Calculations of Ca adsorption on a MgO(100) surface: Determination of the binding sites and growth mode

Abstract

Density functional theory is used to calculate the adsorption of Ca adatoms on a MgO(100) surface. The Ca monomer binds preferentially to the oxygen site on the terrace with an adsorption energy of $0.84\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and diffuses by hopping to a nearest neighboring oxygen site with a barrier of $0.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The binding at F and ${\mathrm{F}}^{+}$ oxygen vacancy defects is weaker than on the terrace. Small Ca clusters on the surface are either unstable (dimers and trimers) or very mobile (tetramer). Therefore, a Ca adatom can diffuse around the surface until it reaches a strong-binding site such as a step $(2.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, kink $(3.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, divacancy $(5.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, or even a Mg vacancy $(10\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. When Ca binds to MgO(100), charge is transferred from Ca to the surface. The magnitude of the charge transfer is different between different binding sites, which can help explain the different adsorption energies. The oxygen bound to a step edge can provide a stronger binding for a Ca adatom $(7.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, but its existence on MgO(100) still remains to be verified. Based on the adsorption energetics and kinetics, further growth of the Ca particles is expected to occur at strong-binding sites and spread onto the terrace in a three-dimensional growth mode. A two-state kinetic growth model is used to compare our density functional theory calculations with measured heats of adsorption by Zhu et al. [J. Am. Chem. Soc. 130, 2314 (2008)]. This study shows that the defects on MgO(100) have different roles in the growth of alkaline earth metals, such as Ca, as compared to late transition metals, such as Pd.