ARTICLES PUBLISHED ONLINE: 15 FEBRUARY 2009 DOI: 10.1038/NMAT2378 The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons Kyle A. Ritter 1,2 * and Joseph W. Lyding 1,3 Graphene shows promise as a future material for nanoelectronics owing to its compatibility with industry-standard lithographic processing, electron mobilities up to 150 times greater than Si and a thermal conductivity twice that of diamond. The electronic structure of graphene nanoribbons (GNRs) and quantum dots (GQDs) has been predicted to depend sensitively on the crystallographic orientation of their edges; however, the influence of edge structure has not been verified experimentally. Here, we use tunnelling spectroscopy to show that the electronic structure of GNRs and GQDs with 2–20 nm lateral dimensions varies on the basis of the graphene edge lattice symmetry. Predominantly zigzag-edge GQDs with 7–8nm average dimensions are metallic owing to the presence of zigzag edge states. GNRs with a higher fraction of zigzag edges exhibit a smaller energy gap than a predominantly armchair-edge ribbon of similar width, and the magnitudes of the measured GNR energy gaps agree with recent theoretical calculations. T he surface structure of bulk, crystalline semiconductors has profound consequences on the development and manufac- turing of electronic devices. Bardeen 1 explained that the pres- ence of surface states results in binding of free carriers and induces the formation of Schottky barriers at semiconductor–metal inter- faces. A deeper understanding of surface states has enabled scientists and engineers to optimize the performance of integrated circuits for 50 years 2 . Analogous to the surface states that exist in bulk crystals, the edge structure of nanometre-sized, two-dimensional materials such as graphene, a one-atom-thick layer of carbon, can significantly influence their electronic structure. Researchers have reported experimental transport measurements 3–8 and theoretical studies 9–11 of graphene quan- tum dots (GQDs) and nanoribbons (GNRs) elucidating their remarkable promise for future nanoelectronic applications. In spite of theoretical calculations that predict a localized metal- lic state for the zigzag edge 12 , all transport measurements of GQDs (ref. 3) and GNRs (refs 4–7) reveal only semiconducting behaviour. Furthermore, the electronic properties of the graphene are independent of crystallographic orientation 4 in contrast to theoretical predictions 9–12 . Recent theoretical studies show that transport effects such as Coulomb blockade 13 or a mobility gap induced by edge disorder 14,15 may affect the accuracy of bandgaps measured under transport conditions and explain the independence of energy gap and crystallographic orientation. By probing the local electronic structure of GQDs and GNRs using ultrahigh-vacuum scanning tunnelling microscopy (UHV-STM), we detect that the crystallographic orientation of the graphene edges significantly influences the electronic properties of nanometre-sized graphene. Enabled by the development of an atomically clean, in situ deposition method 16 , we experimentally determine the energy gap (E g )–size (L) relation for GQDs with 2–20 nm lateral dimensions and correlate the E g measurements with the GQD 1 Beckman Institute for Advanced Science and Technology, University of Illinois, 405 North Mathews Avenue, Urbana, Illinois 61801-2325, USA, 2 Department of Materials Science and Engineering, University of Illinois, 1304 West Green Street, Urbana, Illinois 61801-2920, USA, 3 Department of Electrical and Computer Engineering, University of Illinois, 1406 West Green Street, Urbana, Illinois 61801-2918, USA. *e-mail:ritter@engineering.uiuc.edu. edge structure. Predominantly zigzag-edge GQDs with 7–8 nm average dimensions are metallic and diverge from the E g –L scaling law owing to the presence of metallic zigzag edge states, which spatially decay into the graphene interior with a 1.0–1.2 nm decay length. In addition to GQDs, we study the electronic structure of GNRs with 2–3 nm widths and 20–30 nm lengths. GNRs with a higher fraction of zigzag edges exhibit a smaller energy gap than a predominantly armchair-edge ribbon of similar width and the magnitudes of the measured GNR energy gaps agree with recent theoretical calculations. Unlike previous studies of micrometre-sized, mechanically exfoliated graphene monolayers on SiO 2 (refs 17,18), nanometre- sized graphene monolayer samples do not typically exhibit the 2.5 Å spaced, hexagonal graphene lattice in high-resolution STM topographs. As shown in Fig. 1, we typically observe either a 4.1 Å hexagonal lattice (Fig. 1a–c) or 2.5 Å triangular lattice patterns (Fig. 1d,e) in the STM topograph of the 3-Å-tall monolayer samples. Owing to the close proximity of the graphene edges in our samples, we believe that the electron wavefunction scatters off the edges and the resultant interference pattern generates the observed STM topographic contrasts for the GQDs and GNRs. Our interpretation is supported by previous STM studies where √ 3 × √ 3 R30 ◦ hexagonal superstructures were observed near graphite terrace edges 19–21 and graphene lattice defects 22 , as well as the topographs of 10–15-nm-wide GNRs defined on a highly oriented pyrolytic graphite (HOPG) substrate through ambient STM lithography 23 . Figure 1a and b show the STM topograph and topographic derivative, respectively, of a 2.3-nm-wide, 20-nm-long GNR. Figure 1b delineates the hexagonal pattern along the GNR and Fig. 1c shows a line contour taken from Fig. 1a, the location of which is designated by the grey line. From Fig. 1c, the nearest-neighbour spacing between the hexagons is 4.1 ± 0.3 Å. The hexagonal pattern with a 4.1 Å average lattice spacing is NATURE MATERIALS | VOL 8 | MARCH 2009 | www.nature.com/naturematerials 235