The construction of electronic devices from single molecular building blocks, which possess certain functions such as switching or rectifying and are connected by atomic-scale wires on a supporting surface, is an essential goal of molecular electronics. A key challenge is the controlled assembly of molecules into desired architectures by strong, that is, covalent, intermolecular connections, enabling efficient electron transport between the molecules and providing high stability. However, no molecular networks on surfaces 'locked' by covalent interactions have been reported so far. Here, we show that such covalently bound molecular nanostructures can be formed on a gold surface upon thermal activation of porphyrin building blocks and their subsequent chemical reaction at predefined connection points. We demonstrate that the topology of these nanostructures can be precisely engineered by controlling the chemical structure of the building blocks. Our results represent a versatile route for future bottom-up construction of sophisticated electronic circuits and devices, based on individual functionalized molecules.

A key challenge in the field of nanotechnology, in particular in the design of molecular machines, novel materials or molecular electronics, is the bottom-up construction of covalently bound molecular architectures in a well-defined arrangement. To date, only rather simple structures have been obtained because of the limitation of one-step connection processes. Indeed, for the formation of sophisticated structures, step-by-step connection of molecules is required. Here, we present a strategy for the covalent connection of molecules in a hierarchical manner by the selective and sequential activation of specific sites, thereby generating species with a programmed reactivity. This approach leads to improved network quality and enables the fabrication of heterogeneous architectures with high selectivity. Furthermore, substrate-directed growth and a preferred orientation of the molecular nanostructures are achieved on an anisotropic surface. The demonstrated control over reactivity and diffusion during covalent bond formation constitutes a promising route towards the creation of sophisticated multi-component molecular nanostructures.

The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations as one molecular unit after another is detached from the surface during stretching.

Scanning tunneling microscopy and X-ray spectroscopy measurements are combined to first-principles simulations to investigate the formation of graphene nanoribbons (GNRs) on Au(110), as based on the surface-mediated reaction of 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules. At variance with Au(111), two different pathways are identified for the GNR self-assembly on Au(110), as controlled by both the adsorption temperature and the surface coverage of the DBBA molecular precursors. Room-temperature DBBA deposition on Au(110) leads to the same reaction steps obtained on Au(111), even though with lower activation temperatures. For DBBA deposition at 470 K, the cyclodehydrogenation of the precursors preceds their polymerization, and the GNR formation is fostered by increasing the surface coverage. While the initial stages of the reaction are found to crucially determine the final configuration and orientation of the GNRs, the molecular diffusion is found to limit in both cases the formation of high-density long-range ordered GNRs. Overall, the direct comparison between the Au(110) and Au(111) surfaces unveils the delicate interplay among the different factors driving the growth of GNRs

Totally wired: A particular molecular adsorption geometry can be prepared by adsorbing single conjugated polyfluorene chains partially on a clean Au(111) surface and partially on a thin crystalline NaCl film, thus connecting metallic and insulating surface areas. This configuration allows the electronic characterization of one and the same molecular wire as a function of its atomic-scale environment in a planar configuration

Verdrahtet: Eine spezielle molekulare Adsorptionskonfiguration lässt sich präparieren, indem einzelne konjugierte Polyfluorenketten teils auf einer Au(111)-Oberfläche und teils auf einem dünnen kristallinen NaCl-Film adsorbieren und so die metallischen und isolierenden Oberflächenbereiche verbinden. Diese Konfiguration ermöglicht die elektronische Charakterisierung eines Drahtes als Funktion seiner atomaren Umgebung in planarer Konfiguration

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Molekulare “Drähte” aus einem einzigen linearen Molekül besitzen ein enormes Potenzial für zukünftige Anwendungen in der Nanotechnologie. Dabei stellt der Ladungstransport ein entscheidendes Kriterium für ihre Verwendung in dieser molekularen Elektronik dar. Unserer Gruppe an der Freien Universität Berlin ist es erstmals gelungen, einen einzelnen molekularen Draht von einer Oberfläche hochzuziehen und den dabei durch den Draht fließenden Strom in Abhängigkeit von der Länge zu messen. Dies liefert ein tieferes Verständnis der elektrischen, aber auch der mechanischen Eigenschaften solcher molekularen Drähte.