Fabrication And The Upconversion Luminescence Of BaTiO₃:Yb³⁺, Er³⁺ With Inverse Opal Phtonic Crystals

The nanometer scale with ordered structure could modify the basic properties of material, and this powerful exploration could also be used to make material obtain new properties. In particular, the nano size modify luminescent materials and their spontaneous emission properties. Photonic crystal is a kind of materials with periodic dielectric constant, and it could control the photons flow by electromagnetic wave of the forbidden area. It has huge potential in laser, sensing, display, detection, photocatalysis, solar energy and so on.Traditional fabrication methods of three-dimensional photonic crystal structures were exorbitant cost and had many defects. The prepared processes using traditional methods also were complicated. The self-assembly technique successfully solved these problems, and photonic band gap can be controlled. We used opal photonic crystals (OPCs) as templates, and sol solution were filled these templates. Then the opal templates can be removed to get the inversion structure of opal, which is called inverse opal photonic crystal (IOPC). The theory shows that the IOPCs have better characteristics than the OPCs, because they can better control photonic band gap (PBG) and electromagnetic waves. In this work, we had prepared the monodisperse polystyrene (PS) microspheres by emulsifier-free emulsion polymerization. The synthesized spheres were self-assembled on substrates to fabricate the opal structure by the float controlled vertical deposition technique. We dabbed a drop of BaTiO3: Yb3+, Er3+ precursor solution on the opal photonic crystal, and the voids among these colloidal spheres were filled by the sol solution through spin coating. The samples were heated to remove polystyrene opals. Finally, the IOPC samples were forming. Under the excitation of 980nm laser diode, the samples presented the green and red emissions around 525,550 and 660 nm. It was significant to observe that the 660nm red emission increased obviously while the 525nm and 550nm green emission was inhibited by the PBG effect in the IOPC, then the red light of 660nm wavelength was enhanced. The investigation of excited state process demonstrates that both cross relaxation (CR) and energy back transfer (EBT) from Er3+ to Yb3+ may populate 4I13/2 even more, in case that the spontaneous emissions from relative levels are inhibited by PBG. Subsequently, the population of 4F9/2 also tends to increase, leading to the inceasing of red emission. The decreasing of the branching ratio of 525nm versus 550nm for inverse opal photonic crystals indicates the IOPCs have lower temperarue, owing to their thin wall structure and connected air cavity. Comparing the fluorescence branching ratio of green and red emission, it is observed to have a smaller extreme value when the PBG postion completely overlaps the main emission band. It explained the PBG could inhibit the spontaneous emission.Finality, the research results are summarized and the future research work is prospected.