Among the large variety of bottom-up grown semiconductor nanostructures, semiconductor nanowires are of outstanding interest, since they are promising candidates for integration into future electronic and photonic devices. For the use in devices like transistors, sensors, LEDs, and solar cells, it is essential to know and understand the nature of the nanowires. Furthermore, the selective control of the nanowire properties, by modifying either the wire composition or the surface conditions, allows the development of novel, functional elements for next generation semiconductor devices. This thesis reports on both, the characterisation and the selective modification of semiconductor nanowires. Scanning electron microscopy based methods are shown for the determination of the carrier diffusion lengths in ZnO and GaAs nanowires. The change of the electrical properties of ZnO nanowires during ion beam implantation is investigated as an example for the targeted modification of the nanowire properties. Here, the ion-nanowire interaction enables the detection of single ion impacts. Biofunctionalisation of ZnO nanowires is also shown, demonstrating the suitability of surface-modified nanowires for the use in biosensing devices. The thesis is divided into three parts, addressing the diffusion length measurements, the ion implantation experiments, and the surface biofunctionalisation.