Tailoring the properties of graphene by metal deposition - Towards hydrogen storage applications and beyond

Adsorption or intercalation of foreign elements near the surface of a material is an exciting pathway to tune its properties, particularly for graphene. As a 2D material with outstanding and unique characteristics, arising from its atomically thin nature, graphene has drawn significant attention over the past two decades, with potential applications in fields such as energy storage, catalysis, sensing, and nanoelectronics. Metal functionalization has been envisioned as a powerful tool to advance these applications, and to modify and create new material properties. A key question is how different types of metals interact with graphene, as these interactions critically affect the final properties of the functionalized material.This thesis addresses the question by investigating the deposition of rubidium (Rb), an alkali metal, and platinum (Pt), a transition metal, on buffer layer (i.e., the first carbon layer arranged in a honeycomb lattice but partially bound to the substrate) and monolayer graphene (i.e., the first proper graphene layer only bound via van der Waals forces) epitaxially grown on silicon carbide. Employing surface sensitive techniques, with a systematic investigation of metal coverage as well as deposition and annealing temperature, we examine the Rb and Pt growth and intercalation processes in terms of morphology, chemistry, and electronic changes, often complemented by computational calculations.We reveal distinct behaviors arising from the different bonding interactions of Rb and Pt with the graphene sheet. Rb binds via van der Waals forces, readily intercalating the topmost graphene sheet even at temperatures as low as 100 K, with (2×2) or (√3×√3)R30° ordering between graphene and the buffer layer, but it does not penetrate the buffer layer. In contrast, Pt forms stronger bonds with graphene, readily forms clusters on the graphene, and, upon annealing at elevated temperatures, intercalates below the buffer layer, decoupling it from the substrate. In monolayer graphene regions, Pt additionally forms ordered 2D layers between the graphene and the buffer layer, leading to the formation of a (12×12) moiré pattern.Furthermore, we explore the hydrogen storage potential of these functionalized gra-phene systems. Notably, graphene decorated with Rb or Pt clusters effectively binds molecular hydrogen, exhibiting hydrogen adsorption and desorption at temperatures high enough to enable stable binding of molecular hydrogen at room-temperature. We also demonstrate that metal intercalation and cluster coalescence are detrimental to the hydrogen storage performance, highlighting the critical role of surface morphology in optimizing material properties.These findings provide new insights into metal-graphene interactions, highlighting novel behaviors in metal growth and intercalation, as well as the formation of new ordered 2D structures for both Rb- and Pt-intercalated graphene. This study further underscores the critical role of controlling surface morphology in enhancing hydrogen storage performances, a fundamental aspect also for various other applications.