Back in April, we covered news that graphene was going to make a commercial breakout of sorts as a coating in an LED light bulb to reduce its energy consumption and lengthen its lifetime.
Now researchers at Columbia University from James Hone’s lab, in cooperation with a team at Korea Research Institute of Standards and Science (KRISS), have taken this huge step forward by creating the first on-chip incandescent visible light source using graphene as the filament.
“We've created what is essentially the world's thinnest light bulb,” says Hone, a professor at Columbia Engineering, in a press release. “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.”
In work published in the journal Nature Nanotechnology, researchers suspended graphene above a silicon substrate by attaching it to two metal electrodes and then passed current through the graphene-based filament, causing it to heat up.
In the video below you can see an animated depiction of how the graphene filament operate.
The aim of creating integrated circuits that use photons rather than electrons, sometimes called integrated photonic circuits, depends on being able to generate light on the chip itself. While a number of approaches have been developed for generating this light, this research marks the first time that anyone has done it with the simplest artificial light source: incandescent light.
The main reason that this was never achieved before is because of the amount of heat that incandescent light generates. In order for these micro-scale metal wires to glow in the visible light range, they must be able to withstand temperatures reaching thousands of degrees Celsius. Getting that level of heat to transfer out of the micro-scale wires was always a problem and often led to damaging the surrounding chip.
But graphene makes all the difference. The international team demonstrated heating the graphene-based filament to 2500 degree Celsius, so that it would glow brightly enough to be seen by the naked eye. The material’s success in this application depended on two of its properties: transparency and the fact that it acts as a poorer heat conductor the hotter it gets.
Graphene’s unusual heat conduction was key to keeping the light emitter from destroying the chip it was built on. The lack of conduction confines the heat within a small “hot spot” in the center of the graphene filament.
Graphene’s transparency was behind the discovery that emitted spectrum of light emitted had peaks at certain wavelengths. This, they found, occurred because of interference between the light emitted from the graphene filament and the light reflecting off the silicon substrate beneath it. So it becomes possible to tune the emission spectrum by altering the distance between the filament and the substrate.