Nanographene and Graphene Edges: Electronic Structure and Nanofabrication

Graphene can be referred to as an infinite polycyclic aromatic hydrocarbon (PAH) consisting of an infinite number of benzene rings fused together. However, at the nanoscale, nanographene's properties lie in between those of bulk graphene and large PAH molecules, and its electronic properties depend on the influence of the edges, which disrupt the infinite pi-electron system. The resulting modulation of the electronic states depends on whether the nanographene edge is the armchair or zigzag type, corresponding to the two fundamental crystal axes. In this Account, we report the results of fabricating both types of edges in the nanographene system and characterizing their electronic properties using a scanning probe microscope. We first introduce the theoretical background to understand the two types of finite size effects on the electronic states of nanographene (i) the standing wave state and (ii) the edge state which correspond to the armchair and zigzag edges, respectively. Most importantly, characterizing the standing wave and edge states could play a crucial role in understanding the chemical reactivity, thermodynamic stability and magnetism of nanosized graphene-important knowledge in the design and realization of promising functionalized nanocarbon materials. In the second part, we present scanning probe microscopic characterization of both edge types to experimentally characterize the two electronic states. As predicted, we find the armchair-edged nanographene to have an energetically stable electronic pattern. The zigzag-edged nanographene shows a nonbonding (pi radical) pattern, which is the source of the material's electronic and magnetic properties and its chemical activity. Precise control of the edge geometry is a practical requirement to control the electronic structure. We show that we can fabricate the energetically unstable zigzag edges using scanning probe manipulation techniques, and we discuss challenges in using these techniques for that purpose.