【作者】 郭辉；

【导师】 余华；

【作者基本信息】 南开大学 ， 光学， 2014， 硕士

【摘要】 掺稀土氟化物纳米晶作为一种新型发光材料在照明、显示和生物等领域具有潜在应用价值。在众多领域中,人们主要应用纳米晶中稀土离子的光学性质,而其光学性质又依赖于纳米晶结构。本文采用“热诱导-腐蚀法”制备了掺稀土氟化物纳米晶材料,通过X射线衍射、透射电子显微镜和荧光光谱等手段清晰系统地研究了稀土掺杂浓度对氟化物纳米晶结构的影响,进而基于纳米晶的结构相变实现了对稀土发光的微调控,为掺稀土纳米晶材料的发展提供理论指导。根据对不同稀土浓度掺杂纳米晶的结构和形貌表征,提出了浓度诱导纳米晶的结构相变模型。稀土离子以取代Pb2+离子并以F-离子作电荷补偿的方式掺入纳米晶。在低浓度掺杂时,纳米晶形成立方相Pb3REF9结构,随着掺杂浓度的提高,纳米晶结构逐渐过渡到四方相PbREF5。尤其是本文第一次观察到并证实了中等浓度掺杂下纳米晶同时具有这两种结构的情形。稀土掺杂浓度诱导了纳米晶的结构相变,并导致了稀土离子在纳米晶中的位置对称性从Oh到D4h的破缺。基于所提结构模型,采用Eu3+离子作为荧光探针和Rietveld法结构精修,进一步从实验和理论上验证了相变过程的正确性。纳米晶结构相变可导致稀土离子的不同分布和位置对称性破缺,进而影响稀土间的能量传递和发光特性。通过对不同结构中Yb3+离子合作上转换发光的研究,发现Yb3+离子间距离是影响它们之间敏化作用强弱的主要因素；接着通过对Yb3+-Tb3+共掺纳米晶结构相变的研究提出了两种三中心分布模型,进而基于不同的分布模型建立了Cooperative Energy Transfer (CET)和Accretive Energy Transfer (AET)两种三中心能量传递机制,并通过荧光光谱和速率方程从实验和理论上予以证实。CET过程发生在一个Tb3+离子和两个无相互作用的Yb3+离子之间,而AET过程发生在一个Tb3+离子和两个具有强敏化作用的Yb3+离子之间。通过荧光寿命测量和近红外发光量子效率的计算,发现AET过程中的发光量子效率为134%,远高于CET过程中的104%,说明能够发生AET过程的Yb3+-Tb3+共掺材料更适合应用在太阳能电池方面以提高其转化效率。总之,本文的工作将对进一步研究掺稀土纳米材料的发光特性及其应用提供非常有益的帮助。更多还原

【Abstract】 Rare-earth (RE) ions doped fluoride nanocrystals have been shown to be a novel and promising luminescent material with a perspective application in lighting, display and biological fields. In these areas, people mainly utilize the luminescence of RE3+ions in nanocrystals, however their optical properties strongly depend on the nanocrystal structures. In this thesis, RE3+ions doped nanocrystals have been prepared by thermal induction and corrosion treatment, and the effect of doping concentration on nanocrystal structures are systematically investigated by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and photoluminescence spectroscopy. And then based on structure investigations, it achieves the micro-regulation of nanocrystal structure on RE3+luminescence, which could provide theoretical guidance for the developments of such luminescent nano-materials.In different RE3+concentration doped P-PbF2nanocrystals, a doping concentration induced phase transition model has been proposed on basis of structure and morphology studies. The RE3+-doping mechanism is considered to be RE3+ions substitution for Pb2+ions as well as interstitial F-ions charge compensation. In lowly concentration doped samples, nanocrystal has a cubic structure with Pb3REF9model, while it gradually transforms into a tetragonal PbREFs structure with increasing RE3+doping content. Particularly, the intermediate phase of moderately doped nanocrystal, which contains both phases mentioned above, is observed for the first time. Doping concentration induces the phase transition, consequently, it results in the site symmetry breakdown from Oh to D4h of RE3+ions in nanocrystals. And based on the proposed structure models, it verifies the reasonability of the phase transition by Eu3+ions as fluorescent probes and Rietveld XRD refinements in experiments and theory, respectively.Structure phase transition of nanocrystals can lead to different distributions and site symmetry breakdown of RE3+dopants, further causing the different energy transfer processes and optical properties. Firstly, the dependence of cooperative upconversion luminescence of Yb3+ions on nanocrystal structures has been investigated. It is found that the distance between Yb3+ions is a key factor affecting the sensitization between them. And then, two kinds of three-center (one Tb3+ion and two Yb3+ions) distributions are proposed by analyzing the phase transition of Yb3+-Tb3+co-doped nanocrystals. And two three-center energy transfer mechanisms, Cooperative Energy Transfer (CET) and Accretive Energy Transfer (AET), are established according to different distributions. CET process occurs between one Tb3+ion and two non-interacting Yb3+ions, while AET process involves one Tb3+ion and two strongly interacting Yb3+ions. Experimental results obtained from photoluminescence spectroscopy study are in agreement with the theoretical calculations by applying rate equation in the two models, strongly supporting the proposed three-center energy transfer mechanisms. The sensitization between Yb3+ions only existing in AET process can greatly improve the energy transfer rate, further enhancing the near-infrared luminescence quantum efficiency. The result that the calculated quantum efficiency in AET process is much higher than that in CET process (134%and104%, respectively), can benefit for further increasing the conversion efficiency of c-Si solar cells. In a word, this work could have far reaching significance to the further optical investigations and applications of such RE3+ions doped nano-materials.更多还原