Now Wenbo Ma, Yirong Su and colleagues from China, Italy and South Africa have reported (Nat. Mater. https://doi.org/10.1038/s41563-021-01132-x, 2021) that X-ray-induced triplet excitons can actually enhance emission by exploiting organic thermally activated delayed fluorescence (TADF) molecules. In fact, it turns out that the use of triplets may raise the fundamental limit of scintillator conversion efficiency. The team also used the platform to demonstrate X-ray imaging, of various sample types including an integrated circuit (bottom row of figure), with a resolution down to 16.6 line pairs mm−1.
Yang (Michael) Yang, corresponding author on the work, explained to Nature Photonics that an advantage of organic materials is that their properties and functionalities can be artificially tailored. Yang said that the key to the start of this study was to understand how organic molecules are excited by X-rays. “If that is similar to UV excitation, then an organic dye with very high PLQY [photoluminescence quantum yield] is perhaps the best one we can have, and there are many,” explained Yang. “…the energy of X-ray photons are so high that they can ionize molecules and produce positive ions and secondary electrons. The recombination of ions and secondary electrons is actually very similar to the case of LEDs, and it indicates many triplets will be generated under X-ray illumination […] there are many more triplets than singlets generated by X-rays. With this physical picture in mind, the idea to use TADF for X-ray scintillation is natural; it should be better than conventional fluorescent scintillators.”
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