Molecular self-assembly on graphene - structure and effects

Abstract

Graphene, a two-dimensional allotrope of carbon, has, since its discovery in 2004, taken the world of physics by storm. With its exceptionally high charge-carrier mobility, thermal conductivity, mechanical strength and current density, it has been posited as a serious contender to replace silicon in the semiconductor industry. However, its application in practical electronic circuits require means of opening a sizeable gap in its band-structure and precise control of its doping. Large organic molecules physisorbed on graphene offer a facile route to controllably dope graphene without sacrificing its desirable properties. Under the right conditions these molecules can self-assemble on the surface into periodic, two-dimensional structures and the potential modulation set up thus can potentially lead to opening a band-gap in graphene. Moreover, the electronically inert surface of graphene offers an interesting substrate on which the fundamentals of molecular self-assembly and the electronic properties of the molecules can be studied in detail. This is very important for the potential application of two-dimensional molecular crystals in "bottom-up" fabrication strategies.

In this thesis, the structure and electronic properties of self-assembled layers of organic molecules physisorbed on graphene are studied using ultra-high vacuum, low-temperature scanning tunneling microscopy and spectroscopy. First, the assembly of cobalt phthalocyanine on technologically relevant graphene-on-insulator substrates is examined. A direct parallel is established between assembling motifs on graphene on hexagonal boron nitride and epitaxial graphene on iridium; the higher surface corrugation of graphene on silicon dioxide is found to limit the long-range order of the assembly. Next, going beyond conventional studies of close-packed assembly of molecules interacting via van der Waals forces, assemblies driven by directional intermolecular interactions is studied on graphene on iridium. The 3-fold symmetric molecule benzenetribenzoic acid is seen to assemble into extended honeycomb mesh on graphene, with the network being stabilised by linear hydrogen bonds between the molecules; the periodic nanopores are used to pattern the subsequent deposition of cobalt phthalocyanine. The strong electron acceptor tetrafluorotetracyanoquinodimethane has been proposed as a p-type dopant for graphene; at low coverage, its assembly on graphene on iridium is observed to be markedly site-specific. The molecules are charged and show pronounced structural relaxation, pointing towards a novel bonding mechanism on weakly interacting graphene. Finally, exploratory transport experiments on graphene field-effect transistors decorated with a variety of molecules reveal their effect on the charge-carriers of graphene.