Abstracts

In the field of lanthanide luminescence initially the spectroscopic properties of individual lanthanides were explored and also applied, e.g. in lamp phosphors as Y₂O₃:Eu³⁺ or x-ray phosphors as Gd₂O₂S:Pr³⁺ where the performance relied on a single type of lanthanide ion. Presently it is recognized that cooperation between lanthanides enables new applications and is also interesting for fundamental studies to elucidate the nature and strength of interactions between lanthanides. In this presentation we focus on the role two types of interactions: (1) Cooperative effects and (2) Cross-relaxation in lanthanide-doped luminescent materials.Cooperative absorption, emission and energy transfer were pioneered in the 1970’s with a prominent role for Yb³⁺. After a historical and personal introduction to cooperative effects in lanthanide spectroscopy, the focus is on cooperative energy transfer from a single lanthanide to two or even three neighboring lanthanide ions. Evidence for cooperative energy transfer processes is challenging but can be obtained by modelling luminescence decay curves using the discrete shell model. In addition, correlated photon counting can serve as evidence for the presence (or not!) of cooperative energy transfer and quantum cutting.Cross-relaxation is partial energy transfer between neighboring lanthanide ions of the same or different type. It is often undesired as it can cause effective concentration quenching, e.g. in Pr³⁺-doped phosphors. It can however also be beneficial and cross-relaxation is at the heart of photon avalanching which recently experiences a revival in photon avalanching nanocrystals. Here we use cross-relaxation between neighboring Ho³⁺ ions to probe ion diffusion in NaYF₄ core-shell nanocrystals and show that it serves as a sensitive method to probe the sharpness of core-shell interfaces. Analysis of luminescence decay curves for NaYF₄:12%Ho³⁺ core/NaYF₄ shell nanocrystals gives quantitative insight in diffusion of Ho³⁺ ions into the undoped shell upon heating. Finally, it will be demonstrated how temperature dependent competition between cross-relaxation and multi-phonon relaxation can be used in accurate temperature sensing.The presentation will provide insight in the important role of interactions between luminescent lanthanide ions in present and future applications and shows how the cooperative effects and interactions between lanthanides can be quantified through different time-resolved spectroscopic experiments combined with theoretical modelling.