Effective regulation of upconversion luminescence in NaErF4 and NaErF4@SiO2 core-shell particles.

Upconversion luminescence materials have gained extensive attentions owing to their unique luminescence characteristics of converting near-infrared excitation to visible light emission. However, how to improve the doping concentration of activator (usually <3 mol%) in traditional sensitizer-activator system is still a great challenge due to the luminescence quenching effect. In this work, NaErF 4 crystals with diverse crystal phases and morphologies were prepared by simple hydrothermal method, constructing a 100% doping concentration system of activator (Er3+). With the increase of F− content, the phase of NaErF 4 transfers from cubic to hexagonal, and the morphology changes from nanospheres to hexagonal microflakes, meanwhile, the upconversion luminescence intensity also increases gradually. In addition, in order to further inhibit the luminescence quenching effect of NaErF 4 bare core, a series of NaErF 4 @SiO 2 core-shell samples with different SiO 2 shell thickness were successfully prepared. Compared with NaErF 4 bare core, the morphology and structure of the NaErF 4 @SiO 2 core-shell samples have no obvious change, but the UC luminescence intensity is significantly enhanced. Interestingly, the change of NaErF 4 crystal phase and SiO 2 shell thickness can both bring about multicolor luminescence. In addition, different from the traditional NaYF 4 :Yb, Er co-doping system, the as-prepared hydrophilic NaErF 4 and NaErF 4 @SiO 2 samples showed a greater red to green ratio (R/G > 1), which may expand their potential applications in biological imaging and sensing fields. [ABSTRACT FROM AUTHOR]