Fabrication and characterisation of nanostructures on CaF2

Novel nanostructure fabrication techniques are described in this dissertation. Furthermore, an unusual two dimensional phase segregation on CaF2 surfaces after glancing incidence ion beam irradiation is reported. Atomic force microscope (AFM)-tip assisted self-organisation of clusters on anisotropic surfaces is illustrated. This new technique for the fabrication of periodic structures has been demonstrated using silver clusters grown on vicinal CaF2 (111) surfaces. The scanning action of the tip aligns silver clusters along crystal step edges on the vicinal surface. Nanowire arrays of silver with a uniform periodicity of 20-50 nm and wire widths of 5-7 nm have been achieved over the entire area scanned by the AFM. Calcium remaining on CaF2 surfaces after electron irradiation induced depletion of the surface of fluorine is found to re-organise itself into island structures. These islands show a shape transition, upon reaching a critical size of ∼ 20 nm in diameter, from a compact to an elongated island shape. Crystal step edge erosion of fluorine allows the formation of long ranging nanowires. These wires have a uniform width of ∼ 15 nm. The observed island shape transition and the width of the wires has been explained by the presence of strain in the islands. This is the first experimental observation confirming a theoretical prediction of the reduction of island width with increasing length for strained islands. A novel technique of ion beam assisted fabrication of periodic nanowire arrays over macroscopic areas is reported. Potential sputtering of CaF2 surfaces leads to a calcium enriched surface that organised itself in stress domains due to the lattice misfit between calcium and CaF2. These stress domains can be oriented by directed ion beam irradiation under glancing angles in parallel stripes with a periodicity of 8-12 nm. A phase segregation on the CaF2 (111) surface has been observed upon glancing incidence ion beam irradiation under certain conditions. The surface separates into oval shaped domains of composition or structure different from the remaining surface areas. (author)