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 up -conversion 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 lu-minescence 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. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

In lanthanide-doped upconversion nanoparticles (UCNPs), the cross-relaxation between activator ions, particularly Er3+ ions, has a general adverse effect on up-conversion luminescence (UCL), contrary to previous beliefs. The energy of the excitation light is accumulated into a low-lying excited state of Er3+ due to cross-relaxation, forming a low-lying excited state energy trap that affects the UCL channels. This effect can be utilized to regulate UCL channels and achieve a concentrated UCL band for specific applications.