新型掺杂材料的设计，制备和发光特性
文献类型：学位论文
| 作者 | 刘小峰 |
| 学位类别 | 博士 |
| 答辩日期 | 2010 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 邱建荣 |
| 关键词 | 发光材料 下转换 稀土离子掺杂纳米晶 氮化硼 |
| 其他题名 | Design, synthesis and spectroscopic investigation of novel impurity-activated luminescent materials |
| 中文摘要 | 发光材料是指在外界光，电场，热能等激发下发出光频电磁辐射的固态物质。近十年来，由于能源危机，自然资源短缺以及环境问题的加剧，发光材料的研究也转向于便宜而且高效节能的固态照明材料的开发以及能源相关领域的应用。此外，纳米时代的到来也激起了纳米尺度发光材料的合成及其应用的研究热潮。发光材料领域的这些发展方向引导了本论文研究课题的提出，包括以下三个部分： 一．近红外下转换材料 因具有将一个高能量光子剪裁为两个低能量光子的功能，量子剪裁材料引起人们的兴趣，他们被认为是提高太阳能电池光电转换效率的理想下转换材料。本人的研究关注的是稀土离子掺杂的近红外下转换材料。本研究选取了Yb3+为红外发光离子，RE3+(RE3+=Tb3+, Tm3+, Pr3+)为敏化剂离子。在基质为稀土硼锗酸盐的玻璃中，成功的观察到了RE3+-Yb3+离子对的合作下转换，也就是说这些敏化剂离子吸收的一个高能量光子会导致Yb3+离子发射两个低能量的红外波段的光子。在这三个不同体系中，所得到的最高内部量子效率高达170%。 然而由于以上这些敏化剂离子的4f-4f跃迁的吸收为窄带并且强度较低，对实际应用很不利，这是由于在此种情况下仅有很小一部分的太阳光能被下转换为近红外光。为解决这一问题，本研究选用了Ce3+为敏化剂，钇铝石榴石(YAG)为基质。在YAG中，Ce3+的4f-5d跃迁的能量很宽，并能覆盖Yb3+：2F7/22F5/2跃迁能量的两倍。研究发现在蓝光激发下，YAG:Ce,Yb中Ce3+向Yb3+发生合作能量传递，所得到的最大内部量子效率为150%。这一可被可见光宽带激发的下转换材料在光谱利用带宽上较以往RE(RE=Tb3+, Tm3+, Pr3+)敏化的下转换材料有显著优势。 二．稀土离子掺杂的无机纳米晶 由于在光学性能，化学稳定性以及环境友好性等方面具有优势，稀土离子激活的无机纳米晶近来倍受重视。本研究选取了具有较低声子能量的NaYF4为基质进行掺杂，并通过溶剂热手段合成了单分散的稀土离子掺杂的NaYF4纳米晶。通过合理的控制各种掺杂离子的浓度，在980 nm激发下，成功的在NaYF4:Yb,RE(RE=Er, Tm)纳米颗粒的胶体溶液中观察到了纯的红色，绿色和蓝色的上转换发光。此外，我们通过演示性的实验阐明这种无色透明的上转换纳米颗粒胶体可以用作液态的三维立体显示介质。 在本研究中，还合成了近红外光通讯波段发光的纳米晶溶胶。在实验中，通过调整合成方法以及选用光学惰性的四氯化碳为溶剂，有效的避免了羟基以及有机基团导致的在红外波段的荧光猝灭。在Yb3+-Er3+，Yb3+-Pr3+，和Yb3+-Tm3+离子对激活的NaYF4纳米晶溶胶中，观察到在1300 - 1650 nm间表现出很强的红外发光，并且其荧光寿命接近于对应的体材料。在这一纳米颗粒-溶剂胶体体系中，研究发现不同离子掺杂的纳米晶由于间隔距离太大而不会发生相互作用，从而避免了荧光猝灭。在这种情况下，可以实现在同一波长激发下，在含有不同纳米颗粒的胶体中同时观察到不同离子的多个波段的发光。此外，在本研究中还使用含有0.5 wt.%的NaYF4:Yb,Er纳米晶的溶胶为增益介质，成功的进行了光放大的演示，观察到这一胶体体系在1550 nm处的最大增益系数可达0.58 cm-1。 三．基于氮化硼的可调谐荧光材料及其纳米晶 氮化硼(BN)和一对碳原子为等电子体，近年来他们和碳纳米材料一样也开始引起了许多学者的兴趣。本研究关注的是BN的发光性能。我们通过尿素和硼酸的反应合成了乱层结构的BN(t-BN)。并且通过在原料中引入聚乙二醇实现了BN的碳掺杂。研究表明，可以通过控制合成原料的配比，反应温度等条件，来制备具有可调谐可见荧光的BN:C材料。此外，研究还发现，这一可调谐的荧光具有长余辉特性，并且可能和其中的氮空位有关。对这一材料的光谱研究表明，所观察到的BN:C的可调谐荧光不是源于带间跃迁，而是来自与碳有关的缺陷能级。然而目前对这些结果在理论上的解释仍显不足，还值得深入研究。 此外，由于结构研究表明在尿素和硼酸反应的产物中，发光的BN:C纳米颗粒镶嵌在剩余的氧化硼基体中。基于这一情况，实验中将所得到的产物在水中浸泡除去了氧化硼，并且发现这些纳米颗粒尺寸在5 - 10 nm，他们由于表面被羟基包裹因而呈现很好的水溶性。光谱研究表明这些纳米颗粒也表现为可调谐的荧光，在单光子（紫外）和双光子（800 nm飞秒激光）激发下都有较高的量子效率。进一步测试发现这些纳米颗粒具有很强的抗光降解的能力。这些结果为BN基纳米材料在生物成像以及纳米光电子领域的应用做了铺垫。 |
| 英文摘要 | Luminescent materials represent a category of solid state matter that emits electromagnetic radiation in optical frequency as a result of external stimuli such as photon, electric field, thermal energy and etc. In recent decades, driven by increasing energy crisis, shortage of natural resources and environmental issues, the research into luminescent materials has largely concentrated on the development of cheap and efficient light-emitting medium for new energy-saving light sources and energy-related applications. In addition, the incoming era of nanotechnology has also stimulated great effort in the synthesis and application of luminescent materials at the nanoscale. These driving forces in the field of luminescent materials have directed the three research topics in this thesis: I. Near-infrared (NIR) downconverting materials Near-infrared (NIR) quantum cutting (QC) materials have recently received increasing interest, as they are capable of splitting of a high energy photon into two NIR photons. This feature is attractive for applications as down-converting (DC) materials to enhance the efficiency of silicon-based photovoltaic cells. My work focused on rare-earth activated materials for NIR-QC, using Yb3+ as the NIR emitting ion and trivalent RE ions of Tb3+, Tm3+ and Pr3+ as sensitizers. We have successfully demonstrated cooperative down-conversion with a borogermanate glass host activated with ion pairs of RE3+-Yb3+ (RE=Tb, Tm, or Pr). Namely, absorption of each high energy photon by the sensitizer ion (Tb3+, Tm3+, or Pr3+) resulted in the emission of two NIR photons by electronic transition of Yb3+. The highest quantum efficiency we have obtained was greater than 170%. Since the absorption of the above-mentioned sensitizer ions due to 4f-4f transitions are naturally weak in intensity and narrow in spectral width, this character is therefore unfavorable from viewpoint of practical application because only a very small portion of sunlight can be down-converted into NIR photons with these DC materials. To circumvent this problem, we chose Ce3+ as the sensitizer and yttrium aluminum garnet (YAG) as a crystalline host, where the 4f-5d transition of Ce3+ is well located at a much wide spectral range that covers the twice the energy of Yb3+ 2F7/22F5/2 transition. Under blue light excitation we identified cooperative energy transfer from Ce3+ to Yb3+, and obtained the highest quantum efficiency of 150%. This broadband excitable new DC material is a significant progress towards practical application because of its much broadened light-harvesting bandwidth compared with the previous RE (RE=Tb3+, Tm3+, or Pr3+) sensitized DC materials. II. Rare-earth (RE) doped inorganic nanocrystals (NCs) RE ions activated inorganic compound NCs attract growing attention due to its exceptional optical properties, chemical stability and low toxicity. In our research, NaYF4, a host of low vibration energy, was selected as the matrix for doping with RE ions, and a solvothermal route was employed for the synthesis of mono-dispersive RE-doped NaYF4 NCs. In an attempt to tune the upconversion (UC) emission color of RE-activated NCs, through controlled doping we successfully realized pure blue, green and red UC emission from colloidal solutions of NCs of NaYF4:Yb,RE(RE=Er, Tm) with a NIR laser pumping at 980 nm. Furthermore, our demonstrative experiment suggested that the transparent colloids containing upconverting NCs can be a novel liquid medium for three-dimensional volumetric display. The synthesis of NCs with emission in the NIR spectral range has also been involved in my investigation. In our experiment, the quenching of NIR emission due to hydroxyl and alkyl groups has been effectively mitigated through the application of an appropriate synthetic route and an optically inert solvent of carbon tetrachloride. The colloids of NaYF4 NCs activated with RE ion pairs of Yb3+-Er3+, Yb3+-Pr3+, and Yb3+-Tm3+ gave efficient emission in the NIR, ranging from 1300 nm to 1650 nm, with radiative lifetimes comparable to their bulk counterparts. Within the NCs-colloid system, we identified that the interaction among different RE ions located in different NCs can be completely suppressed as each NC was separated with intervals greater than the Foster distance. Under this circumstance, we observed a broad emission spectrum composed of simultaneous emissions from colloidal solution containing different NCs activated with different ion pairs. Moreover, we also successfully demonstrated optical amplification with a CCl4 solution of 0.5 wt.% NaYF4:Yb,Er NCs. The colloid system showed a highest optical gain coefficient of 0.58 cm-1 at 1550 nm. III. Boron nitride based emission-tunable phosphors and nanocrystals Boron nitride (BN), whose molecule is iso-electronic to a pair of carbon atoms, becomes of interest similar to carbon-based nanostructures that have fascinated numerous researchers in recent years. My research concentrated on the photoluminescence behavior of turbostratic BN (t-BN), which was synthesized with a low temperature liquid route by the reaction of urea with boric acid. Doping of t-BN with carbon was realized by the introduction of a carbon source PEG (polyethylene glycol) to the raw material mixture of boric acid and urea during synthesis. Through adjusting of the synthesis conditions, such as ratios of raw materials and the reaction temperature, we obtained a series carbon doped BN phosphor with tunable emission covering ultraviolet and the entire visible spectral region. Moreover, we found that the tunable luminescence exhibited a long persistent character, which related to the presence carbon-related nitrogen vacancies in BN. The spectroscopic investigation indicated that the observed tunable emission of carbon-doped t-BN was not due to band-to-band transition, but from carbon-related defect levels. However, the understanding of these results from the theoretical aspect is still inadequate and thus I have planned a further investigation. As structural characterization has confirmed that the fluorescent t-BN nanoscale domains were surrounded by the residual boron oxide in the reaction product, based on this fact we developed a simple strategy for the extraction of the fluorescent BN nanoparticles from the product by water treatment to remove the residual B2O3 phase. The obtained nanoparticles were 5-10 nm in size and water-soluble due to surface capping by hydroxyl groups. These nanoparticles exhibited tunable emission with high quantum efficiencies under both single-photon (ultraviolet light) and two-photon (NIR femtosecond laser at 800 nm) excitation. Further test indicated that they were robust against photo-degradation and toxicity-free. These results are expected to pave the way for application of BN NCs in bio-imaging and optoelectronic areas. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15290]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 刘小峰. 新型掺杂材料的设计，制备和发光特性[D]. 中国科学院上海光学精密机械研究所. 2010. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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