On-Surface Synthesis of Two-Dimensional Covalent Organic

On-Surface Synthesis of Two-Dimensional Covalent Organic Structures versus HalogenBonded Self-Assembly: Competing Formation of Organic Nanoarchitectures David Peyrot and Fabien Silly* TITANS, CEA, IRAMIS, SPEC, CNRS, Université Paris Saclay, CEA Saclay, F-91191 Gif sur Yvette, France S Supporting Information *

ABSTRACT: The competition between the on-surface synthesis of covalent nanoarchitectures and the selfassembly of star-shaped 1,3,5-Tris(4-iodophenyl)benzene molecules on Au(111) in vacuum is investigated using scanning tunneling microscopy above room temperature. The molecules form covalent polygonal nanoachitectures at the gold surface step edges and at the elbows of the gold reconstruction at low coverage. With coverage increasing two-dimensional halogen-bonded structures appear and grow on the surface terraces. Two different halogen-bonded nanoarchitectures are coexisting on the surface and hybrid covalent-halogen bonded structures are locally observed. At high coverage covalent nanoarchitectures are squeezed at the domain boundary of the halogen-bonded structures. The competitive growth between the covalent and halogen-bonded nanoarchitectures leads to formation of a two-layer film above one monolayer deposition. For this coverage, the covalent nanoarchitectures are propelled on top of the halogenbonded first layer. These observations open up new opportunities for decoupling covalent nanoarchitectures from catalytically active and metal surfaces in vacuum. KEYWORDS: covalent, halogen-bonding, iodine, Ullmann coupling, STM, ultra high vacuum, temperature, step edge

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specific organic building blocks to generate covalent carbonstructures via intermolecular interactions.5−11 Several reactions have been identified to generate carbon nanoarchitectures via surface-assisted intermolecular covalent C−C coupling. This includes Glaser coupling,12 Bergman cyclization,13 cyclodehydrogenation,14,15 dehydrogenative coupling,16,17 and Ullmann coupling.18−21 The current state-of the-art of surfaceactivated molecular covalent reactions is described in the review papers of Dong et al. and Fan et al.22,23 Porous carbonnanoarchitectures,24−28 graphene nanoribbons,29−31 have thus been successfully synthesized by surface-assisted reactions. The dimensions of such structures are, however, quite limited and the number of defects is usually important because the covalent bond formation is a nonreversible process preventing selfhealing. Molecules with peripheral halogen atoms are promising organic building blocks to engineer different types of twodimensional porous carbon-nanoarchitectures. The molecular

ngineering two-dimensional (2D) carbon-based nanoarchitectures has received tremendous attention due to the unusual exceptional electronic properties of these materials. Graphene is for example expected to revolutionize numerous industrial technologies because of its high mobility.1 The zero band gap property of graphene is, however, a drawback for its integration in semiconductor-based devices. The calculations of Pedersen et al., however, predict that creating a hole-superlattice in a semimetallic graphene layer would lead to a drastic modification of its electronic properties.2 The so-prepared graphene layer would thus become a semiconductor with a significant and tunable energy band gap. This engineering at the atomic scale opens new opportunities for fabricating carbon layers with specific electronic properties. Graphene can be patterned using topdown techniques like electron-beam lithography. However, the generation of large-scale highly ordered superlattice of nanoholes separated by less than 10 nm still remains a challenging task to experimentally achieve.3,4 Alternative methods have been developed. On-surface bottom-up synthesis of patterned graphene nanoarchitectures at atomic scale can for example be achieved by taking advantage of the ability of some © XXXX American Chemical Society

Received: March 21, 2016 Accepted: May 9, 2016

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DOI: 10.1021/acsnano.6b01938 ACS Nano XXXX, XXX, XXX−XXX

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the covalent organic nanoarchitecture lays on top of the halogen-bonded structures.

halogen atoms (X) can stabilize highly ordered organic nanoarchitectures through the formation of intermolecular halogen bonds.32−36 Halogen-bonded self-assembled nanoarchitectures have been successfully created using molecules with bromine37−40 and iodine substituents.41 In addition, these building blocks can also be used to engineer covalent nanoarchitectures taking advantage of on-surface polymerization. 2D covalent-nanoarchitectures have been fabricated using molecules with bromine atoms18,29,34,42−45 as well as molecules with iodine atoms.34,46−48 However, the competing formation of halogen-bonded nanoarchitectures and covalent structures on catalytically active metal surfaces has still to be elucidated to optimize the formation of one specific type of organic structure. This is essential for example to trigger the formation of porous graphene-nanoarchitectures over the formation of self-assembled halogen-bonded films at room temperature. Tailoring the formation of porous 2D carbon-nanoarchitecture is only one of the challenges in carbon-based nanostructure engineering. Intercalating an intermediate layers between the metal surface and the covalent carbon structures is another one. Walter et al. already succeeded in intercalating an intermediate oxygen layer between the metal surface and a graphene layer after exposure to air49 and Li et al. suceeded in intercalating Si heteroatoms between graphene and Ru(0001) in vacuum.50 Eder et al. developed a drop-casting method to deposit 1,3,5Tri(4-iodophenyl)benzene (Figure 1a) covalent aggregates on

RESULTS AND DISCUSSION The chemical structure of the 1,3,5-Tris(4-iodophenyl)benzene molecule is presented in Figure 1a. This 3-fold symmetry molecule is a star-shaped molecule. The molecular skeleton is consisting of a central benzene ring connected to three peripheral 4′-iodophenyl groups. Two molecules are expected to form on metal surface a covalent dimer (Figure 1b) through surface-assisted Ullmann coupling. Initial Growth of the Organic Layer: 0.02−0.10 Monolayer Coverage. The Figure 2a shows the Au(111) surface after low concentration deposition (less than 0.02 monolayer (ML)) of 1,3,5-Tris(4-iodophenyl)benzene molecules. The STM image reveals that molecules preferentially adsorb at the gold step edges. The molecules form small zigzag chains on each side of the step edge. The center-to-center distance of adjacent molecules is 1.3 ± 0.1 nm. This indicates intermolecular covalent bonding, Figure 1b. Bright spots can be observed at the apex of molecular arms (highlighted by black arrows in Figure 2a). It has been previously experimentally observed with STM that halogen atoms, such as bromine and iodine, appear brighter than molecular carbon atoms.41,51 These bright spots therefore reveal that molecular iodine atoms are not systematically dissociated from molecular skeleton when molecules are adsorbed on the Au(111) surface at room temperature, in contradiction with previous reports.45,47,52 STM images of the surface step edges after 0.10 ML deposition are presented in the Figure 2b,c. The STM images show that small porous structures are growing from the Au(111) step edges to the surface terraces. The STM images also reveal that bright spots are now adsorbed on the step edges. This is particularly obvious in the high resolution STM image in Figure 2c. Iodine adatoms created during the polymerization of molecular building blocks appear to preferentially adsorb on the Au(111) step edges. Figure 2d shows that small covalent porous structures are locally observed at the elbows of the gold herringbone reconstruction for 0.10 ML deposition. The formation of covalent structures appears however to be quite limited. Only one or two covalent cycles are usually observed in the elbows of the Au herringbone surface reconstruction. Covalent Organic Polygonal Nanoarchitecture for 0.1−0.2 Monolayer Coverage. Above 0.2 monolayer deposition, a 2D porous nanoarchitecture is observed on the surface. This structure is composed of organic pores (Figure 3) and zigzag chains (Figure 3h). The pores adopt a polygonal geometry. Tetragonal, pentagonal, heptagonal, and octagonal cavities are mainly composing the organic structure, (Figure 3c−g) . The measured center-to-center distance of the starshaped molecular building block is 1.3 ± 0.1 nm. This reveals that the molecules are covalently linked through the dehalogenation of the molecular peripheral iodine atoms. The image in Figure 3b is in addition showing that iodine adatoms are sometimes trapped inside the polygonal-cavities of the covalent nanoarchitecture. Appearance of Halogen-Bonded Nanoarchitectures above 0.2 Monolayer Coverage. The STM images in Figure 4 show molecular self-assembly on the Au(111)−(22 × √3) terraces for 0.2 monolayer deposition. Molecules now form small well-organized domains trapped between the gold reconstruction lines, Figure 4a,b. Ordered bright features can

Figure 1. Scheme of 1,3,5-Tris(4-iodophenyl)benzene (C21H15I3) dimer building block. Carbon atoms are gray, iodine atoms purple, and hydrogen atoms white, respectively.

top of a layer supposed to be made of chemisorbed iodine atoms and disordered molecules adsorbed on Au(111).46 They claimed that the presence of the covalent aggregates in a second layer is unique to their solution approach and can not be successfully achieved in vacuum.47 In this paper, we investigate the self-assembly of 1,3,5-Tris(4iodophenyl)benzene molecules and the formation of covalent nanoarchitectures on Au(111) in vacuum. Scanning tunneling microscopy (STM) reveals that molecules form covalent structures at the Au step edges at low concentration, whereas different halogen-bonded networks appear at higher coverage. The competitive growth of the covalent and halogen-bonded nanoarchitectures leads to formation of a two layer film, where B


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Figure 3. STM image of the 1,3,5-Tris(4-iodophenyl)benzene selfassembled covalent porous nanoarchitecture on Au(111), (a) 21 × 12 nm2, Vs = 0.9 V, It = 20 pA, (b) 75 × 60 nm2, (c) 3 × 3 nm2, (d) 3 × 3 nm2, (e) 3 × 3 nm2, (f) 3 × 3 nm2, (g) 3 × 3 nm2, (h) 3 × 3 nm2, Vs = 1.0 V, It = 25 pA.

bright patterns can be observed in the organic layer, Figure 4c,d. The bright patterns can adopt a well-ordered sine-wave (Figure 5) or bow-tie structure (Figure 6) . The boundaries of the molecular network appear in comparison quite defective. Sine-wave X-bonded Structure. Figure 5a shows a high resolution STM image of the sine-wave organic nanoarchitecture observed in Figure 4. The STM image reveals that the molecules are self-assembled into an ordered porous structure on the surface. Neighboring side-by-side molecules are rotated by an angle of 180°, as it is highlighted by the molecular schemes superimposed to the STM image in Figure 5a. The molecular iodine atoms appear brighter than molecular carbon skeleton in the STM image. The bright sine-wave pattern results from the local arrangement of molecular iodine atoms. Each bright wave corresponds to an infinite iodine synthon X∞. The STM images presented in Figure 5b,c are showing that some molecular arms are locally dehalogenated.

Figure 2. Initial growth of the organic layer on Au(111)−(22 × √(3): (a) 22 × 12 nm2, Vs = 0.9 V, It = 20 pA; (b) 45 × 38 nm2, Vs = 1.3 V, It = 205 pA; (c) 20 × 20 nm2, Vs = 1.3 V, It = 245 pA; (d) 36 × 36 nm2, Vs = 1.3 V, It = 205 pA.

be observed inside the molecular domains. The size of the domain is increasing with molecular concentration. Molecular covalent structures can be observed at the edge of these molecular domains, Figure 4c. For one monolayer deposition (1 ML) the whole Au(111) surface is covered as shown in Figure 4d. Different organic networks are coexisting, different C


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Figure 4. STM images of the Au(111) terraces at increasing molecular coverage, (a) 40 × 31 nm2, Vs = 2.2 V, It = 55 pA, (b) 50 × 35 nm2, Vs = 2.2 V, It = 55 pA, (c) 26 × 12 nm2, Vs = 0.9 V, It = 245 pA. (d) 36 × 36 nm2, Vs = 0.6 V, It = 245 pA.

This does not affect the structure of the organic nanoarchitecture. The missing iodine atoms are highlighted by a white dotted circle superimposed to the STM images. The unit cell of the sine-wave nanoarchitecture is represented by a black dashed line in Figure 5d. The network unit cell of this porous structure is a rectangle with 2.1 ± 0.2 nm and 1.9 ± 0.2 nm unit cell constants. The unit cell is composed of two molecules. The molecular architecture appears to be stabilized by halogen··· halogen bonds between neighboring molecules. The angle between I−C groups of neighboring molecules is 180° and 120°. Bow-Tie X-Bonded Structure. A second well-ordered organic nanoarchitecture is also usually observed on the Au(111) surface. High resolution STM images of this structure are presented in Figure 6. This organic nanoarchitecture is also porous. Six molecular schemes have been superimposed to the STM image in Figure 6b to visualize molecular assembly. Neighboring molecules are rotated by an angle of 180°. The molecules are however not adopting a strict side-by-side arrangement, as previously observed in the sine-wave structure. The molecular iodine atoms are forming 6-synthons (X6) adopting a bow-tie shape. The network unit cell of this structure is a parallelogram with 2.2 ± 0.2 nm and 1.7 ± 0.2 nm unit cell constants and an angle of 75 ± 3° between the axes.

Figure 5. High resolution STM images of the 1,3,5-Tris(4iodophenyl)benzene sine-wave nanoarchitecture, (a) 10 × 8 nm2, Vs = 1.0 V, It = 23 pA, (b) 5 × 3 nm2, Vs = 1.0 V, It = 23 pA, (c) 3 × 5 nm2, Vs = 1.0 V, It = 23 pA, (d) model of the organic nanoarchitecture. Molecular skeleton is represented by an orange star and the iodine atoms are represented by yellow balls. The network unit cell is represented by a dotted black rectangle.

The unit cell is composed of two molecules. The molecular architecture appears to be stabilized by halogen···halogen bonds between neighboring molecules. The angle between I−C groups of neighboring molecules is 60° and 120°. Covalent-Dimer X-Bonded Structure. Another organic nanoarchitecture is locally observed on the Au(111) surface. An STM image of this structure is presented in Figure 7. Molecular iodine atoms are now paired and are aligned on the surface. The high resolution STM image reveals that the building block of this structure is the 1,3,5-Tris(4-iodophenyl)benzene covalent-dimer, previously presented in Figure 1b. The molecular dimers are aligned along their axis on the surface and they are forming parallel chains. Neighboring dimers are binded along their axis through double X···X bonds. The angle between I−C groups is 60°. The model of this arrangement is represented in the Figure 7d. The network unit cell of this D


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Figure 7. STM images of the mixed covalent/X-bonded 1,3,5Tris(4-iodophenyl)benzene nanoarchitecture on Au(111), (a) 10 × 10 nm2, Vs = 1.3 V, It = 205 pA, (b) 7 × 7 nm2, Vs = 1.3 V, It = 205 pA, (c) 7 × 7 nm2, Vs = 1.3 V, It = 205 pA. (d) Model of the molecular arrangement. The network unit cell is represented by a dotted black parallelogram.

7a). This structure is a mixed nanoarchitecture, combining covalent and X···X halogen bonds. The angle between I−C groups of neighboring molecules is 120°. Covalent-Chain Assembly. The STM image presented in Figure 8a reveals the existence of a fourth organic organized nanoarchitecture on the Au(111) surface. This structure is composed of covalent molecular zigzag chains arranged side by side. The zigzag chains result from the sequential covalent binding of molecules alternatively rotated by an angle of 180°. The model of the zigzag chain arrangement is presented in Figure 8b. The network unit cell of this structure is a rectangle with 2.2 ± 0.2 nm and 1.6 ± 0.1 nm unit cell constants. The unit cell is composed of two covalently linked molecules. This structure is, however, only locally observed on the Au(111) surface.

Figure 6. High resolution STM images of the 1,3,5-Tris(4iodophenyl)benzene bow-tie nanoarchitecture, (a) 14 × 14 nm2, Vs = 1.3 V, It = 20 pA, (b) 5 × 6 nm2, Vs = 1.3 V, It = 20 pA. (c) Model of the organic nanoarchitecture. The network unit cell is represented by a dotted black parallelogram.

structure is a parallelogram rectangle with 2.7 ± 0.2 nm and 1.7 ± 0.2 nm unit cell constants and an angle of 43 ± 3° between the axes. The unit cell is composed of one dimer (two covalently linked molecules). Local defects can be observed in the organic layer. These structure defects correspond to dehalogenated dimers (Figure 7b,c) and local bow-tie nanoarchitecture (see superimposed star molecule scheme in Figure E


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Figure 8. (a) STM image of the covalent chain-nanoarchitecture on Au(111), 14 × 12 nm2, Vs = 1.9 V, It = 55 pA. (b) Model of the zigzag chain-nanoarchitecture. The network unit cell is represented by a dotted black square.

Double-Layer Organic Film above 1 Monolayer Deposition. Figure 9 shows the Au(111) surface after 1.2 ML molecular deposition. The Au(111) surface is now fully covered by an organic layer. The STM images show that the first organic layer on the Au(111) surface is essentially composed of the sine-wave and bow-tie nanoarchitectures, whereas the dimer and chain-structures are only locally observed (Figure 9c). The second organic layer appears to be exclusively composed of the covalent polygonal nanoarchitecture that grows at low coverage (Figure 3). The line profile in Figure 9b shows the height difference between the halogenbonded first layer and the supported covalent-nanoarchitectures. The self-assembly of 1,3,5-Tris(4-iodophenyl)benzene molecules on Au(111) surface in vacuum has been investigated using STM. STM images reveal that at low coverage the molecules preferentially adsorb on the Au(111) step edges. The molecules form there covalent structures through iodine dehalogenation. Small covalent structures are also observed at the herringbone elbows of the Au(111) reconstruction. A coverage increase essentially leads to the growth of halogenbonded nanoarchitectures on the surface; the formation of covalent nanoarchitecture is in comparison less favored. In contrast with previous reports,45,47,53 our measurements show that the iodine dehalogenation process is a limited process on Au(111) at room temperature since iodine atoms can be undoubtedly identified with STM on the molecular skeleton in numerous case on Au(111), Figure 2. Our STM images are in

Figure 9. STM images of the Au(111) surface after 1.2 ML molecular deposition, (a) 110 × 60 nm2, Vs = 2.1 V, It = 445 pA. (b) Line profile taken along the blue dotted line in (a); (c) 60 × 60 nm2, Vs = 1.9 V, It = 445 pA.

fact showing that Au(111) step edges are preferential reaction sites for Ullmann coupling reaction. This effect was previously proposed by Saywell et al. for bromine molecules.42 They also observed that Au(111) step edges and kinks act as “active sites” and catalytically induce the cleavage of the molecular halogen atoms. This explains why covalent structures preferentially start growing from Au(111) step edges, Figure 2a,b. The free iodine atoms produced during the Ullmann coupling are preferentially adsorbing on the Au(111) step edges. These atoms then form chains along the step edges, as it can be observed in Figure 2b,c. The saturation of Au(111) step edges with iodine atoms reduces the catalytic activity of the Au(111) surface. At high deposition the formation of covalent nanoarchitectures is highly diminished. The molecules preferentially form halogen-bonded nanoarchitectures on the Au(111) surface. Molecules with missing iodine atoms are only locally observed in these structures, not only at room temperature but also at high temperature (see Figures S3 and S4 in Supporting Information). Increasing further the coverage leads to the competitive growth of the covalent structure and F


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ACS Nano Table 1. Structure, Bonding, and Packing Density of the 1,3,5-Tris(4-iodophenyl)benzene Nanoarchitecturesa

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The carbon skeleton of th molecular building block is represented by an orange star, and the iodine atoms are represented by yellow balls.

the hybrid dimer network, and 1 mol/1.76 nm2 for the hybrid zigzag network. In comparison, the density of the covalent hexagonal network is the smallest with 1 mol/2.16 nm2, see Table 1. As the covalent nanoarchitecture is the less dense, it is pushed away from the Au(111) surface by the growth of the denser nanoarchitectures. In contradiction with Elder et al. statement, we prove that the presence of the covalent aggregates in a second layer is not unique to the solution approach and can also be obtained in vacuum46 The effect of surface temperature on Ullmann coupling reaction efficiency has been investigated. Molecules have been deposited on a hot surface and molecules have also been deposited on a surface at room temperature, followed by a postannealing. The temperature range investigated goes from room temperature to 500 °C. This was done for different surface coverages. STM images, however, reveal that temperature increase has no effect on the formation of covalent nanoarchitectures (see Supporting Information). X-bonded structures are still observed at high temperature and the competitive growth of the different structures is not modified. Therefore, it appears that the formation of covalentarchitectures is intrinsically connected to surface coverage and, therefore, the saturation of surface reaction sites (step edges and elbows of the Au(111) herringbone reconstruction) more than temperature. It should be noticed that for temperatures higher than 500 °C, molecules and iodine adatoms are both desorbing from the Au(111) surface. Bui et al. previously classified the different types of C−X··· X−C bonds depending on the angle between X−C groups.55 The type-I interaction is of van der Waals type. The type-II interaction is an attractive interaction between the nucleophilic (−) and electrophilic (+) areas of halogen atoms. The angle between the molecular X−C group axis is 90−120°. The sinewave structure appears therefore to be stabilized by type-I and

halogen-bonded nanoarchitectures. STM is showing that the halogen-bonded structures are preferentially covering the surface and are pushing away the covalent nanoarchitectures. The covalent nanoarchitectures are then decorating the edges of X-bonded structures. The covalent structures are finally squeezed at the domain boundary of X-bonded structures for 1 ML deposition, Figure 4c. This results in the formation of very defective domain boundaries, Figure 4d. Two X-bonded nanoarchitectures coexist. They are presented in the high resolution STM images in Figure 5 and Figure 6. However, it should be noticed that hybrid X-bonded-covalent nanoarchitectures (Figure 7) and covalent chain nanoarchitectures (Figure 8) are only locally observed; their domain area is never larger than 200 nm2. Above one monolayer deposition, the Au(111) surface is fully covered by an organic layer, essentially composed of the X-bonded nanoarchitectures. A second layer is observed on top of the first X-bonded layer, Figure 9. STM images reveal that the second layer is surprisingly composed of polygonal-covalent nanoarchitectures. As the Au(111) surface is now covered by an organic layer and the step edges are already saturated with iodine adatoms, Ullmann reaction cannot occur anymore for new molecules reaching the gold surface. It appears therefore that the preferential adsorption of the Xbonded nanoarchitectures on the Au(111) surface is propelling the initially formed covalent-structures on top of the X-bonded nanoarchitectures above 1 ML deposition. Figure 3 obviously shows that the packing density of the covalent nanoarchitecture is largely lower than the one of the X-bonded nanoarchitectures, Figure 4. The growth of the most compact structures is usually favored on surfaces because these structures not only favor intermolecular interactions but they also lower the surface free-energy of the sample.54 The structure density is 1 mol/1.99 nm2 for the sine-wave network, 1 mol/1.81 nm2 for the bow-tie network, 1 mol/1.56 nm2 for G


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type-II halogen bonds, whereas the bow-tie and dimer structures appear to be stabilized by type-II halogen bonds only, according to ref 55.

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CONCLUSION To summarize, the on-surface synthesis of covalent nanoarchitectures and the self-assembly of star-shaped 1,3,5-Tris(4iodophenyl)benzene molecules was investigated using scanning tunneling microscopy. STM shows that at low coverage, covalent polygonal nanostructures appear at the Au(111) step edges and at the elbows of the Au(111) surface reconstruction. The iodine atoms generated by molecule dehalogenation diffuse on the surface and are then adsorbed at the surface step edges. At high coverage two-dimensional halogen-bonded nanoarchitectures are preferentially growing on the gold surface. These structures are pushing away the covalent nanoarchitectures at their domain boundaries. Above one monolayer deposition the whole Au(111) surface is covered with an organic layer and the covalent structures are propelled on top of the halogen-bonded organic layer. These observations open up new opportunities for decoupling covalent nanoarchitectures from catalytically active and metal surfaces in vacuum. The electronic decoupling induced by the halogen bonded structures localized between the covalent structures and the surface could be investigated, for example, using low temperature scanning tunneling spectroscopy. EXPERIMENTAL SECTION Experiments were performed in an ultrahigh vacuum (UHV) chamber at a pressure of 10−8 Pa. The Au(111) surface was sputtered with Ar+ ions and then annealed in UHV at 600 °C for 1 h. 1,3,5-Tris(4iodophenyl)benzene molecules (90%, Aldrich), Figure 1a, were evaporated at 180 °C and deposited on the gold surface. Cut Pt/Ir tips were used to obtain constant current STM images at room temperature with a bias voltage applied to the sample. STM images were processed and analyzed using the homemade FabViewer application.56

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01938. The distribution of covalent polygonal nanoarchitecture versus halogen-bonded arrangements on Au(111) for different coverages, the height of the covalent and halogen-bonded molecular structures in the first organic layer, STM images of the halogen-bonded organic structures at high temperatures. (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: +33(0)169088019. Fax: +33(0)169088446. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 259297. The authors thank F. Merlet for precious technical support. H


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On-Surface Synthesis of Two-Dimensional Covalent Organic

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