ZnS-based phosphors have been intensively studied as a commercial phosphor used for display devices. Blue- and green-emitting ZnS-based phosphors, widely used for cathode ray tubes, field emission display, and electroluminescent display, have advantages such as high brightness and high efficiency compared with oxide phosphor. Although its vast applications, its luminescence property is not sufficient to be used in display devices and fluorescent lamps yet, which are being developed nowadays.
To enhance the luminescence property of ZnS phosphors, a new surface treatment method, a combination of stirring and ultrasonication (SUST), was developed. A commercially available blue-emitting ZnS:Ag,Cl phosphor was surface-treated with SUST in KOH solutions. As a result, it showed the improved cathodoluminescence (CL) efficiency by 58%. From surface analyses, it was confirmed that $OH^{-}$ ions were physi-sorbed on the surface of the phosphor and they formed an additional electrical field on the surface into the e-beam induced field present in CL operation mode. The improved CL property was originated from the surface electrostatic field by the electrical double layer with physi-sorbed $OH^{-}$ ions and a model supporting the CL enhancement was proposed.
Additionally, SUST was applied to the commercial phosphors, such as ZnS:Cu,Al, ZnS:Mn, $Y-2O-3:Eu, SrTiO-3:Pr,Al,Ga$ and so on. As a result, CL enhancement was also observed. In detail, the CL increase ratios were different with each other and the increase ratios of CL intensity depended on the amount of surface charge - the maximum amount of adsorbed $OH^{-}$ ions. It means that the strength of surface electrostatic field depends on the property of the phosphor surface.
To verify the proposed model, SUST into ZnS:Mn thin film phosphor was conducted under the assumption - the thin film is the most simplified surface model under CL circumstances. SUST-treated films also showed the CL enhancement by 28 % maximally like the results of powder phosphors. From the contact angle measurements, it was revealed that $OH^{-}$ ions formed the surface charges and its surface energy was very high enough to cause the electrostatic field into the phosphor.
The validity of the verified model of surface electrostatic field was examined into the display devices. To be adapted in the display device, the model for the static electric field was modified, that is, $V-2O-5$ coating on the phosphor surface. It also led to the improvement of CL and EL property of ZnS:Mn phosphor film. Especially, $V-2O-5$ coated ZnS:Mn IELD showed the remarkably reduced results for turn-on voltage in operation.
On the other hand, it has been wanted to improve emission color property of ZnS:Mn thin film phosphor for highly efficient full color displays. For the enhancement of color coordinates of ZnS:Mn, Mg-added ZnS:Mn thin film was investigated as a new red EL phosphor. With varying rapid thermal annealing (RTA) temperature, emission color of $Zn_{0.75}Mg_{0.25}S:Mn$ thin film was easily changed although the use of identical source composition. Its luminescent properties were superior to ZnS:Mn in PL, CL and EL measurements and more reddish emission was noticeable for high color purity. By using X-ray pole figure mapping, it was revealed that changes of the luminescent properties were originated from structural changes in $Zn_{0.75}Mg_{0.25}S:Mn$ thin film phosphor from cubic to hexagonal phases and the growing up of hexagonal phase mainly caused cracks and porous morphology on the surface of thin films. It clearly shows the conviction that ZnMgS:Mn is the candidate for red color emissive phosphor.
