Photoluminescence And High Pressure Structure Of Gd₂O₃:Eu～(3+) Nanocrystals

Nowadays Nano Science and Technology (Nano-ST) has become the focus in the science and technology research field. The fabrication and property analysis of nano materials is the base for the development of Nano-ST. As a red phosphor, Gd₂O₃:Eu³⁺ can be used in displayers. The resolution of displayers is dependent on the phosphor scale, and the smaller the phosphor scale is, the higher the resolution is. At present the phosphor used in displayers has a magnitude of micrometers. So it is important to fabricate nano scale phosphors in order to improve the resolution of displayers. The development of science and technology, especially the microelectronic technology, require smaller devices. If we can fabricate nanorods, the micromation of devices may become true. Up-conversion luminescence is a phenomenon that when long-wavelength light is absorbed by some materials, short-wavelength light can be emitted. So the up-conversion material has a huge potential in application. High pressure is an important approach to change structure of materials, and many novel phenomena appear under high pressure, for example, some elements, not superconductor under ambient pressure, can change into superconductor under high pressure. Based on what is mentioned above, we make every effort to fabricate Gd₂O₃:Eu³⁺ nanoball and nanorod with good morphology, and study the samples'PL (Photoluminescence) properties especiallyup-conversion luminescence under ambient and high pressure. Chemical Precipitation Method is a simple method to synthesize nano materials. In this thesis Gd₂O₃:Eu³⁺ nanoball was prepared by this method through mixing the reactants. TEM(Transmission Electron Microscopy) images of the samples displayed that, with the increasing of the sintering temperature, the diameter of the sample increased from some dozens of nanometers to hundreds of nanometers, and it became more round in shape. SAD(Selected Area Diffraction) spots of this nanoball indicated that it was still cubic phase at sintering temperature of 1050°C. When the temperature was improved, XRD(X-Ray Diffraction) showed that, sample was pure cubic phase at 1300°C, a little monoclinic phase appeared at 1400°C and there was only monoclinic phase at 1500°C. Gd(OH)3 nanorod, which was about 20nm wide and 300nm long, was fabricated through Slow Precipitation Method. It was found that the nanorod could be obtained in a wide range of NaOH concentration, but nanorods with good aspect ratio could only form in suitable concentration. During the sintering process, it was observed that the rod shape of the sample would be destroyed when it was sintered in air ambience, but the rod shape could be kept if it was sintered in oxygen ambience.Gd₂O₃:Eu³⁺ can be used as red phosphors because the predominant emission of cubic Gd₂O₃:Eu³⁺ is located at 611nm, and intensities of other emission peaks are much weaker. In our research, we obtained Gd₂O₃:Eu³⁺ samples with the predominant peaks at about 550nm and 707nm respectively under excitation of 830nm laser. As for the former sample, the phenomenon was due to the existence of Er3+ ions. In other words, the predominant emission peaks were caused by Er3+ ions. The latter sample was much smaller than the former one, so we attribute this abnormal phenomenon to the surface defect. Besides, the luminescence properties of nanoballs were different from that of nanorods. From zero dimension to one dimension, some emission peaks of Gd₂O₃:Eu³⁺ nanocrystal became stronger and other peaks became weaker. The emission life was also different. Longer nanorods had longer emission life. It might be due to that there were fewer defects in Gd₂O₃:Eu³⁺ nanorods than in nanoballs.Gd2O3 crystals can exist in cubic, monoclinic and hexagonal phase. Eu3+ ions, doped in different host, present different luminescence properties. High pressure is an effective approach to change materials'structure, so we can enlarge our research area and deepen our understanding into Gd₂O₃:Eu³⁺. Structure transition of Gd₂O₃:Eu³⁺ was observed by high pressure energy dispersion X-ray diffraction (XRD) and high pressure photoluminescence (PL). The XRD showed that the bulk sample changed from cubic structure to hexagonal one when the pressure was higher than 10.97GPa. As for the nano sample one part also transformed into hexagonal phase at 14.07GPa, while the other part changed into monoclinic phase, which was different from the bulk sample. The hexagonal structure in both samples transformed into monoclinic structure when the pressure was released. This conclusion was confirmed by high pressure PL spectra.After analysis on the details of high pressure PL spectra, we found red-shift and widening effect appeared for emission peaks of Eu3+ ions under high pressure. PL spectra of Gd₂O₃:Eu³⁺ and Y2O3:Eu3+ from ambient pressure to 11.3GPa were investigated in detail. Obviously they were in cubic phase. The emission peaks for 5D0-7F2 transition of Eu3+ ions had the same widening effect in both hosts, because this transition was an electronic dipole transition and it was sensitive to crystal field which can be enhanced under high pressure. For the 5D0-7F1 transition of Eu3+ in Gd2O3 host, the widening effect of emission peaks under high pressure was not so strong, because this transition was magnetic dipole transition, which was insensitive to crystal field. But in Y2O3 host, the widening effect of this transition emission became large again. We thought the reason was that the radius of Eu3+ was clearly larger than the one of Y3+, resulting in the doped Eu3+ ion had a position departure from the equilibrium position of Y3+ ion.In a summary, Gd₂O₃:Eu³⁺ nanoballs and nanorods were fabricated and their photoluminescence properties were discussed in detail, which would be helpful to utilizing this materials and understanding the luminescence process of rare earth ions.