Study On Rare-earth Element Doping, Compositing And Luminescence Property Of Single-crystal Lead Titanate Nanofibers

Perovskite (ABO3) ferroelectric nanostructures have attracted intense attention in high sensitivity sensor, high density storage and mechanical-electrical-optical coupling devices because of their unique structure characteristics and physicochemical property. Further exploring the relationship between the nanostructures, especially one-dimension(1D) perovskite nanostructure, and the properties are highly desired and particularly important for optimizing the functionality and expanding the application in nanodevices.In this dissertation, the structure of ferroelectric oxides, the ferroelectricity and the research history have been reviewed firstly. Furthermore, the upconversion mechanism and the dopant/host design principle have been summarized in detail. In addition, much effort has been made to present an overview of the advantages of perovskite ferroelectric oxides host material and the remarkable potential in ferroelectric modified upconversion (UC) luminescence in rare-earth element doped perovskite oxides. Despite of much effort, the substitution site (A or B site) of rare-earth ions is still uncontrollable, causing the difficulty in pursuing the effects of different chemical environment and substitution site of rare-earth ions on the photoluminescence (PL) and UC properties. Based on the hereinbefore issues, the Er doped pre-perovskite and perovskite single-crystal PTO nanofibers with the single substitution (A site or B site) have been firstly synthesized by combinating a hydrothermal with solid state phase transformation method. Furthermore, the effects of substitution sites, the tetragonality and polarization on the PL and UC properties of doping Er3+ ions have been intensively explored. Moreover, the growth mechanism, the interface structure and the luminescence properties of Er2Ti2O7-PbTiO3 (ETO-PTO) and Tb2Ti207-PbTi03 (TTO-PTO) composite nanofibers have been systematically investigated. The main innovations and results are summarized as follows:(1) The Er doped pre-perovskite and perovskite single-crystal PTO nanofibers with a serial doping concentration (0-4 mol%,) have been successfully synthesized by hydrothermal method and solid state phase transformation method, respectively. The growth direction of Er-doped pre-perovskite and perovskite single-crystal PTO nanofibers are identified to be [001] with the average size around 300nm in diameter. The Er3+ ions occupy Pb2+ sites in pre-perovskite PTO nanofibers, while they occupy Ti4+ sites in perovskite PTO. In addition, the substitution site of Er3+ ions remains unchanged in certain range of doping concentrations (0-4 mol%).(2) The PL and UC measurements indicated that the PL and UC emissions of Er3+ ions which occupied Pb2+ sites in Er-doped pre-perovskite single-crystal PTO nanofibers were absence and this might be caused by the STE of the pre-perovskite single-crystal PTO nanofibers. In contrast, the strong green emission (around 524 nm and 554 nm) and the red emission around 670 nm occurred in the PL and UC processes of Er-doped perovskite PTO nanofibers are corresponding to the characterictic emissions of Er3+ ions. Therefore, it is concluded that the PL and UC properties of Er3+ ions are susceptibly modulated by the chemical environments of the host matrix.(3) By means of in-situ XRD and UC measurements, the variation in the structure and UC property of the 4 mol% Er-doped single-crystal perovskite PTO nanofibers has been revealed. The results showed that the c/a value of the nanofibers decreased from 1.068(0) to 1.062(5) and the bond length difference value of Ti-O bond decreased from 1.15 A to 1.04 A when the temperature increased from 50 K to 300 K. The corresponding green UC emission intensity of Er3+ ions around 523 nm was obviously increased about 43 times while the red UC emission intensity was enhanced about 8 times. The in-situ UC decay curves revealed that the UC lifetimes of 4S3/2 (523 nm) and 4F9/2 (656 nm) level mainly unchanged in the range of 130±10 μs and 270±10μs, respectively. The assisted effect of the low-energy E(1TO) phonon on the UC process of Er3+ ions was proposed to explain the above phenomenon in Er-doped single-crystal perovskite PTO nanofibers.(4) The ETO-PTO and TTO-PTO composite nanofibers have been successfully synthesized by self-templated strategy combined with hydrothermal and solid state phase transformation method. From the point of view of morphology, the average sizes of these composite nanofiberss are around 300nm-600 nm in diameter and tens to hundreds micrometers in length. The microstructure investigation results revealed that the single-crystal pyrochlore ETO and TTO nanoparticles uniformly grow on the surface of PTO nanofibers via the formation of high-quality heterogenous interface. According to the variations in structure of TTO-PTO composite nanofiberss against the hydrothermal reaction time, the dissolved-recrystallization hydrothermal growth mechanism has been proposed.(5) The systematically investigation results on the structures, PL and UC measurements of the ETO-PTO composite nanofibers calcined at different temperatures showed that the formation of disordered heterogenous interface could be promoted by increasing the calcined temperature and caused the increased PL and UC emission intensities in ETO-PTO composite nanofibers.