Reduction of work functions for high current density cathode by introducing fractional monolayer adsorbate on tungsten

Abstract

Thermionic dispenser cathodes are widely used in microwave devices because of their high current densities. Several investigations have shown that the structural arrangement and coverage of atoms or molecules on tungsten surface can dramatically reduce the work function. Theoretically, work function was reduced and increased by electropositive and electronegative adsorbates, respectively. In this work, we study the surface dipole, charge transfer, and work function of a W (001) surface while a fraction of monolayer of alkali metal or alkaline earth metal atoms adsorbed onto it. The change of work function due to the coverage and geometry properties of fractional monolayer adsorbate atoms is investigated using first principles calculations based on the density functional theory. The ab initio calculation shows that the work functions that absorbed alkali-metal atoms with a 50% coverage and oblique arrangement is the lowest one for both local density and generalized gradient approximations.