X-ray scattering studies on self-assembled alloy structures
thesisposted on 15.12.2014, 10:40 by Richard Paul Williams
X-ray scattering has been used to determine the alloy structure of GdFe2 and the real-time formation of nanoparticle assemblies. Initial growth curve analysis on the individual deposition of Gd and Fe allowed the correct stoichiometric ratio of the two species to be co-evaporated and annealed to form the alloy structure GdFe2- The structure of the [1/3 1/3] reconstruction has been determined. The Patterson map obtained from the structure factors at zero perpendicular momentum transfer shows that the Fe atoms are laterally displaced from their predicted positions. Structure factor data for the out-of-plane atomic positions shows that the Gd atoms sit in adsorption sites that are 22% higher than predicted hard sphere positions. Gold nanoparticles of various sizes coated with organic thiol molecules have been studied by X-ray diffraction. The self-assembly process on to a Si(III) substrate was probed in real-time by GISAXS which showed for the first time that the largest particles drop out of solution first and act as seeds for the smaller particles which infringe on the surface minutes later surrounding the larger particle domains. The intermixing of two particles with average size ratio 0.58 showed evidence for a self-assembled bimodal alloy structure which was indicated by missing intensity from a 2 peak Gaussian fit which could be accommodated for by a central 3rd peak. Self-assembly of thiol stabilised gold nanoparticles has been shown to occur at the solvent-air interface. The quality of self-assembly depends strongly on the size of the particles and the evaporation rate of the solvent. A random walk model was used to explain that as the solvent evaporation rate is significantly slowed the nanoparticles are restricted to vertical diffusion only but at room temperatures the particles can diffuse in all directions a result that is further emphasized by the increased level of ordering at cooler temperatures. A complete self-assembly procedure has been suggested where particles above a certain size (1 nm radius) can immediately self-assemble at the solvent-air interface to be later deposited on to the substrate where the thiol ligands can lock the particles into a tight self-assembled layer.