Yb³⁺ and Er³⁺ Codoped BaLiF₃ Nanocrystals for X-ray Dosimetry and Imaging by Upconversion Luminescence

We report on the potential of BaLiF3 as a host matrix for the upconversion sensitizer (Yb3+)-activator (Er3+) pairs as prepared by ball milling. Specifically, the infrared (980 nm)-excited upconversion luminescence (UCL) of BaLiF3:Yb3+, Er3+ samples before and after annealing in air is reported, and it is demonstrated that the upconversion efficiency is enhanced by ∼2 orders of magnitude via a postannealing treatment. The UCL of BaLiF3:Yb3+, Er3+ compared favorably with that of the efficient NaYF4:Yb3+, Er3+ UCL nanophosphors. In addition, the X-ray storage capability of BaLiF3:Yb3+, Er3+ was explored. It was observed that the photoluminescence intensities of Yb3+ (∼980 nm) and Er3+ (∼650 nm) ions decrease gradually with cumulative X-ray doses due to X-ray-induced reduction of Yb3+ to Yb2+ and Er3+ to Er2+, and as a result, the upconversion luminescence decreases. The Yb2+ photoluminescence spectrum upon X-irradiation was observed at 480 nm in accord with earlier reports for the BaLiF3 host. In a unique development, 2D X-ray imaging based on the UCL signal is reported for the first time. These results demonstrate the X-ray recording capability of mechanochemically synthesized BaLiF3:Yb3+, Er3+ nanocrystals and point to possible applications in combining X-ray dosimetry and bioimaging.