The Effects Of High Temperature Treatment On The Upconversion Luminescent Performance Of Rare Earth Doped Nanoparticles

Upconversion luminescence of rare earth doped nanoparticles is that the luminescent center of nanoparticles absorbs two of more low energy photons to generate a high-energy photon,which is an anti-Stokes process.Due to their large anti-Stokes shifts,sharp emission bands,long-lived luminescence,great light stability,low autofluorescence and high sensitivity,rare earth doped upconversion nanoparticles(Re³⁺-doped UCNPs)have held great promise in the biological imaging,diagnostics and therapeutics of disease,solid-state lasers,flat-panel displays,and solar cells.Up to now,a lot of research works have focused on synthesis and application of Re³⁺-doped UCNPs.However,the biggest obstacle that Re³⁺-doped UCNPs are facing is the low upconversion luminescent efficiency,which hinders the development of Re³⁺-doped UCNPs.All kinds of strategies have been explored to improve the upconversion luminescent efficiency,including the choice of host materials,surface passivation,broadband sensitization,photonic-crystal engineering,surface plasmon coupling,etc.Unfortunately,the luminescent efficiency of Re³⁺-doped UCNPs is still very low.Under the same doping concentration of rare earth ions,the luminescent efficiency of bulk Re³⁺-doped upconversion materials is higher than Re³⁺-doped UCNPs.Therefore,it is of great significance to explore the intrinsic reasons why bulk materials and nanomaterials are different in the luminescent efficiency.Compared to nanomaterials,bulk materials are prepared under high temperature.Thus,does high temperature play a key role in improving the luminescent efficiency of Re³⁺-doped UCNPs? However,it is a realistic difficult problem that nanoparticles aggregate into bulks during the annealing process.In this work,firstly,we coated SiO2 around nanoparticles to prevent Re³⁺-doped UCNPs from aggregating during the annealing process.Secondly,we annealed Re³⁺-doped UCNPs coated with SiO2 under high temperature.Finally,with the help of HF aqueous solution,the SiO2 layers could be easily removed.We studied the upconversion luminescent performance of nanomaterials in different stages,and we explored the essential reasons that influence the upconversion luminescent performance.The specific studies were as follows:?1??-NaYF₄:Yb³⁺,Er³⁺ UCNPs were prepared via the methanol-assisted organicphase synthetic method.NaYF₄:Yb³⁺,Er³⁺ UCNPs were synthesized with controllable shapes and sizes by varying the reaction temperature,reaction time,and ratios of ODE/OA.Finally,sphere-like ?-NaYF₄:Yb³⁺,Er³⁺ UCNPs were obtained with a uniform size of 25.5±0.7 nm in diameter.?2?Sphere-like ?-NaYF₄:Yb³⁺,Er³⁺ UCNPs with a uniform size of 25.5±0.7 nm were used as seed crystals to induce the growth of ?-NaLuF₄ layers.?-NaYF₄:Yb³⁺,Er³⁺/Na LuF₄ core-shell UCNPs were synthesized with variable thickness of ?-NaLuF₄ layers by changing the content of ?-NaLuF₄ sacrificial nanoparticles,and the dependence of the upconversion luminescent performance on ?-NaLuF₄ layers was explored.The results showed that the isotropic and thick?5.5 nm??-NaLuF₄ shells dramatically reduced the surface-related quenching effect of ?-NaYF₄:Yb³⁺,Er³⁺ UCNPs,resulting in much brighter upconversion luminescence?3??-NaYF₄:Yb³⁺,Er³⁺/Na LuF₄ core-shell UCNPs with a uniform size of 34.5±0.6 nm were used as seed crystals to induce the growth of SiO2 layers by the microemulsion method.?-NaYF₄:Yb³⁺,Er³⁺/NaLu F₄/SiO2 UCNPs were annealed under high temperature.?-NaYF₄:Yb³⁺,Er³⁺/NaLu F₄ UCNPs were retrieved by selectively removing SiO2 layers of annealed nanomaterials with HF.Finally,we compared the luminescent performance of pre-and post-annealed ?-NaYF₄:Yb³⁺,Er³⁺/Na LuF₄ UCNPs and explored the constraints on the luminescent performance of upconversion nanoparticles.The results showed that the size,shape and crystal structure of the recovered NaYF₄:Yb³⁺,Er³⁺/NaLu F₄ UCNPs could remain unchanged after annealing at 400 ?.However,due to introducing new surface defects on the surface of NaYF₄:Yb³⁺,Er³⁺/NaLuF₄ during SiO2 etching process,the upconversion luminescence of NaYF₄:Yb³⁺,Er³⁺/Na LuF₄ UCNPs could not be improved through this strategy,which showed that the surface defect was an important factor in affecting luminescent performance of upconversion nanoparticles.