New Insights in the Modification of the Work Function of Cathode Materials due to Thin Surface Coatings using Ab-initio Modelling

The changes in the work function upon adsorption of coating layers on materials utilized in both field and thermionic emission applications have been investigated computationally within the framework of Density Functional Theory (DFT) [1] as implemented by the Vienna Ab-initio Simulation Package (VASP) [2]. This approach provides valuable information for understanding the origin of the improvement in the emitted current density by monitoring changes in the electronic structure of a material system upon adsorption of surface species. The presence of surface coatings induces changes in the charge density in the surface and sub-surface regions of the substrate, significantly influencing the value of the work function. The origin of the enhanced emission properties of Cesium-Iodide (Csl) coated Carbon-based field emission cathodes [3] as well as Barium-Oxide (Ba-O) Tungsten thermionic cathode systems has been investigated. The former is a promising cathode material system capable of operating under low global electric fields, demonstrating operation lifetimes in excess of one million pulses, and characterized by good shot-to-shot-reproducibility, low levels of outgassing, and no evident plasma formation upon appropriate conditioning.