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 -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. These nanoparticles contain only a single Tm3+ activator ion, while coating of these cores with a NaGdF4 inert shell was performed to minimize the deleterious influence of surface quenching. The role of cross relaxation in upconversion luminescence was examined by time-resolved luminescence measurements using a combination of standard NIR excitation of the Yb3+ sensitizer and direct UV excitation of the Tm3+ activator. The fine tuning of the number of activators per particle via an optimized synthesis pathway along with an appropriate excitation scheme enabled us to select the operating cross relaxation processes and provide an accurate analysis of the different mechanisms at play in these model nanoparticles.
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Synthesis and emission dynamics of sub-3 nm single-emitter upconversion nanoparticles