Samorí , Paolo : Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics

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Kapitel 7. Conclusions

Chemisorption and physisorption have been used alternatively to design reproducible (macro)molecular architectures from pi-conjugated systems. In the first case Self-Assembled Monolayers of thiol-end functionalized alkanes and alkenes have been grown both on Au and on Ag surfaces. The role of the substrate in the self-assembly has been discussed; for this purpose a novel ultra flat Au surface (Template Stripped Gold) has been developed. Different thicknesses of the organic layer (length of the alkyl chain) and different compositions of the adsorbate (saturated or unsaturated chain) have shown distinct electronic properties of the molecular adlayer.

In the latter case, making use of intramolecular, intermolecular and interfacial forces highly ordered 2D and 3D polymolecular micro- and nano-scopic architectures have been produced.

Scanning Tunneling Microscopy (STM) investigations at the interface between an almost saturated solution and a solid substrate (HOPG) allowed to characterize both the structure and the dynamics of these systems. Phenyleneethynylene trimers pack in an oriented 2D polycrystalline structure. The dynamics of the single molecular nanorods on a several minutes time scale has been recorded. This Ostwald ripening phenomenon is driven by a minimization of the line energies. Such a high resolution imaging allowed to gain insight into the kinetics of this process and to draw conclusions on thermodynamic and kinetic contributions to the total energy governing this grain coarsening. In addition defects within epitaxial crystals like missing molecules have been monitored. The corresponding polydisperse system is the first polymeric system which has been viewed with a sub-molecular resolution allowing to determine phase segregations in polydisperse polymers on the molecular scale. These macromolecules exhibit a nematic-like molecular order at the interface with HOPG. Single rods are oriented along preferential directions according to the threefold symmetry of the substrate. The true molecular lengths for several hundreds of molecules have been determined from STM images. The key result is a narrow macromolecular fractionation at the interface with the solid substrate: only macromolecules with a rod length around the peak of the distribution of molecular weights segregate at the interface with HOPG.

On the other hand, dried macromolecular films of PPE prepared by solution casting have been


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studied with Scanning Force Microscopy (SFM) in Tapping Mode. Varying several parameters during the self-assembly, like the substrate, the solvent, the concentration of the solution and the average length of the macromolecule along the conjugated backbone, allowed to understand and drive the growth of these architectures towards epitaxially oriented micrometer long nanoribbons. These nanostructures are typically two to three monolayers thick with their alkyl chains oriented perpendicular to the substrate. The distribution of ribbon widths is in good agreement with the molecular weight distribution according to the Schulz-Zimm distribution, taking into account a broadening effect due to the SFM tip. This result indicates that SFM offers an valuable alternative route to determine molecular weight distributions for a rigid rod polymer. These nanoribbons are molecular architectures which upon thiol functionalization at their edges are nanostructures ready to bridge Au nanoelectrodes in a molecular nanowire device. Results on the electric properties of PPE molecular aggregates between the two Au nanoelectrodes have been presented. Moreover the electronic structures of phenyleneethynylene derivatives both for the case of a pristine and n-doped thin films have been investigated by means of photoelectron spectroscopies corroborated by theoretical calculations.

Furthermore highly ordered layer architectures of hexakis-dodecyl-hexabenzocoronene have been grown from solutions. Complementary insight into the molecular order in organic dry thin films have been gained with Tapping Mode - Scanning Force Microscopy and angle-resolved photoemission measurements. It is found that this disc-like molecule on a conductive solid flat substrate like highly oriented pyrolitic graphite (HOPG) can self-assemble into monolayers with the pi-conjugated disc like system lying preferentially parallel to the basal plane of the substrate. Varying the procedures of the film preparation from solution it was possible to tune the rate of the molecular self-assembly. At very slow deposition speeds it was possible to produce layers aligned preferentially along the crystallographic axes of the HOPG substrate. This suggests that the growth of this molecular system is a kinetically governed phenomenon which on the crystalline support in equilibrium leads to a hetero-epitaxial type of growth.


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