Low-lying excited state energy trap induced by cross-relaxation – The main origin of concentration quenching in lanthanide upconversion nanoparticles

In lanthanide-doped upconversion nanoparticles (UCNPs), the concentration of emitter ions, also known as activator ions, is usually limited to 1 − 5 mol% due to concentration quenching effects. This circumstance limits the luminescent efficiency of UCNPs’ and their use in a variety of application areas. Earlier studies have attributed the activator concentration quenching to migration of energy to the nanoparticle surface, while indicating that cross-relaxation between activator ions had a minor role therein. In this work, we carried out comparative studies on Er³⁺-doped and Yb³⁺-Er³⁺ codoped UCNPs and could, in contrast to this notion, prove a general adverse effect of cross-relaxation between activator ions, here Er³⁺ ions, on upconversion luminescence (UCL). The direct result of the cross-relaxation is that the energy of the excitation light is accumulated into a low-lying excited state of Er³⁺ in the infrared region, so forming a "low-lying excited state energy trap". As a result, the excitation energy is used for generating down-conversion luminescence or for indirectly facilitating UCL channels that are directly related to the low-lying excited state energy trap. The identified effect can be used to regulate UCL channels to achieve a concentrated UCL band that is more favorable for certain applications, e.g., biological imaging.