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Understanding intercalation of epitaxial graphene

Graphene, an atomically thin sheet of carbon atoms, is a promising candidate for many future applications due to its extraordinary physical properties [ref]. Additionally, these properties can be modified by creation of various graphene hybrid structures where graphene is combined with other substances into a new system. Intercalation of epitaxial graphene is one of the routes to such novel systems. Even though intercalation of epitaxial graphene is studied by many authors, the mechanism governing it i.e. a microscopic view of an atom transport across the graphene sheet is still unclear. Here we present a study of a cesium + graphene/Ir(111) system at various stages. By utilizing scanning tunneling microscopy (STM) we were able to characterize emerging structures at the atomic scale. Complementary, low-energy electron microscopy (LEEM) revealed cesium intercalation and desorption dynamics in real time on micron scales. Angle-resolved photoemission (ARPES) was used to characterize chemical doping of graphene. Density functional (DFT) calculations were performed in order to gain more insight into the driving forces of intercalation. In the experiments we clearly observe a phase separation of cesium deposited on epitaxial graphene. For low coverage, a dilute phase of cesium adatoms is formed on top of graphene. As more cesium is added, an additional phase forms which corresponds to the cesium atoms being intercalated under the graphene in ~15 times denser structure. Therefore intercalation occurs when a critical value of adatom concentration is reached. Only then it pays off energetically to the cesium atoms to go under the graphene at the cost of delamination energy which has its origin in the van der Waals binding of graphene to the iridium. The two cesium phases can coexist and are separated by a sharp interface. Besides topographical differences, the two phases also give rise to different electronic structures of graphene. We found that transport of cesium to the intercalated phase takes place at graphene wrinkles which contain nano-cracks. Our results contribute to the understanding of intercalation under graphene in detail and can be used to form more general picture of intercalation of epitaxial graphene and other modern layered materials.

graphene, iridium, intercalation, cesium, wrinkles, ARPES, LEEM, STM

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