Enhanced Red Emission in Er-Sensitized NaLuF Upconversion Crystals via Energy Trapping.

Luminescence efficiency of trivalent lanthanide-doped upconversion (UC) materials is significantly limited by luminescence concentration quenching. In this work, red UC emission is dramatically enhanced in Er-sensitized NaLuF UC crystals through energy trapping under multiple excitation wavelengths. Cross-relaxation quenching and the energy migration to internal lattice defects are simultaneously suppressed by confining the excitation energy in the Er activator after introducing the Tm or Ho energy trapping center. The enhanced red UC emission (Er: 660 nm) mainly comes from the effective excitation energy confinement by Tm and Ho trapping centers through an easy energy transfer between Er and Tm/Ho: I (Er) → H (Tm) → I (Er) and I (Er) → I (Ho) → I (Er). It is found that the confining efficiency of excitation energy in Er-sensitized NaLuF crystals is higher than that in Yb/Er cosensitized NaLuF crystals, and the luminescence efficiency of Er-sensitized NaLuF crystals is much higher than that of Er-based host sensitization UC crystals (NaErF). Moreover, Er-sensitized UC particles can be efficiently excited by three different wavelengths (808, 980, and 1532 nm), indicating huge advantages for applications in bioimaging, anticounterfeiting, and solar cells.