successfully applied to the growth of AGNRs 11-13 and related structures [14][15][16] . Here, we describe the successful bottom-up synthesis of ZGNRs, which are fabricated by the surface-assisted colligation and cyclodehydrogenation of specifically designed precursor monomers including carbon groups that yield atomically precise zigzag edges. Using scanning tunnelling spectroscopy we prove the existence of edge-localized states with large energy splittings. We expect that the availability of ZGNRs will finally allow the characterization of their predicted spin-related properties such as spin confinement 17 and filtering 18,19 , and ultimately add the spin degree of freedom to graphene-based circuitry.To explore the fundamental electronic and magnetic properties related to zigzag edges and to realize specific carbon nanostructures for the controlled manipulation of their spin states,ZGNRs with atomically precise edges are required. For GNRs with armchair edges, it was demonstrated that atomic precision can indeed be achieved by a bottom-up approach based on the surface-assisted polymerization and subsequent cyclodehydrogenation of specifically designed oligophenylene precursor monomers 11 . The on-surface synthesis has been applied by many groups to fabricate a number of different AGNR structures [11][12][13] , N-doped AGNRs 14,15 as well as AGNR heterostructures 15,16 . It is, however, not directly suited forZGNRs since polymerization of monomers via aryl-aryl coupling does not take place along the zigzag but along the armchair direction (Fig. 1a). In addition, dehydrogenative cyclization of phenyl subgroups is not sufficient to form pure zigzag edges, thus calling for a totally new chemical design. Thereby, additional carbon functions must be placed at the edges of the monomers to complete the tiling toolbox needed for the bottom-up fabrication of arbitrary GNR structures.Here, we report a bottom-up fabrication approach to ZGNRs. In our unique protocol, surfaceassisted polymerization and subsequent cyclization of suitably designed molecular precursors carrying the full structural information of the final ZGNR afford atomic precision with respect to ribbon width and edge morphology. The groundbreaking idea depends upon the choice of a unique U-shaped monomer as 1 shown in Fig. 1b. With two halogen functions for thermally induced aryl-aryl-coupling at the R 1 positions, it allows the polymerization toward a snake-like polymer. It is the beauty of this design that additional phenyl groups at the R 2 position fill the holes in the interior of the undulating polymer. The crucial precursor is monomer 1a which carries two additional methyl groups. In such a case, apart from the 3 polymerization and planarization, an oxidative ring closure including the methyl groups is expected which would then establish two new six-membered rings together with the zigzag edge structure. To our delight, this concept could indeed be synthetically realized under reaction monitoring and structure proof by scanning tunneling microscopy (S...

(K.A.). elusive. The use of template molecules to unambiguously dictate the diameter and chirality of the resulting nanotube 8,13-16 holds great promise in this regard, but has hitherto had only limited practical success 7,17,18 . Here we show that this bottom-up strategy can produce targeted nanotubes: we convert molecular precursors into ultrashort singly capped (6,6) 'armchair' nanotube seeds using surface-catalysed cyclodehydrogenation on a Pt(111) surface, and then elongate these during a subsequent growth phase to produce single-chirality and essentially defect-free SWCNTs with lengths up to a few hundred nanometres. We expect that our onsurface synthesis approach will provide a route to nanotube-based materials with highly Fig. 1) was designed and synthesized by multi-step organic synthesis to tackle this challenge (for details, see Methods). Upon intramolecular CDH it affords seed S1, an ultra-short singly capped (6,6) SWCNT bearing a carbon nanotube segment. The selective growth of (6,6) SWCNTs is illustrated in Fig. 1 and combines two steps: (1) formation of seed S1, and (2) subsequent epitaxial elongation. The first step is realized by depositing precursor P1 on a Pt (111) surface followed by annealing to 770 K under ultrahigh vacuum conditions to induce the surfacecatalysed CDH reaction ( Fig. 2a, b). The second step, epitaxial elongation, is achieved by the incorporation of carbon atoms originating from the surface-catalysed decomposition of a carbon feedstock gas ( Fig. 3a-c). indicates that the different topographic features observed for the adsorbed precursors can be attributed to the different adsorption geometries. Importantly, the stereoisomerism does not affect the CDH process, since all chiral centres will disappear during intramolecular cyclization. Over the last two decades, single-walled carbon nanotubes (SWCNTsAlthough P1 is designed to yield seed S1, the conformational flexibility of the peripheral biphenyl groups leads partially to undesired adsorption geometries. In contrast to the stereoisomers discussed above, these molecules will follow a different CDH pathway, ending in the formation of undesired buckybowls (Extended Data Fig. 2). A statistical analysis of more than 100 precursor monomers observed by STM revealed that more than 50% adopt the desired configurations (Extended Data Fig. 1). Most importantly, the condensation products of precursor molecules exhibiting 'wrong' conformations cannot act as seeds for the subsequent CNT growth process via epitaxial elongation, and thus will not affect the selectivity of SWCNT formation.Surface-catalysed CDH of precursors (P1) into seeds (S1) is induced by annealing at 770 K for 10 min. STM images (Fig. 2d) show that the originally quasi-planar three-fold symmetric molecules transform into dome-shaped species with a prominent increase in apparent height from 2 to 4.5 Å (Fig. 2f). Additional proof of successful dehydrogenation of P1 into S1 derives from the good agreement of high-resolution STM images and simulations of the frontier molecu...

On-surface synthesis is a powerful route toward the fabrication of specific graphene-like nanostructures confined in two dimensions. This strategy has been successfully applied to the growth of graphene nanoribbons of diverse width and edge morphology. Here, we investigate the mechanisms driving the growth of 9-atom wide armchair graphene nanoribbons by using scanning tunneling microscopy, fast X-ray photoelectron spectroscopy, and temperature-programmed desorption techniques. Particular attention is given to the role of halogen functionalization (Br or I) of the molecular precursors. We show that the use of iodine-containing monomers fosters the growth of longer graphene nanoribbons (average length of 45 nm) due to a larger separation of the polymerization and cyclodehydrogenation temperatures. Detailed insight into the growth process is obtained by analysis of kinetic curves extracted from the fast X-ray photoelectron spectroscopy data. Our study provides fundamental details of relevance to the production of future electronic devices and highlights the importance of not only the rational design of molecular precursors but also the most suitable reaction pathways to achieve the desired final structures.

We report on the atomic structure of graphene nanoribbons (GNRs) formed via on-surface synthesis from 10,10'-dibromo-9,9'-bianthryl (DBBA) precursors on Cu(111). By means of ultrahigh vacuum noncontact atomic force microscopy with CO-functionalized tips we unveil the chiral nature of the so-formed GNRs, a structure that has been under considerable debate. Furthermore, we prove that-in this particular case-the coupling selectivity usually introduced by halogen substitution is overruled by the structural and catalytic properties of the substrate. Specifically, we show that identical chiral GNRs are obtained from 9,9'-bianthryl, the unsubstituted sister molecule of DBBA.

Sharpening of optical spectra caused by commensurate growth of an organic adlayer on salt single crystals is reported. The structure is elucidated by atomic force microscopy and advanced potential energy calculations. Continued deposition or annealing induces a rearrangement of the molecular monolayer into 3D crystallites, demonstrating the crucial role of the Coulomb interaction with the substrate to form the unexpected commensurate structure.