Researchers from Tsinghua University in Beijing demonstrated a graphene-based LED that not only can be tuned to emit different colors of light, but can do so across nearly the entire visible spectrum: from blue (450-nm wavelength) to red (750-nm wavelength)—basically all colors but the darkest blues and violets. Such a color tunable LED has never before been realized.
The scientists made the light-emitting material from the interface of two different forms of graphene. These forms are graphene oxide (GO) and reduced graphene oxide (rGO). Placed 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.
The occurrence of this property is especially interesting because, on their own, neither GO nor rGO (or any other known form of graphene) can emit any light at all. According to the scientists, this is because neither material has the right size bandgap - GO has an extremely large bandgap while rGO has a zero bandgap. Instead of having a bandgap somewhere in between GO and rGO, the partially reduced interfacial GO actually 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. Because these rGO nanoclusters are reduced to varying degrees at the interface, they include variations in their energy levels and, consequently, in the color of emitted light. These 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.
In their work, the researchers 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 2 hours in vacuum. The researchers attribute the short lifetime to oxidation in the air and predict that protective coatings may improve this area.
Applications of the new LED are varied and may 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.