission in Graphene
This clip shows the emission of light from graphene, and radiation spectrum engineering by strong optical interference effect. The graphene vibrates during light emission due to the flexural mode of graphene at high temperature. (Credit: Myung-Ho Bae/KRISS)
The team, led by Young Duck Kim, a postdoctoral researcher at Columbia Engineering, attached small graphene strips to metal electrodes and suspended the strips over a substrate. When a current was passed through the filaments, they heated up.
We've created what is essentially the world's thinnest light bulb. This 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
James Hone, Professor of Mechanical Engineering at Columbia
"This 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," said Hone, Wang Fon-Jen Professor of Mechanical Engineering at Columbia Engineering and co-author of the study.
In order to develop fully integrated 'photonic' circuits, it is necessary to produce light in small structures over the surface of a chip. Although researchers have taken several different approaches this, it has not yet been possible to put the incandescent light bulb - the simplest and oldest artificial light source - onto a chip. This is mainly due to the heat of light bulb filaments, which must be thousands of degrees Celsius in order to radiate in the visible range. Metal wires in the micro-scale cannot withstand such high temperatures. Further, the high efficiency of heat transfer from the filament to its surroundings at the microscale can cause damage to the surrounding chip, making such structures impractical.
The team showed that the graphene reached temperatured over 2500 ᵒC by measuring the spectrum of emitted light. This temperature was sufficient for the graphene to glow brightly. "The visible light from atomically thin graphene is so intense that it is visible even to the naked eye, without any additional magnification," explained Young Duck Kim, first and co-lead author on the paper.
At specific wavelengths, peaks could be observed in the emitted light spectrum. The team identified that the peaks were due to interference between the light that was directly emitted from the graphene, and that which reflected off the silicon substrate and was passed back to the graphene. Kim stated, "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."
Graphene's ability to reach such high temperatures without melting the metal electrodes or the substrate is due an interesting characteristic: graphene's conductivity decreases as it heats up. This means that the high temperatures are restricted to a small 'hot spot' in the center.
"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. 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," explained Myung-Ho Bae, a senior researcher at KRISS and co-lead author.
By creating large-scale of arrays of chemical-vapor-deposited (CVD) graphene light emitters, the team demonstrated that this new technique was scalable.
Yun Daniel Park, co-lead author and professor in the department of physics and astronomy at Seoul National University said, "Edison originally used carbon as a filament for his light bulb and here we are going back to the same element, but using it in its pure form - graphene - and at its ultimate size limit - one atom thick."
The team is currently analysing and characterising the performance of the devices, including the time taken to turn on and off to create 'bits' for optical communications, and developing techniques to integrate them into flexible substrates.
Hone added, "We are just starting to dream about other uses for these structures -- for example, as micro-hotplates that can be heated to thousands of degrees in a fraction of a second to study high-temperature chemical reactions or catalysis."
The study, titled 'Bright visible light emission from graphene', was published in the Advance Online Publication (AOP) on Nature Nanotechnology's website.
The work was supported by the Korea Research Institute of Standards and Science as a part of the project 'Convergent Science and Technology for Measurements at the Nanoscale', and funded by the Korea government.