It has been shown recently, that organic nanofibers grown from para-hexaphenyl and from α-sexithiophene molecules can be used as a new type of nanoscopic waveguides. Their growth is due to a self-assembly process, thus large quantities of aligned nanofibers can be fabricated simultaneously. Because of the growth mechanism of the nanofibers, their widths and heights are limited to a few 100 nm and a few 10 nm, respectively. In this paper we show how this kind of control has been obtained via modification of the bare muscovite surface before organic molecule deposition. Introducing e.g. a thin layer of Au islands before nanofiber growth results in an up to 15-fold increase in height, whereas the mean width and the optical properties of the fibers remain almost unchanged. Au films of varying thickness lead to tailor-made height profiles along the fiber. Using atomic force microscopy the details of these Au/organic heterostructures are examined and the growth is compared to growth on untreated mica. By scratching the fibers with an AFM tip grating structures have been written into the fibers.
Single crystalline organic nanoaggregates from organic semiconductors such as para-hexaphenyl and sexithiophene might become building blocks for a new type of organic electronic and optoelectronic devices. For the performance of such devices detailed knowledge about the mechanisms responsible for formation and for alignment of the aggregates on the growth substrate is important. On muscovite mica long, mutually parallel fibers of para-hexaphenyl grow, whereas on alkali halides mainly two different orientations, on phlogopite mica three different orientations exist. For sexithiophene on muscovite mica depending on the growth temperature either three equivalent aggregate orientations exist, or a single one dominates. The interplay between epitaxy and dipole assisted alignment on different growth substrates favors either unidirectional or multidirectional growth.
The growth of nanoscopic oligophenylene and oligothiophene aggregates on muscovite mica by vacuum deposition has been investigated. In the case of para-phenylenes a dipole assisted self assembly generation of needle-like aggregates is observed on mica. At optimum fiber growth temperature phenylene aggregates grow in most cases without a layer of upright oriented molecules. In contrast, vacuum deposition of oligothiophenes results simultaneously in fibers of laying molecules as well as islands of upright molecules. Since both phenylenes and thiophenes are strongly polarizable but differ in the lattice parameters of the resulting crystalline overlayers a direct comparison between the two classes of molecules allows us to study the role of epitaxy on the growth of nanoaggregates. Besides straight
aggregates we also observe thiophene rings on water and methanol treated mica surfaces, which consist of radially oriented, laying molecules.