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Degradation of nanoparticulate-coated and uncoated sulfide-based cathodoluminescent phosphors

Abrams, B.L.; Thomes, William J.

Changes in the cathodoluminescent (CL) brightness and in the surface chemistry of nanoparticulate SiO{sub 2}-coated and uncoated ZnS:Ag, Cl powder phosphor have been investigated using a PHI 545 scanning Auger electron spectrometer (AES), an Oriel optical spectrometer and a JEOL 6400 scanning electron microscope (SEM). The data were collected in a stainless steel UHV chamber with residual gas pressures between 1 x 10{sup -8} and 1 x 10{sup -6} Torr as measured by a Dycor LC residual gas analyzer (RGA). The primary electron current density was 272 {micro}A/cm{sup 2}, while the primary beam energy was varied bwteen 2 and 5 keV. In the presence of a 2keV primary electron beam in 1 x 10{sup -6} Torr of water for both the SiO{sub 2}-coated and the uncoated cases, the amounts of C and S on the surface decreased, that of O increased and the CL intensity decreased with electron dose. This surface chemistry change lead to the development of a surface dead layer and is explained by the electron beam stimulated surface chemical reaction model (ESSCR). The penetration range of the impinging low energy primary electrons is on the order of 10-100 nm creating a reaction region very close to the surface. The ESSCR takes this into account postulating that primary and secondary electrons dissociate physisorbed molecules to form reactive atomic species. These atomic species remove surface S as volatile SO{sub x} or H{sub 2}S. In the case of an oxidizing ambient (i.e. high partial pressure of water), a non-luminescent ZnO layer is formed. this oxide layer has been measured to be on the order of 3-30 nm. In the case where the vacuum of 1 x 10{sup -8} Torr was dominated by hydrogen and had a low water content, there was a small increase in the S signal, no rise in the O Auger signal, but the CL intensity still decreased. This is explained by the ESSCR whereby H removes S as H{sub 2}S leaving elemental Zn, which evaporates due to a high vapor pressure. In the case of ZnS:Ag,Cl coated with SiO{sub 2}, morphological changes were observed on the surface after extended electron beam exposure. Erosion of ZnS occurs more dramatically at an accelerating voltage of 5kV even at the same current density. Uncoated ZnS:Ag,Cl phosphors exhibited similar surface chemical changes to that of SiO{sub 2}-coated ZnS:Ag,Cl but did not degrade to the same extent. Also, no change in the surface morphology was observed. These SEM images as well as reaction rate data suggest that these nanometer sized SiO{sub 2} particles acted as a catalyst for decomposition of the ZnS especially in a reducing ambient (i.e. high hydrogen partial pressure). In order to reduce CL degradation of these and other phosphors, protective coatings were pulse laser deposited onto the phosphor surface. The effectiveness of these coatings was dependent upon both the thickness and the uniformity. Thicknesses of these coatings ranged from 1-5 nm and were uniform as determined using profilometry and TEM.