Robert Röder

Single step integration of ZnO nano- and microneedles in Si trenches by novel flame transport approach: whispering gallery modes and photocatalytic properties.

Direct growth of quasi-one-dimensional nano- and microstructures in desired places of complex shaped substrates using simple growth methods is highly demanded aspect for various applications. In this work, we have demonstrated direct integration of ZnO nano- and microneedles into Si trenches by a novel flame transport synthesis approach in a single fabrication step. Growth of partially and fully covered or filled trenches in Si substrate with ZnO nano- and microneedles has been investigated and is discussed here. Detailed microstructural studies revealed the evolution of the ZnO nano- and microneedles as well as their firm adhesion to the wall in the Si trenches. Micro-photoluminescence measurements at different locations along the length of needles confirmed the good crystalline quality and also the presence of whispering gallery mode resonances on the top of needles due to their hexagonal shape. Faceted ZnO nano- and microstructures are also very important candidates with regard to photocatalytic activity. First, photocatalytic measurements from the grown ZnO nano- and microneedles have shown strong degradation of methylene blue, which demonstrate that these structures can be of significant interest for photocatalysis and self-cleaning chromatography columns.

Low threshold room-temperature lasing of CdS nanowires.

The synthesis of CdS nanostructures (bands, wires, irregular structures) was investigated by systematic variation of temperature and gas pressure, to deduce a comprehensive growth phase diagram. The high quality nanowires were further investigated and show stoichiometric composition of CdS as well as a single-crystalline lattice without any evidence of extended defects. The luminescence of individual nanowires at low excitation shows a strong near band edge emission at 2.41 eV indicating a low point defect concentration. Sharp peaks evolve at higher laser power and finally dominate the luminescence spectrum. The power dependence of the spectrum clearly shows all the characteristics of amplified stimulated emission and lasing action in the nanowire cavity. A low threshold was determined as 10 kW cm(-2) for lasing at room temperature with a slope efficiency of 5-10% and a Q factor of up to 1200. The length and diameter relations necessary for lasing of individual nanowires was investigated.

Intense Intrashell Luminescence of Eu-Doped Single ZnO Nanowires at Room Temperature by Implantation Created Eu–Oi Complexes

Successful doping and excellent optical activation of Eu(3+) ions in ZnO nanowires were achieved by ion implantation. We identified and assigned the origin of the intra-4f luminescence of Eu(3+) ions in ZnO by first-principles calculations to Eu-Oi complexes, which are formed during the nonequilibrium ion implantation process and subsequent annealing at 700 °C in air. Our targeted defect engineering resulted in intense intrashell luminescence of single ZnO:Eu nanowires dominating the photoluminescence spectrum even at room temperature. The high intensity enabled us to study the luminescence of single ZnO nanowires in detail, their behavior as a function of excitation power, waveguiding properties, and the decay time of the transition.

Continuous wave nanowire lasing.

Tin-doped cadmium sulfide nanowires reveal donor-acceptor pair transitions at low-temperature photoluminescence and furthermore exhibit ideal resonator morphology appropriate for lasing at continuous wave pumping. The continuous wave lasing mode is proven by the evolution of the emitted power and spectrum with increasing pump intensity. The high temperature stability up to 120 K at given pumping power is determined by the decreasing optical gain necessary for lasing in an electron-hole plasma.

Highly efficient visible-light driven photocatalysts: a case of zinc stannate based nanocrystal assemblies

The design and discovery of green and highly efficient visible-light driven catalysts hold significant importance towards efficient harvesting, conversion and utilization of the full-spectrum solar energy. Here in this work, we report several wide band gap (>3.1 eV) semiconducting nanostructures counterintuitively showing excellent catalytic activity under solar light towards organic dye degradation. These nanostructured stannates include amorphous ZnSnO3 nanocubes and Zn2SnO4–SnO2 nanocrystal assemblies, which possess the merits of high activity, low cost, absence of toxicity, and ease of synthesis. Hydroxyl radicals (˙OH) are confirmed to be the major active species responsible for the dye degradation reactions. The catalysis tests under monochromatic light and optical characterization revealed remarkable activities in the visible light range due to populated defect states in these semiconductor nanostructures. Finally, the Zn2SnO4–SnO2 hetero-junction nanocrystal assemblies produced by slow-ramping thermal decomposition demonstrated very high photocatalytic efficiency under visible light, with dye molecules almost fully degraded in 20 min. Besides more visible-light-active defect states and larger crystallite size, the coherent (hetero-epitaxial) interfaces and strong type II heterojunction interaction between the spinel Zn2SnO4 and rutile SnO2 nano-grains might be the main reasons for the drastically improved photocatalytic performance of the slow-ramp Zn2SnO4–SnO2.

Amphoteric nature of Sn in CdS nanowires.

High-quality CdS nanowires with uniform Sn doping were synthesized using a Sn-catalyzed chemical vapor deposition method. X-ray diffraction and transmission electron microscopy demonstrate the single crystalline wurtzite structure of the CdS/Sn nanowires. Both donor and acceptor levels, which originate from the amphoteric nature of Sn in II-VI semiconductors, are identified using low-temperature microphotoluminescence. This self-compensation effect was cross examined by gate modulation and temperature-dependent electrical transport measurement. They show an overall n-type behavior with relatively low carrier concentration and low carrier mobilities. Moreover, two different donor levels due to intrinsic and extrinsic doping could be distinguished. They agree well with both the electrical and optical data.

Ultrafast Dynamics of Lasing Semiconductor Nanowires

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double-pump approach. Wide bandgap gallium nitride (GaN), zinc oxide (ZnO), and cadmium sulfide (CdS) nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier-phonon coupling in ZnO leads to the fastest laser onset time of ∼1 ps in comparison to CdS and GaN exhibiting values of ∼2.5 and ∼3.5 ps, respectively. These values are constant between nanowires of different sizes implying independence from any optical influences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from ∼5 to ∼15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization.

Mode Switching and Filtering in Nanowire Lasers

Coherent light sources confining the light below the vacuum wavelength barrier will drive future concepts of nanosensing, nanospectroscopy, and photonic circuits. Here, we directly image the angular emission of such a light source based on single semiconductor nanowire lasers. It is confirmed that the lasing switches from the fundamental mode in a thin ZnO nanowire to an admixture of several transverse modes in thicker nanowires approximately at the multimode cutoff. The mode competition with higher order modes substantially slows down the laser dynamics. We show that efficient photonic mode filtering in tapered nanowires selects the desired fundamental mode for lasing with improved performance including power, efficiency, and directionality important for an optimal coupling between adjacent nanophotonic waveguides.

Gate modulation of below-band-gap photoconductivity in ZnO nanowire field-effect-transistors

We investigated the modulation of the photoconductivity under below-band gap excitation in single ZnO nanowire field effect transistors. Light excitation at 550 nm does not induce any change in the drain–source current when the gate voltage is kept at Vgs = 0 V, but results in a current increase when it is set to Vgs = −50 V. At this negative value of the gate voltage we further investigated the photo-reaction in the below-band-gap range 400–800 nm, observing a qualitative similar profile for all the photo-current curves. These results were attributed to a local effect, suggesting that the change in conductivity is due to the release of electrons from interface states located between the ZnO nanowire active channel and the gate dielectric SiO2.

Phonon-assisted lasing in ZnO microwires at room temperature

We report on room temperature phonon-assisted whispering gallery mode (WGM) lasing in ZnO microwires. For WGM laser action on the basis of the low gain phonon scattering process high quality resonators with sharp corners and smooth facets are prerequisite. Above the excitation threshold power