Material glows in NIR for two weeks
ATHENS, Ga. – Move over, cheesy glow-in-the-dark plastic toys that take forever to “charge up” with light and that fade to black in a blink. A new material can emit a near-infrared glow for two weeks after a single minute of exposure to sunlight, or even fluorescent lights – and its creators envision applications more important than playthings. It could revolutionize medical diagnostics, supply a secret source of illumination for law enforcement and military agencies, or provide the foundation for highly efficient solar cells.
While visible-light emitters are commonplace, there has been little success in creating materials that emit NIR light – until now. The new material from the University of Georgia is composed of trivalent chromium ion, a well-known emitter of NIR light. When exposed to light, its electrons at ground state quickly move to a higher-energy state. As they return to the ground state, energy is released as NIR light. Typically the period of light emission is on the order of a few milliseconds.
Zhengwei Pan, associate professor of physics and engineering, and postdoctoral researcher Feng Liu stand in a darkened room, using only their recently invented ceramic discs, which emit NIR light, as a source of illumination. Their phosphorescent material was mixed into the paint used to create the University of Georgia logo behind them. There is no other source of illumination in the room; without the aid of a night-vision device, the image would be completely dark. At right, 5 g of NIR persistent powder glowing in a glass vial. Images courtesy of Pan and Liu.
The new material uses a matrix of zinc and gallogermanate to host the trivalent chromium ions. Its chemical structure creates a labyrinth of “traps” that capture excitation energy and store it for an extended period. As the stored energy is thermally released back to the chromium ions at room temperature, the compound persistently emits NIR light over a period of up to 360 hours.
“We have successfully discovered a series of trivalent chromium ion-doped gallates that have extraordinary capabilities in excitation energy absorption, storage, NIR light conversion and very long NIR light afterglow,” said Zhengwei Pan, associate professor of physics and engineering at Franklin College of Arts and Sciences; he is also on the engineering faculty. “We expect that this work will not only stimulate the research in persistent luminescent materials, but also several important areas for applications including defense and forensics, solar energy utilization and in vivo bioimaging.”
He explained that the material could be fabricated into nanoparticles that bind to cancer cells, for example, so doctors could visualize small metastases that otherwise might go undetected. For military and law enforcement, the material could be fashioned into ceramic discs that are visible only to those wearing night-vision goggles. It could also provide the foundation for highly efficient solar cells, Pan said.
A cartoon featuring five monkeys was drawn with paint that was made by mixing NIR persistent powder into a water-based indoor wall paint. Courtesy of Grace Pan and Zhengwei Pan.
Besides three years developing the material, the researchers spent an additional year testing it – indoors and out, as well as on sunny, cloudy and rainy days – to prove its versatility. They placed it in freshwater and saltwater and even in a corrosive bleach solution for three months and found no decrease in performance.
Pan said the team will continue to tune the compositions and processing conditions to find materials that are more sensitive to visible sunlight activation. They also plan to explore the applications of the materials in high-efficiency silicon solar cells and deep-tissue bioimaging. Lastly, they will expand the material systems to discover other series of NIR persistent materials.
The research was published online in Nature Materials (doi: 10.1038/nmat3173).
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