Nano-Encapsulated ZnS:Ag Phosphors for Field Emission Flat Panel Display Applications
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Nano-Encapsulated ZnS:Ag Phosphors For Field Emission Flat Panel Display Applications Michael Ollinger, Valentin Craciun, and Rajiv K. Singh Department of Materials Science & Engineering, University of Florida, Gainesville, FL 32611, U.S.A. ABSTRACT ZnS:Ag phosphor particles were encapsulated with nano-meter thick films of indium tin oxide (ITO) in order to slow the degradation process of the phosphor as well as to reduce the amount of sulfur species out-gassing from the phosphor. Cathodoluminescent (CL) degradation measurements were performed at two different vacuum conditions (6.1×10-7 Torr and 1×10-5 Torr). The CL degradation curves showed that the ITO coating improved the brightness lifetime. X-ray photoelectron spectroscopy was used to determine the chemical changes on the coated and uncoated degraded particle surfaces. CL images were used to show the loss of brightness from the surface of the phosphor particles. Then energy dispersive x-ray spectroscopy was used to measure the ratio of the area under the zinc and sulfur peaks on an atomic weight %, which showed a loss in sulfur during the degradation experiments. INTRODUCTION For more than 30 years, the display industry has attempted to create a thin flat low power version of the highly successful cathode ray tube (CRT). The present market leader in flat panel displays (FPDs) is the active matrix liquid crystal display (AMLCD), which has made a significant improvement in the last decade, still has not met all the needs for reduced power consumption, brightness efficiency, video response, viewing angle, operating temperature, full color gamut and scalability. In contrast, field emission displays (FEDs) which are based on field emission, as in CRTs, can meet all these requirements, but they have been plagued by some technological challenges. One of the challenges is the loss of brightness of sulfide based phosphors (ZnS:Ag) upon continuous irradiation with the e-beam. Additionally, the sulfide ions, which out-gas from the phosphor during degradation, poison the cathode emitter tips and thus significantly reduce their lifetime. Field emission displays (FEDs) are one of the many new types of flat panel displays currently under development. They rely on the excitation of a phosphor material by an electron beam to produce photons. A reliability issue encountered in FEDs, which is hindering their commercialization, is the rapid decay of brightness in the sulfide-based phosphors. The loss of brightness has been attributed to electron beam induced surface reactions[1] and charging effects[2] on the surface. Recently, attempts have been made to prevent the degradation of these sulfide-based phosphors through the application of surface coatings[3-5]. In this paper, the effects of coulombic aging on a commercially available ZnS:Ag powder (Osram Sylvania) and the same ZnS:Ag powder coated with a nano-scale layer of indium tin oxide (ITO) at two different vacuum levels will be shown. The ZnS:Ag powder was coated using a unique physical vapor deposition technique based on pu
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