Density functional theory calculations on alkali and the alkaline Ca atoms adsorbed on graphene monolayers

The adsorption of the alkali Li, K, and Na and the alkaline Ca on graphene is studied using periodic density functional theory (DFT) under various adatom coverages. The charge transfers between the adatom and the graphene sheet and the almost unchanged densities-of-states spectra in the energy region near and below the Fermi level support an ionic bond pattern between the adatom and the graphene atoms. However, the presence of small orbital overlap between the metal and the nearest graphene atom is indicative of small covalent bonding. Van der Waals interactions are examined through a semiempirical correction in the DFT functional and by comparing adatom-graphene calculations between 3% and 1.4% adatom coverages. Optimized adatom-graphene geometries identify the preferred adatom sites, whereas the adatom-graphene strength is correlated with the adsorption energy and the adatom distance from the graphene plane. Calculated electronic properties and structural parameters are obtained using hybrid functionals and a generalized gradient approximation functional paired with basis sets of various sizes. We found that due to long range electrostatic forces between the alkali/alkaline adatoms and the graphene monolayer, the adatom-graphene structural and electronic properties could be well-described by specific DFT functionals paired with high-quality adatom basis sets. For Li, K, and Na adsorbed on graphene, increased adatom surface coverage weakens the adatom-graphene interaction. However, this statement does not apply for Ca adsorbed on graphene. In this case, the Ca adsorption strength, which is stronger at higher coverages, is opposite to increases in the Ca-4s orbital population. (C) 2017 Elsevier B.V. All rights reserved.