Thin SiO₂ coating on ZnS phosphors for improved low-voltage cathodoluminescence properties

Abstract

A significant improvement (40-60%) was reported in the low voltage (100-1000V) cathodoluminescence efficiency of ZnS phosphors coated with SiO 2 by the sol-gel technique. The properties of the coatings were found to be critically dependent upon the precursor concentration, pH value and the temperature of the solution with optimum performance being obtained for a SiO 2 concentration of 1.0 wt%, pH values between 7-9, and a solution temperature of 83°C. The efficiency curves exhibited a characteristic voltage dependence which was analyzed by a one-dimensional numerical model. Enhanced low voltage efficiency was attributed to a reduction of surface recombination and the actual shape of the efficiency curve was determined by the interplay between the reduction of surface recombination and energy losses in the SiO 2 coating.The physical mechanism of phosphor excitation in the field emission display (FED) and conventional cathode ray tube (CRT) displays is the same. However, FEDs operate at much lower voltages, which makes it necessary to use higher current densities in order to maintain the same output luminance.1 At low acceleration voltages, the electron penetration depth is small compared to the phosphor particle size, and therefore, phosphors exhibit low efficiency due to the loss processes associated with the surface. However, there are many advantages that can be achieved by very low voltage (<500 V) operation; more simple FED structures can be utilized because the need for focusing electrodes is eliminated, arcing is reduced, and switched anode operation becomes feasible. Low-voltage operation can also reduce electron beam stimulated chemical reactions which have been associated with phosphor and cathode degradation in FEDs. 2Because of the small penetration depth at low voltages, the loss processes are believed to be associated with surface recombination, and thus, the surface coating and encapsulation of phosphors is an important technique to improve low-voltage cathodoluminescence (CL) and chemical stability and to reduce outgassing. Coatings can also reduce phosphor surface degradation (such as oxidation) associated with display fabrication processes thus reducing the surface dead-layer thickness and passivating surface recombination centers. Recently, Bechtel et al. reported a significant improvement in maintenance of ZnS:Ag by applying a thin calcium polyphosphate coating.3 However, this work focused on the midvoltage region (approximately 4 kV) in which the surface effect is less critical. Also, no improvement in efficiency was reported. It is the aim of this paper to present a coating technique that can improve the CL efficiency at voltages less than 2 kV by reducing surface-related losses and at the same time protect the phosphors by completely encapsulating the phosphor particles. SiO 2 was used as the coating material because of its favorable band alignment when forming a heterojunction with ZnS.4 Furthermore, SiO 2 , which consists of light elements and has low density, exhibits minimal losses in e...

Improved moisture resistance of SrS:Eu2+ phosphors with nanoscale SiO2 coating

“…The emission spectra exhibited a single peak under the excitation of l exc ¼460 nm for both uncoated and coated phosphors. There has been a report about 60% enhancement in cathodoluminescence efficiency by thin SiO 2 coating on ZnS phosphors [18], but in this study, the PL emission intensity remained almost the same after coating indicating that the non-luminescent SiO 2 coating layer has little effect on the PL performance. The degradation behavior of the uncoated and coated SrS:Eu 2 + phosphors in moist air is shown in Fig.…”

Structural and optical properties of ZnS/niobate composites synthesized by exfoliation/self-assembly processing

“…Even with the existence of traces of oxygen, the surface of ZnS nanoparticles would be oxidized into ZnO dead layers, which reduced luminescence intensity significantly [4], or even may degrade the performance of the devices over years of operation [5]. One strategy is to employ a protective layer on ZnS surface to enhance the chemical stability and avoid the agglomeration and oxidation of ZnS at high temperatures [6,7]. A typical example is coating ZnS with silica [8][9][10][11], and reported to be more stable against electron beam or UV light irradiation compared to uncoated ZnS [12,13], or embedding ZnS in polymers [14,15].…”

Synthesis and characteristics of silica-modified ZnS nanoparticles by sol–gel-hydrothermal method

“…Even with the existence of traces of oxygen, the surface of ZnS nanoparticles would be oxidized into ZnO dead layers which reduced luminescence intensity significantly [7,8]. Silica as protective layers on ZnS surface could isolate the surfaces, enhance the chemical stability and avoid the oxidation of ZnS at high temperatures [9]. The silica coated ZnS had been reported to be more stable against electron beam, compared to uncoated ZnS [10].…”