Adsorption Mechanisms of Alkali Metal, Alkaline Earth Metal, and Halogen Atoms on Van der Waals Materials

We studied the alkali metal (IA), alkaline earth metal (IIA), and halogen (VIIA) adatoms on the surface of monolayer h-BN, graphene, and 1T-MoS2 using density functional method plus van der Waals (vdW) corrections. h-BN stands for the vdW materials with a finite band gap, while the last two materials stand for the ones with a zero band gap. The magnitude of the charge transfer between the adatoms and the surface at equilibrium is found to be determined by two competitive electrostatic interactions, namely, the nucleus-electron interaction and the adatom-substrate interaction. It makes the binding strength with the surface vary with the atomic radius nonmonotonically for the metal adatoms in the same group but monotonically for the halogen adatoms. The binding energy of the adatoms at equilibrium is contributed by the vdW interaction E-vdw and the energy cost of the charge transfer E-ct. The magnitude of E-vdw increases with the radius of the adatoms, whereas E-ct can be estimated by scaling energy that just depends on the radius of the adatom and the energy level relative to that of the substrate. These findings would help us to understand the mechanism of adsorption and study the adsorption or intercalation further.