Synthesis and Emission Dynamics of Sub‐3 nm Upconversion Nanoparticles

• Published in: Advanced optical materials 2024-08, Vol.12 (24), p.n/a
• Main Authors: Amouroux, Baptiste, Eftekhari, Ali, Roux, Clément, Micheau, Jean‐Claude, Roblin, Pierre, Pasturel, Mathieu, Gauffre, Fabienne, Würth, Christian, Resch‐Genger, Ute, Sliwa, Michel, Bouchet, Aude, Coudret, Christophe
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Cross relaxation; Emission; Emitters; Excitation; Gadolinium; Luminescence; Nanoparticles; or physical chemistry; Quenching; single‐emitter particles; surface quenching rate constant; Synthesis; temperature cycling; Theoretical and; Thulium; ultrasmall upconversion nanoparticles; Upconversion; Ytterbium
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.202303283

Reducing the size of upconversion nanoparticles (UCNPs) down to a few nm yields luminescent materials containing a very small number of emitters. Considering the bottom limit of one activator per particle ultrasmall UCNPs offer an unprecedented platform to study the contributions of the different energy transfers at play in upconversion luminescence. Maintaining detectable emission despite the limited number of emitting ions and the high surface‐to‐volume ratio requires suitable particle architectures. Na(Gd‐Yb)F4:Tm3+ emissive sub‐3 nm diameter β‐phase UCNPs are prepared using a gadolinium‐rich composition in situ mixing of the precursors and a microwave high‐temperature cycling sequence allowing precise control of the particle size and dispersity. These cores are coated with a NaGdF4 inert shell to minimize the deleterious influence of surface quenching (SQ). Time‐resolved luminescence measurements combining standard NIR excitation of the Yb3+ sensitizer and direct UV excitation of the Tm3+ activator are performed to quantify cross relaxation and surface quenching processes. The fine tuning of the number of activators per particle via an optimized synthesis pathway along with the use of an appropriate excitation scheme enabled to provide an accurate analysis of the different mechanisms at play in these model nanoparticles and to characterize the structure of the core‐shell architecture. Reducing the size of upconversion nanoparticles to a few nanometers produces materials containing a very small number of emitters. This simplifies the analysis of the contributions of the different energy transfers at play in upconversion luminescence. The synthesis of these ultrasmall nanoparticles requires a sophisticated, optimized and reproducible protocol so that the size and crystallinity can be well controlled.