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Changes in the cathodoluminescent (CL) brightness and in the surface chemistry of
nanoparticulate SiO2-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 1x10-8 and 1x10-6 as measured by a Dycor LC residual gas analyzer (RGA).
The primary electron current density was 272 µA/cm2, while the primary beam
energy was varied between 2 and 5 keV. In the presence of a 2keV primary
electron beam in 1x10-6 Torr of water for both the SiO2-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 SOx or H2S. 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
1x10-8 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
H2S leaving elemental Zn, which evaporates due to a high vapor pressure.
In the case of ZnS:Ag,Cl coated with SiO2, 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 SiO2-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 SiO2 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.
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