Papers by Author: Andreas Markwitz

Abstract: We report the structural and electrical properties of ion beam sputtered ZnO films vacuum annealed at varying temperatures. XRD results revealed that the films grow along the c-axis. The crystallite sizes increase from ~8 to ~30 nm upon annealing at 800 ^ºC. Annealing aided to recover the compressive strain and regain the standard lattice parameter values. The RMS surface roughness increased to ~5.0 nm after annealing at 800 ^ºC as observed in AFM micrographs. Increased resistivity on the annealed films suggested that the oxygen vacancies are compensated by de-trapped oxygen at grain boundaries.

Abstract: Zinc oxide (ZnO) nanorods have been synthesized via the arc discharge method. Different cathode materials, graphite and copper, were applied to modulate the morphology and UV & humidity sensing properties of the as-synthesized ZnO nanorods. Compared with ZnO nanorods synthesized by graphite cathode, shorter length and other spherical and cubical structures were also detected for those ZnO nanorods synthesized by copper cathode. A better UV-sensitive photoconduction and higher humidity sensitivity were detected for ZnO nanorods synthesized by graphite cathode than those obtained by copper cathode, which is considered to be due to the higher aspect ratio for long ZnO nanorods. The simplicity of the synthesis route coupled with the modulation of morphology and sensing properties of ZnO nanorods make the arc discharge method a very promising way to produce high quality ZnO nanorods with adjustable morphologies.

Abstract: We have fabricated surface magnetic iron nanoclusters using low energy Fe⁺ implantation and electron beam annealing. We find that changing the fluence has a significant effect on the nanocluster growth, structural and magnetic properties. Low fluences lead to small nanoclusters and superparamagnetism, while high fluences result in larger chain-like nanoclusters that have high remnant magnetizations and a significantly reduced saturation field. Our results show that the nanostructure and the magnetic properties can be tuned by varying the Fe⁺ fluence, which means that a reliable method can be used to make surface nanoclusters for a variety of applications (e.g. large magnetoresistance sensors with no hysteresis).

Abstract: Low-energy 14N+ ions were implanted with 23 keV under normal incidence into C-axis (0001) sapphire at room temperature. DYNAMIC-TRIM calculations were performed to calculate the N depth profiles for the various fluences from 1x1016 to 1017 cm-2. Electron Beam Annealing (EBA) was performed at a sample temperature of 700 °C for 10 min to allow the implanted and substrate atoms in the implanted layer to move to energetically preferable positions. Nuclear Reaction Analysis revealed the implanted nitrogen ion concentrations. Atomic Force Microscopy and Scanning Electron Microscopy show some nanostructures at the surface of the sapphire substrate exhibiting an average width of 139 ± 25 nm and height of 37 ± 7 nm using the lowest fluence of 1x1016 ions cm-2. Notably for samples implanted with the highest fluence of 1x1017 ions cm-2, bubble/holes like structures appeared after EBA due to out-diffusion of nitrogen that causes blistering and exfoliation effects.