Search

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces.

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces., This work was financially supported by the European Research Council Consolidator Grant NanoSurfs (no. 615233), the Advanced Grant (no. 694097), the Horizon 2020 research and innovation program 2D ink (no. 664878) and the National Science Foundation of China (no. 11974403 and Sino-German Project no. 51761135130). W.A. acknowledges funding by the DFG via a Heisenberg professorship. M.R., R.B., and X.F. thank the German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden (cfaed)” and EnhanceNano (No. 391979941). A.P.P. and A.Ru. thank the Cluster of Excellence "Advanced Imaging of Matter (AIM)" and Grupos Consolidados (IT1249-19). M.G. acknowledges funding by the H2020-MSCA-IF−2014 program under GA no. 658070 (2DNano).

The templated synthesis of porphyrin-based oligomers and heterosystems is of considerable interest for materials with tunable electronic gaps, photovoltaics, or sensing device elements. In this work, temperature-induced dehydrogenative coupling between unsubstituted free-base porphine units and their attachment to graphene nanoribbons on a well-defined Ag(111) support are scrutinized by bond-resolved scanning probe microscopy techniques. The detailed inspection of covalently fused porphine dimers obtained by in vacuo on-surface synthesis clearly reveals atomistic details of coupling motifs, whereby also putative reaction intermediates are identified. Moreover, the covalent attachment of porphines at preferred locations of atomically precise armchair-type graphene nanoribbons is demonstrated.

Surface-assisted covalent linking of precursor molecules enables the fabrication of low-dimensional nanostructures, which include graphene nanoribbons. One approach to building functional multicomponent systems involves the lateral anchoring of organic heteromolecules to graphene. Here we demonstrate the dehydrogenative coupling of single porphines to graphene edges on the same metal substrate as used for graphene synthesis. The covalent linkages are visualized by scanning probe techniques with submolecular resolution, which directly reveals bonding motifs and electronic features. Distinct configurations are identified that can be steered towards entities predominantly fused to graphene edges through two pyrrole rings by thermal annealing. Furthermore, we succeeded in the concomitant metallation of the macrocycle with substrate atoms and the axial ligation of adducts. Such processes combined with graphene-nanostructure synthesis has the potential to create complex materials systems with tunable functionalities.

Nanographenes, namely polycyclic aromatic hydrocarbons (PAHs) with nanoscale dimensions (>1 nm), are atomically precise cutouts from graphene. They represent prime models to enhance the scope of chemical and physical properties of graphene through structural modulation and functionalization. Defined nitrogen doping in nanographenes is particularly attractive due to its potential for increasing the number of π-electrons, with the possibility of introducing localized antiaromatic ring elements. Herein we present azomethine ylide homocoupling as a strategy to afford internally nitrogen-doped, non-planar PAH in solution and planar nanographene on surfaces, with central pyrazine rings. Localized antiaromaticity of the central ring is indicated by optical absorption spectroscopy in conjunction with theoretical calculations. Our strategy opens up methods for chemically tailoring graphene and nanographenes, modified by antiaromatic dopants., Polyaromatic hydrocarbons can be precisely manipulated to yield ever more complex and discrete graphene analogs, such as nanographenes. Here, the authors use azomethine ylide homocoupling to insert an antiaromatic pyrazine ring into the core of a nanographene, and characterize the molecule’s unique electronic character.

The engineering of nanoarchitectures to achieve tailored properties relevant for macroscopic devices is a key motivation of organometallic surface science. To this end, understanding the role of molecular functionalities in structure formation and adatom coordination is of great importance. In this study, the differences in formation of Cu-mediated metal–organic coordination networks based on two pyridyl- and cyano-bearing free-base porphyrins on Ag(111) are elucidated by use of low-temperature scanning tunneling microscopy (STM). Distinct coordination networks evolve via different pathways upon codeposition of Cu adatoms. The cyano-terminated module directly forms 2D porous networks featuring fourfold-coordinated Cu nodes. By contrast, the pyridyl species engage in twofold coordination with Cu and a fully reticulated 2D network featuring a pore size exceeding 3 nm2 only evolves via an intermediate structure based on 1D coordination chains. The STM data and complementary Monte Carlo simulations reveal that these distinct network architectures originate from spatial constraints at the coordination centers. Cu adatoms are also shown to form two- and fourfold monoatomic coordination nodes with monotopic nitrogen-terminated linkers on the very same metal substrate—a versatility that is not achieved by other 3d transition metal centers but consistent with 3D coordination chemistry. This study discloses how specific molecular functionalities can be applied to tailor coordination architectures and highlights the potential of Cu as coordination center in such low-dimensional structures on surfaces.

We have examined the geometric and electronic structures of iron phthalocyanine assemblies on a Cu(111) surface at different sub- to mono-layer coverages and the changes induced by thermal annealing at temperatures between 250 and 320 °C by scanning tunneling microscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. The symmetry breaking observed in scanning tunneling microscopy images is found to be coverage dependent and to persist upon annealing. Further, we find that annealing to temperatures between 300 and 320 °C leads to both desorption of iron phthalocyanine molecules from the surface and their agglomeration. We see clear evidence of temperature-induced homocoupling reactions of the iron phthalocyanine molecules following dehydrogenation of their isoindole rings, similar to what has been observed for related tetrapyrroles on transition metal surfaces. Finally, spectroscopy indicates a modified substrate-adsorbate interaction upon annealing with a shortened bond distance. This finding could potentially explain a changed reactivity of Cu-supported iron phthalocyanine in comparison to that of the pristine compound.

Silver opens up interesting perspectives in the fabrication of complex systems based on heteroepitaxial layers after the growth of a silicene layer on its (111) face has been proposed. In this work we explore different synthesis methods of hexagonal boron nitride (h-BN) and graphene sheets on silver. The resulting layers have been examined by high-resolution scanning tunneling microscopy. A comparison of the interfacial electronic band structure upon growth of the distinct two-dimensional (2D) layers has been performed by scanning tunneling spectroscopy and complementary first-principle calculations. We demonstrate that the adsorption of the 2D layers has an effect on the binding energy of the Shockley state and the surface potential by lowering the local work function. These effects are larger in the case of graphene where the surface state of Ag(111) is depopulated due to charge transfer to the graphene. Furthermore, we show that the electronic properties of the h-BN/silver system can be tuned by employing different thicknesses of silver ranging from a few monolayers on Cu(111) to the single crystal Ag substrate.