  The interaction of metals with carbon materials, specifically with graphene,
is of importance for various technological applications. In particular,
intercalation of alkali metals is believed to provide a means for tuning the
electronic properties of graphene for device applications. While the
macroscopic effects of such intercalation events can readily be studied,
following the related processes at an atomic scale in detail and under
well-defined experimental conditions constitutes a challenge. Here, we
investigate in situ the adsorption and nucleation of the alkali metals K, Cs,
and Li on free-standing graphene by means of low-energy electron point source
microscopy. We find that alkali metals readily intercalate in between bilayer
graphene. In fact, the equilibrium distribution of K and Cs favours a much
higher particle density in between the bilayer than on the single layer
graphene. We obtain a quantitative value for the difference of the free energy
of binding between these two domains. Our study is completed with a control
experiment introducing Pd as a representative of the non-alkali metals. Now, we
observe cluster formation in equal measure on both, single and bilayer
graphene, however no intercalation. Our investigations thus constitute the
first in situ study of metal atom sorption of different specificity on
free-standing graphene.
