Supplementary Information. Edge Structures for Nanoscale Graphene Islands on Co(0001) Surfaces

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1 Supplementary Information Edge Structures for Nanoscale Graphene Islands on Co(0001) Surfaces Deborah Prezzi 1,4, Daejin Eom 2,3,4, Kwang T. Rim 2, Hui Zhou 3, Michael Lefenfeld #2, Shengxiong Xiao 2, Colin Nuckolls 2, Tony F. Heinz &*3,5, George W. Flynn *2 and Mark S. Hybertsen *6 1 Nanoscience Institute CNR, S3 Center, I Modena, Italy 2 Department of Chemistry, Columbia University, New York, New York 10027, United States 3 Department of Physics, Columbia University, New York, New York 10027, United States 4 Center for Electron Transport in Molecular Nanostructures, Columbia University, New York, New York 10027, United States 5 Department of Electrical Engineering, Columbia University, New York, New York 10027, United States 6 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States AUTHOR ADDRESS: tony.heinz@columbia.edu, gwf1@columbia.edu, and mhyberts@bnl.gov. Contents: 1. Details of structures analyzed with DFT (Figs. S1, S2) 2. Details of calculated edge formation energies (Table S1) 3. Bias dependent simulated STM images (Fig. S3) 4. Additional STM images (Figs. S4, S5, S6) 5. Simulated STM images for triangular nanostructures (Fig. S7) 6. References 1

2 1. Details of structures analyzed with DFT All the structures considered in our study are displayed in Figures S1-S2. Figure S1. Ball-and-stick models for relaxed graphene nanoribbons. a, Clean and b, hydrogen-passivated armchair-edged GNRs. c, Clean and d, H-passivated zigzag-edged GNRs. e, The zigzag GNR in c is modified to form a Klein termination on the left edge (i.e. one additional C atom is bonded to the C edge atom). f, 57-reconstruction of the zigzag edge, characterized by the alternation of pentagons and heptagons. For each structure, both top and lateral views are shown. C atoms are in yellow, H in cyan, Co in blue. Figure S2. Ball-and-stick models for relaxed triangular graphene nanotructures. Clean zigzag-edged GNSs with edge atoms in a, hollow and b, on-top registry. c, The GNS in b is modified to form a Klein termination (i.e. one additional C atom is bonded to the C edge atom). We also considered H-passivated versions of a and b. C atoms are in yellow, H in cyan, Co in blue. 2

3 2. Details of calculated edge formation energies Even though graphene edges are expected to be unpassivated under the experimental growth conditions, we took into account both clean and hydrogen-passivated systems, for completeness. To compare the energy for systems involving H in different configurations, we included the zero point energy of H in all relevant cases, that is 0.136, (0.315) and ev/h atom for H-H 1, zigzag (armchair) H-C 2 and H-Co. In the absence of data for Co, the zero-point motion of Co(0001):H was taken to be the same as that for Ni(111):H, having found that E ZPE varies very little for H on a number of different metals for which it occupies the hollow site 3-4. Moreover, in order to compare the formation energy of clean and H passivated edges, a reservoir for H must be chosen. There are two natural possibilities: H 2 in the gas phase and H chemisorbed on the Co(0001) surface. The measured energy gain upon dissociative adsorption is 0.70 ± 0.08 ev per molecule or 0.35 ev per H atom 5. In view of the lower energy for H on Co(0001) and the role of C-H bond cleavage in the graphene island growth process, we chose the Co(0001):H system as the reservoir for H. The energy for Co(0001):H is calculated for a series of supercells to estimate the adsorption energy in the low density limit, finding a value of 0.86 ev per atom. The computed edge formation energies are reported in Table S1. Table S1. Edge formation energies (ev/å). Isolated On Co(0001) with H # w/o H with H # w/o H top ZZ hollow ZZ-Klein 0.29 ZZ AC ** (#) The reservoir for the isolated systems is H 2 in the gas phase, while we chose the Co(0001):H as the reservoir for the systems on substrate. (**) Constrained on-top geometry. Relaxed bridge position: 0.43 ev/å. 3

4 3. Bias dependent simulated STM images Figure S3. Bias dependence of simulated STM images. An expanded view of the simulated STM images for the GNR illustrated on the right and in Fig. 5 of the main text for a series of four bias windows indicated in ev above the four images. 4. Additional STM images: Figure S4. Full STM topographic image of the graphene island on the Co(0001) surface shown in part in Fig. 5 of the main text. Data taken at 4.9 K with Vsample = V and It = 2 na. The image size is 9.8 x 5 nm2. Specific straight edges are denoted following Fig. 5. In addition, there are some disordered edges and defective regions within the body of the island. 4

5 Figure S5. STM topographic images of four graphene islands on the Co(0001) surface. Data taken at 4.9 K with Vsample = V and It = 3 na. The image size is 10 x 6 nm2. Although the bias conditions are slightly different, the three substantial islands all exhibit the distinguishable edge structures identified in Fig. 5 in the main text. In particular, the triangular island in the upper left only shows B-type edges that we identify as being of the Klein type. Figure S6. STM images of two triangular graphene islands on the Co(0001) surface. Data taken at 4.9 K with: (a) Vsample = V and It = 3 na and (b) Vsample = V and It = 5 na. Both image sizes are 4 x 4 nm2. The island in (a) represents a zoom on the upper left island in Fig. S5. 5

6 5. Simulated images for triangular nanostructures Figure S7. Simulated STM images for small, triangular graphene nanostructures on the Co(0001) surface. (a) Zigzag terminated triangular island together with simulated images for bias windows and ev. (b) Klein terminated triangular island together with simulated images for bias windows and ev. Depending on bias window, finite size and interference effects change the apparent image. 6. References 1. American Insitute of Physics Handbook, 3rd ed. (McGraw-Hill, New York, 1972), Chap. 7, p Yamada, M.; Yamakita, Y.; Ohno, K., Phonon Dispersions of Hydrogenated and Dehydrogenated Carbon Nanoribbons. Phys. Rev. B 2008, 77, Christmann, K., Interaction of Hydrogen with Solid Surfaces. Surf. Sci. Rep. 1988, 9, Barteau, M. A.; Broughton, J. Q.; Menzel, D., Determination of Hydrogen Atom Binding Sites on Ru(001) by Hreels. Surf. Sci. 1983, 133, Bridge, M. E.; Comrie, C. M.; Lambert, R. M., Hydrogen Chemisorption and the Carbon Monoxide-Hydrogen Interaction on Cobalt (0001). J. Catal. 1979, 58,