Surface Modification Of Phosphors By Transparent Conducting Oxide Films

Luminescence materials are widely used in technological field such as, display,raster display, luminescence resources, nuclear physics, detection and log of radiationfield and so on. There are prior significance especially in display, raster display andluminescence resources, which are close related to daily life and society economy.Surface coatings have proved an effective solution against agglomeration ofpowders, unstable surface electrical and chemical performances in as-preparedphosphors. Several studies have been conducted on the preparation of compositeparticles consisting of cores covered with shells of different chemical composition.Since the properties of such dispersions (magnetic, optical, electric, adsorptive, etc.) canbe altered by appropriate coatings, these powders may be useful in many applications.As the development of surface coating techniques, the coating purpose transfers fromsimple protective coating to functional modification. At present, surface coatings focuseon two aspects, one is improvement of luminescence performances, the other is to meetthe ever-increasing demand for new optical devices.In this paper, we have carried out surface modification by transparent conductingoxide films. The main results are as follows:1. A square wave was used to electrolyze zinc nitrate aqueous solution forpreparation of a transparent zinc oxide (ZnO) film by cathodic deposition. If the cathodesubstrate is an indium tin oxides (ITO) coated glass piece, the ZnO film showed highoptical transmission around 80 % in the visible light region. It was found that a suitableproportion of Ce dopant in zinc oxide film could improve its transmission. Similartechnique has been used to prepare a ZnO film on a porous silicon, giving a greenphotoluminescence at room temperature after one hour annealing at 1000 ℃ in oxygenatmosphere. The structural and optical characterizations of the deposited films wereperformed with x-ray diffraction, scanning electron microscope, and measurements ofoptical transmission. The electrodeposite reaction mechanism was discussed.2. We have developed a surface coating technique for obtaining CaSiO₃:Pb,Mnphosphors with excellent photoluminescence property and low electrical resistivity.CaSiO3:Pb,Mn phosphor particles with red-luminescence were prepared by aqueous co-deposition reaction. Surface modification of CaSiO3:Pb,Mn phosphors was carried outby coating transparent conducting films of Al-doped zinc oxide which are formed byhomogeneous phase co-precipitation and heat treatment process. The coated phosphorswere characterized by X-ray Diffraction (XRD), Photoluminescence (PL), transmittanceelectron microscopy (TEM) and conductance measurements. It is generally acceptedthat the phosphors electrical properties are strongly dependent on the coating processand heat-treatment conditions. The results showed that the conductivity of ZnO:Al-coated phosphors, n(Zn)/n(Ca) = 10%, n(Al)/n(Zn) = 5%, has been obviously improvedin the case of remaining the corresponding photoluminescence intensity the same as theas-prepared phosphors. For lowering the resistivity of phosphors, experimentalconditions have been optimized, which include hydrolyzing temperature of 75 ℃,hydrolyzing duration of 1.5 h, heating treatment at 500 ℃ for 45 min.3. An investigation is reported of the coating of ZnS:Mn by Sb-doped SnO2 usingan coprecipitation process. ZnS:Mn phosphor particles were prepared by solid reactionwith ZnS and MnCl2·4H2O. Surface modification of the ZnS:Mn powders was carriedout by coating transparent conductive films of Sb-doped tin oxides which were formedby co-precipitation and heat treatment process. Tin tetrachloride and antimonytrichloride were used as the precursor materials for the co-precipitation. The influencesof coating molar ratio, Sb concentration in the coatings, annealing temperature and timeon the resistivity of coated ZnS:Mn phosphors were investigated. The optimumcoprecipitation processing parameters and annealing conditions were determined:n(Sn)/n(Zn) = 5 %, n(Sb)/n(Sn) = 2.5 %, heating treatment at 500 ℃ for 30 min, in N2atmosphere. The phosphors were characterized by X-ray Diffraction (XRD),transmission electron microscopy (TEM), Photoluminescence(PL) observation andconductance measurements. An improvement in phosphors conductivity was observedwhile the respective photoluminescence intensity is analogous to the as-preparedZnS:Mn phosphors.