Confining Excitation Energy in Er3+‐Sensitized Upconversion Nanocrystals through Tm3+‐Mediated Transient Energy Trapping

A new class of lanthanide‐doped upconversion nanoparticles are presented that are without Yb³⁺ or Nd³⁺ sensitizers in the host lattice. In erbium‐enriched core–shell NaErF₄:Tm (0.5 mol %)@NaYF₄ nanoparticles, a high degree of energy migration between Er³⁺ ions occurs to suppress the effect of concentration quenching upon surface coating. Unlike the conventional Yb³⁺‐Er³⁺ system, the Er³⁺ ion can serve as both the sensitizer and activator to enable an effective upconversion process. Importantly, an appropriate doping of Tm³⁺ has been demonstrated to further enhance upconversion luminescence through energy trapping. This endows the resultant nanoparticles with bright red (about 700‐fold enhancement) and near‐infrared luminescence that is achievable under multiple excitation wavelengths. This is a fundamental new pathway to mitigate the concentration quenching effect, thus offering a convenient method for red‐emitting upconversion nanoprobes for biological applications.