Dendrimers on substrates: Interfacial interactions, domain formation and physical intermixing.

摘要

Dendrimers are three-dimensional, globular, highly-branched macromolecules made up of a focal point surrounded by repetitive units all enclosed by a terminal group 'shell'. Both their chemistry and size (in the range about ∼1 - ∼10 nm) are precisely tailorable and some dendrimers are known to readily form monolayers on technologically useful substrates. For these reasons they are of interest both scientifically and practically in the context of nanostructure and ultra-thin film studies. This dissertation focuses on three interrelated issues: (1) interfacial interactions (the role of dendrimer monolayers in mediating the properties of subsequently deposit thin films), (2) submonolayer dendrimer domain formation, and, (3) physical intermixing between dendrimer domains and metal overlayers.; In the dendrimer mediation section, the effects of dendrimer monolayers on thin metal overlayers, subsequently deposited by physical vapor deposition, are studied focusing on tribomechanical behavior and molecule-metal reactions. Dendrimer monolayer mediation of Cu and Cr overlayer films is found to dramatically influence nanoscratch mechanisms in these systems. Physical intermixing of metal atoms with dendrimer molecules to form a nanocomposite with distinct properties (rather than chemical interaction, grain size effects, etc.) is proposed as the probable explanation for these observations. In the domain formation section, a simple aerosol spray process is developed and used to prepare thin dendrimer-containing liquid layers from which submonolayer dendrimer domains form during the evaporation of solvent. An understanding of the relevant physical interactions (e.g., dewetting, boundary energy, long-range repulsion, dendrimer-substrate and dendrimer-dendrimer bonding, domain thermal stability, etc.) which control the form and coverage-dependent evolution of experimentally observed domains is developed. The importance of electrostatic interaction revealed in this section motivated a study of charge patterning by scanning probe microscopy methods in granular materials (Appendix A). Finally, submonolayer dendrimer domain patterns with uniform monolayer thickness are used as heterogeneous substrates for the deposition of Au overlayers. Direct evidence is found for the extent and type of physical intermixing of non-reaction Au with dendrimer monolayer domains by atomic force microscopy based step-height and force measurements as a function of Au thicknesses.