A multidentate polymer microreactor route for green mass fabrication of mesoporous NaYF4 clusters.

We report a "multidentate polymer microreactor" method for the creation of secondary structures of colloidal nanocrystals. Using NaYF4:Yb,Er as an example, we demonstrate that the use of sodium polyacrylate (PAAS) as a "multidentate polymer microreactor" allows the controllable growth of primary nanocrystals and induces aggregation of the nanocrystals into well-defined mesoporous clusters.

Highly Regular, Uniform K3ScF6:Mn4+ Phosphors: Facile Synthesis, Microstructures, Photoluminescence Properties, and Application in Light-Emitting Diode Devices.

A new generation of red phosphors of complex fluoride matrices activated with Mn4+ has gained a broad interest in getting high color quality and low color temperature of solid-state white light-emitting diodes (WLEDs). However, besides their instability toward moisture, the extremely irregular and nonuniform morphologies of these phosphors have limited their practical industry applications. In the present study, a novel type of K3ScF6:Mn4+ red phosphor with highly regular, uniform, and high color purity was obtained successfully through a facile coprecipitation route under mild conditions. The crystal structure was identified with aids of the powder X-ray diffraction, Rietveld refinement, and density functional theory calculations. The prototype crystallizes in the space group Fm3 m with a cubic structure, and the lattice parameters are fitted well to be a = b = c = 8.4859(8) Å and V = 611.074(2) Å3. The Mn4+ ions occupy Sc3+ sites and locate at the centers of the distorted ScF6 octahedrons. A wide band gap of approximately 6.15 eV can provide sufficient space to accommodate impurity energy levels. Unlike most other Mn4+ ion-activated fluoride phosphors, the as-prepared K3ScF6:Mn4+ phosphors demonstrate highly uniform and regular morphologies with shapes transforming from cube to octahedron with increasing Mn4+ ion concentration. Under blue light excitation, the as-prepared K3ScF6:Mn4+ sample exhibits intense sharp-line red fluorescence (the strongest peak located at 631 nm) with high color purity. An excellent recovery in luminescence upon heating and cooling processes implies high stability of K3ScF6:Mn4+. Furthermore, a warm WLED fabricated with blue GaN chips merged with the mixture of K3ScF6:Mn4+ and the well-known commercial YAG:Ce3+ yellow phosphors exhibits wonderful color quality with lower correlated color temperature (3250 K) and higher color-rendering index ( Ra = 86.4). These results suggest that the K3ScF6:Mn4+ phosphor possesses stupendous potentiality for practical applications.

Formation of colloidal nanocrystal clusters of iron oxide by controlled ligand stripping.

We report a "ligand stripping" method for the creation of secondary structures of colloidal nanocrystals. Using iron oxide as an example, we demonstrate that the use of diols as "stripping agents" allows the controllable removal of the original capping ligands and induces aggregation of nanocrystals into well-defined clusters.

Upconversion in NaYF(4):Yb, Er nanoparticles amplified by metal nanostructures.

Upconversion (UC) fluorescence in NaYF(4):Yb, Er nanoparticles amplified by metal nanostructures was compared in two nanostructure geometries: gold nanoshells surrounding nanoparticles and silver nanostructures adjacent to the nanoparticles, both placed on a dielectric silica surface. Enhanced UC luminescence signals and modified lifetimes induced by these two metals were observed in our study. The UC luminescence intensities of green and red emissions were enhanced by Ag nanostructures by a factor of approximately 4.4 and 3.5, respectively. The corresponding UC lifetimes were reduced ∼ 1.7-fold and ∼ 2.4-fold. In NaYF(4):Yb, Er nanoparticles encapsulated in gold nanoshells, higher luminescence enhancement factors were obtained (∼9.1-fold for the green emission and ∼ 6.7-fold for the red emission). However, the Au shell coating extended the red emission by a factor of 1.5 and did not obviously change the lifetime of green emission. The responsible mechanisms such as plasmonic enhancement and surface effects are discussed.

Formation mechanism of CaTiO3 hollow crystals with different microstructures.

The crystal growth of CaTiO(3) hollow crystals with different microstructures has been investigated. In a water-free poly(ethylene glycol) 200 (PEG-200) solution, CaTiO(3) nanocubes formed first. The nanocubes underwent an oriented self-assembly into spherical particles, enhanced by the surface-adsorbed polymer molecules. Since the growth of nanocubes and their aggregation took place simultaneously, the nanocubes in the outer shells were larger than those in the cores. Disappearance of the small nanocubes in the cores of the spheres during an Ostwald ripening process led to spherical hollow crystals. Addition of a small amount of water (1.25 vol %) in the polymer solution enhanced surface recrystallization of the aggregated spheres, forming a cubic morphology. The orthorhombic distortion of the perovskite CaTiO(3) structure did not have a significant effect on the nanocube aggregation, resulting in a domain structure in the shells. Single-crystalline hollow cubes were produced with a slightly higher water content, e.g., 5 vol %. This process of (1) aggregation of nanocubes and (2) surface crystallization followed by (3) surface-to-core extension of recrystallization gives a good example of the reversed crystal growth route in ceramic materials. The proposed formation mechanism of the hollow CaTiO(3) crystals would enable us to control the microstructures of these materials and to explain the formation of many other hollow crystals.