Physiochemical properties of greatly enhanced photoluminescence of aqueous dispersible upconversion CaF2:Yb/Er nanoparticles

Abstract

Crystal phase morphological structure and optical properties of the as-prepared upconversion CaF₂:Yb/Er(core) and sequential coating of an inert crystalline material and silica layers surrounding the seed core-nanoparticles (NPs) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), optical absorption, FTIR spectroscopy 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 and aqueous dispersibility. The surface coating on the 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 a characteristic optical absorption band in UV/Visible region. As a result, optical band gap gradually decreases after sequential growth of the inert shell and amorphous silica due to an increase in the crystalline size. Comparative upconversion luminescence analysis showed that after inert shell growth, the upconversion intensity was greatly improved, and such an improvement was found to arise from efficient suppression of surface-related deactivation from the core nanocrystals. Interestingly, growth of an inert (CaF₂) shell over the seed core NPs shows intense upconversion emission lines under 980 nm NIR laser excitation, highlighting their promising applications, such as multi-analyte biolabels, staining, displays and other photonic based technological applications.