Understanding and Preventing Photoluminescence Quenching to Achieve Unity Photoluminescence Quantum Yield in Yb:YLF Nanocrystals

Abstract

Ytterbium-doped LiYF4 (Yb:YLF) is a commonly used material for laser applications, as a photon upconversion medium, and for optical refrigeration. As nanocrystals (NCs), the material is also of interest for biological and physical applications. Unfortunately, as with most phosphors, with the reduction in size comes a large reduction of the photoluminescence quantum yield (PLQY), which is typically associated with an increase in surface-related PL quenching. Here, we report the synthesis of bipyramidal Yb:YLF NCs with a short axis of ∼60 nm. We systematically study and remove all sources of PL quenching in these NCs. By chemically removing all traces of water from the reaction mixture, we obtain NCs that exhibit a near-unity PLQY for an Yb3+ concentration below 20%. At higher Yb3+ concentrations, efficient concentration quenching occurs. The surface PL quenching is mitigated by growing an undoped YLF shell around the NC core, resulting in near-unity PLQY values even for fully Yb3+-based LiYbF4 cores. This unambiguously shows that the only remaining quenching sites in core-only Yb:YLF NCs reside on the surface and that concentration quenching is due to energy transfer to the surface. Monte Carlo simulations can reproduce the concentration dependence of the PLQY. Surprisingly, Förster resonance energy transfer does not give satisfactory agreement with the experimental data, whereas nearest-neighbor energy transfer does. This work demonstrates that Yb3+-based nanophosphors can be synthesized with a quality close to that of bulk single crystals. The high Yb3+ concentration in the LiYbF4/LiYF4 core/shell nanocrystals increases the weak Yb3+ absorption, making these materials highly promising for fundamental studies and increasing their effectiveness in bioapplications and optical refrigeration.

Nucleation and Growth of Bipyramidal Yb:LiYF4 Nanocrystals─Growing Up in a Hot Environment

“…For this, the use of yttrium-based hosts is ideal as Y 3+ has a similar size and oxidation state as the elements in the lanthanide series. LiYF 4 stands out because it fulfills all these conditions and can be prepared with a very high optical and crystal quality. , An additional benefit over NaYF 4 , another common host material for lanthanide-doped NCs, − is the more facile shell growth for LiYF 4 as no phase changes , occur. This furthermore allows subsequent growth of LiYF 4 NCs to the preferred size by simply adding additional precursors .…”

“…Much work has been done on optimizing the synthesis of doped LiYF 4 NCs through a variety of routes, often yielding NCs with a bipyramidal shape ( vide infra ). ,− Little is known, however, about the complex nucleation and growth mechanisms of this material, with only a few works focusing on the nucleation stage. ,,, When ammonium fluoride is used as the fluoride precursor, instead, reports mention that LiF or YF 3 , nucleate first. Reports using the thermal decomposition of trifluoroacetate (TFA) as the fluoride precursor show the initial formation of semispherical LiYF 4 particles. , None of these reports however discusses how these initial particles grow into a monodisperse ensemble of bipyramidal NCs.…”

“…In this work, we focus on the nucleation and growth of ytterbium-doped LiYF 4 (Yb:YLF), as most of the studied lanthanide-doped LiYF 4 (Ln:YLF) NCs contain a significant fraction of Yb 3+ either as the luminescent ion , or as activator for other lanthanide ions. , Yb:YLF itself is an interesting material for a broad range of optical applications. As bulk Yb:YLF, it is used in lasers, − for photon upconversion (when codoped with other lanthanide ions such as Pr 3+ , Ho 3+ , Er 3+ , or Tm 3+ ) − and optical refrigeration. ,− When synthesized as NCs, its applicability in biological ,− and physical studies ,,,,, increases significantly.…”

“…As bulk Yb:YLF, it is used in lasers, − for photon upconversion (when codoped with other lanthanide ions such as Pr 3+ , Ho 3+ , Er 3+ , or Tm 3+ ) − and optical refrigeration. ,− When synthesized as NCs, its applicability in biological ,− and physical studies ,,,,, increases significantly. This becomes especially relevant when NC samples with a monodisperse size distribution ,, and excellent optical properties ( e.g . a high photoluminescence quantum yield , ) are synthesized.…”

“…This becomes especially relevant when NC samples with a monodisperse size distribution ,, and excellent optical properties ( e.g . a high photoluminescence quantum yield , ) are synthesized. Using Yb 3+ additionally allows us to study the uniformity of its distribution and observe differences in the nucleation and growth of lanthanide-doped NCs, and hence if and how doping LiYF 4 with lanthanide ions affects the formation of the NCs.…”