Fluorescent carbon nanodots: making foldable displays

Imagine reading a newspaper with an LED-like display that folds to fit in your pocket. Or materials that can visualize individual cell processes in real-time. That’s part of the promise of Doo-Young Kim’s research.

Doo-Young is a Ph.D., and an Associate Professor in the department of chemistry at the University of Kentucky. He conducts research into carbon nanodots. It’s a substance that is beginning to play a greater role in your life, whether you know it or not.

Carbon nanodots are tiny particles made of carbon that scientists can make from various sources such as bulk carbon, carbohydrates, or even biomass, a total mass of organisms.

“These particles are relatively easy to synthesize and the cost of preparation can be really cheap,” Doo-Young said.

After appropriate surface treatment, carbon nanodots can emit strong fluorescent light. And the color of emission can be tuned from ultraviolet to red.  These particles are useful in cell imaging, optoelectronics, solar cells, catalysis, and many other areas. And because it’s non-toxic, it can also be used in medicine. Florescence for chemical analysis reveals information on biological samples.  

Carbon nanodots are stacks of a few graphene layers. Graphene is a continuous two-dimensional carbon honeycomb. Due to the confined size, carbon nanodots have finite band-gap which can absorb and emit light. By modifying the size and surface chemistry, scientists can tune the color of fluorescence from carbon nanodots.

The graphene does not fluoresce since it’s a metallic substance. It has high electrical conductivity, useful in many applications. But graphene’s interesting optical property emerges when the size of the graphene becomes smaller.

“Think about hundreds of carbon atoms which are arranged to form two dimensional sheets with a finite size,” he said. “It becomes semi-conductive and can emit strong fluorescence. A carbon nanodot, the graphene with finite size, shifts the molecules from metallic properties to semiconductor properties.”

There are a few useful applications for carbon nanodots. One is already showing great promise and is being extensively explored. That is the use of carbon nanodots in bioimaging and photo-induced therapeutics. Carbon nanodots are introduced into biological cells. It’s applied to “stain” the cell and track the biological components. It colors the specific cellular components and tracks its locations and movements with certain carbon nanodots, fluorescing at different colors.  

Another medical application is cancer treatment. If you excite the carbon nanodots with a specific light source, it generates toxic chemicals locally, such as singlet oxygen species. Certain species have been identified as being able to damage a cancer cell. This area, so called, Photodynamic Therapy, is being intensely studied.

“My research is not just about studying basic research, but to make the carbon nanodot more efficient to produce singlet oxygen species,” Kim said.

The biological applications and therapeutic research are popular, and Doo-Young is optimistic about the effectiveness of this technology. But another area has faced a slower adoption. That is in the area of displays. An LED, or light emitting diode, is a display technology that uses relatively expensive silicon to manufacture and requires a more sophisticated optimization.

Scientists have been trying to utilize carbon nanodots as active components for display devices, so called organic light emitting diodes, or OLEDs. OLEDs take advantage of the carbon nanodot’s fluorescing properties. These are being used for displays in televisions and monitors. The advantages include a lower cost to produce and flexibility of the material. OLED-based displays can be made in a curved shape, but durability and its susceptibility to the environment have been a hindrance.

Doo-Young just received funding from the Samsung Advanced Institute of Technology to conduct the fundamental research to study the optical properties of carbon nanodots.

“My job is to understand how the structure of the carbon nanodots affects the functions and properties of the carbon nanodots,” he said. “We try to understand how the size and structure of the carbon nanodot influences the fluorescence and other properties.”

OLED as a display technology is already being used commercially by many companies using organic emitting molecules, including Samsung and LG, which are the leaders in this area. They continue to search for new materials with better performance and less expensive manufacturing costs. Equipped with new technologies based on bright, durable, and color-tunable OLEDs, Doo-Young and his colleagues are optimistic there will be completely foldable electronic display devices in the near future.

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