Case Studies in Ion Beam Assisted Nanostructure Engineering

Abstract: Beams of energetic ions can be used for material analysis and modification. It provides us with a tool featuring unique control over the area, depth and amount of damage in the material. This property of ion beams can be used to generate desired changes in material properties or form nanostructures with specific characteristics in the material. While the modification of materials through the irradiation of GeV energy ions has been extensively researched, it is worth considering MeV ion irradiation for ion beam-based material modification. Accelerators with the ability to deliver ions in the MeV energy regime, such as tandem accelerators with a few MV terminal voltage and cyclotrons, are more accessible, easier to maintain and already available in industrial spaces. Thus, a comprehensive study on whether the damage caused in different material systems by MeV ion irradiation can be used for nanostructure engineering needs to be performed. In this thesis, the formation of nanostructures by MeV ion irradiation in two different materials is studied. The dependence of the nanostructures formed on the ion parameters such as ion type and energy is investigated for two material systems. In the first study, we investigate the formation of nanostructures on the surface of a model system, i.e., single crystals of CaF2. In the second study, we investigate the formation of etchable ion tracks in polyimide membranes. Nanoscale pores and channels, that can be formed from these etchable ion tracks, are expected to be the basis of next-generation detectors, biosensors and DNA/RNA sequencing. In the case of both materials, three distinct regions of ion-induced damage are identified after irradiation. The thresholds dividing these distinct regions of damage are dependent on ion velocity. Thus, electronic stopping power thresholds for the formation of nanostructures in materials are observed to be lower for MeV ions than for GeV ions.