Control of Crystallisation Using Surface Topography

The purpose of this research project was to realise the control of crystallisation using surface topography. Calcium carbonate crystallisation was primarily used as this is a model system for crystallisation studies. The first experimental chapter involves the precipitation of calcium carbonate on a plasma-treated poly(dimethylsiloxane) (PPDMS) substrate. Exposure to atmospheric air-plasma results in oxidation of the surface of the poly(dimethylsiloxane), causing a superficial silica-like (SiOx) surface layer to form. Cracks in this surface, which arise because of its rigidity compared to the underlying bulk, were found to act as effective nucleation sites for calcium carbonate precipitation. Crystals were often observed only at the cracks and were absent elsewhere on the substrate. The surface chemistry was altered using a carboxylate terminated alkylthiol self-assembled monolayer (SAM), formed from the monomer 16-mercaptohexadecanoic acid. The presence of this monomer on the PPDMS substrate caused a significant increase in crystal population and enhanced the localising effect of the cracks. Further control of crystallisation was achieved by varying the initial ion ratio (Ca2+:CO32-) and by the presence of magnesium ions in solution. The substrate also proved effective at causing the localised precipitation of other inorganic materials: strontium carbonate, barium carbonate, manganese carbonate, basic copper carbonate and calcium oxalate. Through the application of contact masks or a tensile stress during plasma-treatment, it was possible to control the distribution of the cracks across the surface and therefore pattern the crystallisation of calcium carbonate. The second research chapter is a quantitative analysis of the system discussed in the first chapter. Here, an image analysis software, ImageJ, was used to obtain data from electron micrographs of the PPDMS substrate after crystallisation. Calcium carbonate had been precipitated on the PPDMS substrate once the surface chemistry had been modified using a variety of differently terminated alkylthiol SAMs. The research revealed that not only is it possible to control crystal populations at the surface features, but also the crystal polymorph. In the third research chapter, silicon wafers that had been spincoated with reduced-graphene oxide were used as substrates for calcium carbonate precipitation. Interestingly, the vast majority of crystals that had formed on these wafers were aragonite, a metastable phase of calcium carbonate. In order to understand why this was so, a series of investigations followed that were aimed at elucidating if it was a topographical effect or due to the presence of impurities. The data suggested that the large proportion of aragonite was due to topographical effects. The final research chapter involved cleanroom microfabrication techniques common to the semiconductor industry to construct surface structures on silicon wafers that were used as substrates. Crystallisation was locallised to these features, proving that it is possible to design surface topographies that can be used to control crystallisation.