Preparation And Up-conversion Luminescence Properties Of Rare Earth Doped Tungstates

In this thesis,three kinds of rare earth-doped composite tungstates,Ho³⁺and Yb³⁺co-doped Gd₆WO₁₂,Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁and Er³⁺and Yb³⁺co-doped La₁₄W₈O₄₅ up-conversion luminescence materials were successfully synthesized by hydrothermal method or coprecipitation method combined with calcination.The effects of experimental conditions and doping ion concentration on the phase structure and luminescent properties of the samples were researched.three rare earth tungstates including Gd₆WO₁₂,La₁₀W₂₂O₈₁ and La₁₄W₈O₄₅ were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,ultraviolet-visible diffuse emission（UV-vis）and up-conversion luminescence（UPL）had been used to characterized the phase structure,morphology and optical properties of samples.The following conclusions are obtained:Submicron Er³⁺and Yb³⁺co-doped La₁₄W₈O₄₅ composite tungstate up-conversion luminescence materials were successfully synthesized by hydrothermal method combined with calcination treatment.A series of rare earth co-doped samples were synthesized by changing the preparation process conditions,such as p H,calcination temperature,the doping amount of Er³⁺or Yb³⁺and the ratio of n（RE）/n（Na₂WO₄）.The effects of the above preparation conditions on the phase structure and up-conversion luminescence properties of the prepared products were studied.The experimental results show that the pure phase orthorhombic Er³⁺and Yb³⁺co-doped La₁₄W₈O₄₅ crystals can be successfully synthesized under the condition of calcination temperature of 800℃,n（RE）/n（Na₂WO₄）ratio of 2 and p H of 10.Under the excitation of 980 nm infrared light,the Er³⁺and Yb³⁺co-doped La₁₄W₈O₄₅ composite tungstate can emit emission peaks with center wavelengths at 526 nm,548 nm and 660 nm,corresponding to the characteristic transition bands of ²H_11/2→⁴I_15/2,⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2energy levels of Er³⁺,respectively,and the green emission band at 548 nm plays a dominant role in the up-conversion emission spectra,confirming that the prepared samples can emit strong green light at room temperature.Concentration quenching phenomenon occurs when the doping amount of Er³⁺and Yb³⁺is more than 1%and 5%,respectively.The relationship between the up-conversion luminescence intensity of the sample and the pump power proves that the ²H_11/2→⁴I_15/2,⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2energy level transitions of Er³⁺in La₁₄W₈O₄₅ sample belong to two-photon absorption mechanism.Ultraviolet-visible diffuse reflectance spectrum shows that the Er³⁺and Yb³⁺co-doped La₁₄W₈O₄₅ crystal is a direct semiconductor with an optical band gap of 3.84 e V.The color coordinates of the samples show that Er³⁺and Yb³⁺co-doped La₁₄W₈O₄₅ crystal is a potential and excellent green up-conversion luminescent material.Ho³⁺and Yb³⁺co-doped Gd₆WO₁₂ composite tungstate phosphors were successfully synthesized by co-precipitation method combined with calcination.The effects of preparation conditions,such as p H,calcination temperature,doping amount of Ho³⁺or Yb³⁺and the ratio of n（RE）/n（Na₂WO₄）,on the phase structure and up-conversion luminescence properties of the obtained samples were studied.The results show that when the p H is 13 and the ratio of n（RE）/n（Na₂WO₄）is 2,pure tetragonal of Ho³⁺and Yb³⁺co-doped Gd₆WO₁₂ crystals can be prepared by co-precipitation combined with calcination at 900℃.Under the excitation of 980 nm infrared light,the green emission band plays a dominant role in the up-conversion emission spectra of Ho³⁺and Yb³⁺co-doped Gd₆WO₁₂ luminescence materials.The emission peaks with center wavelengths at 553 nm and 663 nm,corresponding to the ⁵S₂/⁵F₄→⁵I₈and⁵F₅→⁵I₈transition of Ho³⁺,respectively.When the doping amount of Ho³⁺is more than 1%,the concentration quenching phenomenon occurs in the samples through the electric dipole-electric quadrupole interaction（d-p）.The dependence relationship between the up-conversion emission intensity of the sample and the pump power verifies that the energy level transitions corresponding to the emission wavelength of Ho³⁺at 553 nm and 663 nm in Gd₆WO₁₂ crystal belong to the two-photon absorption mechanism.The UV-Vis diffuse reflectance spectrum of sample shows that Gd₆WO₁₂:Ho³⁺,Yb³⁺crystal is a direct semiconductor with an optical band gap of 3.65 e V.The CIE color coordinate results confirmed that the color coordinates of Ho³⁺and Yb³⁺co-doped Gd₆WO₁₂ composite tungstate all fell in the green region.Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁ up-conversion phosphors were successfully synthesized by hydrothermal method conbined with high temperature heat treatment.A series of Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁ samples were synthesized by changing the preparation conditions such as p H,calcination temperature,doping amount of Tm³⁺or Yb³⁺and the ratio of n（RE）/n（Na₂WO₄）.The effects of the above conditions on the phase structure and up-conversion luminescent properties of the prepared Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁ samples were studied,and the up-conversion luminescence mechanism of the samples was also explored.The experimental results confirmed that the pure phase Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁ crystals with orthorhombic strucure and space group of Pbcn（60）can be successfully prepared under the condition of n（RE）/n（Na₂WO₄）ratio of 1 and keeping the p H at 5 after hydrothermal treatment and then sintering at 900℃..The up-conversion emission spectra of La₁₀W₂₂O₈₁:Tm³⁺,Yb³⁺composite tungstates is composed of emission peaks with central wavelengths at 477 nm（¹G₄→³H₆）,650 nm（¹G₄→³F₄）and 695nm（³F₃→³H₆）under the excitation of an external laser source at 980 nm.When the doping amount of Tm³⁺and Yb³⁺is more than 0.1%and 5%,respectively,the concentration quenching phenomenon appears.The calculation results confirm that the up-conversion emission quenching mechanism of Tm³⁺and Yb³⁺in the sample is electric dipole-electric dipole interaction（d-d）and electric quadrupole-electric quadrupole interaction（p-p）,respectively.Logarithmic graphs of up-conversion emission intensity and pump power of the sample show that the ¹G₄→³H₆and ¹G₄→³F₄ transitions of Tm³⁺in the La₁₀W₂₂O₈₁ sample belong to the two-photon absorption mechanism.Ultraviolet-visible diffuse reflectance spectrum shows that Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁ crystal is a direct semiconductor with an optical band gap of 3.41 e V.The color coordinates of the samples all fall in the blue region,indicating that Tm³⁺and Yb³⁺co-doped La₁₀W₂₂O₈₁ composite tungstate is a potential blue up-conversion luminescence material.