(678b) Organic Semiconducting Nanostructures Obtained by Self-Assembling Processes: a Scanning Probe Microscopy Characterization

Well-defined conjugated materials play an important role in the growing field of organic electronics because their precise chemical structure and conjugation length give rise to well-defined properties and facilitate control over their supramolecular organization. The building of nanoscopic and mesoscopic architectures represents a starting point for the construction of (supra)molecular electronics or even circuits, through surface patterning with nanometer-sized objects. It clearly appears that the solid-state properties of organic electronic materials are determined not only by those of individual molecules but also by those of ensembles of molecules. The ability to control the supramolecular architectures is thus essential for optimizing the properties of conjugated materials for their use in ?supramolecular electronics?; this is primordial for technological applications in nanoelectronics. In this work, we report on the observation by atomic force microscopy (AFM) of 1D and 2D nanoscale architectures obtained in the solid-state from solutions of molecularly-dissolved conjugated materials (oligomers, polymers, and block copolymers), and demonstrate that they can organize onto a surface over lengthscales from nanometers to several microns, forming semiconducting nano-objects by pi-stacking processes. For instance, we established a clear correlation between the molecular structures, the mesoscopic structures, and the optoelectronic properties of thiophene-, phenylene-, phenylenevinylene-, and fluorene-based conjugated oligomers and polymers. We observe that the formation of unidimensional supramolecular organization (i.e., fibrils) is predominant only when the molecular interactions are stronger than the molecule/surface interactions; otherwise, 2D-morphologies (i.e., monolayers) dominate. It appears that the presence of bulky side chains, or chiral centers, or hydrogen bonding groups is drastically affecting the thin film morphology. During these processes, the interplay between the conjugated molecules, the solvent and the substrate surface is one key parameter governing the formation of the supramolecular assemblies. Moreover, molecular modeling calculations are essential for a better understanding on how the molecules are organized within these nanostructures and therefore rationalize the experimental data. Some examples will be presented. Finally, a novel approach using spatial deposition by soft-lithography-derived techniques is presented. In combination with scanning probe microscopy-derived techniques, it allows to create organized semi-conducting architectures. For this purpose, we propose here to use an AFM tip as a pencil to organize at the local scale the nanoobjects along a given axis. Field effect transistors (FET's) incorporating such semiconducting materials patterned between two electrodes show very promising results.