Investigations Of Rare-earth Doping Concentration Induced Phase Transition For Energy Transfer In Fluoride Nanocrystals

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.