Directed Coassembly of Oriented PbS Nanoparticles and Monocrystalline Sheets of Alkylamine Surfactant

We demonstrate control over the orientation of PbS nanoparticles by way of directed assembly, which in turn affects the crystal structure of alkylamine surfactants such as octadecylamine (ODA, C₁₈H₃₇NH₂) and hexadecylamine (HDA, C₁₆H₃₃NH₂). This directed assembly method results in the arrangement of PbS nanoparticles with a well-defined epitaxial orientation on lamellar alkylamine sheets, which undertake a new crystal structure to accommodate these relations. Understanding these surfactant–nanoparticle inter-relations is very instrumental in understanding surfactant-assisted nanoparticle synthesis and assembly

Origin of the Contact Angle Hysteresis of Water on Chemisorbed and Physisorbed Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or “ionic” bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including “dynamic” contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant–solid surface interactions, the organization of surfactant-coated nanoparticles, etc