Synthesis and emission dynamics of sub-3 nm upconversion nanoparticles

Reducing the size of upconversion nanoparticles (UCNPs) down to a few nm yields unique luminescent materials containing a very small number of emitters. Considering the bottom limit of one activator per particle, such ultrasmall UCNPs offer an unprecedented platform to study the contributions of the different energy transfer processes at play in upconversion luminescence, especially the role of cross relaxation. Maintaining detectable emission despite the limited number of emitting ions and the high surface-to-volume ratio requires suitable particle architectures. The preparation of Na(Gd-Yb)F4:Tm emissive sub-3 nm diameter ꞵ-phase UCNPs was achieved using a gadolinium-rich composition, in situ mixing of the precursors NaOH and NH4F, and a microwave high-temperature cycling sequence that allowed precise control of the particle size and dispersity. Coating strategy of these cores with a NaGdF4 inert shell was performed 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 were performed to evaluate cross relaxation and surface quenching processes in the luminescence properties of these UCNPs. 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 us to select the operating cross relaxation (CR) processes, to provide an accurate analysis of the different mechanisms at play in these model nanoparticles, and to characterize the structure of the core-shell ultrasmall UCNPs.