Enhanced Red Emission in Er3+-Sensitized NaLuF4 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 Er3+-sensitized NaLuF4 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 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 NaLuF4 crystals is higher than that in Yb3+/Er3+ cosensitized NaLuF4 crystals, and the luminescence efficiency of Er3+-sensitized NaLuF4 crystals 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.