Polarization and self-assembly at metal-organic interfaces: Models and molecular-level processes

This dissertation contributes to the understanding of driving forces of self-assembly processes leading to the growth and self assembly of metal nanostructures for nanoelectronic, sensor, and drug delivery applications. It remains difficult to image and characterize molecular-level processes by measurement alone so that we have employed molecular simulation to gain more insight into two aspects: (1) the contribution of induced charges in metal nanostructures to the interaction with biomolecules and ionic liquids, (2) and the mechanism of gold nanorod growth. Polarization at {111} and {100} surfaces of gold in contact with water and peptides of different charge state contributes to the binding strength and plays a role in the assembly of metal nanostructures. We computed the attractive energy using the concept of image charges and molecular dynamics simulation in all-atomic resolution. Attractive interactions amount to between -50 and -70 mJ/m2, corresponding to -0.6 kcal/mol per water molecule and variable amounts per amino acid in peptides depending on the charge state (0 to -10 kcal/mol). The main contribution to adsorption on {111} surfaces stems from soft epitaxial interactions and the net contribution of polarization to adsorption is smaller. On surfaces with poor epitaxial fit of the solute molecules such as {100} surfaces, the main contribution to adsorption can be made by polarization, particularly for charged peptides and ionic liquids. Therefore, the magnitude of interfacial polarization can modulate the binding strength of biomolecules or surfactants versus solvent to metal surfaces.;The mechanism of shape regulation in the growth of gold nanorods in ionic liquids (ILs) such as 1-Ethyl-3-methylimidazolium ethyl sulfate is initiated by silver underdeposition and modulated by induced surface charge which decreases in the order {110}, {100}, and {111} for gold surfaces. Attractive effect of induced charges is quantified based on our previous work on polarization at metal-biomolecular interfaces in solution. Higher surface corrugation increases the induced charge so that the side surfaces of gold nanorods synthesized in imidazolium based ionic liquids are formed by {110} and {100} faces, in which a higher contribution of induced charges by -2.3 kcal/mol-molecule compared to {111} surfaces accounts for the thermodynamic stability observed in TEM. In a nonionic solvent like water, surface energies of all three faces {110}, {100}, and {111} are about the same and lead to different surface bounds of gold nanorods in combination with capping agents. The adsorption energy of IL molecules on the gold surfaces changes as a function of the number of molecules per unit area depending on packing constraints and network strength between cations and anions. The adsorption energy per molecule saturates once flat-on molecular multilayers are formed. We suggest that the balance between induced charges, soft epitaxy, and silver underdeposition contributes to the anisotropic growth of gold nanorods in a given solvent.