Spatially resolving edge states of chiral graphene nanoribbons

Abstract

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...

Electronic and optical properties of graphene antidot lattices: comparison of Dirac and tight-binding models

“…Furthermore, they showed that the edge states become spin polarized when S ≥ 6. Edge states have also been observed experimentally using scanning tunnelling spectroscopy on GNRs fabricated by "unzipping" carbon nanotubes [29]. Edge states modify the electronic properties of GNRs and figure 2 shows that they also modify the electronic properties of GALs.…”

Edge states in graphene-like systems

“…Unlike the work of Tau et al 56 , a detailed evolution of the I(V ) curves along the ribbons is not provided. In addition, edge magnetism is definitely not expected to survive up to room temperature 71 , al-though the effect of long-range order depletion due to quantum and thermal fluctuations on the spectral function of the quasiparticles has not been studied.…”

“…Graphene ribbons are interesting by their own sake, and there has been enormous progress in the fabrication of ribbons with smooth edges [53][54][55][56][57] , so that inter-edge coupling is also an interesting topic [30][31][32]35,37,38,40,42,51,57,58 .…”

Chirald-wave superconductivity in doped graphene

“…For example, magnetism has experimentally been reported both in nanographene [58,59,60], and in graphite in the presence of disorder [61] or grain boundaries [62], although pristine graphene has not been found to be either magnetic [63] or gapped [64,20]. Theoretically, on-site Coulomb repulsion exceeding U > 3.9t has been found to give an antiferromagnetic state in undoped graphene in quantum Monte Carlo simulations [15,17].…”