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 preparationjust the right amounts of sintering temperature and timethey 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 said. "We will continuously tune the parameters so that we may find a much better one."
The researchers spent an additional year testing the materialindoors and out, as well as on sunny days, cloudy days and rainy daysto 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 said, "so this means that it is a good candidate for making solar cells significantly more efficient."
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