Experimental approach to electronic and magnetic properties of nanographene

Abstract

When an in?nite graphene sheet is cut into nano-size graphene frag- ments (nanographene) or semi-in?nite graphene sheets, important questions arise regarding the creation of graphene edges. This is evident in early theoretical [1-6] and experimental works [7-13], several of which predate the discovery of graphene in 2004 by many years [14]. Depending on the direction of the line along which a graphene sheet is cut, two distinct types of edge can be created, as shown in Fig. 6.1: armchair and zigzag edges [13]. The electronic structure of nanographene depends crucially on the geometrical shape of the edges. In fact, according to theoretical [1-6] and experimental studies [7-13], a nonbonding p electron state called an edge state is created along zigzag edges, whereas no such state appears in armchair edges. The presence of an edge state can be H H H H H H H H H H physically understood as a consequence of broken symmetry in Dirac fermions at zigzag edges (see Chapters 2 and 4). The presence or absence of edge states is relevant to the discussion in Chapter 5 regarding aromaticity in condensed polycyclic aromatic hydro- carbon molecules [15-22], of which graphene can be considered an inde?nitely large version. In these molecules, nonbonding p electron states are created with the same geometrical dependence as in nanographene and graphene edges. Chemists describe the presence of edge states as a result of the degradation of aromaticity.