DIRECTING NUCLEATION AND GROWTH OF MOLECULAR CRYSTALS ON ORDERED SUBSTRATES: THE ROLE OF EPITAXIAL INTERACTIONS

摘要

Much of the interest in molecular materials, whether in thin film or bulk form, stems from the ability to employ molecular-level "crystal engineering" strategies, which aim to rationally manipulate crystal packing and control physical and electronic properties. However, understanding the assembly of molecules, particularly on surfaces, during nucleation of tliese phases is imperative if the crystallization process is to be directed rationally. We describe herein strategies for directing the organization of molecular assemblies during nucleation and growth of thin films and bulk crystals of organic materials. The formation of these nuclei and their supramolecular structure depend upon a subtle balance of intermolecular forces within the nuclei and nuclei-substrate epitaxial interactions. In some cases, the formation of tliese nuclei have been observed directly, and their molecular level structure characterized, using scanning tunneling microscopy (STM) and in-situ real-time atomic force microscopy (AFM). Organic crystals can be used as substrates for directing nucleation of other organic crystalline materials by "ledge-directed epitaxy." The examples presented illustrate the influence of the substrate on nucleation rate, orientation of nuclei, monolayers, multilayers, and crystals with respect to the nucleating substrate, polymorphism, and quality.