Raphael Niepelt

Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions.

Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths.

Stable enhancement of near-band-edge emission of ZnO nanowires by hydrogen incorporation

We report on the photoluminescence properties of ZnO nanowires treated with a mild Ar plasma. The nanowires exhibited stable and strong enhancement of the near-band-edge emission and quenching of the deep level emission. The low temperature PL revealed a strong hydrogen donor-bound-exciton line in the plasma-treated samples indicating unintentional incorporation of hydrogen during the plasma treatment. To confirm the results, hydrogen was implanted into the ZnO nanowires with a low ion energy of 600 eV and different fluences. The observed result can be related to the passivation of deep centers by hydrogen. The absolute photoluminescence intensity measured by an integrating sphere showed stable and strong UV emission from the treated samples even after several weeks.

Alignment of Semiconductor Nanowires Using Ion Beams

Gallium arsenide nanowires are grown on 100 GaAs substrates, adopting the epitaxial relation and thus growing with an angle around 35 degrees off the substrate surface. These straight nanowires are irradiated with different kinds of energetic ions. Depending on the ion species and energy, downwards or upwards bending of the nanowires is observed to increase with ion fluence. In the case of upwards bending, the nanowires can be aligned towards the ion beam direction at high fluences. Defect formation (vacancies and interstitials) within the implantation cascade is identified as the key mechanism for bending. Monte Carlo simulations of the implantation are presented to substantiate the results.

Ion beam doping of semiconductor nanowires

Semiconductor nanowires are of major importance within the area of nanotechnology, and are usually synthesized using the so-called vapor-liquid-solid mechanism. Controlled doping, a necessary issue in order to realize devices, is an unsolved problem and an extremely difficult task if using such a growth mechanism. We use an alternative route for modifying the electrical, optical and magnetic properties of semiconductor nanowires: ion implantation. Several independent studies on ion beam doping of semiconductor nanowires will be presented.

Biofunctionalization of zinc oxide nanowires for DNA sensory applications

We report on the biofunctionalization of zinc oxide nanowires for the attachment of DNA target molecules on the nanowire surface. With the organosilane glycidyloxypropyltrimethoxysilane acting as a bifunctional linker, amino-modified capture molecule oligonucleotides have been immobilized on the nanowire surface. The dye-marked DNA molecules were detected via fluorescence microscopy, and our results reveal a successful attachment of DNA capture molecules onto the nanowire surface. The electrical field effect induced by the negatively charged attached DNA molecules should be able to control the electrical properties of the nanowires and gives way to a ZnO nanowire-based biosensing device.

Protein Adsorption on Nano-scaled, Rippled TiO2 and Si Surfaces

We synthesized nano-scaled periodic ripple patterns on silicon and titanium dioxide (TiO2) surfaces by xenon ion irradiation, and performed adsorption experiments with human plasma fibrinogen (HPF) on such surfaces as a function of the ripple wavelength. Atomic force microscopy showed the adsorption of HPF in mostly globular conformation on crystalline and amorphous flat Si surfaces as well as on nano-structured Si with long ripple wavelengths. For short ripple wavelengths the proteins seem to adsorb in a stretched formation and align across or along the ripples. In contrast to that, the proteins adsorb in a globular assembly on flat and long-wavelength rippled TiO2, but no adsorbed proteins could be observed on TiO2 with short ripple wavelengths due to a decrease of the adsorption energy caused by surface curvature. Consequently, the adsorption behavior of HPF can be tuned on biomedically interesting materials by introducing a nano-sized morphology while not modifying the stoichiometry/chemistry.

Persistent ion beam induced conductivity in zinc oxide nanowires

We report persistently increased conduction in ZnO nanowires irradiated by ion beam with various ion energies and species. This effect is shown to be related to the already known persistent photo conduction in ZnO and dubbed persistent ion beam induced conduction. Both effects show similar excitation efficiency, decay rates, and chemical sensitivity. Persistent ion beam induced conduction will potentially allow countable (i.e., single dopant) implantation in ZnO nanostructures and other materials showing persistent photo conduction.

DNA hybridization assay at individual, biofunctionalized zinc oxide nanowires

Reliable and efficient identification of DNA is a major goal in on-site diagnostics. One dimensional nanostructures like nanowires (NW) represent potential sensor structures due to their extreme surface-to-bulk ratio, enabling enhanced biomolecule binding which results in optimal signals. While silicon NW are already well studied, NW made from other materials with promising properties like ZnO are not yet established as NW sensor material for bioanalytics. Here we demonstrate the DNA functionalization of ZnO NW even at the single NW level and their successful application in a DNA hybridization assay.

Determination and influence evaluation of the acoustic impedance ratio for thermal co-evaporation

Metal-halide perovskites are promising materials for applications like lasers and solar cells. In this work, we show the importance of an accurate determination of the source material parameters (acoustic impedance ratio and density) for thermal co-evaporation of soft materials like perovskites. We use here methylammonium iodide and lead(II)iodide for the exemplary deposition of methylammoniumlead(II)triiodide. We measure the thickness of the deposited layers by scanning electron microscopy cross sections and monitor the frequency change of the quartz crystal microbalances. We use a model with a one-dimensional acoustical composite resonator for the correct determination of the acoustic impedance ratio, resulting in values of 0.025 ± 0.002 for methylammonium iodide and of 0.11 ± 0.01 for lead(II)iodide. We use the resulting material parameters to deposit a layer of crystalline methylammonium lead triiodide with an accurately controlled stoichiometry of MAPbIx with x = 3.2 ± 0.2. We show the impact assuming false acoustic impedance ratios by simulating the actual evaporation rates of the source materials. We show that the ratio of the evaporation rates changes significantly during the deposition process. This results in a strong stoichiometry gradient in the perovskite layer and a mismatch in the average stoichiometry for a typical absorber thickness of 600 nm.Metal-halide perovskites are promising materials for applications like lasers and solar cells. In this work, we show the importance of an accurate determination of the source material parameters (acoustic impedance ratio and density) for thermal co-evaporation of soft materials like perovskites. We use here methylammonium iodide and lead(II)iodide for the exemplary deposition of methylammoniumlead(II)triiodide. We measure the thickness of the deposited layers by scanning electron microscopy cross sections and monitor the frequency change of the quartz crystal microbalances. We use a model with a one-dimensional acoustical composite resonator for the correct determination of the acoustic impedance ratio, resulting in values of 0.025 ± 0.002 for methylammonium iodide and of 0.11 ± 0.01 for lead(II)iodide. We use the resulting material parameters to deposit a layer of crystalline methylammonium lead triiodide with an accurately controlled stoichiometry of MAPbIx with x = 3.2 ± 0.2. We show the impact assumin...