title: A single-step synthesis and the kinetic mechanism for monodisperse and hexagonal-phase NaYF4:Yb, Er upconversion nanophosphors.
abstract: A single-step synthesis for monodisperse and hexagonal-phase (beta) NaYF(4):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(CF(3)COO)/Re(CF(3)COO)(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.
authors.affiliation: Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA. jshan@princeton.edu
authors.forename: Jingning
authors.initials: J
authors.lastname: Shan
authors.forename: Yiguang
authors.initials: Y
authors.lastname: Ju
mesh_terms: D004871:Erbium / Q000737:chemistry*; D005459:Fluorides / Q000138:chemical synthesis / Q000737:chemistry*; D007700:Kinetics; D046529:Microscopy, Electron, Transmission; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D019301:Oleic Acid / Q000737:chemistry; D010316:Particle Size; D010720:Phosphines / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics; D015018:Ytterbium / Q000737:chemistry*; D015019:Yttrium / Q000737:chemistry*
chemical_list: D010720:Phosphines; C525362:sodium yttriumtetrafluoride; D019301:Oleic Acid; D015019:Yttrium; D004871:Erbium; D015018:Ytterbium; D005459:Fluorides