Controlled Synthesis Of Phase,Size,Morphology And Research On The Luminescent Properties Of Rare Earth Doped MYF₄（M=Li,Na,K） Nanomaterials

The rare earth elements have excellent characteristics in optics,electricity and magnetics owing to their own special 4f electrical configuration.In recent years,enormous researches and attentions have been paid to the lanthanide-doped fluoride upconverting luminescent materials because they have low phonon energies,long excited-state lifetimes,low toxicity,narrow emission bandwidth and other comprehensive advantages,which results in the widespread application domains of the materials,including 3D display,solid-state laser,solar cells,biological imaging and biomarkers,optical storage,anti-counterfeit printing and so on.For up-conversion luminescent materials,the matrix material,a main body of the matrix materials,served as a nonluminous energy-level but can provide a suitable crystal field for the luminescence center.In the trivalent rare earth ions,Y³⁺,La³⁺,Lu³⁺,etc.are suitable as a host material because they lack of electrons for their 4f energy level or are fully-filled with the electrons for the 4f sublayer,and exhibit optical inertia since they have a closed shell structure.Furthermore,upconversion matrix materials also require low phonon energy,high chemical stability and mechanical strength.In this treatise,LiYF₄、NaYF₄、KYF₄ were chosen as the matrix materials,Yb³⁺ and Er³⁺ as the sensitizer and activator respectively.Controlled synthesis of crystal forms,sizes,and morphologies of LiYF₄,NaYF₄,KYF₄ nanmaterials were realized by adjusting the ratio of MF/RE（LiF,Na F,KF）,reaction time,reaction temperature,etc.,and the effect of the above conditions on their luminescence properties was explored.The specific findings are as follows:（1）Preparation of rare earth trifluoroacetate precursors by hydrothermal synthesis for the first time（2）Precise control of sizes of β-NaYF₄:20%Yb³⁺/2%Er³⁺ nanoparticles（27-33nm）was achieved in the high-boiling organic solvents（oleic acid,octadecene）via thermal decomposition method,and the effect of Na F/RE ratio,reaction temperature,and reaction time on the mechanism of nucleation and growth of NaYF₄:20%Yb³⁺/2%Er³⁺ nanomaterials was discussed.It is further concluded that the following processes are mainly involved in the growth of NaYF₄:20%Yb³⁺/2%Er³⁺ nanomaterials.（1）Nucleation,（2）Growth of the α-NaYF₄ nanoparticles,（3）Phase transition（Ostwald ripening）,（4）Size shrinkage of the β-NaYF₄,（5）Growth of the β-NaYF₄ nanoparticles（3）Controlled synthesis of from YF₃:20%Yb³⁺/2%Er³⁺ to LiYF₄:20%Yb³⁺/2%Er³⁺ in mixture solutions of oleic acid and n-hexanol through solvothermal method was realized,and the effect of ratio of LiF/RE and reaction temperature on the mechanism as mentioned above has been discussed systematically.Subsequently,the optical properties of YF₃:20%Yb³⁺/2%Er³⁺ and LiYF₄:20%Yb³⁺/2%Er³⁺ were characterized by upconversion emission spectra and fluorescence lifetime spectra.（4）Controlled synthesis of the crystal form（α,β）and morphology（nanorods,nanospheres,nanoprismks）of NaYF₄:20%Yb³⁺/2%Er³⁺ nanomaterial are realized using solvothermal synthesis in a mixture of oleic acid and n-hexanol by precisely adjusting the ratio of Na F/RE.In addition,the growth mechanism of β-NaYF₄ nanorods was studied.It was concluded that growth process of the nanorods mainly included the following processes: nucleation,growth（α-NaYF₄）,the process of Ostwald Ripening（orientation growth）,and regrowth.The luminescent color of the sample can be adjusted by the tuning of ionic species of the activator,,and the lifetime of the ⁴S_3/2 level of Er³⁺,the ¹G₄ level of Tm³⁺,and the ⁵S₂ level of Ho³⁺ were observed by using the fluorescence lifetime spectrum.（5）Controlled synthesis of from KY₃F₁₀:20%Yb³⁺/2%Er³⁺ to K₂YF₅:20%Yb³⁺/2%Er³⁺,YF₃:20%Yb³⁺/2%Er³⁺ to KY₃F₁₀:20%Yb³⁺/2%Er³⁺ was achieved in the mixed solvents of oleic acid and n-hexanol by adjusting the ratio of KF/RE and temperature,respectively.The effect of reaction temperature and reactant monomer concentration on the mechanism of phase transition of the material was researched.