Synthesis And Properties Of Monodisperse Assemblies Of NaLnF₄（Ln=Y、Eu、Ce、Tm）Nanocrystals

Sodium lanthanide fluoride (NaLnF4) nanocrystals have attracted considerable attention due to their low phonon energies, high refractive indexes and excellent thermal and optical stabilities, thus enabling their use in a wide range of application from display devices, optical communication to short wavelength solid-state lasers. In recent years, these nanocrystals have gained much popularity especially in the material science community due to their high efficiency in up-converting near-infrared (NIR) to visible light upon doping with various lanthanides. In contrast to traditional down-conversion materials, up-conversion materials offer many advantages for use in biological applications such as high detection sensitivity, low fluorescence background, minimum photo-damage and high light-penetration depth in tissues.This article describes the synthesis and properties of monodisperse assemblies of NaYF.4nanocrystals. We have developed a one-step, solution-phase approach to synthesise assembled NaYF4nanocrystals. These self-assembly nanocrystals are in cubic phase and well crystallized, determining from their X-ray powder diffraction. Further more, these NaYF4assemblies are composed of some tiny single-crystal NaYF4nanocrystals. Interestingly, NaYF4nanocrystals in each assembly are attached together along one specific crystalline orientation, as revealed by the electron microscopy analysis. We have conducted careful observation on the reaction process. It revealed that these hierarchical structures are spontaneously formed via a self-assembly and sequential Ostwald ripening process. Characterized by the BET analysis, these assemblies obviously have a large surface area due to their special structural geometry. The assembled nanostructures presented here can be doped with different lanthanide ions to display strong emissions through down-or up-converting process. As such, they can serve as efficient hosts for both types of luminescent materials. In order to harness the NaYF4assemblies for biomedical applications, we also performed silica coating on these assemblies by modifying the well-known Stober method. Surface modification with silica is a powerful tool to enhance the chemical and physical stability of luminescent nanocrystals and conjugate them to biomolecules.Through the similar method, we have also prepared other assemblies of NaLnF4(Ln=Eu、Ce、Tm) nanocrystals at large scale. These assemblies were also well characterized by various apparatus and techniques. Photoluminescent spectra demonstrated that these NaLnF4assemblies could give strong down-and up-conversion emissions and were expected to be useful in a broad range of fields.