Covalent Molecular Architectures and Dithienylethene Switches on Metal Surfaces: a Scanning Tunneling Microscopy Study

The subject of this work is the study of molecular structures on gold surfaces at the single-molecule level with scanning tunneling microscopy (STM). The focus lies on one- and two-dimensional covalently coupled structures. A method for their formation has been devised only recently, therefore their properties as well as the coupling process are in need of investigation. These networks possess promising characteristics, among them the stability required for inclusion into a device. One-dimensional oligo-terfluorene chains can serve as a prototype for molecular wires, whose most important characteristic is their electric conductance. To facilitate its measurement, a novel configuration is realized, in which individual chains are partly decoupled from the gold surface in a horizontal geometry. This is facilitated by the introduction of thin layers of insulating material, i.e. sodium chloride. It is shown that the polymerization of the terfluorenes can be performed in the presence of the NaCl. On the other hand, the crystalline growth of the insulating islands can be continued next to the polymer chains. The decoupled configuration is realized by manipulation with the STM tip and on a preparative route. Tunneling spectroscopy along individual chains reveals their partial decoupling, i.e. that of the segments that are adsorbed on top of the insulator, whereas the rest of the same chain shows the spectrum typically observed on the gold surface. Furthermore, to increase the control of the coupling process a novel approach was implemented that facilitates a hierarchical polymerization procedure. To that end, porphyrin building blocks equipped with different substituents were employed to realize the stepwise supply of reactive sites. The entire process was monitored by variable-temperature STM from the intact monomers, via chain intermediates, to the final 2D network structures. The prearrangement of the chain intermediates by a self-templating effect on Au(111) and by the interaction with the corrugated Au(100) surface were found to lead to enhanced network regularity, which is a major challenge for covalent coupling due to its non-reversibility. The process was furthermore employed for the formation of copolymers made of porphyrin and terfluorene building blocks. Due to the controlled provision of reactive sites, the resulting network structures are characterized by a high degree of selectivity. Molecular switches offer the exciting prospect to control the flow of electrical signals through a network. Dithienylethenes (DTE) are promising candidates as conductance switches, because the ring-opening/-closing isomerization has a strong influence on the HOMO-LUMO gap of these molecules. In this study, different derivatives of DTE have been investigated on Au(111) as monomeric units. Subsequently, coupling into both homo- and also co-polymeric structures was performed, which constitutes the first demonstration of the inclusion of such functional units in a covalent structure at a surface. By means of tunneling spectroscopy the state of the units could be unambiguously determined. Thus, is was found that the molecules are in the ring-open form upon evaporation. It was shown that reversible isomerization of the DTE units inside the chains can be induced by application of bias pulses from the STM tip. Finally, the chains are used to form single-molecule junctions between STM tip and sample to perform conductance measurements. These indicate that switching is possible in this configuration and corroborate the expected dependence of the conductance on the state of the switch

Nanorings and rods interconnected by self-assembly mimicking an artificial network of neurons

[EN] Molecular electronics based on structures ordered as neural networks emerges as the next evolutionary milestone in the construction of nanodevices with unprecedented applications. However, the straightforward formation of geometrically defined and interconnected nanostructures is crucial for the production of electronic circuitry nanoequivalents. Here we report on the molecularly fine-tuned self-assembly of tetrakis-Schiff base compounds into nanosized rings interconnected by unusually large nanorods providing a set of connections that mimic a biological network of neurons. The networks are produced through self-assembly resulting from the molecular conformation and noncovalent intermolecular interactions. These features can be easily generated on flat surfaces and in a polymeric matrix by casting from solution under ambient conditions. The structures can be used to guide the position of electron-transporting agents such as carbon nanotubes on a surface or in a polymer matrix to create electrically conducting networks that can find direct use in constructing nanoelectronic circuits.The research leading to these results has received funding from ICIQ, ICREA, the Spanish Ministerio de Economia y Competitividad (MINECO) through project CTQ2011-27385 and the European Community Seventh Framework Program (FP7-PEOPLE-ITN-2008, CONTACT consortium) under grant agreement number 238363. We acknowledge E. C. Escudero-Adan, M. Martinez-Belmonte and E. Martin from the X-ray department of ICIQ for crystallographic analysis, and M. Moncusi, N. Argany, R. Marimon, M. Stefanova and L. Vojkuvka from the Servei de Recursos Cientifics i Tecnics from Universitat Rovira i Virgili (Tarragona, Spain).Escarcega-Bobadilla, MV.; Zelada-Guillen, GA.; Pyrlin, SV.; Wegrzyn, M.; Ramos, MMD.; Giménez Torres, E.; Stewart, A.... (2013). 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Unveiling thermal transitions of polymers in subnanometre pores

The thermal transitions of confined polymers are important for the application of polymers in molecular scale devices and advanced nanotechnology. However, thermal transitions of ultrathin polymer assemblies confined in subnanometre spaces are poorly understood. In this study, we show that incorporation of polyethylene glycol (PEG) into nanochannels of porous coordination polymers (PCPs) enabled observation of thermal transitions of the chain assemblies by differential scanning calorimetry. The pore size and surface functionality of PCPs can be tailored to study the transition behaviour of confined polymers. The transition temperature of PEG in PCPs was determined by manipulating the pore size and the pore–polymer interactions. It is also striking that the transition temperature of the confined PEG decreased as the molecular weight of PEG increased

Manipulating the Conformation of Single Organometallic Chains on Au(111)

The conformations of organometallic polymers formed via the bottom-up assembly of monomer units on a metal surface are investigated, and the relationship between the adsorption geometry of the individual monomer units, the conformational structure of the chain, and the overall shape of the polymer is explored. Iodine-functionalized monomer units deposited on a Au(111) substrate are found to form linear chain structures in which each monomer is linked to its neighbors via a Au adatom. Lateral manipulation of the linear chains using a scanning tunneling microscope allows the structure of the chain to be converted from a linear to a curved geometry, and it is shown that a transformation of the overall shape of the chain is coupled to a conformational rearrangement of the chain structure as well as a change in the adsorption geometry of the monomer units within the chain. The observed conformational structure of the curved chain is well-ordered and distinct from that of the linear chains. The structures of both the linear and curved chains are investigated by a combination of scanning tunneling microscopy measurements and theoretical calculations

Micrometre-long covalent organic fibres by photoinitiated chain-growth radical polymerization on an alkali-halide surface

On-surface polymerization is a promising technique to prepare organic functional nanomaterials that are challenging to synthesize in solution, but it is typically used on metal substrates, which play a catalytic role. Previous examples on insulating surfaces have involved intermediate self-assembled structures, which face high barriers to diffusion, or annealing to higher temperatures, which generally causes rapid dewetting and desorption of the monomers. Here we report the photoinitiated radical polymerization, initiated from a two-dimensional gas phase, of a dimaleimide monomer on an insulating KCl surface. Polymer fibres up to 1 μm long are formed through chain-like rather than step-like growth. Interactions between potassium cations and the dimaleimide’s oxygen atoms facilitate the propagation of the polymer fibres along a preferred axis of the substrate over long distances. Density functional theory calculations, non-contact atomic force microscopy imaging and manipulations at room temperature were used to explore the initiation and propagation processes, as well as the structure and stability of the resulting one-dimensional polymer fibres

Nanophysik: Strommessung an einzelnen molekularen Drähten

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

Conductance of a single conjugated polymer as a continuous function of its length

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