Combinatorial Approach For Red VUV Phosphors And The Spectra Study Of GdP₃O₉:Dy³⁺

Driven by the advances in the plasma display panel (PDP) technique and Hg-freefluorescent lamps, the demand on high efficient vacuum ultraviolet (VUV) phosphorshas increased dramatically in the past decade. Most of the present phosphors for PDPapplication are borrowed from the commercial phosphors of UV fluorescent lamps orCRT. Since the performance of these phosphors, such as energy efficiency,luminescence efficiency and color purity, are not optimized for PDP application, it isnecessary to search for new high efficient VUV phosphors. From1995, thecombinatorial method was introduced into inorganic materials science. This methodhandles a large number of samples simultaneously, and consists of two main steps:parallel synthesis and high-throughput characterization. The application ofcombinatorial method dramatically increases the efficiencies of the material studiesand the possibility of finding new materials.In this thesis, we focus on the new VUV phosphors. It consists of three chapters.Chapter1is the review of the VUV phosphors and the combinatorial materialsscience. Chapter2is the combinatorial approach for new red VUV phosphors.Chapter3is the study of GdP₃O₉:Dy³⁺0.01's luminescence properties and theanomalous emission phenomenon of Dy³⁺in GdP₃O₉.The chapter1consists of two parts. In the first part, the concept, principle andhistory of combinatorial materials science are introduced and the development of thecombinatorial library design, synthesis, characterization and data mining are reviewed.In the second part, the quantum cutting process in VUV phosphors was introduced.The present commercial VUV phosphors for PDP application were also listed and theshortcomings of these phosphors were analyzed. The development of theory for VUVluminescent materials was reviewed and the P. Dorenbos's prediction for5d levelposition of trivalent lanthanides in inorganic compounds was also introduced in thispart.In chapter2, the combinatorial method was adopted for new red VUV phosphorsscreening. At first, we had set up a scanning multi-inkjet delivery system for thesolution-based combinatorial synthesis of material libraries; secondly, a high-throughput parallel screening apparatus for VUV photo-luminescence based on a home designed cathode gas (Xe/He or Xe/Ne) discharge lamp was designed andconstructed; finally, by fabricating a series of RE doped borophosphate phosphorslibraries and characterizing their VUV photo-luminescence with these machines, wedemonstrated the applicability of the combinatorial strategy in searching for highefficient VUV phosphors and found out a series of clue phosphors such asGdSrB_0.8P_1.2O_5.5⁷.7:Eu³⁺0.1.In Chapter3, the UV&VUＶ spectra of GdP₃O₉:Dy³⁺0.01and the crystal structurewere measured. From the emission and excitation spectra of GdP₃O₉:Dy³⁺0.01in140-600nm spectral region, an interesting phenomenon was found that the relativeintensity of the yellow and blue emissions of Dy³⁺changes from¹.4to².4whenthe excitation wavelength moves from ultraviolet into vacuum ultraviolet.Accompanying the change, a weak broader emission emerges in the spectrum from290nm to490nm.In order to interpret the observed anomalous emission, a hypothesis excitonmediated model was proposed. When the excitation is lower than194nm, the electronwas first excited to4f levels of Gd³⁺, then transfers to the4K15/2state of Dy³⁺by crossrelaxation and further relaxes to the⁴F_9/2state of Dy³⁺nonradiatively. Finally, ittransits to either the6H15/2or the⁶H_13/2states and emits a blue or a yellow photonrespectively. This refers as the normal emission and its Y/B ratio is determined by theligand, especially the symmetry of the local coordination. When the electron isexcited to the4f85d state of Dy³⁺or into the conduction band, the situation is changed.Due to the large radius and ellipticity, the electron cloud of the5d has the probabilityto overlap the electron cloud of the valence O2p and Dy4f0. So, the electrostaticinteraction has the possibility to expel the4f0electron from the valence band to the⁴F_9/2state of Dy³⁺where it finally transits to either the6H15/2or the⁶H_13/2states. At thesame time, the hole left in the valence band forms an exciton with the5d electron atthe HS state. What is different from the normal emission is that there is an exciton inthe nearby of the emitting transition. Because of the hypersensitivity of the transitions(⁴F_9/2→⁶H_13/2), the transition probability and consequently the Y/B ratio can be alteredby the dynamic exciton.In this model, the broader emission is assigned to the dynamic exciton emission.Due to the indirect transition property of the5d HS to hole, the broader band emissionin GdP₃O₉:Dy³⁺0.01is currently weak. However, if a direct exciton emission materialcould be found, then the inter-band emission could forms a quantum cutting system. Two photons, one from the⁴F_9/2emission and the other from the exciton emission,could be realized by the absorption of one VUV photon. From this work, we can findthat the luminescence properties of VUV phosphors are decided by not only the ligandof the dopted rare earth ions but also the location of their energy levels in the band.