Research On Oxybromide Upconversion Phosphors And Silicate Wled Phosphors

Because of the special electronic structure of rare earth ions (REI), the REI-doped luminescent materials have been proved to be outstanding luminescent materials than other type materials. Owning to their high brightness, long lifetime and non-radiation substance, more attention has been paid to the investigations of REI activated luminescent material. For examples, upconversion materials show extensively applications in three-dimensional displays, infrared radiation detectors, laser anti-counterfeiting brands and so on. The investigation on the fabrication and luminescent properties of white LED (WLED) phosphor, such as REI activated alkaline earth silicate phosphor, has aroused scientists' interest due to the development of elaboration technology of WLED and the extension of applied fields. The thesis contains two parts:(A) upconversion in REI doped Oxybromide-based phosphors, and (B) investigation on Sr2SiO4:Eu2+ WLED phosphors.In Chapter 1, the research progress and the upconversion mechanisms of upconversion phosphors were reviewed.In Chapter 2, pure orthorhombic Gd3O4Br:Er3+ upconversion phosphors were synthesized by a solid-state reaction method and the structural properties of Gd3O4Br:Er3+ were investigated by X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The results show that Gd3O4Br has low phonon cutoff energy, indicating that Gd3O4Br:Er3+ may have high luminescent efficiency. Intense green (514-582nm) and strong red (645-692nm) upconverted luminescence of Gd3O4Br:Er3+ were observed under 980 nm laser excitation. The bright green emission is visible to the naked eyes even for 1 mW of the pump power (980 nm) for Gd3O4Br:Er3+ (0.1%) samples, indicating that Gd3O4Br:Er3+ may be used as upconversion phosphors. In Chapter 3, pure and Er3+ doped GdOBr phosphors were synthesized by a simple solid-state reaction method. X-ray diffraction, Raman spectroscopy and photoluminscent spectroscopy were used to characterize the structural and photoluminscent properties of GdOBr:Er3+. Raman spectra show that tetragonal GdOBr has low phonon energy, indicating that GdOBr:Er3+ may have high upconversion efficiency. Under 980 nm and (or) 785 nm laser excitation, GdOBr:Er3+ samples present strong green (2H11/2,4S3/2→4I15/2) and red (4F9/2→4I15/2) upconverted emissions. The upconversion mechanisms were studied in detail through laser power dependence and excited state absorption process is discussed as the possible upconversion mechanisms. More interesting, the bright green upconverted emission is visible to the naked eyes for GdOBr:Er3+(1%) samples even excited by 1 mW 980 nm laser. Such phenomenon indicates that GdOBr:Er3+ may be used as upconversion phosphors.In Chapter 4, the investigation on Sr2SiO4:Eu2+ based white LED phosphors is introduced briefly. Sr2SiO4:Eu2+ phosphors were prepared by a conventional solid-state reaction method using NH4F as a flux. The effect of NH4F flux on the structural and luminescent properties of Sr2Si04:Eu2+ was investigated in detail. X-ray diffraction (XRD) results showed that pure monoclinicβ-Sr2SiO4 phase can be elaborated at low annealing temperature (1100℃) by adding appropriate amount of NH4F. The Sr2Si04:Eu2+ phosphors show two emission spectra peaked at 540 and 474 nm originated from the 5d-4f transition of Eu2+ ion doped in two different Sr2+ sites in the host lattice. The intensities of Eu2+ emissions increased with increasing NH4F amount up to 6 wt%. By compared NH4F flux with other fluxes used in preparation of Sr2Si04:Eu2+ phosphors, it is found that NH4F flux is more superior to NH4Cl and Li2CO3 flux. Our investigation indicates that Sr2SiO4:Eu2+ is a good light-conversion phosphor candidate for near-UV chip and NH4F is a good flux for Sr2SiO4:Eu2+.In Chapter 5, the conclusions of the thesis were summarized.