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.

We experimentally demonstrate dramatically enhanced light-matter interaction for molecules placed inside the nanometer scale gap of a plasmonic waveguide. We observe spontaneous emission rate enhancements of up to about 60 times due to strong optical localization in two dimensions. This rate enhancement is a nonresonant nature of the plasmonic waveguide under study overcoming the fundamental bandwidth limitation of conventional devices. Moreover, we show that about 85% of molecular emission couples into the waveguide highlighting the dominance of the nanoscale optical mode in competing with quenching processes. Such optics at molecular length scales paves the way toward integrated on-chip photon source, rapid transfer of quantum information, and efficient light extraction for solid-state-lighting devices.

Mixed chalcopyrite semiconductors like Cu(In,Ga)S2 and Cu(In,Ga)Se2 are characterized by the coexistence of different local atomic arrangements around the S or Se anion. The resulting anion displacement strongly influences the material bandgap. We studied the atomic-scale structure of Cu(In,Ga)S2 as a function of composition using x-ray absorption spectroscopy and valence force field simulations. Applying a specially developed model for not fully random cation distributions, we find that structural relaxation of the anion with respect to In and Ga contributes significantly more to the bandgap bowing observed for Cu(In,Ga)S2 and Cu(In,Ga)Se2 than relaxation with respect to Cu and group-III atoms.

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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.The one dimensional geometry of nanowires leads to an immense surface to volume ratio and the path of any current through the wire is always close to the surface. Sensing applications [1,2] benefit directly from this pronounced surface influence on electrical characteristics. Metal oxides in general and ZnO thin films in particular have already shown a good ultra-violet (UV) [3] and gas-sensing [4] ability due to the instability of oxygen at the surface [5][6][7]. As the surface influence is increased in nanowires, greatly enhanced sensitivity is expected from nanowire devices [1,6]. Although the conductivity of ZnO can easily be increased by three orders of magnitude by UV excitation, the current only returns to the relaxed value very slowly due to the known Persistant Photo Conduction (PPC) effect [6,[8][9][10].In this work, we report that an analogue Persistant Ion beam induced Conduction (PIC) arises during ion irradiation. PIC may allow single ion detection in individual ZnO nanowires, as well as countable doping with single dopants into ZnO nanowires. The strongly localized energy deposition of a single ion impact can result in a change in the properties of the entire device, providing a localized probe to understand the underlying excitation mechanisms.The typical lifetime of charge carriers in ZnO nanostructures is in the order of 100 ps [11]. To contribute to a persistent current increase, excited charge carriers have to populate distinct, stable states. According to Prades et al. [6] these are surface-oxygen bound states. Further discussion on the origin and properties of PPC can be found in literature [1, 6-10, 12, 13]. For PIC discussed in this work, ions instead of UV photons generate electron hole pairs in the semiconductor. By qualitative comparison we show that PIC and PPC rely on the same mechanism.The ZnO nanowires used for this work were grown via vapor-liquid-solid growth [14][15][16] and have a typical diameter range of 150 to 250 nm. They were imprinted onto the desired substrate by lightly pushing the substrate across the densly covered growth sample. Via photolithography and electron-beam evaporation, Ti/Au 50/50 nm contact pads were defined onto the sparsely * andreas.johannes@uni-jena.de covered substrate. Some wires span the 5 µm space between contact pads (see inset figure 1). Superfluous wires were easily cut with a focused ion beam to reliably manufacture single, contacted nanowires with this randomsuccess method. The nanowire devices were then contacted a...