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

For ZnO nanowires of 150 to 200 nm diameter standing on a flat substrate, the development of the surface contour/morphology and the local elemental composition under 175 keV Mn irradiation has been investigated both experimentally and by means of three-dimensional dynamic Monte Carlo computer simulation. The simulation results reveal a complex interplay of sputter erosion, implant incorporation, resputtering and atomic mixing, which is discussed in detail. The sputter-induced thinning of the wire is in good quantitative agreement with the experimental results obtained from pre- and post-irradiation scanning electron microscopy. The experiments also confirm the predicted sharpening of the tip, neck formation at the bottom interface, and ultimately the detachment of the nanowires from the substrate at high ion fluence. Additional good agreement with experimental results from nano-x-ray fluorescence is also obtained for the continuously increasing Mn/Zn atomic ratio within the nanowires as a function of ion fluence. The simulation yields a great deal of additional information that has not been accessible in the experiments. From this, preferential sputtering of O compared with 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 of the irradiation of nanowires at a suitable combination of wire diameter and ion energy.