Self Assembly of Anisotropic Organic Molecules: Diffusion versus Sticking Anisotropy

Abstract

The molecular/crystal orientation and morphology of active molecular structures is a key determinant for the function of nanoscaled organic devices. In π-conjugated systems, both charge transport and optical properties will strongly depend on the molecular orientation due to the highly anisotropic charge carrier mobility in these organic crystals and the anisotropic absorption and luminescence behavior of the molecules. Although the importance of organic on inorganic interface formation and thin film growth is widely acknowledged, little is known regarding the growth kinetics. A better understanding of the processes driving molecular self-assembly is necessary if the self-assembly process is to be controlled. Moreover, it is interesting as the anisotropy of the molecular building blocks presents a fundamental difference from what is known from inorganic growth. Here we show that either sticking or diffusion anisotropy can control the growth depending on preparation conditions. This is illustrated by an investigation into the growth of sexiphenyl (6P) on the anisotropic TiO₂(110)-(1×1) surface for temperatures between 80K and 400K using in-situ UHV photoemission, x-ray absorption spectroscopy, synchrotron x-ray diffraction and ex-situ atomic force microscopy. For 6P adsorption even at 80K we found that the molecules orient parallel to the TiO₂ oxygen rows and form small crystallites. At 300K this molecular orientation is retained and large micrometer sized 6P(203) oriented needles running perpendicular to oxygen substrate rows are formed. In contrast, for growth at elevated temperatures the 6P molecular axis is near perpendicular to the surface and large islands elongated parallel to the substrate rows are formed. These differences in crystallite orientation and morphology can be explained by the domination of the growth kinetics by either sticking or diffusion anisotropy depending on growth temperature.