On-surface synthesis of graphene nanoribbons with zigzag edge topology

Abstract

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...

Transport gap engineering by contact geometry in graphene nanoribbons: Experimental and theoretical studies on artificial materials

“…34 Zigzag ribbons are predicted to be metallic for all ribbon widths. First samples of small graphene nanoribbons have been synthesized recently [10][11][12][13][14][15][16][17] indicating the predicted behavior. Here, we present emulation experiments of the electronic transport through small nanoribbons with specific edges and atomically precise connections to source and drain leads.…”

“…The realization of such contact geometries may be extremely difficult in real graphene. However, considering the recent progress, [10][11][12][13][14][15][16][17] we think that this can be possible in the near future. Our microwave emulation experiments thus may be viewed as a testing ground for new concepts.…”

Ferromagnetism and Half‐Metallicity in a High‐Band‐Gap Hexagonal Boron Nitride System

“…[17] Recently,R uffieuxe tal. [18] synthesised atomically precise zigzag graphene nanoribbons and predicted that such graphenebased systems are convenient for use in spintronics applications. Like graphene, hexagonal boron nitride (h-BN) is an importantm aterial due to its stability,w ide band gap and multifunctional properties.…”

Synthesis and characterization of triangulene

“…Compound 2 was deposited on Cu(111), NaCl(100), and Xe(111) surfaces to generate triangulene (1) by means of atomic manipulation. STM/AFM is an ideal combination to study on-surface synthesis ranging from individual molecules 9,10 to graphene nanoribbons 11,12 . The chemical structure of reactants and products can be resolved by means of AFM with functionalized tips 13 .…”