Metal adsorption and nucleation on free-standing graphene by low-energy electron point source microscopy (graphene intercalation)

TL;DR

This study uses low-energy electron microscopy to observe how alkali metals intercalate and nucleate on free-standing graphene, revealing differences between alkali and non-alkali metal behaviors at the atomic level.

Contribution

First in situ investigation of metal atom sorption on free-standing graphene, distinguishing intercalation behaviors of alkali versus non-alkali metals.

Findings

Alkali metals intercalate between bilayer graphene, not single layers.

Higher particle density of K and Cs in bilayer regions.

Non-alkali metal Pd forms clusters without intercalation.

Abstract

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.