K.D.G.I. Jayawardena

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.

Triboelectric nanogenerators: providing a fundamental framework

A new model which comprehensively explains the working principles of contact-mode triboelectric nanogenerators (TENGs) based on Maxwells equations is presented. Unlike previous models which are restricted to known simple geometries and derived using the parallel plate capacitor model, this model is generic and can be modified to a wide range of geometries and surface topographies. We introduce the concept of a distance-dependent electric field, a factor not taken into account in previous models, to calculate the current, voltage, charge, and power output under different experimental conditions. The versatility of the model is demonstrated for non-planar geometry consisting of a convex–concave surface. The theoretical results show excellent agreement with experimental TENGs. Our model provides a complete understanding of the working principles of TENGs, and accurately predicts the output trends, which enables the design of more efficient TENG structures.

Multi-Functional Carbon Fibre Composites using Carbon Nanotubes as an Alternative to Polymer Sizing

Carbon fibre reinforced polymers (CFRP) were introduced to the aerospace, automobile and civil engineering industries for their high strength and low weight. A key feature of CFRP is the polymer sizing - a coating applied to the surface of the carbon fibres to assist handling, improve the interfacial adhesion between fibre and polymer matrix and allow this matrix to wet-out the carbon fibres. In this paper, we introduce an alternative material to the polymer sizing, namely carbon nanotubes (CNTs) on the carbon fibres, which in addition imparts electrical and thermal functionality. High quality CNTs are grown at a high density as a result of a 35 nm aluminium interlayer which has previously been shown to minimise diffusion of the catalyst in the carbon fibre substrate. A CNT modified-CFRP show 300%, 450% and 230% improvements in the electrical conductivity on the ‘surface’, ‘through-thickness’ and ‘volume’ directions, respectively. Furthermore, through-thickness thermal conductivity calculations reveal a 107% increase. These improvements suggest the potential of a direct replacement for lightning strike solutions and to enhance the efficiency of current de-icing solutions employed in the aerospace industry.

Control of nanocrystal surface defects for efficient charge extraction in polymer-ZnO photovoltaic systems

Factors determining the photovoltaic device performance of blends of regioregular poly(3-hexylthiophene) (rr-P3HT) and ZnO nanostructures are reported. A decrease in the crystallinity of rr-P3HT upon the formation of ZnO (through hydrolysis) is observed through optical absorption spectroscopy. Increasing the humidity level for the ZnO formation leads to a decrease in the photoluminescence of the rr-P3HT:ZnO blend together with improved photovoltaic device performance. This is attributed to more efficient charge extraction due to a decrease in the radiative trap sites on the ZnO surface as a result of decreasing ZnO surface area with increasing humidity level.

Laser-assisted hydrothermal growth of size-controlled ZnO nanorods for sensing applications

Pulsed laser irradiation is used to seed the low-temperature hydrothermal growth of ZnO nanorods. UV laser irradiation produces ZnO nanoparticles in solution that act as nucleation seeds for the subsequent hydrothermal growth of the nanorods. By systematically varying the seed density and/or the concentration of the reactants, the diameter of the nanorods can be controlled over a wide range with a narrow size distribution. The nanorods are linked into multi-pod structures, due to nucleation at a central seed, but ultrasonic processing of the solutions is shown to yield isolated nanorods. Three-dimensional networks of these multi-pod structures are fabricated by drop-casting the solutions onto inter-digitated electrodes. These devices are used to detect ethanol, water vapour and UV light exposure.

Band alignment effects at the metal electrode interface of poly(3-hexylthiophene):zinc oxide hybrid photovoltaics

Photoelectron spectroscopy is used to investigate the role of titanium oxide as an interfacial layer between a hybrid regioregular poly(3-hexylthiophene):zinc oxide photoactive layer and the Al back contact. The inspection of chemical bonds through X-ray photoemission spectroscopy core peaks indicates that the inner structure of the rr-P3HT:ZnO photo-active layer is preserved, subsequent to the deposition of the TiOx layer. Furthermore, the band alignment of rr-P3HT:ZnO/TiOx and TiOx/Al interfaces gives rise to the enhancement in device efficiency from 1.08% to 1.22% upon incorporating the TiOx layer, which is associated with the additional open circuit voltage obtained in the interface of P3HT:ZnO/TiOx.

X-ray induced photocurrent characteristics of CVD diamond detectors with different carbon electrodes

Diamond has unique properties which make it suitable for a broad range of radiation detection applications ranging from particle timing and spectroscopy, to neutron, UV and X-ray sensors. In X-ray dosimetry, the atomic number of diamond (Z = 6) close to that of the human tissues (Z = 7.42) allows to mimic the real absorbed dose avoiding off-line recalculations. Moreover, its low atomic number and the capability to withstand high radiation fluxes make possible its use as beam monitor without altering significantly the properties of the interacting beam. To preserve the tissue equivalence of the diamond and minimize the perturbation and absorption of the incident beam, diamond detectors based on low thickness and low atomic number electrodes become a requirement. In this paper we present the X-ray detection characteristics of electronic grade CVD diamond sensors prepared in house with thin amorphous carbon electrodes fabricated by Pulsed Laser Deposition (PLD) technique in the fluence range of 2.3–3.6 Jcm−2. The devices showed a linear dependence of the induced photocurrent respect to the dose rate. Also, best dynamic response and better stability of the signals were achieved for applied bias up to ±50 V with signal to noise ratio (SNR) of ~ 300.

Hybrid and nanocomposite materials for flexible organic electronics applications

Flexible organic electronics have progressed from organic-only semiconductor devices, based on thin films of organic materials (small molecules and polymers) to hybrid and nanocomposite materials, a family of truly advanced materials designed at the nanoscale that offers enhancements in device performance and a reduction in production costs over traditional inorganic predecessors. These hybrid and nanocomposite materials are attractive because of the potentially wide range of available organic semiconductors (both small molecule and polymeric) and nanoparticle types (carbon allotrope, metal oxide, metal nanostructure, etc.). Here, we emphasize the variety and potential of these materials and introduce some of the production methods, properties, and limitations of their use in flexible electronics applications.

High sensitivity organic inorganic hybrid X-ray detectors with direct transduction and broadband response

X-ray detectors are critical to healthcare diagnostics, cancer therapy and homeland security, with many potential uses limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitate thick crystals (~1 mm–1 cm), resulting in rigid structures, high operational voltages and high cost. Here we present a disruptive, flexible, low cost, broadband, and high sensitivity direct X-ray transduction technology produced by embedding high atomic number bismuth oxide nanoparticles in an organic bulk heterojunction. These hybrid detectors demonstrate sensitivities of 1712 µC mGy−1 cm−3 for “soft” X-rays and ~30 and 58 µC mGy−1 cm−3 under 6 and 15 MV “hard” X-rays generated from a medical linear accelerator; strongly competing with the current solid state detectors, all achieved at low bias voltages (−10 V) and low power, enabling detector operation powered by coin cell batteries.X-ray detectors based on low-cost organic semiconductors have inherently low detector sensitivity due to poor X-ray to charge conversion and charge collection. Here, the authors demonstrate a flexible, high-sensitivity X-ray detector based on an organic bulk heterojunction-nanoparticle composite.