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 gaps 1,2 and the presence of one-dimensional (1D) edge states [3][4][5] with unusual magnetic structure 6,7 . 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 unclear [8][9][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 substrate 11,12 . 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 ∼3 Å above the mid-plane terrace of the GNR and a width of ∼30 Å (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) is reminiscent of curved edge structures observed previously near graphite step-edges 13 . We rule out that these GNRs are collapsed nanotubes by virtue of the measured ratio (observed to be π) of GNR width to nanotube height for partially unzipped CNTs. We further rule out that the curved GNR edges observed here are folded graphene boundaries by means of a detailed comparison of terminal curved edges and actual folded edges (see Supplementary Information). Low-temperature STM images (Figs 1c and 2a) show finer structure in both the interior GNR terrace and the edge re...

The edges of graphene nanoribbons (GNRs) have attracted much interest due to their potentially strong influence on GNR electronic and magnetic properties. Here we report the ability to engineer the microscopic edge termination of high quality GNRs via hydrogen plasma etching. Using a combination of high-resolution scanning tunneling microscopy and first-principles calculations, we have determined the exact atomic structure of plasma-etched GNR edges and established the chemical nature of terminating functional groups for zigzag, armchair and chiral edge orientations. We find that the edges of hydrogen-plasma-etched GNRs are generally flat, free of structural reconstructions and are terminated by hydrogen atoms with no rehybridization of the outermost carbon edge atoms. Both zigzag and chiral edges show the presence of edge states.Keywords: graphene nanoribbon, synthesis, scanning tunneling microscopy, firstprinciples calculations 3 The edges of graphene exhibit several unique features, such as the presence of localized edge states, and are anticipated to provide an important means of controlling the electronic properties of this two-dimensional material. [1][2][3] In particular, edges oriented along the high-symmetry zigzag direction or along any low-symmetry (chiral) direction give rise to unique localized edge states 4 -9 that are predicted to result in magnetic ordering. 1-3 Such edge-dependent behavior is expected to be even more pronounced in ultra-narrow strips of graphene, dubbed nanoribbons, where edges make up an appreciable fraction of the total nanostructure volume, thus creating new nanotechnology opportunities regarding novel electronic and magnetic nanodevices. 2,3 Edge states in chiral nanoribbons have been experimentally observed, 10, 11 but it has so far not been possible to control and correlate nanoribbon edge electronic structure with specific chemically defined terminal edge groups.Here we report a scanning tunneling microscopy (STM) study of graphene nanoribbons (GNRs) that are treated by hydrogen plasma etching. We find that hydrogen plasma etches away the original edge groups and develops segments with different chiralities along the edge. We have closely examined three different types of representative GNR edge segments: zigzag segments, (2,1) chiral edge segments, and armchair segments. Comparison between our experimental data and first-principles simulation of energetically most favorable structures shows good agreement. For example, we find that the edge carbon atoms of our etched GNRs are terminated by only one hydrogen atom, and that both zigzag and chiral edges show the presence of edge states. The edges of hydrogen-plasma-etched GNRs are seen to be generally free of structural reconstructions and curvature 10,11 , with the outermost carbon edge atoms being 4 in the sp 2 hybridization state. Hydrogen plasma etching thus enables the engineering of GNR edges from an unknown terminal group (with associated edge curvature 10,11 ) to a flat edge morphology with known atomic te...

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