Gareth Redmond

Microcavity effects and optically pumped lasing in single conjugated polymer nanowires

Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications. Conjugated polymer lasers, in a variety of resonator geometries such as microcavity, micro-ring, distributed feedback and photonic bandgap structures, have been fabricated using a range of coating and imprinting techniques. Currently, one-dimensional nanowires are emerging as promising candidates for integrated, subwavelength active and passive photonic devices. We report the first observation of optically pumped lasing in single conjugated polymer nanowires. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry-Pérot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action.

Oxidative stress and toxicity of gold nanoparticles in Mytilus edulis

Gold nanoparticles (AuNP) have potential applications in drug delivery, cancer diagnosis and therapy, food industry and environment remediation. However, little is known about their potential toxicity or fate in the environment. Mytilus edulis was exposed in tanks to 750 ppb AuNP (average diameter 5.3 ± 1 nm) for 24h to study in vivo biological effects of nanoparticles. Traditional biomarkers and an affinity procedure selective for thiol-containing proteins followed by two-dimensional electrophoresis (2DE) separations were used to study toxicity and oxidative stress responses. Results were compared to those obtained for treatment with cadmium chloride, a well known pro-oxidant. M. edulis mainly accumulated AuNP in digestive gland which also showed higher lipid peroxidation. One-dimensional SDS/PAGE (1DE) and 2DE analysis of digestive gland samples revealed decreased thiol-containing proteins for AuNP. Lysosomal membrane stability measured in haemolymph gave lower values for neutral red retention time (NRRT) in both treatments but was greater in AuNP. Oxidative stress occurred within 24h of AuNP exposure in M. edulis. Previously we showed that larger diameter AuNP caused modest effects, indicating that nanoparticle size is a key factor in biological responses to nanoparticles. This study suggests that M. edulis is a suitable model animal for environmental toxicology studies of nanoparticles.

Gold nanoparticles and oxidative stress in Mytilus edulis

Little is known about potential environmental impact of nanoparticles. Gold nanoparticles can cause unexpected biological responses. Here, Mytilus edulis were exposed (24h) to gold-citrate nanoparticles (GNP), menadione and both compounds simultaneously (GNP/menadione). Protein ubiquitination and carbonylation were determined in gill, mantle and digestive gland, along with traditional oxidative stress biomarkers; catalase activity and neutral red retention time assay (haemolymph). 2DE gels were performed on gill proteins (menadione; GNP/menadione). Our results reveal that GNP may induce oxidative stress.

Probing intrinsic transport properties of single metal nanowires: Direct-write contact formation using a focused ion beam

The transport characteristics of 70-nm-diameter platinum nanowires (NWs), fabricated using a pore-templated electrodeposition process and individually contacted using a focused ion beam (FIB) method, are reported. This approach yields nanowire devices with low contact resistances (∼400Ω) and linear current–voltage characteristics for current densities up to 65kA∕cm2. The intrinsic nanowire resistivity (33±5μΩcm) indicates significant contributions from surface- and grain-boundary scattering mechanisms. Fits to the temperature dependence of the intrinsic NW resistance confirm that grain-boundary scattering dominates surface scattering (by more than a factor of 2) at all temperatures. Our results demonstrate that FIB presents a rapid and flexible method for the formation of low-resistance ohmic contacts to individual metal nanowires, allowing intrinsic nanowire transport properties to be probed.

Exposure of the blue mussel, Mytilus edulis, to gold nanoparticles and the pro-oxidant menadione

Relatively little is known about how gold nanoparticles (GNP) might interact in vivo with marine organisms. Mytilus edulis was exposed (24h) to approximately 15 nm GNP, menadione and both compounds simultaneously (GNP/menadione). GNP was detected by inductively coupled plasma-optical emission spectroscopy mainly in digestive gland of samples exposed to GNP though not GNP/menadione, perhaps due to impaired feeding. Thioredoxin reductase activity and malondialdehyde levels were determined in all tissues. Thioredoxin reductase inhibition was detected only in digestive gland exposed to menadione whilst malondialdehyde levels did not vary in response to treatment in all tissues. GNP caused a decrease in the reduced/oxidized glutathione ratio in digestive gland, but no difference was found in other tissues or for other treatments. One dimensional electrophoresis of proteins containing thiol groups was performed in all tissues and revealed a reduction in protein thiols for all treatments in digestive gland. Two dimensional electrophoresis of digestive gland extracts, from GNP and control groups, showed decreased levels of thiol proteins in response to GNP which we attribute to oxidation. Our results suggest that GNP causes a modest level of oxidative stress sufficient to oxidize thiols in glutathione and proteins but without causing lipid peroxidation or induction of thioredoxin reductase activity.

Low-temperature electron mobility in Trigate SOI MOSFETs

Evidence of one-dimensional subband formation is found at low temperature in trigate silicon-on-insulator MOSFETs, resulting in oscillations of the I/sub D/(V/sub G/) characteristics. These oscillations correspond to the filling of energy subbands by electrons as the gate voltage is increased. High mobility, reaching 1200 cm/sup 2//Vs, is measured in the subbands at T=4.4 K. Subband mobility decreases as temperature is increased. Conduction in subbands disappears for temperatures higher than 100 K or for drain voltage values that are significantly larger than kT/q.

Temperature effects on trigate SOI MOSFETs

Trigate silicon-on-insulator (SOI) MOSFETs have been measured in the 5-400 K temperature range. The device fin width and height is 45 and 82 nm, respectively, and the p-type doping concentration in the channel is 6/spl times/10/sup 17/ cm/sup -3/. The subthreshold slope varies linearly with temperature as predicted by fully depleted SOI MOS theory. The mobility is phonon limited for temperatures larger than 100 K, while it is limited by surface roughness below that temperature. The corner effect, in which the device corners have a lower threshold voltage than the top and sidewall Si/SiO/sub 2/ interfaces, shows up at temperatures lower than 150 K.

Near-infrared electroluminescent devices based on colloidal HgTe quantum dot arrays

Crystalline 4.6 nm HgTe quantum dots, stabilized by 1-thioglycerol ligands, were synthesized by wet chemical methods. Room-temperature photoluminescence spectra of the dots, both in solution and as solid arrays, exhibited near-infrared emission. Light-emitting devices were fabricated by deposition of quantum dot layers onto glass∕indium tin oxide (ITO)∕3,4-polyethylene-dioxythiophene-polystyrene sulfonate (PEDOT) substrates followed by top contacting with evaporated aluminum. Room-temperature near-infrared electroluminescence from 1mm2 ITO∕PEDOT∕HgTe∕Al devices, centered at ∼1600nm, with an external quantum efficiency of 0.02% and brightness of 150nW∕mm2 at 50 mA and 2.5 V was achieved.

A versatile multi-platform biochip surface attachment chemistry

A versatile DNA spotting and immobilization method for covalent attachment of amino-modified probe oligonucleotides in microarray format at glass, native silicon dioxide and CVD silicon nitride substrates is reported. Optimal probe spot printing and attachment buffers are identified for each substrate. Relative areal densities of immobilized probes as measured by epi-fluorescence microscopy vary with substrate type reflecting differences in surface morphology and chemistry. Target oligonucleotide hybridization occurs at glass and nitride supported probe microarrays in an efficient and reproducible manner with excellent measured fluorescence signal-to-background.

Two-dimensional polymer photonic crystal band-edge lasers fabricated by nanoimprint lithography

We report on the fabrication and characterization of two-dimensional polymer photonic crystal band-edge lasers operating in the visible range. The components have been fabricated in a dye chromophore-loaded polymer matrix by nanoimprint lithography. High-symmetry band-edge modes are used to generate laser emission. The experimental lasing frequencies are in good agreement with those calculated using a two-dimensional plane wave algorithm. These results demonstrate the potential of nanoimprint lithography for the fabrication of two-dimensional planar photonic crystal structures in an active medium in a one-step process.