NaYF₄:Yb,Er/NaYF₄ Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield

Abstract

Upconversion core/shell nanocrystals with different mean sizes ranging from 15 to 45 nm were prepared via a modified synthesis procedure based on anhydrous rare-earth acetates. All particles consist of a core of NaYF :Yb,Er, doped with 18 % Yb and 2 % Er , and an inert shell of NaYF , with the shell thickness being equal to the radius of the core particle. Absolute measurements of the photoluminescence quantum yield at a series of different excitation power densities show that the quantum yield of 45 nm core/shell particles is already very close to the quantum yield of microcrystalline upconversion phosphor powder. Smaller core/shell particles prepared by the same method show only a moderate decrease in quantum yield. The quantum yield of 15 nm core/shell particles, for instance, is reduced by a factor of three compared to the bulk upconversion phosphor at high power densities (100 W cm ) and by approximately a factor of 10 at low power densities (1 W cm ).

Long‐Term Colloidal and Chemical Stability in Aqueous Media of NaYF4‐Type Upconversion Nanoparticles Modified by Ligand‐Exchange

“…Upconversion nanoparticles (UCNPs) established in the last years as versatile luminescent probes due to intense progress in the synthesis enabling complex particle architectures of controlled dimensions . As a consequence, also the photophysical principles of photon upconversion, including luminescence efficiency affected by surface quenching and passivation processes have been excessively studied .…”

“…Most upconversion nanoparticles described in literature are consisting of a NaYF 4 host lattice as this materials has a preferable low phonon‐energy (≈300 cm −1 ) compared to other host materials like fluorides (LiYbF 4, ≈460 cm −1 ), oxides (Y 2 O 3 , ≈591 cm −1 ), or vanadates (YVO 3 , ≈890 cm −1 ). Therefore, it is not surprising that this host material together with Yb 3+ and Er 3+ has been used to design the most efficient upconversion nanoparticles . However, the synthesis of those nanoparticles is often based on thermal decomposition of lanthanide oleates or acetates in high boiling organic solvents, ending up with a hydrophobic surface ligand (e.g., oleic acid) .…”

Atomic‐Level Passivation of Individual Upconversion Nanocrystal for Single Particle Microscopic Imaging

“…Here, 484-fold luminescence enhancement in LuF 3 :Yb 3+ , Er 3+ rhombic flake UCNPs is achieved, thanks to the Yb 3+ -mediated local photothermal effect, and their original morphology, size, and good dispersibility are well preserved. [15][16][17] To date, many surface passivation strategies, [18] like annealing treatment [17,19] and coating technology, [20,21] have been proposed for improving upconversion luminescence (UCL), however most of these efforts may not necessarily lead to the optimal enhancement at the single-particle level. The confocal luminescence images obtained by superresolution stimulated emission depletion also show the great enhancement of individual LuF 3 :Yb 3+ , Er 3+ nanoparticles; the high signal-to-noise ratio images indicate that the laser treatment technology opens the door for single particle imaging and practical application.…”

“…[15][16][17] To date, many surface passivation strategies, [18] like annealing treatment [17,19] and coating technology, [20,21] have been proposed for improving upconversion luminescence (UCL), however most of these efforts may not necessarily lead to the optimal enhancement at the single-particle level. [15][16][17] To date, many surface passivation strategies, [18] like annealing treatment [17,19] and coating technology, [20,21] have been proposed for improving upconversion luminescence (UCL), however most of these efforts may not necessarily lead to the optimal enhancement at the single-particle level.…”

Colloidal Crystals of NaYF4 Upconversion Nanocrystals Studied by Small‐Angle X‐Ray Scattering (SAXS)

“…Synthesis of β‐NaYF 4 :Yb,Er Particles (Sample B) : Anhydrous yttrium, ytterbium, and erbium acetate were prepared as previously reported . 1556.4 mg (5.85 mmol) of yttrium acetate, 525.4 mg (1.50 mmol) of ytterbium acetate, and 51.7 mg (0.15 mmol) erbium acetate were dissolved in 18.75 mL of oleic acid and 18.75 mL of octadecene by heating the mixture at 110 °C under vacuum (<0.1 mbar) for 60 min in a 250 mL vessel.…”