Cathodoluminescence Studies of Phosphor Particles

Cathodoluminescence (CL) studies carried out in a field emission scanning electron microscope are reported.  CL images of phosphor powders frequently manifested a bright ring around the periphery of the particles.  Additionally, particles resting on top of others were noted to be substantially brighter than under-lying ones.  Similar effects are often seen in secondary electron images and are usually ascribed to the electron beam spending more time in the surface region near the edges and therefore generating more secondary electrons close to the surface that are able to escape into vacuum.  This explanation, however, does not explain why we see such effects in CL microscopy.  To exclude the possibility of charging artefacts conductive ZnO:Zn phosphor powders (~1-2µm) were studied and these also exhibited the effect.  Studies have been made of model systems with both luminescent and non-luminescent materials adjacent to one another and on the basis of these results the origins of these phenomena are believed to be due to the combined effects of electrons backscattered out of the crystals onto adjacent particles, together with light absorption by the substrate.

Figure 1.  Cathodoluminescent micrograph of ZnO:Zn phosphor particles.