【作者】 吕强；

【导师】 赵连城； 国凤云；

【作者基本信息】 哈尔滨工业大学 ， 材料物理与化学， 2008， 博士

【摘要】 上转换发光材料的特点是所吸收的光子能量低于发射的光子能量(反Stokes定律),是一种颇具发展潜力的功能材料,在夜视系统、彩色显示、传感器、上转换激光器和生物医学等国民经济和国防建设领域有广阔的应用前景。鉴于掺杂稀土离子作为分立发光中心以及纳米材料具有较大的比表面积,纳米化上转换发光材料的表面效应会对其上转换发光产生一定影响,严重时甚至会猝灭荧光,而采用核-壳结构是解决这一问题的有效方法。本论文是以Y2O3:Tm3+-Yb3+纳米材料为范例,通过研究核-壳型Y2O3:Tm3+-Yb3+纳米材料的制备、结构及其上转换发光特性,为进一步提高核-壳型纳米材料的上转换发光性能提供实验和理论基础。为了检验纳米材料的表面效应对上转换发光的影响,采用Pechini型sol-gel方法合成了平均粒径为3.6～35 nm的Y2O3:Tm3+ -Yb3+纳米材料;微观结构研究表明,延长烧结时间和提高烧结温度会导致纳米材料的尺寸增大;光谱学研究表明,纳米材料的上转换发光强度依赖于纳米材料的尺寸。鉴于掺杂稀土离子的发光机制,一个外球壳与纳米材料中稀土离子摩尔浓度比的实验模型揭示,尺寸增大会使纳米材料中可参与上转换发光的稀土离子数目逐渐增加,这直接导致上转换发光强度的增加。依据此模型预测,能够产生上转换发光的纳米材料极限尺寸为4 nm。为了削弱表面效应对上转换发光特性的影响,采用St?ber方法制备了SiO2或TiO2包覆Y2O3:Tm3+-Yb3+纳米材料;微观结构研究表明,壳层厚度随包覆反应时间或包覆次数的增加而增加;光谱学研究表明,SiO2或TiO2包覆Y2O3:Tm3+-Yb3+纳米材料的上转换发光强度明显高于Y2O3:Tm3+-Yb3+纳米材料的发光强度;Eu3+离子掺杂纳米材料的光辐射量子效率在一定范围内也会随包覆反应时间的延长而逐渐提高。上转换发光增强的机理可解释为当包覆壳层后,壳层与内核纳米材料之间所形成的协作配位场“激活”了镶嵌在纳米材料表面处于“休眠”状态的稀土离子,使其成为新的分立发光中心,而大量新分立发光中心的出现则导致上转换发光强度的增大。进一步研究表明,壳层厚度对上转换发光过程会产生积极和消极两个方面的影响,积极影响是能调节和改善核-壳界面之间协作配位场的强度,有利于上转换发光的增强,而消极影响则是衰减入射泵浦光的强度和吸收分立发光中心辐射的上转换发光强度。积极和消极影响的相互竞争决定了核-壳型纳米材料上转换发光的强度。在壳层厚度相似的条件下,不同壳层材料吸收系数的差异是导致其上转换发光强度存在差异的主要原因。为了弱化上转换纳米材料的团簇特性,并使纳米材料具有一定的生物功能性,采用接枝改性法在SiO2包覆Y2O3:Tm3+-Yb3+纳米材料的表面接枝了三种活性有机官能团。结构研究表明,氨基、羧基和醛基官能团已经分别接枝在纳米材料的表面;光谱学研究表明,与Y2O3:Tm3+-Yb3+纳米材料相比,表面改性纳米材料的上转换发光强度均有不同程度的提高;与SiO2包覆Y2O3:Tm3+-Yb3+纳米材料相比,表面改性Y2O3:Tm3+-Yb3+纳米材料的上转换发光强度又略有降低,这表明附着在SiO2外壳上的有机官能团成为上转换发光强度提高的制约因素,因为有机官能团中的高振动能量C-H或C-C振子能够吸收一定的光子能量从而降低激发光的强度,同时也会降低上转换发光强度。Zeta势(ζ电位)和自然沉降实验结果表明,表面改性纳米材料在极性溶液中的分散稳定性均优于Y2O3:Tm3+-Yb3+纳米材料。综上所述,本文采用一个简单模型分析了纳米材料尺寸对上转换发光强度的影响,并预测稀土掺杂纳米材料上转换发光的极限尺寸。提出核-壳型纳米材料上转换发光的增强机理,通过实验验证了两个相关的推论。提出壳层厚度对上转换发光产生影响的原因是源自两个作用相互竞争的结果,即基质材料表面处的稀土离子由“休眠”状态转变为“激活”状态,和包覆壳层对入射泵浦光的吸收作用和对辐射上转换发光的再吸收作用。实现了Eu3+掺杂纳米材料的双光子同时吸收上转换发光,采用J-O理论定量地探索壳层厚度对上转换发光性能产生的影响。分析不同壳层材料对上转换发光强度产生影响的原因。氨基、羧基和醛基改性纳米粒子可获得较高的分散稳定性和上转换发光强度。更多还原

【Abstract】 Upconversion luminescence materials capable of converting infrared radiation into visible light in an efficient way are potential candidates for photonic applications in several areas such as detection of infrared radiation, color displays, sensors, upconversion laser and biomedicine. However, due to the larger specific surface area of nanoparticles and the isolated luminescence centers of rare earth ions, numerous surface effects, such as lattice distortion, broken bond, surface adsorbate and other surface defect inevitably degenerate the fluorescent properties of rare earth ions doped nanoparticles, and even quench the fluorescence. A convenient strategy of growing an undoped shell around the nanoparticle surfaces, i.e. the so-called core-shell structure, can resolve this problem. In this paper, in order to improve the photoluminescence properties of rare earth ions doped nanoparticles, we reported the synthesis, structure and upconversion luminescence properties of core-shell type Y2O3:Tm3+-Yb3+ nano-particles.To verify the influence of surface effects on upconversion luminescence of Y2O3:Tm3+-Yb3+ nanoparticles, nanoparticles with average sizes of 3.6～35 nm were synthesized using the Pechini type sol-gel method. Their microstructures show that both prolonging the sinterring time and increasing the sinterring temperature can result in the increase of nanoparticle sizes. The optical spectral results indicate that the upconversion luminescence intensities of Y2O3: Tm3+-Yb3+ nanoparticles are closely related to the sizes of nanoparticles. Considering the luminescence mechanism of rare earth ions doped nanoparticles, the molar concentration ratio of rare earth ions in outside spherical shell to the entire sphere suggests that the increase of nanoparticle sizes can increase the number of rare earth ions participating in upconversion processes, directly resulting to the enhancement of upconversion luminescence intensities. From the model above, we can predict that the limit size of rare earth ions doped nanoparticles for upconversion luminescence is about 4 nm.To decrease the influence of surface effects on upconversion luminescence of Y2O3:Tm3+-Yb3+ nanoparticles, SiO2 or TiO2 coated Y2O3:Tm3+-Yb3+ nano- particles were prepared using the St?ber method. Their microstructures show that the shell thicknesses depend on the coating time and the number of the coated-layer. The optical spectral results indicate that the upconversion luminescence intensities of SiO2 or TiO2 coated Y2O3:Tm3+-Yb3+ nanoparticles are higher than those of non-coated nanoparticles, while the radiative quantum efficiencies of Eu3+ ions doped Y2O3 nanoparticles also increase with prolonging the coating time. The improved mechanism in upconversion luminescence intensities is that the cooperation ligand fields between shell and core structures can activate the“dormant”rare earth ions near or on the surfaces of nanoparticles; and new isolated luminescence centers of rare earth ions on the surfaces of nanoparticles will be formed; a lot of new luminescence centers result in the increase of upconversion luminescence intensities of the coated Y2O3:Tm3+-Yb3+ nano-particles. A competition process between two mechanisms was proposed to explain the effects of different thickness shells on upconversion luminescence intensities. One mechanism is the role conversion of rare earth ions on the nanoparticles’surfaces, which is from the“dormant”state to the“activated”state due to the cooperation ligand fields. The other is the absorption effects of shells on incident pump light and the reabsorption effects of shells on upconversion luminescence. The differences of the absorption coefficients for different shell materials can result in different upconversion luminescence intensities when they have similar shell thicknesses.To weaken the agglomeration of upconversion nanomaterials and bio- functionalize the upconversion nanomaterials, SiO2 coated Y2O3:Tm3+-Yb3+ nanoparticles were modified using the ligand-exchanging method. The microstructures show that amine, carboxyl and aldehyde functional groups have been modified on the surfaces of nanoparticles. The optical spectral results indicate that the upconversion luminescence intensities of surface-modified nanoparticles are higher than those of Y2O3:Tm3+-Yb3+ nanoparticles, while the upconversion luminescence intensities of surface-modified nanoparticles are slightly lower than those of SiO2 coated Y2O3:Tm3+-Yb3+ nanoparticles. Organic functional groups become the limiting factors for enhancing upconversion luminescence intensities, because the organic ligands with high energy C-H and C-C vibrational oscillators on the nanoparticle surfaces can absorb certain photon energies to decrease pump light and upconversion luminescence intensities. The results of Zeta potential (ζ) and plain sedimentation suggest that the stabilities of surface-modified nanoparticles in polarity solutions are better than those of non-modified nanoparticles.In summary, a simple model is utilized to demonstrate the influence of quantum size effect on the upconversion luminescence intensities of nano-particles,and then predict the limit size of rare earth ions doped nanoparticles for upconversion luminescence. The mechanism of the enhancement of upcon-version luminescence for core-shell type nanoparticles is presented, and two relative deductions are confirmed by spectral experiments. A competition process between two mechanisms is proposed to explain the effects of different thickness shells and different shell materials on upconversion luminescence intensities: the role conversion of rare earth ions on the nanoparticles’surfaces from the“dormant”state to the“activated”state, and the absorption effects of shells on incident pump light and the reabsorption effects of shells on upconversion luminescence. A two-photon simultaneous absorption upconversion lumine-scence of SiO2-coated Y2O3 nanoparticles doped with Eu3+ ions is obtained, and a Judd–Ofelt theory is used to quantificationally probe the influence of shell thicknesses on upconversion luminescence intensities. The influence of different shell materials on upconversion luminescence intensities is analyzed. After the surface modification with amine, carboxyl and aldehyde functional groups, the upconversion nanomaterials with better stabilities in polarity solutions and higher luminescence intensities are obtained.更多还原