Study On Controllable Crystal Phase And Luminescence Properties By Ion Doping

With the rapid development of nanoscience and nanotechnology,rare-earth-doped luminescence crystals have recently emerged as the new generation of functional nanomaterials,because they exhibit potential applications ranging from biological imaging and chemical sensing to anti-counterfeiting and volume display.In particular,their exceptional optical properties such as long luminescence lifetimes,large anti-Stokes shifts and sharp emission band,have been studied for several decades.Despite their remarkable potential utility,the limited luminescence efficiency was still far from perfect for their practical application.The purpose of this paper is to achieve the improved luminescence efficiency of rare-earth-doped upconversion/downconversion crystals by doping heterogeneous ions.The main contents of this thesis are as follows:The samples of YF₃:20 mol%Yb³⁺,2 mol%Er³⁺,x mol%Cu²⁺?x=0-40?were prepared by a hydrothermal method.With increasing Cu²⁺ion doping concentration,the samples were gradually changed from orthogonal phase YF₃ to cubic phase NaYF₄.The Cu²⁺ion occupies the Y³⁺ion crystal lattice site leading to positive vacancy and lower energy barrier in the reaction of forming?-NaYF₄.In addition,we have also systematically investigated the phase transformation from?-NaYF₄ to??+??-NaYF₄.With the extension of the reaction time,surface energy of the[101?0]crystal plane decreases dramatically while that of the[0001]crystal plane increases accordingly,which may be the main reason for forming??+??-NaYF₄.?-NaYF₄:20 mol%Yb³⁺,2 mol%Er³⁺upconversion microcrystals doped with Cu²⁺ion were prepared by a simple solvothermal method.X-ray diffraction?XRD?and transmission electron microscopy analysis?TEM?data demonstrated that with increasing Cu²⁺ion doping concentrations,the phase and morphology of the samples did not change significantly.The upconversion emission intensities in the UV/violet and visible regions increased at first and then decreased with the increase of the dopant concentration of Cu²⁺from 0 to 40 mol%.The?-NaYF₄:20 mol%Yb³⁺,2 mol%Er³⁺microcrystals doped with 5mol%Cu²⁺exhibited the maximum upconversion emission intensity.It may be attributed to the substitution of Y³⁺ion by low-valence Cu²⁺ion,which may generate F^-vacancie.It decreased the crystal symmetry around the Er³⁺ion,and favored the improvement of the upconversion luminescence.But F^-vacancies also acted as quenching centers,which might reduce the upconversion luminescence intensity.A series of Sc₂O₃:2 mol%Eu³⁺,x mol%Ca²⁺?x=0,10,30,40,50,60,70,80,90?oxides were synthesized by a high temperature solid phase method.In our paper,the red-shifting of Eu³⁺-O^2-charge transfer band peaks position from 285 nm to 298 nm was investigated in CaSc₂O₄:Eu³⁺matrix.The main reason is that a small amount of Sc³⁺ion are replaced by Ca²⁺ion with low electronegativity.Meanwhile the intensity ratios of?⁵D₀?⁷F₄?/?⁵D₀?⁷F₂?transition emissions of Eu³⁺ion were increased due to excessive doping of Ca²⁺ion with lower electronegativity,the chemical environment of Eu³⁺ion were more polarized.A series of Eu³⁺ion doped Re₂O₃(Re=Sc³⁺,Lu³⁺,Y³⁺,Gd³⁺,La³⁺)phosphors have been synthesized via a simple homogeneous precipitation method and using urea as precipitating agent.The positon shifting of Eu³⁺-O^2-charge transfer band from the ligands O^2-to the central ion Eu³⁺and the ⁵D₀?⁷F₂ red luminescence of Eu³⁺ion in Eu³⁺doped Re₂O₃ phosphors has been investigated.In addition,the morphologies of Eu³⁺-doped Sc₂O₃,Lu₂O₃,Y₂O₃,Gd₂O₃ and La₂O₃ phosphors are irregular-shaped,approximate spherical and spindle-shaped,respectively.