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 that emit near-infrared light as a source of illumination. Their phosphorescent material also was mixed into the paint that was used to create the UGA 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. (Imaging parameters are auto, ISO 200, 3 to 4 sec of exposure time using a night vision monocular). Image: Zhengwei Pan/UGA

Materials that emit visible light after being
exposed to sunlight are commonplace and can be found in everything from
emergency signage to glow-in-the-dark stickers. But until now, scientists have
had little success creating materials that emit light in the near-infrared
range, a portion of the spectrum that only can be seen with the aid of night
vision devices.

In a paper published in the early online edition
of Nature Materials, however, University
of Georgia scientists
describe a new material that emits a long-lasting, near-infrared glow after a
single minute of exposure to sunlight. Lead author Zhengwei Pan, associate
professor of physics and engineering in the Franklin College of Arts and
Sciences and the Faculty of Engineering, says the material has the potential to
revolutionize medical diagnostics, give the military and law enforcement
agencies a “secret” source of illumination and provide the foundation
for highly efficient solar cells.

“When you bring the material anywhere outside
of a building, one minute of exposure to light can create a 360-hr release of
near-infrared light,” Pan says. “It can be activated by indoor
fluorescent lighting as well, and it has many possible applications.”

The material can be fabricated into nanoparticles
that bind to cancer cells, for example, and doctors could visualize the
location of small metastases that otherwise might go undetected. For military
and law enforcement use, the material can be fashioned into ceramic discs that
serve as a source of illumination that only those wearing night vision goggles
can see. Similarly, the material can be turned into a powder and mixed into a
paint whose luminescence is only visible to a select few.

The starting point for Pan’s material is the
trivalent chromium ion, a well-known emitter of near-infrared light. When
exposed to light, its electrons at ground state quickly move to a higher energy
state. As the electrons return to the ground state, energy is released as
near-infrared light. The period of light emission is generally short, typically
on the order of a few milliseconds. The innovation in Pan’s material, which
uses matrix of zinc and gallogermanate to host the trivalent chromium ions, is
that 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 near-infrared light over period of up to two weeks.

In a process that Pan likens to perfecting a
recipe, he and postdoctoral researcher Feng Liu and doctoral student Yi-Ying Lu
spent three years developing the material. Initial versions emitted light for
minutes, but through modifications to the chemical ingredients and the
preparation—just the right amounts of sintering temperature and time—they were
able to increase the afterglow from minutes to days and, ultimately, weeks.

“Even now, we don’t think we’ve found the
best compound,” Pan says. “We will continuously tune the parameters
so that we may find a much better one.”

The researchers spent an additional year testing
the material—indoors and out, as well as on sunny days, cloudy days, and rainy
days—to prove its versatility. They placed it in freshwater, saltwater, and
even a corrosive bleach solution for three months and found no decrease in
performance.

In addition to exploring biomedical applications, Pan’s team aims to use it
to collect, store, and convert solar energy. “This material has an
extraordinary ability to capture and store energy,” Pan says, “so
this means that it is a good candidate for making solar cells significantly
more efficient.”

SOURCE – University of Georgia