Enhanced Red Emission in Er3+-Sensitized NaLuF4Upconversion 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 Er3+-sensitized NaLuF4UC 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 Er3+activator after introducing the Tm3+or Ho3+energy trapping center. The enhanced red UC emission (Er3+: 660 nm) mainly comes from the effective excitation energy confinement by Tm3+and Ho3+trapping centers through an easy energy transfer between Er3+and Tm3+/Ho3+: 4I11/2(Er3+) → 3H5(Tm3+) → 4I13/2(Er3+) and 4I11/2(Er3+) → 5I6(Ho3+) → 4I13/2(Er3+). It is found that the confining efficiency of excitation energy in Er3+-sensitized NaLuF4crystals is higher than that in Yb3+/Er3+cosensitized NaLuF4crystals, and the luminescence efficiency of Er3+-sensitized NaLuF4crystals is much higher than that of Er3+-based host sensitization UC crystals (NaErF4). Moreover, Er3+-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.