UV-Exposed Nanotubes Emit Red Light

By simply attaching light-emitting chemicals to carbon nanotubes and exposing them to UV, researchers have enabled the nanotubes to emit red light.

To the human eye, carbon nanotubes usually appear as a black powder. They can hardly be forced to emit light, as they are excellent electrical conductors and capture the energy from other luminescent chemical species placed nearby. The good charge conductivity combined with high luminescence properties could make these new nanotubes attractive to OLED-based technologies.

Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw contributed to the development of the new photonic material as part of the international FINELUMEN project, coordinated by Dr. Nicola Armaroli from Italy’s Institute for the Organic Synthesis and Photoreactivity (ISOF-CNR) in Bologna.

In the visible light, the carbon nanotubes usually appear as a black powder (top picture). After coating with europium containing lanthanide complexes, developed at the Institute of Physical Chemistry of the PAS in Warsaw, the powder irradiated with a UV lamp emits red light (bottom picture). (Images: IPC PAS/Grzegorz Krzyzewski)

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

Carbon nanotubes can be envisaged as a graphite sheet rolled up into a seamless cylinder. The surface area of each nanotube is relatively high and allows it to attach to many other molecules, including those capable of emitting light.

“Attachment of light-emitting complexes directly to the nanotube is, however, not favorable because the latter, as a black absorber, would highly quench the luminescence,” said Valentina Utochnikova, a PhD student at IPC PAS.

To reduce undesired effect of light absorption, the nanotubes are subjected to a thermal reaction of 140 to 160 ºC in a solution of ionic liquid modified with a terminal azido function. The reaction yields nanotubes coated with molecules acting as anchors/links. On one side, the anchors are attached to the surface of the nanotube, and on the other, they can attach molecules capable of displaying visible light. The free terminal of each link bears a positive charge. The prepared nanotubes are subsequently transferred into another solution containing a negatively charged lanthanide complex composed mainly of europium.

Valentina Utochnikova, a PhD student, presents a sample of carbon nanotubes with europium complexes developed at the Institute of Physical Chemistry of the PAS in Warsaw. When exposed to UV irradiation, the powder emits red light.

“Lanthanide compounds contain elements from the VI group of the periodic table and are very attractive for photonics, as they are characterized by a high luminescence quantum yield and a high color purity of the emitted light,” Utochnikova said.

After dissolving 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. Upon completion of the reaction, the modified nanotubes are washed and dried. The final product is a sooty powder. If the powder is exposed to UV irradiation, however, the lanthanide complexes anchored to nanotubes immediately begin to emit red light.

The concept of how to modify the nanotubes and the reagents — ionic liquid and lanthanide complex for carbon nanotube coating — was developed by Pietraszkiewicz’s research group at IPC PAS, but the modification of nanotubes and spectral studies were done by groups from the University of Namur, Belgium and ISOF-CNR.

The photonic material can be used, among others, to detect molecules, including those of biological importance. The identification would take place by analyzing how the luminescence of nanotubes changes upon deposition of molecules.

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