Yb³⁺ and Er³⁺ codoped BaLiF₃ nanocrystals for X-ray dosimetry and imaging by upconversion luminescence

Abstract

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.