Resolving Atomic Connectivity in Graphene ... - ACS Publications

Letter pubs.acs.org/NanoLett

Resolving Atomic Connectivity in Graphene Nanostructure Junctions Thomas Dienel,*,† Shigeki Kawai,‡,§ Hajo Söde,† Xinliang Feng,∥,⊥ Klaus Müllen,∥ Pascal Ruffieux,† Roman Fasel,†,# and Oliver Gröning*,† †

nanotech@surfaces Laboratory, Empa - Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland ‡ Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland § PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Japan ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ⊥ Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany # Department of Chemistry and Biochemistry, University of Bern, Freiestasse 3, 3012 Bern, Switzerland S Supporting Information *

ABSTRACT: We report on the structural characterization of junctions between atomically well-defined graphene nanoribbons (GNRs) by means of low-temperature, noncontact scanning probe microscopy. We show that the combination of simultaneously acquired frequency shift and tunneling current maps with tight binding (TB) simulations allows a comprehensive characterization of the atomic connectivity in the GNR junctions. The proposed approach can be generally applied to the investigation of graphene nanomaterials and their interconnections and is thus expected to become an important tool in the development of graphene-based circuitry. KEYWORDS: Two-dimensional material, graphene nanoribbon, noncontact atomic force microscopy, nc-AFM, scanning tunneling microscopy, tight binding

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atomic structure of the formed junction. However, noncontact atomic force microscopy (nc-AFM) with functionalized tips has proven successful to image intramolecular structures with ultimate resolution.9−12 Indeed, mapping of the frequency shift of a tuning fork carrying a CO-functionalized tungsten tip at constant height above the surface plane seemingly reveals the inner structure of the AGNR junction (compare Figure 1c).13,14 One can readily distinguish the hexagonal rings within the broad structure on the right and within the two 7-AGNRs running toward the left and top edges of the image (compare works of refs 12 and 15). The former reveals a fully conjugated 14-AGNR (compare Figure 1e) formed by cross-dehydrogenative coupling of two 7AGNRs along their long axis (coaxial fusion of armchair edges, compare Supporting Information Figure S1). The hexagons show subtle variations in brightness (Figure 1d) due to the herringbone reconstruction of the underlying Au(111) surface (height variation of approximately 20 pm,16 compare gray arrows in Figure 1d, enhanced color scale and height profile in Supporting Information Figure S3). Having independent electrical access to the CO-functionalized metal tip allows for the simultaneous detection of the

he controlled nanostructuring of graphene sheets is a key technology in view of the application of graphene-based materials in future electronic devices. Only recently, the fabrication of ultranarrow GNRs was demonstrated by a bottom-up approach using specifically designed molecular precursors.1 Such GNRs can be covalently joined to fabricate larger units,2 create contacts (junctions)1 and heterojunctions,3,4 and may ultimately form elaborate circuits. The synthesis of atomically precise GNRs is usually achieved under ultrahigh vacuum conditions in a two-step surfacecatalyzed process: initial aryl−aryl coupling of the dehalogenated precursors results in a one-dimensional polymer chain, and subsequent cyclodehydrogenation forms a fully conjugated ribbon (compare Figure 1a).5,6 Both processes are controlled by thermal activation and are mediated by the surface of the underlying substrate.1,7 Here, we use a Au(111) substrate and 10,10′-dibromo-9,9′-bianthryl as a precursor, which leads to socalled 7-AGNRs that are characterized by the armchair structure of the long side edge and 7 rows of carbon atom pairs parallel to the long axis of the ribbon. If the produced 7AGNRs come into close proximity to each other, they can undergo cross-dehydrogenative coupling to form more complex structures.2,8 Figure 1b shows a scanning tunneling microscopy (STM) image of a junction between two regular 7-AGNRs and a markedly broader ribbon extending to the right. Unfortunately, ordinary STM does not provide direct insight into the © XXXX American Chemical Society

Received: April 11, 2015 Revised: July 3, 2015

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DOI: 10.1021/acs.nanolett.5b01403 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Formation and STM/AFM characterization of armchair graphene nanoribbons and junctions. (a) Chemical structure of the precursor 10,10′-dibromo-9,9′-bianthryl and the resulting armchair nanoribbon of width N = 7 (7-AGNR). (b) STM image (bias = 0.2 V, set point = 2 pA) of a junction between AGNRs of different widths on Au(111). The lower right corner shows a NaCl island with an adsorbed single CO molecule. (c) Constant-height nc-AFM frequency shift image of the junction between two 7-AGNRs and one 14-AGNR shown in b. (d) STM image (bias = 0.2 V, set point = 2 pA) of a 14-AGNR with indicated contrast of Au(111) reconstruction (gray arrows, compare Supporting Information Figure S3 for different color scale). (e) Frequency shift image of a 14-AGNR (white boxed region in d) measured by nc-AFM in constant-height mode. The overlay in the right part indicates the carbon backbone of the 14-AGNR. (f) Constant height current image (√I is displayed, see Supporting Information) of the 14-AGNR depicted in panel e, acquired simultaneously with the frequency shift image (bias = 5 mV). The overlay in the right part illustrates the highest symmetry Clar structure of the 14-AGNR.

tunneling current during a nc-AFM scan (displayed as √I in Figure 1 f, bias voltage = 5 mV). The current pattern shows a hexagonal tiling with a very pronounced superstructure in amplitude, which is absent in the simultaneously acquired AFM signal (Figure 1e,f). At very low bias, the tunneling current reflects to first approximation the local density of states (LDOS) at the Fermi energy and correspondingly the current image shows the √3 × √3 superstructure,17−19 linked to the highest-symmetry Clar structure of the 14-AGNR.20,21 As apparent from Figure 1b,c, one can also observe the orthogonal fusion of GNRs. Here the armchair edge of the 14AGNR is brought into contact with the terminal end of a 7AGNR, where a short zigzag edge is present. Because of the structural mismatch between the zigzag and the armchair edges such junctions contain imperfections, that is, deviations from the perfect graphene honeycomb lattice. To understand the development of such defects, we consider a junction between two (fully hydrogen-terminated) 7-AGNRs that is formed by a single carbon−carbon bond. Generally, this will lead to the situation that the hydrogen atoms at opposing neighboring carbon sites are brought extremely close to each other. In turn, steric repulsion will either inhibit formation of the bond just considered or induce the formation of further carbon−carbon bonds via cross-dehydrogenative coupling until an energetically favorable situation without excessive steric repulsion or structural distortion is reached. Figure 2a,b shows two

illustrative examples, where only two bonds between the participating AGNRs are formed. In the first case (Figure 2a), the whole junction (including both AGNRs) belongs to a single graphene domain. Consequently, the two AGNRs enclose an angle of 60° (possible steric hindrance highlighted in blue, compare Supporting Information Figure S4). An entirely orthogonal junction with the formation of a pentagon would create steric hindrance to both sides of the junction (compare blue circles in Figure 2b, Supporting Information Figure S6), and render its formation unlikely. Instead, the system adopts a much more favorable configuration by rotating away from the orthogonal orientation (rotation of about 8° in experiment, simulation of GNR junction in the gas phase revealed 10.5° (AMBER force field)22). In this situation, a junction between the zigzag terminus of a 7-AGNR and an armchair edge can consist of up to four C−C bonds defining a hexa-, a penta-, and a heptagon next to each other (compare Figure 2d and structural model in Figure 2e). The coappearance of penta- and heptagon rings is a well-known feature of defect structures in carbon nanotubes and graphene sheets.23,24 While isolated single pentagon defects lead to a concave curvature of the graphene sheet, an isolated heptagon defect induces a convex− concave (that is, saddle-like) curvature. The concomitant appearance of the two defects can cancel the curvature to a large extend, for example, in Stone−Wales defects.25 In a linear repeated fashion, the sequence of the hexa-penta-heptagon B


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Figure 2. Junction between armchair and zigzag edge. (a,b) Geometrically constructed junctions of two 7-AGNRs, connected by two bonds (marked in red). Close proximity of neighboring hydrogen atoms inducing high steric repulsion is highlighted by blue ellipses. (c) STM image of a junction between two 7-AGNRs on Au(111) (bias = 0.2 V, set point = 2 pA). (d) Junction of panel c (white outline) imaged by constant-height nc-AFM (frequency shift image). (e) Lattice structure of a 7-AGNR junction with four closed bonds (marked in red) in relation to a grain boundary between two graphene domains (graphene lattice in gray). Heptagons and pentagons are indicated in green and violet, respectively. (f) Constant height current image (√I, see Supporting Information) of the junction in panel c (measured simultaneously, bias voltage 2 mV). (g,h) Current images (√I) simulated by TB calculations based on the 7-AGNR junction in panel e with four closed C−C bonds, assuming an s-wave (g) or a p-wave tip (h), respectively (GNRs are indicated in white).

fork sensor to resonance frequency shifts is limited to a rather small range in tip−sample separation.11,13 This is clearly seen in Figure 2d, where the atomic connectivity of the GNR junction can only partially be discerned (Laplace filter applied in Supporting Information Figure S2). In such cases, where a given structure cannot be imaged with the desired resolution, the simultaneous detection of the current signal while scanning at close distance provides further insight into the formed bonds. Figure 2f shows such a current map (√I, bias voltage = 2 mV) simultaneously acquired with the constant height frequency shift image shown in Figure 2d. The current exhibits high amplitude close to the junction at the original zigzag edge and stretches significantly into the regular

motif is known to define a planar grain boundary between two graphene sheets that are rotated against each other by 21.8°.26,27 Figure 2e shows a sketch of such a grain boundary with highlighted penta- and heptagons and the overlaid structure of the junction between two 7-AGNRs (colored orange). The formation of nonhexagonal ring defects induces strain within the graphene sheet, respectively in the created junction.28 The resultant bending of the neighboring rings leads to a slight distortion and consequently results in a measurable deviation from the planar configuration of the junction on the substrate (Figure 2d and Supporting Information Figure S2b). Nonplanarity is, however, the major obstacle for constantheight mapping of surfaces, as the high sensitivity of the tuning C


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Figure 3. Double junction between three 7-AGNRs. (a) STM topography image (bias voltage 200 mV, current set point 2 pA). (b) Zoomed view of double junction in panel a, imaged by constant-height AFM with a CO-functionalized tip (frequency shift image, z offset = −178 pm to original current set point in panel a). (c) Structural model of the double junction with four interribbon bonds for the left junction and three bonds at the right junction. Heptagons and pentagons are indicated in green and violet, respectively. (d) Map of the tunneling current (√I, see Supporting Information) close to the Fermi energy (bias voltage 2 mV) obtained at constant height (measured simultaneously with frequency shift image in panel b). (e,f) Current images (√I) simulated by TB calculations based on the AGNR junction geometry sketched in c assuming an s-wave tip (e) and a p-wave tip (f), respectively. GNRs are indicated in white.

simulations reveals that the experimentally observed pattern is dominated by an s-wave tunneling process in this case. All three, experiment and simulations, show a well-defined scattering pattern, that is, streaks along straight lines across the whole ribbon. These streaks are characteristic for the specific bonding configuration of the junction. With an increasing number of formed carbon−carbon bonds an increase of scattering into the armchair region of the bound GNR can be observed (compare Supporting Information Figure S6) and the impact of a single bond becomes assignable (see Supporting Information for a hypothetical junction and corresponding Figures S7−S9). Comparison of TB simulations for different junction geometries with the experimentally acquired tunneling current map thus allows for a straightforward identification of atomic connectivity within the junction, even in cases where the resolution of nc-AFM is limited due to not fully planar geometries. An intriguing question is the possible electronic cross talk between additional junctions. In Figure 3, we present the case of a double junction, where two 7-AGNRs are attached with their zigzag edges to either side of a 7-AGNR running from bottom-left to top-right. The STM image is essentially featureless, indicating only subtle distortions, and a single bright protrusion (see gray arrow in Figure 3a). Following the discussion above, this protrusion can be assigned to one (or more) terminal hydrogen atom(s) in a congested configuration, that is, under significant steric repulsion. While the resolution of the corresponding AFM image (Figure 3b) is too low to reveal the exact bond configuration at the junctions, the ultrahigh sensitivity of the LDOS (and thus the sample current)

armchair region of the vertically oriented ribbon. Regarding the LDOS of a 7-AGNR, two regions can be distinguished: (i) the body of the GNR, where no electronic states are present near the Fermi energy due to the band gap of ∼2.4 eV on Au(111),29,30 and (ii) the zigzag ends, where we expect halffilled end states, also called Tamm states.31,32 The appearance of the zigzag terminus of a free 7-AGNR, namely the Tamm states, strongly depends on the bond configuration of the contributing carbon atoms, for example, the corresponding fingerprint vanishes completely upon a change of only one C atom from sp2 to sp3 hybridization.32 Here at the junction, interribbon bond formation by crossdehydrogenative coupling does not change the hybridization state of the carbon atoms. The Tamm state survives within the electronic bandgap of the 7-AGNR and extends into the armchair region of the attached GNR. The underlying hybridization can be re-enacted by TB simulations of the given bonding configuration at the junction. Although different bond configurations could be assumed, only one arrangement will provide a sufficient description of the observed current map. The strong impact of different arrangements on the observable hybridization in junctions based on a single bond is illustrated in Supporting Information Figure S5. The fact that we are tunneling through a CO-functionalized tip requires that different orbital configurations are taken into account (see Supporting Information).10 For the junction shown in Figure 2c,d, four bonds, as sketched in Figure 2e, are required to reproduce the experimental current map. The corresponding TB current maps are shown in Figure 2g,h, assuming an s-wave or a p-wave tip, respectively. The comparison with the TB D


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structures. S.K., T.D., and H.S. carried out the scanning probe measurements. O.G. performed the TB calculations. T.D. and O.G. prepared the figures and wrote the paper. All authors discussed the results and commented on the manuscript.

on atomic connectivity again helps to unambiguously determine the arrangement of interribbon bonds. The current map at constant height (Figure 3d) shows a large number of subtleties of varying intensity, strong at the two junctions and in between with a standing-wave like pattern. All this can be reproduced with high precision by TB calculations (Figure 3f), for exactly one configuration: a hexa-penta-heptagon junction with the AGNR on the left side, and three bonds (hexapentagon junction) with the AGNR on the right, as illustrated in Figure 3d. The latter leaves two terminal hydrogen atoms opposing each other, which leads to the observed feature in the STM topography (constant current, compare gray arrow in Figure 3a) and a bright feature in the constant-height current map. It is interesting to note that in this case the tunneling process is dominated by the p-wave character as opposed to the s-wave one in Figure 2.10 Also in this example of electronically interacting junctions, comparison of TB simulations for different junction geometries with the experimentally acquired tunneling current map thus allows for a straightforward determination of the atomic connectivity within both junctions. In conclusion, we have investigated the formation of graphene nanoribbon junctions with STM, nc-AFM, and simultaneously acquired constant-height current maps. Upon bond formation between the terminal zigzag edge and the lateral armchair edge of AGNRs, the electronic state intrinsic to the terminal zigzag edge (Tamm state) hybridizes with the connected GNR. The features of the corresponding current maps can be retrieved in fine detail from TB simulations, which reveal a high sensitivity of the spatial charge density distribution on the precise atomic connectivity at the junction. By comparing the overall appearance of TB simulations for different bond configurations with the experimentally determined current map, the junction geometry can be unambiguously determined. This allows a detailed characterization of GNR junctions also in cases where nc-AFM does not provide atomic level insight due to structural nonplanarity within the junction. We anticipate the combined nc-AFM frequency shift and current mapping to become a generally applicable method for the investigation of graphene nanostructures and their junctions.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the Swiss National Science Foundation (SNSF), by the Office of Naval Research BRC program, and by the Japan Science and Technology Agency (JST) “Precursory Research for Embryonic Science and Technology (PRESTO)” for the project “Molecular technology and creation of new function”.

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ASSOCIATED CONTENT

S Supporting Information *

Substrate and sample preparation, experimental setup, details on the tight binding simulations and used parameters. Figures: prealignment of GNRs, contrast enhancement, impact of surface reconstruction on the nc-AFM images, TB model cases for junction formation between zigzag and armchair edge of 7-AGNRs, and TB model for single bond sensitivity in wide junctions. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.nanolett.5b01403.

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AUTHOR INFORMATION

Corresponding Authors

*(T.D.): E-mail: [email protected]. Tel.: +41 58 765 46 06. *(O.G.): E-mail: [email protected]. Tel.: +41 58 765 46 69. Author Contributions

R.F., S.K., and P.R. conceived the experiments. T.D. and H.S. performed the on-surface preparation of the investigated E


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