Strain Driven Work Function Reduction in the XI/W (X = Li, Na, K, Rb, and Cs) Systems

A first-principles approach was used to study the interaction between an alkali metal iodide XI (X = Li, Na, K, Rb, and Cs) overlayer and the (100), (110), and (111) surfaces of W. It was shown that the XI adsobate forms tilted dipoles on the W surface and significantly reduces the work function. These findings contradict experimentally proposed models that suggest that the surface electrostatic energy of the dipoles causes the dipoles to be normal to the W surface. A novel model, based on the internal strain caused by the alkali metal's different atomic sizes, is introduced that can explain the dipole moment orientation, strength, and work function changes for all XI/W systems. Furthermore, by calculating the elastic formation energy of the XI adsorbates, the new model is used to explain the formation energy trends for all systems.