Development of Core Technology for Graphene Quantum-dot Displays
Opening the era of paper-thin displays
“Graphene quantum dots made using the graphite intercalation compound method produce high efficiency light-emitting diodes with over 1,000 cd/m2 of luminance. While the maximum brightness on mobile phone displays is typically just several hundred cd/m2, this study produced much higher luminance graphene quantum-dot LEDs thanks to the high quantum efficiency, proving the practical applicability of the technology. This study has great significance in its application to high-efficiency LED devices and marks the beginning of greater enhancements to the luminance of LED devices. It is expected to contribute to technological advances in a variety of fields such as displays, solar panels, and bio-imaging.”
The world's first graphite-derived high-quality graphene quantum dots
The era of paper-thin displays is here. A joint project between KINC Professor Jeon Seokwoo's team, together with Professor Cho Yong-hoon from the Physics department and Professor Yoo Seunghyup from the Electrical Engineering department, has succeeded in developing light-emitting diodes (LEDs) based on graphene quantum dots (GQDs).
Quantum dots refer to semiconductor crystals of around 10 nm in size, and research is ongoing to produce quantum dots from a variety of materials such as cadmium (Cd), indium (In), and lead (Pb). However, the majority of quantum dots are made of toxic heavy metals and are vulnerable to oxygen and moisture in the air, limiting their practical applicability. Recent efforts have focused on developing quantum dots that are environmentally safe and stable under atmospheric exposure.
Over the years, to understand the fluorescent mechanism, Professor Jeon Seokwoo and his group have reported controlled oxidation of graphene quantum dots (GQDs) synthesized by etching from single-layer, CVD-grown graphene. This method is a controllable fabrication method for 10 and 20 nm graphene quantum dots (GQDs) from single-layer CVD grown graphene using a self-assembled block copolymer. Recently, Jeon Seokwoo's group also introduced a novel graphite intercalation compounds (GICs) approach to generate graphene flakes by using graphite intercalation compounds. Through this method, high quality graphene flakes with extremely low oxygen contents were fabricated. Based on these results, Jeon Seokwoo's group suggests an easy and mass-producible route to obtain high-quality GQDs with controlled oxidation by graphite intercalation compounds (GICs). The proposed method is cost-effective, eco-friendly, and can easily be scaled up, as it allows the direct fabrication of GQDs using water without a surfactant or chemical solvent.
Professor Jeon Seokwoo's group also succeeded in emitting fluorescence in the visible spectrum from the quantum dots, identifying its blue light-emitting properties. They found that the unique properties of GQDs allow them to emit light when modified, and successfully utilized them to produce high-brightness LEDs. Furthermore, they used the filtration method to control the size and luminance of the graphene quantum dots, creating graphene quantum dots capable of emitting a spectrum of light from blue to yellow.
Traditionally graphene quantum dots have been produced by the method of oxidation and further cutting reactions of graphite. However, numerous oxygenous functional groups and defects are inevitably generated both on the basal plain and at the edges of GQDs, significantly making it difficult to control the emission wavelength of GQDs. This type of oxidation thwarts both fundamental understanding of the origin of luminescence and attempts to realize practical applications. On the other hand, GQDs made using the intercalation compound method were then used as emitters in organic light-emitting diodes (OLEDs) in order to identify the GQD’s key optical properties. After carefully designing the layer configuration so that electron and hole injection could be balanced, the constructed GQD LEDs exhibited luminance of 1,000 cd/m2, which is well over the typical brightness levels of the portable displays used in smartphones.
It took Professor Jeon Seokwoo's team a year to produce the graphene quantum-dot LED. They also had to overcome technical challenges to the project. For example, much trial and error went into configuring the specific dispersion in the PVK polymer, a crucial part of the process of applying GQDs to the light-emitting layer. Through tenacious research, the team was able to enhance the LED’s dispersal and luminance. Current efforts are focused on the optimization of light emission and device structure. Using the core technology for graphene quantum-dot displays, it will someday be possible to produce not just paper-thin displays but also displays on flexible materials such as curtains. Furthermore, future explorations should be continuously focusing on extending their PL spectra to all visible wavelengths, NIR and even IR, all of which will greatly contribute to for numerous applications from bioimaging to optoelectronic devices.
This research by Professor Jeon Seokwoo’ group has been published as a cover article of the international optical journal, Advanced Optical Materials, and was selected as the best paper of 2014, garnering international interest and recognition.
Prof. Jeon, Seokwoo
2014 Annual Report