A study of material condensation on liquid substrates
Date of Issue2016-05-27
School of Materials Science and Engineering
This thesis aims to establish the influence of a liquid substrate on material deposition. The influence of a liquid substrate is explored by a comparative study with an amorphous solid counterpart. Simultaneous gold vapor deposition onto both silicone oil and amorphous carbon was carried out and the morphologies were found to be highly dissimilar. A uniform distribution of condensate following the conventional growth mechanism was observed on the solid substrate. However, on the other hand, low coverage with large empty regions were prevalent on the liquid substrate. This thesis deduces that the effect is due to structural differences between the two substrates. The magnitude of the response of the liquid to the depositing atoms causes the depositing atoms to undergo either desorption or diffusion. The resulting morphology is related to the affinity of the condensing species and momentum transfer accompanying deposition. In the case of gold vapor deposition, weak interacting forces, combined with higher atomic mass of gold, leads to additional desorption events, giving rise to low coverage condensate on the liquid substrate. Delayed secondary nucleation, along with multilayer growth, is also observed. Therefore, under weak binding forces, a liquid substrate prefers condensate growth over coverage. Furthermore, this mechanism is verified by simulating via the kinetic Monte Carlo technique, the nucleation and growth on a liquid substrate. The simulated morphologies under liquid substrate conditions match well with experimentally observed morphologies. The deposition of copper on the surface of silicone oil resulted in monodisperse nanoparticles uniformly distributed over the surface. This distinct morphology is attributed to differences in the response of the liquid substrate (additional desorption / diffusion events). A novel phenomenon called shared coarsening, for synthesis of uniformly sized particles on a 2D surface is proposed for particle growth on liquid surfaces. It is the simultaneous coarsening of a smaller particle by several similarly sized larger particles, resulting in monodispersity. Increased diffusion mobility of adatoms on the liquid surface along with an appropriate spatial distribution of the particles is critical for shared coarsening. Thus, a facile nanoparticle synthesis approach has been developed. A condensate - substrate interaction model has been developed which defines the morphological outcome. Deposition of silver and aluminum onto silicone oil has been studied. The effects of the condensate mass on the perturbation of the liquid molecular arrangement, along with their affinity towards the substrate are discussed. The limitation of a molecular liquid substrate to provide particle stability is overcome by fabrication of highly monodispersed gold nanoparticles on the surface of an ionic liquid. The ionic liquid yields shared coarsening, resulting in spherical particles with uniform size distribution. Thus our model and experiments were utilized to synthesize novel monodispsered nanoparticles. In summary, vapor condensation onto liquid substrates was studied by determining the effect of the kinetics of nucleation and growth on condensate morphology. The new concepts introduced, along with the synthesis method developed here are thus a step forward to understand and exploit material condensation on liquid substrates.