Nanostructuring in 2D and 3D: from the structure and dynamics of single (macro)molecules to supramolecular architectures

Paolo Samorí¹, Viola Francke², Klaus Müllen² and Jürgen P. Rabe¹ ¹Department of Physics, Humboldt University Berlin, Invalidenstr. 110, 10115 Berlin, Germany ² MPI for Polymer Research, Ackermannweg 10, 55021 Mainz, Germany The growth of p-conjugated polymers on insulating solid substrates represent a strategy for building well defined and stable nanostructures with chemical functionalities and physical properties, which are of potential use for active components in electronic devices. The transport phenomena in these molecular devices depends on the interplay between electronic structure and order in the molecular assembly. An important topic is therefore to control intramolecular, intermolecular and interfacial forces in order to design highly ordered 2D and 3D polymolecular architectures.We report on the self-assembly of soluble alkylated oligomeric and polymeric para-phenyleneethynylenes in monolayers and we demonstrate how their growth from solution can be controlled in order to produce well defined micro- and nano-scopic architectures.

Chemical formulae: Trimer 1 and polymer 2 of para-(dihexyl-phenyleneethynylene) both with end-thiol functionalities capped by carbamoyl groups. Scanning Tunneling Microscopy (STM) investigation at the interface between an almost saturated solution and a solid substrate (HOPG) enabled us to characterize both the structure and the dynamics of these systems. Phenyleneethynylene trimers pack in an oriented 2D polycrystalline structure (Fig. 2a). 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 enabled us to gain insight into the kinetics of this process and to draw some preliminary 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 (Fig. 1a). The corresponding polydisperse system has been viewed for the first time with a sub-molecular resolution. These macromolecules exhibit a nematic-like molecular order at the interface with HOPG (Fig 1b). 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 very precise macromolecular fractionation at the interface with the solid substrate: only macromolecules with a rod lengths around the peak of the distribution of molecular weights are crystallised on HOPG.

Fig 1: STM constant current image of (a) trimer 1 and (b) polymer 2 at the interface between an organic solution and the basal plane of HOPG. The trimer in (a) shows an epitaxial polycrystalline structure.

The arrow indicates a defect within a single crystal as two missing molecules. The 7.9 nm long polymer in (b) self-assembles into a nematic texture. 2D-Fast Fourier Transform indicates how the order at the molecular level is induced by the symmetry of the (001) surface of HOPG.On the other hand, dried macromolecular films of PPE prepared by solution casting have been 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 us to understand and drive the growth of these architectures towards expitaxially oriented micrometer long nanoribbons [1] (Fig. 2). These nanostructures are typically two monolayers thick with their alkyl chains oriented perpendicular to the substrate. The distribution of ribbon widths is in good agreement with the molecular weights 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 alternative valuable route to determine molecular weight distributions for a rigid rod polymer [2]. These nanoribbons are molecular architectures which upon thiol functionalization at their edges are nanostructures ready to bridge Au nanoelectrodes in a molecular nanowire device.

Fig. 2: Tapping Mode SFM image of macromolecular nanoribbons of 2 on mica

References

[1] P. Samorí, V. Francke, T. Mangel, K. Müllen, J.P. Rabe Optical Materials 1998, 9, 390.
[2] P. Samorí, V. Francke, K. Müllen, J.P. Rabe submitted for publication 1998.