Surface chemical analysis and ab initio investigations of CsI coated C fiber cathodes for high power microwave sources

CsI coated C fiber cathodes are promising electron emitters utilized in field emission applications. Ab initio calculations, in conjunction with experimental investigations on CsI-spray coated C fiber cathodes, were performed in order to better understand the origin of the low turn-on E-field obtained, as compared to uncoated C fibers. One possible mechanism for lowering the turn-on E-field is surface dipole layers reducing the work function. Ab initio modeling revealed that surface monolayers of Cs, CsI, $Cs2O$⁠, and CsO are all capable of producing low work function C fiber cathodes $(1eV<Φ<1.5eV)$⁠, yielding a reduction in the turn-on E-field by as much as ten times, when compared to the bare fiber. Although a CsI-containing aqueous solution is spray deposited on the C fiber surface, energy dispersive x-ray spectroscopy and scanning auger microscopy measurements show coabsorption of Cs and I into the fiber interior and Cs and O on the fiber surface, with no surface I. It is therefore proposed that a cesium oxide $(CsxOy)$ surface coating is responsible, at least in part, for the low turn E-field and superior emission characteristics of this type of fiber cathode. This $CsxOy$ layer could be formed during preconditioning heating. $CsxOy$ surface layers cannot only lower the fiber work function by the formation of surface dipoles (if they are thin enough) but may also enhance surface emission through their ability to emit secondary electrons due to a process of grazing electron impact. These multiple electron emission processes may explain the reported 10–100 fold reduction in the turn-on E-field of coated C fibers.