【作者】 郑克志；

【导师】 秦伟平；

【作者基本信息】 吉林大学 ， 物理电子学， 2011， 博士

【摘要】 光频上转换是物理学中获得短波长发光十分重要的方法；对上转换发光材料及其机理的研究一直是固体发光领域的前沿和热点课题。蓝紫色和紫外上转换材料和器件在激光、通信、能源、医疗、催化和军事等领域都有着十分重要的应用前景。利用稀土离子具有的丰富能级,原则上人们可以通过上转换的方式将不同低能频域的光转换为所需要的高能量光子,以满足实际应用中的需要。目前,有关红外光激发下的上转换发光的研究主要集中在可见波段,而紫外上转换发光的研究相对较少。究其原因,主要是由于上转换过程中低的转换效率造成的。上转换过程的这个特点导致实验中获得的上转换发光大多来自两光子或三光子过程,有些材料在高的激发功率的激发下可能出现弱的四光子上转换发光。但是更高阶数的上转换过程却不容易被观测到,上转换发光的这个特点严重地限制了它在很多领域中的应用。针对上述问题,本文以具有优异上转换发光性能的六角相NaYF4为研究对象,首次获得了部分稀土离子的高阶多光子上转换发光。同时,我们围绕高阶多光子上转换发光过程中光与物质相互作用这一基本科学问题,从稳态光谱到荧光动力学过程、从电子跃迁与晶格弛豫到能量传递、从上转换材料到其中的物理机制,展开了系统的研究并取得了一些创新性的研究结果。另外,在稀土上转换白光材料方面,我们也取得了创新性的成果,主要结果如下：1、用水热法合成了NaYF4:Yb3+/Er3+微米晶。在近红外光激发下,发现了样品中Er3+离子覆盖近红外到紫外波段的上转换发光,首次观察到了Er3+离子的五光子和六光子上转换发光过程。首次在实验上证明了室温下Er3+离子红光发射的三光子过程。动力学分析的结果证明了共掺杂体系中Yb3+到Er3+离子的能量传递过程。Yb3+到Er3+离子的持续的能量传递是布居高能态Er3+离子的关键。探讨了Er3+离子高能态布居的功率密度依赖特性,与低功率密度激发相比,在高的激发光功率密度激发下Er3+离子的紫外发射来自于更高阶的上转换过程。来自Er3+离子2I11/2和4D7/2能级的上转换发光表现出功率和温度依赖特性,证明了存在于两能级间的多声子弛豫过程。2、用水热法合成了NaYF4:Yb3+/Er3+/Gd3+和NaYF4:Yb3+/Tm3+/Gd3+微米晶。在980 nm激光激发下,首次在NaYF4基质中观察到了Gd3+离子的紫外上转换发光。Er3+离子和Tm3+离子在两个共掺杂体系中分别起到稀土离子间能量传递的“桥联”作用；Er3+和Tm3+离子到Gd3+离子的有效的能量传递是布居激发态Gd3+离子的关键。浓度变化实验及动力学分析的结果证明了Er3+和Tm3+离子到Gd3+离子的能量传递并给出了一些可能的能量传递途径。功率依赖的发光强度分析表明980 nm激光激发下共掺杂体系中Gd3+离子的上转换发光来自于五光子和六光子上转换过程。3、在红外光(1560 nm)激发下,在NaYF4:Yb3+/Er3+微米晶中首次观察到了Er3+离子的紫外(244 nm,256 nm,276 nm,288 nm,306 nm,317 nm和335 nm)上转换发光。光谱分析证实了存在于Er3+和Yb3+离子之间的能量传递过程。Er3+离子的激发态吸收和Yb3+到Er3+离子的能量传递是布居高能态Er3+离子的两个重要途径。红外光激发下Er3+离子的紫外发射来自高阶上转换过程,功率依赖的发光强度分析指出了迄今为止有记录的最高阶的上转换过程——十一光子过程。4、在1560 nm激光激发下,在β-NaYF4:Yb3+/Gd3+/Er3+微米晶中首次观测到了Gd3+离子的紫外(276.8 nm,279.6 nm,306 nm和311 nm)上转换发光。Er3+离子在共掺杂体系中的“桥联”作用是布居高能态Gd3+离子的关键因素。光谱分析和动力学分析结果证明了Er3+到Gd3+离子的能量传递。Gd3+离子61J多重态内相邻能级间的布居由热布居决定。发光强度与功率依赖关系证明了Gd3+离子的上转换发光来自八光子和九光子过程。5、用湿化学方法伴随后退火的方法合成了稀土离子掺杂的Gd2O3纳米管。扫描电镜结果证明了样品管状结构的形成。在980 nm近红外光激发下,首次在Gd203基质材料中获得了室温下明亮的上转换白色发光。当980 nm激光二极管激发功率密度在很大范围内变化时,利用样品上转换发射谱计算得到的色坐标值变化很小,并全部位于1931色度图白色区域内,证明样品是一种优异的白光材料。在Yb3+-Er3+-Tm3+共掺杂的Gd2O3纳米管中,白色上转换发光中的蓝光和绿光成分分别来自于Tm3+离子和Er3+离子的贡献,而红光发射来自于Er3+和Tm3+离子的共同贡献。蓝色,绿色和红色上转换发光分别来自于两光子和三光子过程。动力学分析证明了存在于Tm3+离子和Er3+离子间的能量传递过程。更多还原

【Abstract】 Frequency upconversion is one of the major routes for achieving short-wavelength light. The studies around upconversion luminescence materials and upconversion mechanism became research focus in the past decades. Blue, violet, and ultraviolet upconversion luminescence have been widely investigated throughout the scientific community owing to great fundamental interests and numerous applications in the fields of laser, communication, energy, medical treatment, photocatalysis, military, etc. Multiphoton upconversion is an anti-stokes process, which can convert low energy photon to high energy photon for satisfying requirements of real applications. Up to now, most studies about rare earth upconversion were limited in visible portion, and only a few papers have reported their ultraviolet upconversion luminescence. All these can attribute to the low transform efficiency during upconversion processes. Most upconversion luminescence of rare earth ions in the experiments came from two-photon or three-photon process, and some materials may present weak four-photon upconversion process under high power light excitation. However, higher-order upconversion processes are difficult to obtain. This bottleneck severely limited the use of upconversion technology in many fields. In view of the above questions, hexagonal NaYF4, with good upconversion performance, was selected as research subject in this paper. High-ordre multiphoton upconversion luminescence of some rare-earth ions were obtained firstly in our experiments. The light-matter interaction, electron transition, lattice relaxation, energy transfer, multiphoton upconversion luminescence and its interior mechanism in rare-earth ions doped materials were studied systematically through laser spectroscopic measurements and dynamic analysis and some of innovative results were obtained. Additionally, innovative research results were obtained in the field of rare-earth doped white upconversion luminescence. The major achievements obtained are as follow:1. NaYF4:Yb3+/Er3+microcrystals were prepared by using hydrothermal method. Under near-infrared light excitation, near-infrared to ultraviolet upconversion luminescence of Er3+were observed. Five-photon and six-photon upconversion processes of Er3+were obtained for the first time. Three-photon upconversion process of red emission of Er3+were confirmed experimentally for the first time. In Yb3+-Er3+ codoped system, energy transfer from Yb3+to Er3+played important roles in populating high-energy excited states of Er+. The power density-dependent populating processes of Er3+were discussed. Compared with low power density excitation, the ultraviolet emissions of Er3+came from higher-order processes under high power density excitation. The ultraviolet emissions came from 2I11/2 and 4D7/2 states of Er3+present power and temperature dependence, which demonstrated the existence of nonradiative relaxation process between these two states.2. NaYF4:Yb3+/Er3+/Gd3+and NaYF4:Yb3+/Tm3+/Gd3+microcrystals were prepared by using hydrothermal method. Under 980 nm excitation, ultraviolet upconversion luminescence of Gd3+were observed firstly in NaYF4 matrix. In the codoped system, Er3+and Tm3+act as "bridging" ions during energy transfer processes. Energy transfers from Er3+to Gd3+ions and from Tm3+to Gd3+ions played important roles in populating excited Gd3+. Experiments on concentration variation and dynamic analysis revealed the energy transfer processes between Er3+and Gd3+ and between Tm3+and Gd3+. Some of possible energy transfer routes were proposed based on experimental results. Power dependence of luminous intensity revealed the upconversion emissions of Gd3+came from five-photon and six-photon processes.3. Under 1560 nm infrared light excitation, ultraviolet (244 nm,256 nm,276 nm, 288 nm,306 nm,317 nm, and 335 nm) upconversion emissions of Er3+were observed firstly in NaYF4:Yb3+/Er3+microcrystals. Energy transfers between Er3+and Yb3+ were confirmed by spectral analysis. Excited state absorption of Er3+and energy transfers from Yb3+to Er3+are two major routes in populating high-energy excited states of Er3+. Ultraviolet emissions of Er3+came from high-order multiphoton upconversion processes. The highest-order upconversion process—eleven-photon upconversion process was observed in our experiments.4. Under 1560 nm excitation, ultraviolet (276.8 nm,279.6 nm,306 nm, and 311 nm) upconversion emissions of Gd3+were observed firstly inβ-NaYF4:Yb3+/Gd3+/Er3+microcrystals. In the codoped system, the "bridging" effect of Er3+played important roles in populating high-energy states of Gd3+ions. Energy transfers from Er3+to Gd3+were confirmed by spectral analysis and dynamic analysis. The thermal population between 6IJ multiplet is the main mode in producing their neighboring states. Power dependence of luminous intensity demonstrated the eight-photon and nine-photon upconversion processes of Gd3+.5. Rare-earth ions doped Gd2O3 nanotubes were synthesized by using a simple wet-chemical route at low temperature and ambient pressure followed by a subsequent heat treatment. SEM results depicted the hollow and tubular column feature of the nanotubes clearly. Under 980 nm excitation, room-temperature upconversion white luminescence was achieved in Gd2O3 matrix for the first time. The calculated CIE color coordinates fall well within the white region and shift only slightly when the power density changed in a wide region. In the Yb3+-Er3+-Tm3+codoped Gd2O3 nanotubes, blue and green upconversion emissions came from Tm3+and Er3+ respectively. Both Tm3+and Er3+ are effective to induce the red upconversion luminescence. Blue, green and red upconversion luminescence came from two-photon and three-photon processes, respectively. Dynamic analysis revealed the energy transfer from Tm3+ to Er3+ clearly.更多还原