1.INTRODUCTION
Afterglow phosphors are capable of a long-term storage for varied excitation photons, including X-ray, ultraviolet, and visible light.[1-6] Their potential application in information encryption has been widely demonstrated in a two-dimensional (2D) manner such as emergence sign,[7] anti-counterfeiting pattern,[8] and X-ray imaging screen.[9]Up to date, many afterglow materials have been developed such as SrAl2O4: Eu2+, Dy3+,[10] CaS: Eu2+, Sm3+,[11]SrSi2AlO2N3:Eu2+, Ln3+.[12] These traditional phosphors usually have a high lattice energy, which requires a high temperature calcination procedure to form the target phase. Unfortunately, the calcination process not only increases the production cost but also brings a severe safety risk to the manufacturer. Moreover, it causes unwanted agglomeration of powders that leads to a severe scattering issue.[13, 14] This poses a great challenge to the transparency of product and the ensuing application of afterglow phosphors in 3D information storage or volumetric display.
To circumvent the scattering issue, transparent afterglow materials have emerged in the last decade. For instance, Claire et al. synthesized a polycrystalline ZnGa2O4 ceramic by combining high-energy ball milling, solid-state reaction and spark plasma sintering methods. The maximum of transmittance reached 78% in the near infrared region.[15] Tang et al. successfully embedded fluoride nanocrystals into an amorphous glass matrix, which exhibited a high transmittance up to 80%.[16] One major caveat of these nanocrystal-based strategies was the low photoluminescence quantum yield and ensuing poor afterglow intensity as a consequence of the abundant quenching sites on the surface. Recently, our group has developed several transparent afterglow crystals based on chloride double perovskite.[5, 17-20] Through proper doping of activator ions, the single crystals exhibited a tunable afterglow emission ranging from red to near-infrared light. Thanks to their low phonon energy, the chlorides showed a high quantum efficiency up to 82% and an afterglow duration of 2 hours. In addition, the doping of Tb ions in Cs2NaScCl6 not only boosted the PL QY up to 98.2%, but also activated an intense green afterglow up to 12 hours after ceasing X-ray excitation.[20] Despite these progress, ultraviolet chargeable transparent crystals of long-term afterglow remained elusive.
In this work, a transparent CsCdCl3 crystal was grown by programmable cooling method in a hydrothermal reactor. The PL intensity of pristine crystals was significantly by 4 folds by elevating the temperature from 25 oC to 200 oC. Such abnormal behavior was investigated and ascribed mainly to the thermal enhanced absorption. After Mn2+ doping, the PL QY exhibited a dramatical increase to nearly unity. Importantly, an ultra-long afterglow over 12-hour was activated under UV excitation.