First color-tunable graphene-based LED is demonstrated
The LED can emit from blue (450-nm wavelength) to red (750-nm wavelength). Only the darkest blues and violets are not emitted.
The color-tunable LED is made from graphene which is a material showing promise in the manufacture of next generation batteries and solar cells. Graphene-based LEDs have never been realized until now. The Tsinghua University research marks the advent of the first-ever graphene-based LED in addition to being the first color-tunable LED.
Applications of the LED include high-quality, color-tunable LED displays for TVs and mobile devices, color-tunable LED light fixtures, and the potential for a variety of future graphene-based photonic devices.
The researchers, led by Professor Tian-Ling Ren at Tsinghua University in Beijing, made the light-emitting material from the interface of two different forms of graphene. These forms are graphene oxide (GO), which is produced from inexpensive graphite, and reduced graphene oxide (rGO), which is a more pristine form of GO.
Lying at the interface of the GO and rGO is a special type of partially reduced GO that has optical, physical, and chemical properties that lie somewhere in between those of GO and rGO. The most important ‘blended’ property of the interfacial layer is that it has a series of discrete energy levels, which ultimately allows for the emission of light at many different energies, or colors.
On their own, neither GO nor rGO can emit any light at all because neither material has the right size ‘bandgap’. GO has an large bandgap while rGO has a zero bandgap.
Instead of having a bandgap somewhere in between GO and rGO, the partially reduced interfacial GO has many different intermediate bandgaps as a result of how the blending occurs — not as a smooth transition, but in the form of rGO nanoclusters embedded within the GO layer. The rGO nanoclusters are reduced to varying degrees at the interface, they exhibit variations in their energy levels and, consequently, in the color of emitted light. The energy levels can be easily modulated by changing the applied voltage or by chemical doping, which selectively stimulates a single color of luminescence and enables tuning of the LED’s color.
The researchers discovered that a combination of GO and rGO can create a conductive and wide bandgap material.
The discovery is the first observation of luminescence in a graphene-based system and enables graphene to be used as a light source in future graphene-based photonic devices. A color-tunable LED is ideal for high-quality LED displays and light fixtures and because the color changes can be madein response to certain chemicals, the devices could also be used as sensors.
The use of a graphene-based, color-tunable LED means that full-color and flexible displays can be realized and makes a wide range of new consumer and medical electronics applications a possibility.
The researchers designed, fabricated, and tested 20 graphene-based LEDs. Overall, the devices demonstrated good brightness but low efficiency, which they plan to improve. Another drawback of the current prototype is a very short emission lifetime of less than a minute or so in ambient conditions and about two hours in vacuum. The researchers attribute the short lifetime to oxidation in the air and predict that protective coatings may improve this area.
Despite the room for improvement, the researchers expect the graphene-based LEDs to have encouraging commercial prospects due to several advantages, including their precise color tunability, compact structure, and straightforward fabrication.
Xiaomu Wang, et al. "A spectrally tunable all-graphene-based flexible field-effect light-emitting device." Nature Communications. DOI: 10.1038/ncomms8767