Physiochemical properties of greatly enhanced photoluminescence of aqueous dispersible upconversion CaF2:Yb/Er nanoparticles
The crystalinity, phases, morphological structure and optical properties of as-prepared upconversion CaF2:Yb/Er(core) and sequential coating of an inert crystalline and silica layers surrounding the seed core-nanoparticles(NPs) were thoroughly characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), optical absorption, FTIR and upconversion photoluminescence spectroscopy. Owing to the unique properties of CaF2 host matrix, we realized their practical applications in biomedical science to improve the upconversion luminescence property along with aqueous dispersibility. The surface coating on seed core particles will significantly influence the structural, optical band gap energy and upconversion luminescence properties. These NPs were well-dispersed in aqueous and non-aqueous solvents to form clear colloidal solutions. The colloidal solutions of three samples show well characteristic optical absorption band in UV/Visible region. As a result, optical band gap energy gradually decreases after sequential growth of inert shell and amorphous silica due to increase the crystalline size. Comparative upconversion luminescence analysis revealed that after inert shell growth the upconversion intensity was greatly improved such an improvement arises from efficient suppression of surface-related deactivation from the core nanocrystals. Interstingly, growth of an inert shell before silica layer deposition, the CaF2:Yb,Er@CaF2(core/shell) and CaF2:Yb,Er@CaF2@SiO2 (core/shell/SiO2) NPs show intensive up-conversion emission under 980 nm NIR laser excitation, showing great promise in applications such as multi-analyte biolabels, staining, displays and other photonic based technological applications.