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キーワード:
STM; polymerization; molecular conductance; molecular switch; molecular junction; dithienylethene; hierarchical growth
要旨:
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.
