The Influence Of Surface Modification On Luminescent Properties Of Rare Earth Doped Upconversion Materials

Since the1960s, luminescent materials have been unprecedented developed.Rare earth doped upconversion luminescent materials have attracted widespreadattention. Due to potential applications at three-dimensional display, solid state lasers,biomarkers, and new energy, rare earth doped upconversion luminescent materialshave deeply developed at synthesis and theory research. Among a variety of rare earthdoped luminescent materials, fluoride materials are the most attractive material matrixbecause of low phonon energy. NaYF₄: is recognized as the best upconversionluminescence matrix material, and a lot of research have been reported on NaYF₄withdifferent sizes, different morphologies and luminescence properties. But highthreshold is limited its broader application. Therefore in recent years a variety ofmethods have been studied to improve the luminescence properties of rare earthdoped materials. Surface modification is an effective method. Due to the influence ofsurface modification, luminescent intensity has been significantly improved,expanding its potential applications. Using surface modification to improve rare-earthdoped conversion luminescence properties of luminescent materials has become a hotpoint in this field.The main contents are as follows:（1）Sodium citrate was chosen as a surfactant to synthesize β-NaYF₄:Yb³⁺,Er³⁺micro-nanocrystalline by hydrothermal method. SEM、XRD and TEM indicated thatNaYF₄:Yb³⁺, Er³⁺was hexagonal with smooth surface, well morphology, uniform sizeand good dispersion. By adjusting the amount of the reaction of sodium citrate andsodium nitrate we controlled the size and morphology of β-NaYF₄:Yb³⁺,Er³⁺micro-nanocrystal. We synthesized hexagonal NaYF₄:Yb³⁺, Er³⁺with the diameter of 180nm. We tested the upconversion luminescence properties of luminescent materialsand analyzed the energy transfer and non-radiative relaxation process.（2）By heterogeneous coating and homogeneous coating, we synthesized thecomposite SiO₂/β-NaYF₄:Yb³⁺,Er³⁺and NaYF₄/β-NaYF₄:Yb³⁺, Er⁽3+0. We studied theluminescence properties of the two composites, and found the NaYF4/β-NaYF₄:Yb³⁺,Er3+upconversion luminescence enhanced about30%.（3）We used sodium citrate as reducing agent and also assurfactant in the reactionto synthesize Au@β-NaYF₄:Yb³⁺,Er³⁺composite material with excellent conversionluminescence properties in99℃water bath. TEM and FT-IR indicated Aunanoparticles successfully modified on the surface of β-NaYF₄:Yb³⁺,Er³⁺. TEM andSEM indicated the composite was a little bigger than before and Au nanoparticleswere about10nm. Pumped by the980nm near-infrared laser, conversion luminescenceintensity of the composites was enhanced more than100times. In particular,⁴G_11/2�'⁴I_15/2emission of Er³⁺ions was enhanced more than1000times under lowpump. Lifetimes of the levels of Yb³⁺and Er³⁺ions in the composites were muchlonger than before. This phenomenon is attributed to field enhancement effect causedby the Au nanoparticles. We simulated the local field near the Au nanoparticles byFDTD method. Due to excellent up-conversion luminescence properties and lowpump threshold Au@β-NaYF₄:Yb³⁺, Er³⁺have a wide range of potential applicationsin the field of optical materials, biomarkers and three-dimensional display.