The Studies On Structural And Optical Properties Of TiO₂ And Rare-earth Doped TiO₂ Based Materials

Semiconductors light emitting devices are widely used in many fields. The widebandgap semiconductors light emitting materials have been paid more and moreattention since GaN light emitting devices were successfully developed. TiO₂, as widebandgap semiconductors has potential applications in the fields of light-emittingmaterials, solar cells, photocatalysis, and spin electronics. In this work, TiO₂ was usedas matrix, TiO₂ and Eu³⁺ ions doped TiO₂ nanofibers were prepared byelectrospinning method. Eu³⁺ ions doped TiO₂-SiO₂ composite powders and thin filmsas well as Er³⁺ ions doped TiO₂-SiO₂ composite thin films were fabricated by sol-gelmethod. Modern analysis techniques including FE-SEM, TEM, XRD, Raman, FT-IRand UV-Vis were used to investigate the characteristics of these materials. The factorsinfluencing on the luminescence properties and the luminescence mechanisms of thematerials were studied by photoluminescence (PL) and photoluminescence excitation(PLE) spectra. The main results of the research work are as follows:1. TiO₂ nanofibers were fabricated by electrospinning and the effect of annealingtemperature on the morphology, structure and PL properties of the nanofibers wasinvestigated. With increasing of the annealing temperature, the average diameters ofthe nanofibers changed from 70nm to 40nm, the surface of nanofibers became muchrougher and the crystalline phase transformed from anatase to rutile. The nanofibersannealed at 400℃show a visible emission at 550nm, which was attributed to theradiative recombination of self-trapped excitons. The intensity of the visible peakdecreased with the increase of annealing temperature. The nanofibers annealed at 600℃and 800℃show an infrared emission at 820nm which was attributed to thedefect states associated with Ti³⁺ ions. The intensity of the infrared peak increased asthe annealing temperature increases. These results indicate that for the differentphases, the different defect centers act as radiative and non-radiative centers.2. Eu³⁺ ions doped TiO₂ nanofibers were obtained by electrospinning method andtheir PL properties were investigated. The PL spectra of the materials showed strong red emission. PL intensity in visible range due to Eu³⁺ ions increased at first but thendecreased as the concentration of Eu³⁺ ions increases. And it reached maximum whenthe concentration of Eu³⁺ ions was 3mol%. This is called concentration quench effect.The annealing temperature effect on the PL properties of the nanofibers was alsostudied. The intensity of visible emission due to Eu³⁺ ions reached maximum whenthe annealing temperature was 600℃. The PL intensity due to defect statesassociated with Ti³⁺ ions of host TiO₂ appeared at 820nm and was stronger thanundoped nanofibers. There was an existence of energy transfer between the TiO₂ hostand Eu³⁺ ions. When the annealing temperature was lower, the energy transferoccured from TiO₂ host to Eu³⁺ ions, and led to the increase of the visible emission.The energy back transfer from Eu³⁺ ions to defect level associated with Ti³⁺ ionsdominanted the emissions at higher annealing temperature and led to the increase ofinfrared 820nm emission.3. Eu³⁺ ions doped TiO₂-SiO₂ composite powders were prepared by sol-gelmethod and the effects of annealing temperature and the TiO₂ content on the PLproperties of the powders were investigated. The PL spectra and PLE spectra wererecorded at room temperature. With the increasing of annealing temperature, theintensity of the PL increased initially (up to 900℃). When the annealing temperatureis above 900℃, decreased PL intensity can be observed. With the annealingtemperature increasing, the spatial separation between Eu³⁺ ions becomes smaller andcross-relaxation rate is higher, which increased the probability of the nonradiative ofthe optically active ions to the ground electronic state. Therefore, the fluorescenceintensity decreased at higher annealing temperature. For the samples with differentTiO₂ concentrations, the intensity of the Eu³⁺ emission increased with TiO₂concentration (up to 80%). After the TiO2 concentration was above 80%, the intensityof the Eu³⁺ ions emission decreased. These results indicated that the solubility of Eu³⁺in TiO₂-SiO₂ was limited and the TiO₂ concentration of 80% was the saturation. Whenthe TiO₂ concentration was above 80%, the Eu³⁺ ions formed aggregates and clusterswhich was responsible for the decrease of the PL intensity.4. Eu³⁺ ions doped TiO₂-SiO₂ composite thin films with different annealing temperature were fabricated by sol-gel method. The intensity of visible emission dueto Eu³⁺ ions reached maximum when annealing temperature was at 700℃. Theintensity of the infrared emission at 820nm due to the defect states associated withTi³⁺ ions increased with the increase of annealing temperature. On the one hand, thenumber of Ti³⁺ ions increased with the increased annealing temperature, which causedthe increasing of defect levels; on the other hand, the energy back transfer from Eu³⁺ions to defect level associated with Ti³⁺ ions. These two facts lead to the increase ofthe emission intensity at 820nm with the increase of annealing temperature.5. Er³⁺ doped TiO₂-SiO₂ composite thin films were fabricated by sol-gel methodand the effects of annealing temperature and the ratio of Ti and Si atoms on the PLproperties of the thin films were investigated. Three luminescence bands could bedetected at 523, 545 and 660nm in visible region which corresponded to the intra-4ftransitions of ²H_11/2 -⁴I_15/2, ⁴S_3/2-⁴I_15/2, and ⁴F_9/2-⁴I_15/2, respectively. The intensity of theemission bands at visible region due to Er³⁺ ions increased with the increaseof of Ti/Si, and the peaks split into several subpeaks superimposed on the bands at 523, 545and 660nm. This splitting could be attributed to the transition between the Starksublevels of the upper states (²H_11/2, ⁴S_3/2, ⁴F_9/2) and the ground state ⁴I_15/2, due to thecrystalline environment of the rare earth. With the increase of annealing temperature,the intensity of emission bands at visible region increased and the splitting of theemission bands became more obviously. Neither temperature quenching effect norconcentration quench effect was found here, which could be due to that Er³⁺ ions haddifferent solubility in different host materials.