Shaping and compositional modification of zinc oxide nanowires under energetic manganese ion irradiation

The development of the surface contour and the local elemental composition of ZnO nanowires of 150 to 200 nm diameter under 170 keV Mn irradiation is addressed both experimentally and by means of three-dimensional dynamic Monte Carlo computer simulation using the binary collision approximation. A random rotation of the incident beam around the nanowire axis mimics the experimental irradiation under sample rotation. The simulation results demonstrate a complex interplay of sputter erosion, implant incorporation and resputtering, as well as atomic mixing, which is discussed in detail. The sputter-induced thinning of the wire is in good quantitative agreement with experimental results obtained from pre- and post-irradiation scanning electron microscopy. The experiments also confirm the predicted sharpening of the top and neck formation at the bottom interface. Due to the latter, the wire detaches from the substrate at high ion fluence. Good agreement with experimental results from nano-X-ray fluorescence is also obtained for the continuously increasing Mn/Zn ratio as function of ion fluence. The simulation yields manifold additional information which has not been accessible to the experiments, such as fractional sputtering data and three-dimensional elemental composition profiles at increasing ion fluence. From these, preferential sputtering of O vs. Zn is deduced. A significant contamination of the wires with substrate material arises from ion mixing at the wire/substrate interface rather than from redeposition of sputtered substrate atoms. Surprising hollow profiles are observed. Their formation is attributed to a special mechanism of collisional transport which is characteristic for the irradiation of nanowires at a suitable combination of wire diameter and ion energy.