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  • Ostwald-ripening and particle size focussing of sub-10 nm NaYF₄ upconversion nanocrystals.

Ostwald-ripening and particle size focussing of sub-10 nm NaYF₄ upconversion nanocrystals.

Nanoscale (2014-10-28)
Thorben Rinkel, Jörg Nordmann, Athira Naduviledathu Raj, Markus Haase
ABSTRACT

We have studied the growth behaviour of sub-10 nm NaYF₄ upconversion nanocrystals of the hexagonal β-phase and the cubic α-phase. Ostwald-ripening of such particles in oleic acid/octadecene solvent results in broadening of the particle size distribution if the colloid contains particles of one crystal phase only. Narrow size distributions are formed only if β-phase particles grow in the presence of an excess of α-phase particles. Such binary mixtures of α-phase and β-phase particles form intrinsically when colloids of α-phase particles are heated for a sufficiently long time, because seeds of the β-phase nucleate in the solution after some time at high temperatures. Since the number of seeds determines the final size of the β-phase product, control of the nucleation is crucial for controlling the final particle size. We show that the number of β-phase seeds strongly depends on the composition of the α-phase known to form solid solutions Na₁-xYF₄-x in the range from x = 0 to x = 4/9. Sodium-deficient α-phase particles form a negligible number of β-phase seeds whereas α-phase particles with high sodium content yield a very large number of seeds. By taking advantage of this dependence and modifying the synthesis of the α-phase particles accordingly, small phase-pure β-NaYF₄:Yb,Er particles with a size smaller than 6 nm can be prepared in oleic acid/octadecene just as well as much larger particles.

MATERIALS
Product Number
Brand
Product Description

Oleic acid, European Pharmacopoeia (EP) Reference Standard
Erbium, foil, not light tested, 50x50mm, thickness 0.025mm, as rolled, 99%
Sigma-Aldrich
Oleic acid, SAJ first grade, ≥70.0%
Supelco
1-Octadecene, analytical standard, ≥99.0% (GC)
Sigma-Aldrich
1-Octadecene, ≥95.0% (GC)
Sigma-Aldrich
Oleic acid, natural, FCC
Sigma-Aldrich
Oleic acid, meets analytical specification of Ph, Eur., 65.0-88.0% (GC)
Sigma-Aldrich
Oleic acid, technical grade, 90%
Supelco
Oleic acid, analytical standard
Sigma-Aldrich
1-Octadecene, technical grade, 90%
Sigma-Aldrich
Oleic acid, ≥99% (GC)
Sigma-Aldrich
Sodium oleate, ≥95% (capillary GC)
Sigma-Aldrich
Sodium oleate, ≥99%
Sigma-Aldrich
Oleic acid, BioReagent, suitable for cell culture
Supelco
Oleic acid, Selectophore, ≥99%
Erbium, foil, 25x25mm, thickness 1.0mm, as rolled, 99%
Erbium, rod, 50mm, diameter 2.0mm, cast, 99%
Erbium, foil, 25x25mm, thickness 0.125mm, as rolled, 99%
Erbium, rod, 50mm, diameter 10.0mm, cast, 99%
Erbium, rod, 50mm, diameter 6.35mm, cast, 99%
Erbium, rod, 100mm, diameter 10.0mm, cast, 99%
Erbium, wire reel, 100mm, diameter 1.0mm, hard, 99.9%
Erbium, rod, 100mm, diameter 6.35mm, cast, 99%
Erbium, foil, not light tested, 25x25mm, thickness 0.005mm, as rolled, 99%
Erbium, foil, 50x50mm, thickness 1.0mm, as rolled, 99%
Erbium, foil, 50x50mm, thickness 0.125mm, as rolled, 99%
Erbium, wire reel, 50mm, diameter 1.0mm, hard, 99.9%
Sigma-Aldrich
Ammonium fluoride, SAJ special grade, ≥97.0%
Sigma-Aldrich
Ytterbium, powder, ≥99.9% trace rare earth metals basis
Sigma-Aldrich
Ammonium fluoride, ≥99.99% trace metals basis