Athavan Nadarajah

Flexible inorganic nanowire light-emitting diode.

We report a highly flexible light-emitting device in which inorganic nanowires are the optically active components. The single-crystalline ZnO nanowires are grown at 80 degrees C on flexible polymer-based indium-tin-oxide-coated substrates and subsequently encapsulated in a minimal-thickness, void-filling polystyrene film. A reflective top contact serving as the anode in the diode structure is provided by a strongly doped p-type polymer and an evaporated Au film. The emission through the polymer side of this arrangement covers most of the visual region. Electrical and optical properties as well as performance limitations of the device structure are discussed.

Controlled Spatial Switching and Routing of Surface Plasmons in Designed Single-Crystalline Gold Nanostructures

Electron emission microscopy is used to visualize plasmonic routing in gold nano-structures. We show that in single-crystalline gold structures reliable routing can be achieved with polarization switching. The routing is due to the polarization dependence of the photon-to-plasmon coupling, which controls the mode distribution in the plasmonic gold film. We use specifically designed, single-crystalline planar structures. In these structures, the plasmon propagation length is sufficiently large such that significant plasmon power can be delivered to the near-field region around the end tips of the router. Solid state devices based on internal electron excitation and emission processes appear feasible.

Laser annealing of photoluminescent ZnO nanorods grown at low temperature

We report on pulsed laser annealing as an efficient method for improving the structural and optical properties of ZnO nanorods grown at low temperature. We find that the excitonic luminescence at 390 nm can be improved by as much as a factor of 8. We also show that laser annealing can be carried out in typical device structures, in particular by illumination through glass/indium-tin-oxide substrates and through polymer/indium-tin-oxide substrates.

Selective growth of single-crystalline ZnO nanowires on doped silicon

We report the growth of single-crystalline ZnO nanowires on n- and p-type Si wafers by electrodeposition. On strongly doped n-type Si high-quality nanowires can be grown under similar conditions as used for metallic substrates. For low electron concentrations occurring in weakly n-type or in p-type wafers, nanowire growth is inhibited. This difference allows selective growth in strongly n-type areas. The inhibited growth on weakly n-type and p-type wafers can be improved by applying stronger cathodic electrode potentials or by illuminating the growth area. The wires on n‐Si show efficient electroluminescence covering the visible and extending into the ultraviolet spectral range.

Nanostructured Semiconductor Heterojunctions from Quantum Dot Layers

We report the deposition of conformal thin layers of 10–50 nm thickness from II-VI quantum dot suspensions on ZnO nanowire substrates. Smooth polycrystalline films of high electronic quality can be obtained from CdSe quantum dots after annealing at moderate temperatures. The electronic properties are adequate for detector and solar cell applications. The growth and annealing temperatures permit deposition on light-weight and flexible substrates. Some elemental diffusion of Se across the CdSe/ZnO interface occurs in the film formation. A comparison with CdS/ZnO junctions indicates that the low Se diffusion rates are essential for efficient charge transfer.

ZnO nanowires for LED and field‐emission displays

— The use of ZnO nanostructures in various display applications is reported. Single-crystalline vertically oriented nanowires with typical diameters of 100 nm and a length of 1–2 μm were grown at deposition temperatures below 100°C. Homogeneous growth over areas up to 50 cm2 on Si as well as on various metallic, transparent, and flexible substrates were obtained. Visible electroluminescence in the region between 400 and 900 nm and narrow-line near-ultraviolet (UV) electroluminescence is demonstrated. The physical conditions leading to single-crystalline growth at low temperature, the role of defects, and the possibility of doping are discussed. These issues present the main challenges on the road towards high emission rates in LED operation. Under certain conditions, sharply tipped wires can be grown that hold promise for field-emission applications.

Improved performance of nanowire-quantum-dot-polymer solar cells by chemical treatment of the quantum dot with ligand and solvent materials.

We report a nanowire-quantum-dot-polymer solar cell consisting of a chemically treated CdSe quantum dot film deposited on n-type ZnO nanowires. The electron and hole collecting contacts are a fluorine-doped tin-oxide/zinc oxide layer and a P3HT/Au layer. This device architecture allows for enhanced light absorption and an efficient collection of photogenerated carriers. A detailed analysis of the chemical treatment of the quantum dots, their deposition, and the necessary annealing processes are discussed. We find that the surface treatment of CdSe quantum dots with pyridine, and the use of 1,2-ethanedithiol (EDT) ligands, critically improves the device performance. Annealing at 380 °C for 2 h is found to cause a structural conversion of the CdSe from its initial isolated quantum dot arrangement into a polycrystalline film with excellent surface conformality, thereby resulting in a further enhancement of device performance. Moreover, long-term annealing of 24 h leads to additional increases in device efficiency. Our best conversion efficiency reached for this type of cell is 3.4% under 85 mW cm(-2) illumination.

Terahertz characterization of zinc oxide nanowires using parallel-plate waveguides

We present a new material parameter extraction method utilizing parallel-plate waveguides (PPWG) with terahertz time-domain spectroscopy (THz-TDS). It can be applied to natively grown samples as thin as a micron, it is repeatable and accurate, and it enables the characterization of thin films and nanostructures on conductive substrates for a variety of materials and growth conditions. The dielectric response and the complex conductivity are measured and fit with theoretic models for chemical-bath deposited zinc oxide nanowires.

Effect of annealing on ZnO nanowires grown at low temperature

Our research demonstrates that thermal and pulsed laser annealing improve the structural and optical properties of ZnO nanowire films grown electrochemically from aqueous electrolytes at a temperature below 90° C. The structural and optical properties of the grown ZnO nanowires were characterized by transmission electron microscopy and by their photo- and electroluminescence. Our results indicate that the as-grown nanowire structures have considerable internal lattice strain. However, moderate thermal annealing and laser annealing induce strain relaxation, considerably improving optical and electrical properties, and thereby allowing the fabrication of devices, such as solar cells and light emitting diodes.

Improved transport with 1,2-ethanedithiol treatment in the preparation of quantum-dot-nanowire solar cells

We describe the design, fabrication, and characterization of heterojunction solar cells consisting of CdSe quantum dot films sandwiched between a hole conducting polymer layer and n-type ZnO nanowires. We also present a detailed analysis of the quantum dot layer deposition processes, solution-phase ligand exchange of this quantum dot layer, the conformality of this layer on deeply nanostructured samples, and the effect of a surfactant-aided thermal anneal process. Annealing creates a structural conversion of the quantum dot layers into an extremely thin continuous polycrystalline film with typical grain diameters of 30–50 nm. This transition is accompanied by a loss of quantum confinement and a significant improvement of the charge transport in the layer. The optimized annealing and deposition processes result in solar cells with an open-circuit voltage up to 0.4 V, a short circuit current of ∼10 mA/cm2, an external quantum efficiency of 65%, and an energy conversion efficiency above 2%.