On-chip fabrication of high performance nanostructured ZnO UV detectors.

Developing rationally controlled bottom-up device fabrication processes is essential for the achievement of high performance optimal devices. We report a controlled, seedless and site-selective hydrothermal technique to fabricate high-performance nanostructured ZnO UV-detectors directly on-chip. We demonstrate that by controlling the nanowire growth process, via tuning the experimental parameters such as the concentration of reactants and the growth time, and by introducing a refresh of the growth solution, the device structure efficiency can be enhanced to significantly improve its performance. The on-chip fabricated bridging nanosyringe ultraviolet detector demonstrates improved sensitivity (~10(5)), nanowatts detectability, and ultrafast response-time (90 ms) and recovery-time (210 ms). The improvement in response-time and recovery-time is attributed to the unique nanowire-nanowire junction barrier dominated resistance and the direct contact between ZnO and Au electrodes. Furthermore, the enhanced sensitivity and nanowatts detectability of the bridging nanosyringe device are due to the reduction in dimensionality and ultrahigh surface-to-volume ratio. This work paves the way toward low cost, large scale, low temperature, seedless and site-selective fabrication of high performance ZnO nanowire sensors on flexible and transparent substrates.

Hybrid graphene-metal oxide solution processed electron transport layers for large area high-performance organic photovoltaics.

Solution processed core-shell nano-structures of metal oxide-reduced graphene oxide (RGO) are used as improved electron transport layers (ETL), leading to an enhancement in photocurrent charge transport in PCDTBT:PC70 BM for both single cell and module photovoltaic devices. As a result, the power conversion efficiency for the devices with RGO-metal oxides for ETL increases 8% in single cells and 20% in module devices.

From 1D and 2D ZnO nanostructures to 3D hierarchical structures with enhanced gas sensing properties.

Facile and low cost hydrothermal routes are developed to fabricate three-dimensional (3D) hierarchical ZnO structures with high surface-to-volume ratios and an increased fraction of (0001) polar surfaces. Hierarchical ZnO nanowires (ZNWs) and nanodisks (ZNDs) assembled from initial ZnO nanostructures are prepared from sequential nucleation and growth following a hydrothermal process. These hierarchical ZnO structures display an enhancement of gas sensing performance and exhibit significantly improved sensitivity and fast response to acetone in comparison to other mono-morphological ZnO, such as nanoparticles, NWs, or NDs. In addition to the high surface-to-volume ratio due to its small size, the nanowire building blocks show the enhanced gas sensing properties mainly ascribed to the increased proportion of exposed active (0001) planes, and the formation of many nanojunctions at the interface between the initial ZnO nanostructure and secondary NWs. This work provides the route for structure induced enhancement of gas sensing performance by designing a desirable nanostructure, which could also be extended to synthesize other metal oxide nanostructures with superior gas sensing performance.

Laser patterning of transparent conductive metal nanowire coatings: simulation and experiment.

Transparent and electrically conductive metal nanowire networks are possible replacements for costly indium tin oxide (ITO) films in many optoelectronic devices. ITO films are regularly patterned using pulsed lasers so similar technologies could be used for nanowire coatings to define electrode structures. Here, the effects of laser irradiation on conducting silver nanowire coatings are simulated and then investigated experimentally for networks formed by spray deposition onto transparent substrates. The ablation threshold fluence is found experimentally for such nanowire networks and is then related to film thickness. An effective model using finite-element heat transfer analysis is examined to look at energy dissipation through these nanowire networks and used to understand mechanisms at play in the laser-material interactions. It is demonstrated that the three-dimensional nature of these coatings and the relative ratios of the rates of lateral to vertical heat diffusion are important controlling parameter affecting the ablation threshold.

Role of the Exposed Polar Facets in the Performance of Thermally and UV Activated ZnO Nanostructured Gas Sensors.

ZnO nanostructures with different morphologies (nanowires, nanodisks, and nanostars) were synthesized hydrothermally. Gas sensing properties of the as-grown nanostructures were investigated under thermal and UV activation. The performance of the ZnO nanodisk gas sensor was found to be superior to that of other nanostructures (Sg ∼ 3700% to 300 ppm ethanol and response time and recovery time of 8 and 13 s). The enhancement in sensitivity is attributed to the surface polarities of the different structures on the nanoscale. Furthermore, the selectivity of the gas sensors can be achieved by controlling the UV intensity used to activate these sensors. The highest sensitivity value for ethanol, isopropanol, acetone, and toluene are recorded at the optimal UV intensity of 1.6, 2.4, 3.2, and 4 mW/cm2, respectively. Finally, the UV activation mechanism for metal oxide gas sensors is compared with the thermal activation process. The UV activation of analytes based on solution processed ZnO structures pave the way for better quality gas sensors.

Organic solar cells with plasmonic layers formed by laser nanofabrication.

A method for the synthesis of metal nanoparticle coatings for plasmonic solar cells which can meet large scale industrial demands is demonstrated. A UV pulsed laser is utilized to fabricate Au and Ag nanoparticles on the surface of polymer materials which form the substrates for plasmonic organic photovoltaic devices to enhance their performance. Control of the particles' size and density is demonstrated. The optical and electrical effects of these embedded particles on the power conversion efficiency are examined rigorously using both experimental and computer simulation. Gold nanoparticles of particular size and spatial distribution enhance the device efficiency. Based on our findings, we propose design considerations for utilizing the entire AM1.5 spectrum using plasmonic structures towards enhancing the efficiency of polymer solar cells using broad spectrum plasmonics.

Laser implantation of plasmonic nanostructures into glass.

A laser direct-writing method producing high-resolution patterns of gold, silver and alloy plasmonic nanoparticles implanted into the surface of glass substrates is demonstrated, by scanning a pulsed UV laser beam across selected areas of ultra-thin metal films. The nanoparticles are incorporated beneath the surface of the glass and hence the patterns are scratch-resistant. The physical mechanisms controlling the process are investigated and we demonstrate that this technique can be used to fabricate a wide range of plasmonic optical structures such as wavelength selected diffraction gratings and high-density substrates for lab-on-chip surface-enhanced Raman spectroscopy.

Laser erasable implanted gratings for integrated silicon photonics.

In this work we experimentally demonstrate laser erasable germanium implanted Bragg gratings in SOI. Bragg gratings are formed in a silicon waveguide by ion implantation induced amorphization, and are subsequently erased by a contained laser thermal treatment process. An extinction ratio up to 24dB has been demonstrated in transmission for the fabricated implanted Bragg gratings with lengths up to 1000µm. Results are also presented, demonstrating that the gratings can be selectively removed by UV pulsed laser annealing, enabling a new concept of laser erasable devices for integrated photonics.

The incorporation of mono- and bi-functionalized multiwall carbon nanotubes in organic photovoltaic cells.

We have successfully prepared mono- and bi-functionalized multiwall carbon nanotubes (MWCNT) with thiophene, amine and thiophene-amine groups. The dispersion of nanotubes has been enhanced and stable optimized dispersions in organic solvents were obtained. These functionalized nanotubes have been successfully incorporated into bulk heterojunction (BHJ) organic photovoltaic (OPV) cells with a poly (3-hexyl thiophene) (P3HT) and [6, 6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photoactive blended layer. The incorporation of MWCNT with different functional groups, in the active layer, results in different cell performance with respect to a reference cell. A maximum power conversion efficiency of 2.5% is achieved with the inclusion of thiophene functionalized nanotubes. This improvement in the device performance is attributed to an extension of the exciton dissociation volume and charge transport properties through the nanotube percolation network in P3HT/CNT, PCBM/CNT or both phases. This is believed to be due to more efficient dispersion of the functionalized nanotubes within the photoactive composite layer.

Engineering the plasmon resonance of large area bimetallic nanoparticle films by laser nanostructuring for chemical sensors.

Large area fabrication of metal alloy nanoparticles with tunable surface plasmon resonances on low-cost substrates is reported. A UV excimer laser was used to anneal 5 nm thick Ag Au bilayer films deposited with different composition ratios to create alloy nanoparticles. These engineered surfaces are used to investigate how the wavelength of the surface plasmon resonance affects the optical detection capability of chemical species by surface-enhanced Raman spectroscopy.

Laser ablation direct writing of metal nanoparticles for hydrogen and humidity sensors.

A UV pulsed laser writing technique to fabricate metal nanoparticle patterns on low-cost substrates is demonstrated. We use this process to directly write nanoparticle gas sensors, which operate via quantum tunnelling of electrons at room temperature across the device. The advantages of this method are no lithography requirements, high precision nanoparticle placement, and room temperature processing in atmospheric conditions. Palladium-based nanoparticle sensors are tested for the detection of water vapor and hydrogen within controlled environmental chambers. The electrical conduction mechanism responsible for the very high sensitivity of the devices is discussed with regard to the interparticle capacitance and the tunnelling resistance.

Secondary nanotube growth on aligned carbon nanofibre arrays for superior field emission.

We report substantial improvement of the field emission properties from aligned carbon nanotubes grown on aligned carbon nanofibres by a two-stage plasma enhanced chemical vapour deposition (PECVD) process. The threshold field decreased from 15.0 to 3.6 V/microm after the secondary growth. The field enhancement factor increased from 240 to 1480. This technique allows for superior emission of electrons for carbon nanotube/nanofibre arrays grown directly on highly doped silicon for direct integration in large area displays.

Enhancement of polymer luminescence by excitation-energy transfer from multi-walled carbon nanotubes.

Carbon nanotubes have been shown to efficiently quench luminescence from conjugated polymers when incorporated in a composite. However, shown here is an up to 100-fold increase in the visible photoluminescence signal from fluorescent chromophores in nylon 10,10 by incorporating multi-walled carbon nanotubes (MWCNTs). Using 325- and 488-nm excitation the optical absorption by MWCNTs embedded within the polymer matrix is demonstrated, followed by efficient excitation-energy transfer to emissive chromophores intrinsic to the polymer but only when the MWCNTs are acid functionalized. Furthermore, the MWCNTs are shown to significantly retard photobleaching of fluorescent centers in the nylon composites. These remarkable properties greatly advance the prospects of utilizing MWCNTs in organic solar cells and electroluminecent devices to improve performance.

Pulsed laser ablation and deposition of thin films.

Pulsed laser ablation is a simple, but versatile, experimental method that finds use as a means of patterning a very diverse range of materials, and in wide areas of thin film deposition and multi-layer research. Superficially, at least, the technique is conceptually simple also, but this apparent simplicity hides a wealth of fascinating, and still incompletely understood, chemical physics. This overview traces our current physico-chemical understanding of the evolution of material from target ablation through to the deposited film, addressing the initial laser-target interactions by which solid material enters the gas phase, the processing and propagation of material in the plume of ejected material, and the eventual accommodation of gas phase species onto the substrate that is to be coated. It is intended that this Review be of interest both to materials scientists interested in thin film growth, and to chemical physicists whose primary interest is with more fundamental aspects of the processes of pulsed laser ablation and deposition.