Observation of Ultrathin Sc-Containing Surface Layer on Life-Tested Scandate Cathodes

To explain enhanced levels of electron emission from scandate cathodes, models invoking surface layers ( $\sim$ 100 nm thick) of Ba-Sc-O, or alternatively a Ba-Sc-O monolayer, have been frequently employed to describe the lowered work function that is thought to lead to these improved emission capabilities. However, limited direct experimental proof has been provided, primarily involving Auger electron spectroscopy (AES) for depth profile analyses of elemental composition. In this article, we report direct observations of an Sc-containing surface layer on scandate cathodes, as indicated by independent, complementary measurements using scanning nanobeam AES and electron energy loss spectroscopy (EELS) techniques. The ultrathin Sc-containing layer on these cathodes is no more than 3-4 nm thick and is likely even thinner than this. The surface layer may consist of amorphous or nanoscale polycrystalline materials, as suggested by scanning nanobeam electron diffraction and analytical mapping.