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.