Surface chemistry on Graphene : Chemisorption, catalysis and molecular manipulation

Abstract

This thesis presents a dissertation on the chemical functionalization of epitaxial graphene grown on Ru(0001) (gr/Ru(0001)) and its role as catalyst agent in surface reactions. The experiments were performed in an Ultra High Vacuum (UHV) system equipped with a low temperature Scanning Tunnelling Microscope (STM). A myriad of applications have been proposed for graphene, a two-dimensional carbon allotrope with unique structural and electronic properties. Developing these applications usually requires the chemical functionalization of graphene, which is a challenging task due to its low reactivity. In this work two important elements are combined: i) Epitaxial graphene grown on a metallic substrate, Ru(0001), which spatially modulates the reactivity of carbon atoms by means of a moiré pattern. ii) Molecular radicals, being excellent candidates to form a covalent bond with the graphene carbon atoms. Herein cyanomethyl radicals (CH2CN ) are created out of a partial pressure of acetonitrile (CH3CN) molecules inserted in the UHV system. Upon the exposure of the gr/Ru(0001) sample to molecular radicals, the CH2CN groups covalently attach to the graphene surface. Setting a suitable sample temperature, a covalent patterning of graphene is achieved with an extremely high yield and long-range order. Taking advantage of a graphene layer with an unique chemical environment, it is explored the on surface reaction between CH2CN groups and another molecule: The 7,7,8,8-Tetracyano-p-quinodimethane (TCNQ) molecule. It is demonstrated that the gr/Ru(0001) surface is essential for this reaction to occur due to its role as catalyst agent. In particular, it was found that graphene makes a threefold contribution: i) Holding the chemisorbed CH2CN groups and adsorbing TCNQ molecules. ii) Acting as a good leaving group regarding the CH2CN species. iii) Providing the necessary charge for the reaction. By means of STM manipulation techniques it is possible to break the C-C bond formed between CH2CN and TCNQ, restoring the properties of the pristine moieties. These results demonstrate the reversibility of the reaction. The product molecule, adsorbed on the graphene layer, is called CM-TCNQ and presents interesting properties that are investigated by means of manipulation at the single molecule level.