Spatially resolving edge states of chiral graphene nanoribbons

A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometre scale with different edge geometries. A fundamental shape relevant to this question is the graphene nanoribbon (GNR), a narrow strip of graphene that can have different chirality depending on the angle at which it is cut. Such GNRs have been predicted to exhibit a wide range of behaviour, including tunable energy gaps1,2 and the presence of one-dimensional (1D) edge states3–5 with unusual magnetic structure6,7. Most GNRs measured up to now have been characterized by means of their electrical conductivity, leaving the relationship between electronic structure and local atomic geometry unclear8–10. Here we present a sub-nanometre-resolved scanning tunnelling microscopy (STM) and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. The GNRs used here were chemically synthesized using carbon nanotube (CNT) unzipping methods that allow flexible variation of GNR width, length, chirality, and substrate11,12. Our STS measurements reveal the presence of 1D GNR edge states, the behaviour of which matches theoretical expectations for GNRs of similar width and chirality, including width-dependent energy splitting of the GNR edge state. The chirality of a GNR is characterized by a chiral vector (n,m) or, equivalently, by chiral angle θ , as shown in Fig. 1a. GNRs having different widths and chiralities were deposited on a clean Au(111) surface and measured using STM. Figure 1b shows a room temperature image of a single monolayer GNR (GNR height is determined from linescans, such as that shown in Fig. 1b inset; some multilayer GNRs were observed, but we focus here on monolayer GNRs). The GNR of Fig. 1b has a width of 23.1 nm, a length greater than 600 nm, and exhibits straight, atomically smooth edges (the highest quality GNR edges, such as those shown in Figs 1 and 2, were observed in GNRs synthesized as in ref. 11). Such GNRs are seen to have a ‘bright stripe’ running along each edge. This stripe marks a region of curvature near the terminal edge of the GNR that has a maximum extension of ∼3Å above the mid-plane terrace of the GNR and a width of ∼30Å (see line scan in Fig. 1b inset). Such edge-curvature was observed for all high-quality GNRs examined in this study (more than 150, including GNRs deposited onto a Ru(0001) surface). This