The dipole model at the atomic scale: Explaining variations in work function due to configurational and compositional changes in Ba/Sc/O adsorbates on W (001), (110), and (112)

The adsorption of alkaline-earth metals (and/or their oxides) on transition metal surfaces reduces the work function significantly, an effect that has been widely applied in thermionic dispenser cathodes to improve electron emission. Even slight changes in adsorbate composition and configuration on the emitting surface can lead to remarkable differences in work function. In this work, atomistic calculations of the structure, work function, and stability of a range of Ba/Sc/O-adsorbed W surfaces relevant to scandate cathodes are used to develop an atomic-scale understanding of the mechanisms leading to changes in work function. While a range of theoretical explanations have been put forward to explain how alkaline-earth ions or oxide layers modify the work function, the present results strongly support the idea that surface dipole effects are the critical mechanism controlling work function changes. These results provide signifiant insight into efforts to design new and novel low-work function surfaces and materials.