Eu~（3+）、Tm~（3+）、Er~（3+）和Yb~（3+）共掺杂氟化物纳米晶上转换发光及其动力学的研究
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摘要
本文围绕稀土Eu~(3+), Tm~(3+), Er~(3+)和Yb~(3+)共掺杂的氟化物纳米晶的研究得到了如下一些创新成果:
1)利用微乳液法合成了直径大约为2 nm的YF3:Yb/Tm/Eu纳米须;用油酸作为修饰试剂,合成了平均尺寸为6.7 nm的NaYF_4: Yb/Er/Eu纳米晶,在这些样品中首次在室温条件下观察到了Eu~(3+)的一些不寻常的紫色和紫外上转换发射,并通过不同掺杂样品光谱及荧光动力学过程的研究分析了稀土离子间能量传递过程。
2)选择PVP作为修饰试剂合成了水溶性的尺寸为30 nm左右的NaYF_4:Yb/Tm/Eu和NaYF_4:Yb/Er/Eu纳米晶,在这两种三掺体系中,其单个纳米晶就可以被394 nm和980 nm激光分别激发,产生两种颜色的特征发射。我们进一步以SiO2为壳层对这两种材料进行了包覆,改善了纳米微粒的环境稳定性和发光性能。并且讨论了这种材料在双色生物标记中的可能应用。
3)用水热法合成了Yb/Tm共掺LaF3及YF3上转换发光材料,当Yb~(3+)掺杂从10%提高到20%时,就使基质LaF3由六角相转变为斜方相。在相同的实验条件下,我们研究了晶相依赖的Tm~(3+)上转换发光特性及其动力学过程。
4)以油酸作为修饰试剂,合成了尺寸为200 nm NaYF_4: Yb/Er微米晶。修饰后的微米晶可以在非极性溶剂中形成澄清透明的胶体溶液。同时测量了这种油酸修饰的微米晶的变温上转换发射光谱,研究表明:Er~(3+)的上转换光谱对温度有很强的依赖关系。
Upconversion refers to nonlinear optical processes characterized by the successive absorption of two or more pump photons via intermediate long-lived energy states followed by the emission of the output radiation at a shorter wavelength than the pump wavelength. Principles and applications of such upconversion phosphors were presented in several reviews up to the 1970s by Auzel, Garlick, and Wright. Since then, upconversion has become a pervading effect in all RE-doped materials under high-density infrared excitation. Due to their unique and excellent optical properties, rare-earth fluoride upconversion nano-materials have important applications in high-resolution displays, solid lasers, integrated optical systems, infrared imaging, biological analysis and medical diagnosis and have attracted much attention. Establishing well-controlled synthesis strategies to these materials, understanding their growth and the relationships of their structures, morphologies and optical behaviors are crucial to the production of nanophosphors with designated optical properties. In this thesis, we reported some distinct upconversion emissions of several kinds of fluoride nanocrystals doped with Eu~(3+), Tm~(3+), Er~(3+) and Yb~(3+) ions, and characterized their properties.
(1) YF3:Yb/Tm/Eu nanobundles with a mean diameter of ~2 nm were synthesized by microemulsion method. Besides the UC emissions from Tm~(3+) ions, not only the unusual ~5D_2→~7F_3, ~5D_3→~7F_J emissions of Eu~(3+) were observed, but also the ultraviolet (UV) UC fluorescence from ~5H_(3-7), ~5G_(2-6), and ~5L_6 levels were recorded under room temperature. To the best of our knowledge, it is the first time to report the ~5H_(3-7), ~5G_(2-6), ~5L_6→~7F_0 transitions, which expand the vast space for the research, development and application of Europium. Furthermore, we investigated the excitation power dependence of upconversion luminescence intensities. The upconversion emission peaked at 326.7 nm came from the transitions of ~5H_J→~7F_0. Its n value was 4.8±0.3, indicating that populating the 5HJ levels needed five 980 nm photons. Also, we found that populating the ~5D_0 level was the result of three photons and four photons processes. These unique optical properties were attributed to the bridging function of Tm~(3+) ions in populating high-energy states of Eu~(3+) ions. The population of the Eu~(3+) ions occured through the energy transfer from Tm ~(3+) ions and resulted in the reduction of upconversion intensity of the Tm~(3+) ions. Through adjusting the doping proportion of Tm ~(3+) to Eu~(3+) ions, strong emissions of both Tm~(3+) and Eu~(3+) ions can be obtained. In addition, we investigated the temporal evolution of upconversion luminescence in the nanocrystals. Temporal evolutions of ~5H_J, ~5G_J, and ~5L_6 levels of Eu~(3+) ions were reported for the first time. The analyses of lifetimes indicated that the lifetimes were increasing along the ladder of the energy levels ~5H_J, ~5L_6, ~5D_3, ~5D_2, ~5D_1, and ~5D_0 except for ~5G_J. The ~5G_J levels presented a short lifetime due to the narrow energy gap to ~5L_J, which increased the nonradiative relaxation rate from ~5G_J to ~5L_J and destroyed the monotony along the energy ladder. This phenomenon indicates that in the trivalent europium ions, the higher the energy level locates, the more unstable the level is. These high-energy states prefer to de-excite to low-energy levels radiatively or nonradiatively.
(2) NaYF_4: Yb/Er/Eu NCs with an average size about 6.7 nm were synthesized using oleic acid as capping ligand. Under 980 nm excitation, besides the UC emissions from Tm~(3+) ions, ~5D_3→~7F_3, ~5D_2→~7F_0,1,2 and more abundant ~5H_(3-7)→~7F_(0-3) transitions were reported for the first time under room temperature. Furthermore, we study the integrated emission intensity ratios of ~4G_(11/2), ~2H_(11/2), ~4F_(9/2) levels of Er~(3+) ions between NaYF_4: 20%Yb~(3+), 1.5%Er~(3+), 1% (or 4%)Eu~(3+) and NaYF_4: 20%Yb~(3+), 1.5%Er~(3+) NCs under 980 nm excitation, and conclude the energy transfer law of ~4G_(11/2_, ~2H_(11/2), ~4F_(9/2) levels of Er~(3+) ions. When the excited Er~(3+) ions serve as activators, it is easier to transfer their energy to acceptors after they are excited to the longer lifetime level.
(3) Water-soluble PVP-stabilized NaYF_4: Yb/Er/Eu and NaYF_4: Yb/Tm/Eu NCs were synthesized by hydrothermal method. The NCs were coated with a very thin silica shell. And amino groups were introduced to the surface of silica shells by copolymerization of 3-aminopropyl (triethoxy) silane. The core/shell NCs can be dispersed in ethanol and water to form stable colloidal solution. This kind of distinct tri-doped NCs can emit dual-color characteristic luminescence under 980 nm and 394 nm excitation.
(4) Three kinds of Tm~(3+) / Yb~(3+) codoped fluoride microphosphors with similar sizes were synthesized by hydrothermal method. The results indicate 20% Yb~(3+) doping is sufficient for hexagonal LaF_3 microparticles to crystallize completely in the orthorhombic phase. And crystal symmetry has obvious effect on the luminescence spectra of Tm~(3+) ions and lifetimes of the radiative levels of Tm~(3+) ions.
(5) Hexagonal-phase NaYF_4: Yb/Er microcrystals with an average size about 200 nm were synthesized using oleic acid as capping ligand. The microcrystals are capable of being dispersed in nonpolar organic solvents to form fully transparent and stable colloidal solutions. Under 980 nm excitation, the Yb~(3+)/Er~(3+) codoped microcrystals colloidal solution presents bright and almost-monochromatic green upconversion fluorescence. Temperature-change spectra indicate: thermalization effects between the ~2H_(11/2) and ~4S_(3/2) levels separated by 765 cm~(-1) in NaYF_4: Yb/Er microcrystals were the reason of the 520 nm emissions. And the green emission intensity of Er~(3+) ions had a maximum at 160K under 980 nm excitation, which was attributed to the thermal activated distribution of electrons and the thermal quenching effect.
1)利用微乳液法合成了直径大约为2 nm的YF3:Yb/Tm/Eu纳米须;用油酸作为修饰试剂,合成了平均尺寸为6.7 nm的NaYF_4: Yb/Er/Eu纳米晶,在这些样品中首次在室温条件下观察到了Eu~(3+)的一些不寻常的紫色和紫外上转换发射,并通过不同掺杂样品光谱及荧光动力学过程的研究分析了稀土离子间能量传递过程。
2)选择PVP作为修饰试剂合成了水溶性的尺寸为30 nm左右的NaYF_4:Yb/Tm/Eu和NaYF_4:Yb/Er/Eu纳米晶,在这两种三掺体系中,其单个纳米晶就可以被394 nm和980 nm激光分别激发,产生两种颜色的特征发射。我们进一步以SiO2为壳层对这两种材料进行了包覆,改善了纳米微粒的环境稳定性和发光性能。并且讨论了这种材料在双色生物标记中的可能应用。
3)用水热法合成了Yb/Tm共掺LaF3及YF3上转换发光材料,当Yb~(3+)掺杂从10%提高到20%时,就使基质LaF3由六角相转变为斜方相。在相同的实验条件下,我们研究了晶相依赖的Tm~(3+)上转换发光特性及其动力学过程。
4)以油酸作为修饰试剂,合成了尺寸为200 nm NaYF_4: Yb/Er微米晶。修饰后的微米晶可以在非极性溶剂中形成澄清透明的胶体溶液。同时测量了这种油酸修饰的微米晶的变温上转换发射光谱,研究表明:Er~(3+)的上转换光谱对温度有很强的依赖关系。
Upconversion refers to nonlinear optical processes characterized by the successive absorption of two or more pump photons via intermediate long-lived energy states followed by the emission of the output radiation at a shorter wavelength than the pump wavelength. Principles and applications of such upconversion phosphors were presented in several reviews up to the 1970s by Auzel, Garlick, and Wright. Since then, upconversion has become a pervading effect in all RE-doped materials under high-density infrared excitation. Due to their unique and excellent optical properties, rare-earth fluoride upconversion nano-materials have important applications in high-resolution displays, solid lasers, integrated optical systems, infrared imaging, biological analysis and medical diagnosis and have attracted much attention. Establishing well-controlled synthesis strategies to these materials, understanding their growth and the relationships of their structures, morphologies and optical behaviors are crucial to the production of nanophosphors with designated optical properties. In this thesis, we reported some distinct upconversion emissions of several kinds of fluoride nanocrystals doped with Eu~(3+), Tm~(3+), Er~(3+) and Yb~(3+) ions, and characterized their properties.
(1) YF3:Yb/Tm/Eu nanobundles with a mean diameter of ~2 nm were synthesized by microemulsion method. Besides the UC emissions from Tm~(3+) ions, not only the unusual ~5D_2→~7F_3, ~5D_3→~7F_J emissions of Eu~(3+) were observed, but also the ultraviolet (UV) UC fluorescence from ~5H_(3-7), ~5G_(2-6), and ~5L_6 levels were recorded under room temperature. To the best of our knowledge, it is the first time to report the ~5H_(3-7), ~5G_(2-6), ~5L_6→~7F_0 transitions, which expand the vast space for the research, development and application of Europium. Furthermore, we investigated the excitation power dependence of upconversion luminescence intensities. The upconversion emission peaked at 326.7 nm came from the transitions of ~5H_J→~7F_0. Its n value was 4.8±0.3, indicating that populating the 5HJ levels needed five 980 nm photons. Also, we found that populating the ~5D_0 level was the result of three photons and four photons processes. These unique optical properties were attributed to the bridging function of Tm~(3+) ions in populating high-energy states of Eu~(3+) ions. The population of the Eu~(3+) ions occured through the energy transfer from Tm ~(3+) ions and resulted in the reduction of upconversion intensity of the Tm~(3+) ions. Through adjusting the doping proportion of Tm ~(3+) to Eu~(3+) ions, strong emissions of both Tm~(3+) and Eu~(3+) ions can be obtained. In addition, we investigated the temporal evolution of upconversion luminescence in the nanocrystals. Temporal evolutions of ~5H_J, ~5G_J, and ~5L_6 levels of Eu~(3+) ions were reported for the first time. The analyses of lifetimes indicated that the lifetimes were increasing along the ladder of the energy levels ~5H_J, ~5L_6, ~5D_3, ~5D_2, ~5D_1, and ~5D_0 except for ~5G_J. The ~5G_J levels presented a short lifetime due to the narrow energy gap to ~5L_J, which increased the nonradiative relaxation rate from ~5G_J to ~5L_J and destroyed the monotony along the energy ladder. This phenomenon indicates that in the trivalent europium ions, the higher the energy level locates, the more unstable the level is. These high-energy states prefer to de-excite to low-energy levels radiatively or nonradiatively.
(2) NaYF_4: Yb/Er/Eu NCs with an average size about 6.7 nm were synthesized using oleic acid as capping ligand. Under 980 nm excitation, besides the UC emissions from Tm~(3+) ions, ~5D_3→~7F_3, ~5D_2→~7F_0,1,2 and more abundant ~5H_(3-7)→~7F_(0-3) transitions were reported for the first time under room temperature. Furthermore, we study the integrated emission intensity ratios of ~4G_(11/2), ~2H_(11/2), ~4F_(9/2) levels of Er~(3+) ions between NaYF_4: 20%Yb~(3+), 1.5%Er~(3+), 1% (or 4%)Eu~(3+) and NaYF_4: 20%Yb~(3+), 1.5%Er~(3+) NCs under 980 nm excitation, and conclude the energy transfer law of ~4G_(11/2_, ~2H_(11/2), ~4F_(9/2) levels of Er~(3+) ions. When the excited Er~(3+) ions serve as activators, it is easier to transfer their energy to acceptors after they are excited to the longer lifetime level.
(3) Water-soluble PVP-stabilized NaYF_4: Yb/Er/Eu and NaYF_4: Yb/Tm/Eu NCs were synthesized by hydrothermal method. The NCs were coated with a very thin silica shell. And amino groups were introduced to the surface of silica shells by copolymerization of 3-aminopropyl (triethoxy) silane. The core/shell NCs can be dispersed in ethanol and water to form stable colloidal solution. This kind of distinct tri-doped NCs can emit dual-color characteristic luminescence under 980 nm and 394 nm excitation.
(4) Three kinds of Tm~(3+) / Yb~(3+) codoped fluoride microphosphors with similar sizes were synthesized by hydrothermal method. The results indicate 20% Yb~(3+) doping is sufficient for hexagonal LaF_3 microparticles to crystallize completely in the orthorhombic phase. And crystal symmetry has obvious effect on the luminescence spectra of Tm~(3+) ions and lifetimes of the radiative levels of Tm~(3+) ions.
(5) Hexagonal-phase NaYF_4: Yb/Er microcrystals with an average size about 200 nm were synthesized using oleic acid as capping ligand. The microcrystals are capable of being dispersed in nonpolar organic solvents to form fully transparent and stable colloidal solutions. Under 980 nm excitation, the Yb~(3+)/Er~(3+) codoped microcrystals colloidal solution presents bright and almost-monochromatic green upconversion fluorescence. Temperature-change spectra indicate: thermalization effects between the ~2H_(11/2) and ~4S_(3/2) levels separated by 765 cm~(-1) in NaYF_4: Yb/Er microcrystals were the reason of the 520 nm emissions. And the green emission intensity of Er~(3+) ions had a maximum at 160K under 980 nm excitation, which was attributed to the thermal activated distribution of electrons and the thermal quenching effect.
引文
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