Red-light-emitting carbon nanotubes could brighten OLEDs

Ashley N. Paddock, ashley.paddock@photonics.com

Carbon nanotubes with light-emitting chemicals attached glow with red light when exposed to ultraviolet wavelengths.

Inducing carbon nanotubes to emit light is difficult, as they are excellent electrical conductors and capture energy from luminescent chemical species placed nearby. Good charge conductivity combined with high luminescent properties could make these red-emitting nanotubes attractive to organic LED-based technologies.

Scientists at the Institute of Physical Chemistry of the Polish Academy of Sciences contributed to the development of the new photonic material as part of the international Finelumen project.

“We take part in the project as a research group specializing in studies on lanthanide compounds,” said professor Marek Pietraszkiewicz. “We decided to combine their high luminescent properties with excellent mechanical and electrical characteristics of nanotubes.”

Carbon nanotubes resemble honeycomb-shape hexagons rolled into a seamless cylindrical tube. The surface area of each nanotube is relatively high, which allows it to attach to many other molecules, including those capable of emitting light. However, attaching a light-emitting complex directly to the nanotube is unfavorable because, as a black absorber, the nanotube would highly quench the luminescence, the scientists explain.

To reduce the light-absorption effect, the nanotubes were subjected to a thermal reaction of 140 to 160 °C in a solution of ionic liquid modified with a terminal azido function. This yielded nanotubes coated with molecules that served as anchors. The anchors attached to the surface on one side of the nanotube, whereas on the other side, the anchors were able to attach molecules capable of displaying visible light. The free terminal of each link bore a positive charge. The prepared nanotubes were subsequently transferred into another solution containing a negatively charged lanthanide complex composed mostly of europium.

Containing elements from the VI group on the periodic table, lanthanide compounds are attractive for photonic applications because they have a high luminescence quantum yield and a high color purity of the light emitted.

Once dissolved in solution, negatively charged europium complexes are spontaneously caught by positively charged free terminals of anchors attached to nanotubes resulting from electrostatic interaction. As a result, each nanotube is durably coated with molecules that can emit visible light. The modified nanotubes are washed and dried once the reaction is completed. The final product is a sooty powder that emits red light when exposed to UV radiation.

The scientists are making the materials, with potential for increased use in bioimaging, optoelectronic devices and sensors, as versatile as possible.

“Photoluminescent carbon nanotubes may find biomedical applications as taggants, bioprobes and markers,” Pietraszkiewicz said. “Also, there is a potential in organic electronics – in particular, in OLED [organic LED] and [organic light-emitting transistor] technologies, since carbon nanotubes are good electron conductors and have been proved to be superior materials for layers in OLED prototype devices.”