Jan-Peter Richters

Influence of polymer coating on the low-temperature photoluminescence properties of ZnO nanowires

We report on low temperature photoluminescence studies of ZnO nanowires embedded in different polymers. Comparing the spectra of as-grown and embedded ZnO nanowires, we find a decrease of the deep-level emission and an increase of the near band-edge emission after the embedding process. The near band-edge emission of the embedded ZnO nanowires is dominated by a surface exciton band. The observed effects are independent of the selected polymer. The decrease of the deep-level emission scales with the balling abilities of the different polymers. We propose a model to explain the spectral changes.

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.

Enhancement of the near-band-edge photoluminescence of ZnO nanowires: Important role of hydrogen incorporation versus plasmon resonances

We investigated the photoluminescence properties of ZnO nanowires coated with Au, Ag, and Pt nanoparticles deposited by dc sputtering. A strong enhancement of the near-band-edge emission was observed in all metal-coated samples but also if the samples were treated with Ar plasma without any nanoparticle deposition. High-resolution photoluminescence spectroscopy revealed hydrogen-donor-bound-exciton emission in all samples indicating unintentional hydrogen incorporation. A shorter decay time of the near-band-edge emission was observed in all cases. The results indicate that unintentional hydrogen incorporation plays a dominant role when metal deposition is performed by sputtering.

Ultrafast exciton dynamics in ZnO: Excitonic versus electron-hole plasma lasing

The use of ZnO bulk and especially nanolayer and nanowire structures for novel device applications has led to a renewal of interest in high-electron-density processes in ZnO, such as those occurring during lasing in ZnO. Using a pump-probe reflectometry technique, we investigate the ultrafast exciton dynamics of bulk ZnO under femtosecond laser excitation close to lasing conditions. Under intense excitation by 266-nm femtosecond (fs) pump pulses, the exciton resonance becomes highly damped and does not recover for several picoseconds. This slow recovery indicates a significant screening of the Coulomb interaction. Even below the lasing thresholds typically found for ZnO nanolayers and nanowires, we observe damping of the exciton resonance for several picoseconds, which indicates that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is electron-hole plasma recombination.

Modulation of carrier density in ZnO nanowires without impurity doping

ZnO nanowire based field effect transistor devices show the distinct performance depending on whether comparably oxygen-rich or oxygen-poor conditions were used for nanowire growth. Higher on-state current flows through the ZnO nanowire channel grown under oxygen-poor condition. A possible origin of this characteristic is discussed based on a photoluminescence analysis of the nanowire samples. The observed effect should be taken into account for ZnO nanowire based devices and applications.

Functional ZnO/polymer core-shell nanowires fabricated by oxidative chemical vapour deposition

Functional ZnO-nanowire/polymer core-shell heterostructures were realized using oxidative chemical vapour deposition (oCVD). This dry and versatile technique allows uniform coating of semiconductor nanowires with polymers and simultaneous doping control of the shell. Here, 100 nm thick, p-doped shells of poly(3,4-ethylenedioxythiophene) (PEDOT) were deposited around n-conductive ZnO nanowires. Energy-dispersive x-ray spectroscopy confirms the incorporation of Br dopants into the PEDOT shell, and the resulting p-conductivity of the polymer shell is demonstrated by electrical measurements on nanowire arrays. Photoluminescence spectroscopy points to reactions of Br with the ZnO surface but proves that the nanowires show only little degradation of their optical properties.

Modal gain and its diameter dependence in single-ZnO micro- and nanowires

Nanowires can successfully be used as building blocks for nanoscaled laser devices. Calculations predict an extremely large modal gain for nanowires made up of semiconductors such as GaN or ZnO. We determine experimentally the modal gain of single-ZnO nano- and microwires to approach 5000 cm?1 under particular size conditions. We demonstrate the distinct and sensitive dependence of the modal gain on the wire diameter and discuss optimizations for lasing of these wires.