Journal
NANOTECHNOLOGY
Volume 20, Issue 27, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0957-4484/20/27/275603
Keywords
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Funding
- National Science Foundation MRSEC Program through the Princeton Center for Complex Materials [DMR-0819860]
- AFSOR
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A single-step synthesis for monodisperse and hexagonal-phase (beta) NaYF4:Yb, Er upconversion nanophosphors (UCNPs) with a consistent hexagonal prism shape in the size range from 18 to 200 nm was achieved. The kinetic mechanisms for the particle phase transition and growth were examined. The beta-UCNPs were obtained via co-thermolysis of trifluoroacetate precursors in octadecene (ODE) with combined ligands of oleic acid (OA) and trioctylphosphine (TOP). The experimental results showed that the combined OA-TOP ligand was crucial for changing the surface energy and controlling the particle shape over a broad size range. It was found that the particle sizes could be controlled by varying the molar ratios of Na(CF3COO)/Re(CF3COO)(3) (Re = Y, Yb, and Er). A high Na/Re ratio accelerated the cubic-phase (alpha) -> beta transition and promoted the growth of smaller beta-UCNPs. The formation of beta-UCNPs was classified into kinetic and diffusion controlled stages, depending on the reaction temperature and the dominant crystalline phases formed in each stage. In stage I, 250-310 degrees C, NaF generation was the limiting step and alpha-UCNPs were formed via a 'burst of nucleation'. In stage II, above 310 degrees C, the alpha-UCNPs formed were re-dissolved and the growth of beta-UCNPs was a diffusion controlled process governed by the Gibbs-Thompson effect. A quasi-steady-state species assumption for NaF and a chemical potential equilibrium in the solution were introduced to explain the particle size dependence on Na/Re ratios. The study of UC luminescence showed that the UC intensity was proportional to the sizes of the beta-UCNPs.
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