Optical and electrical interactions in self-assembled metal nanoparticle superstructures

Abstract

Self-assembly of molecules and supramolecules is one of the fundamental phenomena in chemistry, physics, biology and material science. For example biological systems, like lipid bilayers of cell membranes and tertiary protein structures are formed by spontaneous self-assembly. Conformation and properties of these assemblies can be affected by changing the local environment of the structures. In the case of biological molecules, such an example would be protonation or deprotonation by changes in pH. When changing the conformation, one often changes the collective properties of the molecular assemblies. In this thesis, the formation of functional nanoscale devices is approached from the self-assembly of molecules and metallic monolayer capped nanoparticles into superstructures consisting of numerous nanoparticles. Stabilisation of the individual nanosized particles is based on bonding between noble metals and thiol ligands. The desired chemical characteristics and functionality of the nanoparticles is achieved by choosing the capping ligand layer and thus, directing the interactions between the nanoparticles. Both formation and functionality of the superstructures are studied in this thesis. Syntheses of silver and gold nanoparticles capped with different ligands are included. Both the individual nanoparticles and the colloidal superstructures formed by them were characterised by transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta-potential measurements and UV-vis spectroscopy. Characterisation of the electrical properties of the self-assembled structures were carried out by scanning electrochemical microscopy (SECM). The thesis is divided in three parts, considering first the formation of colloidal nanoparticle superstructures in solution, then a photoresponsive switching nanoparticle structure and finally electron transport processes in nanoscale films. In the first part, formation of nanoparticle aggregates via chemical and electrostatic interactions are studied. The second part consists of assembly and characterisation of a nanoswitch built from nanoparticles and photoisomerisable azobenzene molecules. In the last section of the thesis, electron transport processes in two self-assembled nanoscale films are studied with SECM. The first system is a molecular self-assembled monolayer and the second a film consisting of gold nanoparticles.