Self-Assembly by Design: From Structure and Function to Nanoscale Construction
Open Access
- Author:
- Hohman , James Nathan
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 10, 2011
- Committee Members:
- Paul S Weiss, Dissertation Advisor/Co-Advisor
Paul S Weiss, Committee Chair/Co-Chair
David Lawrence Allara, Committee Chair/Co-Chair
Thomas E Mallouk, Committee Member
Jun Huang, Committee Member - Keywords:
- self-assembly
nanoscience
nanotechnology
self-assembled monolayer
nanoparticle
supramolecular chemistry
chemical patterning
microcontact printing
soft lithography - Abstract:
- This thesis is devoted to the discovery and application of fundamental design principles for molecular self-assembly. Self-assembled monolayer (SAMs) of organic thiols on Au{111} are examined, with an emphasis on structure/function relationships, particularly those related to the interplay between molecular geometry and intermolecular forces and how this translates to the structure and properties of supramolecular assemblies. The quadripartite SAM is composed of a substrate support, a 'head group' functionality that binds molecules to the substrate, a backbone that self-organizes with neighboring molecules, and a 'tail group' that dictates the chemistry of the interface. Each of these components is altered in turn and exploited to test fundamental assumptions and to construct new classes of materials. The tail group is first replaced with a bulky cage molecule, the adamantane diamondoid, which by design simplifies the supramolecular assembly. The spherical cages generate distinct two-dimensional (2D) structures that determine the contribution of geometric factors to the structure and properties of an assembly. Cage molecules of similar size, based on carborane frameworks, are examined in detail. The different orientations of carboranethiol isomers' molecular dipole moments influence the properties of their assemblies, including the stability of SAMs and the coverage of carboranethiolates in competitive binding conditions. Building on the adamantane framework, the head group attachment atom is changed from sulfur to selenium, resulting in a complex, dynamic double lattice of 1-adamantaneselenolate on Au{111}. There are two binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Our approach to produce SAMs on technologically important substrates takes advantage of the many strategies previously developed for gold-thiol self-assembly. Direct assembly of alkanethiols on germanium is impeded by the presence of the native germanium oxide. Depositing SAMs from a mixture of water and ethanol provides an environment where both adsorbate and oxide are soluble and single-step self-assembly is enabled. Drawing from our work on the analysis of 2D assemblies, a SAM was designed that deposits on and controls the morphology of liquid-phase eutectic gallium-indium (EGaIn) nanoparticles. Particle formation is directed by molecular self-assembly and assisted by sonication. As the bulk liquid alloy is ultrasonically dispersed, fast thiolate self assembly at the EGaIn interface protects the material against oxidation. The ligand shell has been designed to include intermolecular hydrogen bonds, which induce surface strain, and assists cleavage of the alloy particles to the nanoscale. Finally, the composition of a surface at the micro- and nanoscale is controlled by the chemical patterning of SAMs using microcontact printing. The dynamic molecular ink concentration at a polymer stamp/substrate interface during long-duration patterning experiments is emphasized. By exerting control over molecular flux, one can likewise control the molecular-scale order and the rate of molecular exchange of SAMs. These techniques are extended to germanium, and used to achieve the first report of the direct chemical patterning of oxide-free germanium by submerged microcontact printing.