Vapour-liquid-solid growth of ZnO-ZnMgO core-shell nanowires by gold-catalysed molecular beam epitaxy

Nanowire heterostructures, combining multiple phases within a single nanowire, modify functional properties and offer a platform for novel device development. Here, ZnO/ZnMgO core-shell nanowires are grown by molecular beam epitaxy. At growth temperatures above 750 °C, Mg diffuses into ZnO making heterostructure growth impossible; at lower shell-growth temperatures (500 °C), the core-shell structure is retained. Even very thin ZnMgO shells show increased intensity photoluminescence (PL) across the ZnO band-gap and a suppression in defect-related PL intensity, relative to plain ZnO nanowires. EDX measurements on shell thickness show a correlation between shell thickness and core diameter which is explained by a simple growth model.

Effect of lithographically-induced strain relaxation on the magnetic domain configuration in microfabricated epitaxially grown Fe81Ga19

We investigate the role of lithographically-induced strain relaxation in a micron-scaled device fabricated from epitaxial thin films of the magnetostrictive alloy Fe₈₁Ga₁₉. The strain relaxation due to lithographic patterning induces a magnetic anisotropy that competes with the magnetocrystalline and shape induced anisotropies to play a crucial role in stabilising a flux-closing domain pattern. We use magnetic imaging, micromagnetic calculations and linear elastic modelling to investigate a region close to the edges of an etched structure. This highly-strained edge region has a significant influence on the magnetic domain configuration due to an induced magnetic anisotropy resulting from the inverse magnetostriction effect. We investigate the competition between the strain-induced and shape-induced anisotropy energies, and the resultant stable domain configurations, as the width of the bar is reduced to the nanoscale range. Understanding this behaviour will be important when designing hybrid magneto-electric spintronic devices based on highly magnetostrictive materials.

A Direct Mapping of Max k-SAT and High Order Parity Checks to a Chimera Graph

We demonstrate a direct mapping of max k-SAT problems (and weighted max k-SAT) to a Chimera graph, which is the non-planar hardware graph of the devices built by D-Wave Systems Inc. We further show that this mapping can be used to map a similar class of maximum satisfiability problems where the clauses are replaced by parity checks over potentially large numbers of bits. The latter is of specific interest for applications in decoding for communication. We discuss an example in which the decoding of a turbo code, which has been demonstrated to perform near the Shannon limit, can be mapped to a Chimera graph. The weighted max k-SAT problem is the most general class of satisfiability problems, so our result effectively demonstrates how any satisfiability problem may be directly mapped to a Chimera graph. Our methods faithfully reproduce the low energy spectrum of the target problems, so therefore may also be used for maximum entropy inference.

Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.

Model-independent quantitative measurement of nanomechanical oscillator vibrations using electron-microscope linescans

Nanoscale mechanical resonators are highly sensitive devices and, therefore, for application as highly sensitive mass balances, they are potentially superior to micromachined cantilevers. The absolute measurement of nanoscale displacements of such resonators remains a challenge, however, since the optical signal reflected from a cantilever whose dimensions are sub-wavelength is at best very weak. We describe a technique for quantitative analysis and fitting of scanning-electron microscope (SEM) linescans across a cantilever resonator, involving deconvolution from the vibrating resonator profile using the stationary resonator profile. This enables determination of the absolute amplitude of nanomechanical cantilever oscillations even when the oscillation amplitude is much smaller than the cantilever width. This technique is independent of any model of secondary-electron emission from the resonator and is, therefore, applicable to resonators with arbitrary geometry and material inhomogeneity. We demonstrate the technique using focussed-ion-beam-deposited tungsten cantilevers of radius ~60-170 nm inside a field-emission SEM, with excitation of the cantilever by a piezoelectric actuator allowing measurement of the full frequency response. Oscillation amplitudes approaching the size of the primary electron-beam can be resolved. We further show that the optimum electron-beam scan speed is determined by a compromise between deflection of the cantilever at low scan speeds and limited spatial resolution at high scan speeds. Our technique will be an important tool for use in precise characterization of nanomechanical resonator devices.

InAs(1-x)P(x) nanowires grown by catalyst-free molecular-beam epitaxy

We report on the self-catalysed growth of vertical InAs(1-x)P(x) nanowires on Si(111) substrates by solid-source molecular-beam epitaxy. High-resolution transmission electron microscopy revealed the mixed wurtzite and zincblende structure of the nanowires. Energy dispersive x-ray spectroscopy and x-ray diffraction measurements were used to study the phosphorus content x in the InAs(1-x)P(x) nanowires, which was shown to be in the range 0-10 %. The dependence of phosphorus incorporation in the nanowires on the phosphorus flux in the growth chamber was investigated. The incorporation rate coefficients of As and P in InAs(1x)P(x) nanowires were found to be in the ratio 10 ± 5 to 1.

Fano resonance resulting from a tunable interaction between molecular vibrational modes and a double continuum of a plasmonic metamolecule

Coupling between tunable broadband modes of an array of plasmonic metamolecules and a vibrational mode of carbonyl bond of poly(methyl methacrylate) is shown experimentally to produce a Fano resonance, which can be tuned in situ by varying the polarization of incident light. The interaction between the plasmon modes and the molecular resonance is investigated using both rigorous electromagnetic calculations and a quantum mechanical model describing the quantum interference between a discrete state and two continua. The predictions of the quantum mechanical model are in good agreement with the experimental data and provide an intuitive interpretation, at the quantum level, of the plasmon-molecule coupling.

Minimization of the contact resistance between InAs nanowires and metallic contacts

We investigate different processes for optimizing the formation of Ohmic contacts to InAs nanowires. The nanowires are grown via molecular beam epitaxy without the use of metal catalysts. Metallic contacts are attached to the nanowires by using an electron beam lithography process. Before deposition of the contacts, the InAs nanowires are treated either by wet etching in an ammonium polysulfide (NH(4))(2)S(x) solution or by an argon milling process in order to remove a surface oxide layer. Two-point electrical measurements show that the resistance of the ammonium polysulfide-treated nanowires is two orders of magnitude lower than that of the untreated nanowires. The nanowires that are treated by the argon milling process show a resistance which is more than an order of magnitude lower than that of those treated with ammonium polysulfide. Four-point measurements allow us to extract an upper bound of 1.4 × 10(-7) Ω cm(2) for the contact resistivity of metallic contacts on nanowires treated by the argon milling process.

Felling of individual freestanding nanoobjects using focused-ion-beam milling for investigations of structural and transport properties

We report that, to enable studies of their compositional, structural and electrical properties, freestanding individual nanoobjects can be selectively felled in a controllable way by the technique of low-current focused-ion-beam (FIB) milling with the ion beam at a chosen angle of incidence to the nanoobject. To demonstrate the suitability of the technique, we report results for zigzag/straight tungsten nanowires grown vertically on support substrates and then felled for characterization. We also describe a systematic investigation of the effect of the experimental geometry and parameters on the felling process and on the induced wire-bending phenomenon. The method of felling freestanding nanoobjects using FIB is an advantageous new technique enabling investigations of the properties of selected individual nanoobjects.

The role of chiral local field enhancements below the resolution limit of Second Harmonic Generation microscopy

While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems.

Superconductivity of freestanding tungsten nanofeatures grown by focused-ion-beam

Freestanding tungsten composite nanofeatures were grown by focused-ion-beam-induced deposition with 1 pA ion beam current. The temperature dependent electrical measurements show that the flying nanoscale tungsten is conducting with a room temperature resistivity of 550 micro omega cm. It is also superconducting with a Tc, above 5.1 K and can be repeatedly thermally cycled. Our results suggest that FIB direct-writing of three-dimensional tungsten composites might be a potential approach to fabricate vertical nanodevices and mask-free superconducting devices.

Dissipative enhancement of the supercurrent in Tl2Ba2CaCu2O8 intrinsic Josephson junctions

We have measured dissipation-induced localization of the reaction coordinate for a metastable-state decay process in a model system with moderate damping. Specifically, the supercurrent in an array of Tl2Ba2CaCu2O8 intrinsic Josephson junctions is larger when all the junctions are in the zero-voltage state than when one or more junctions are in the voltage state since the dissipation is larger in the former case.

Studies on the fate of flocoumafen in the Japanese quail (Coturnix coturnix japonica)

1. 14C-Flocoumafen, administered to Japanese quail as a single oral or i.p. dose, was rapidly and extensively eliminated in excreta; most was eliminated within 24 h. Extensive metabolism of the rodenticide was seen, with at least 8 metabolites detected; unchanged flocoumafen comprised 9% dose. The elimination kinetics and metabolic profiles were qualitatively similar after oral and i.p. dosing. 2. The major metabolites (60% dose) were labile to beta-glucuronidase, liberating aglycones with identical chromatographic mobilities to those of the unchanged flocoumafen isomers. 3. Radioactivity was retained mostly in the liver; largely as unchanged flocoumafen associated with the mitochondrial and microsomal fractions. Elimination of radioactivity from most tissues was biphasic with an initially rapid depletion (5 days) followed by a slow terminal elimination phase. The elimination half life from liver was greater than 100 days. 4. Livers of quail receiving extended dietary exposure to flocoumafen at 5, 15 and 50 ppm had concentrations of flocoumafen (1.0 nmol/g) that were independent of dose, indicating a capacity-limited binding site. These hepatic concentrations were similar to those after a single oral dose and were also similar to those in rats. The data indicate the presence in quail liver of a saturable high affinity flocoumafin binding site with similar characteristics and capacity to that in the rat. 5. The selective toxicity of flocoumafen to rats (highly toxic) and quail (moderately toxic) appears to arise from differences in metabolism rather than from anticoagulant binding in the liver. When hepatic binding sites of rats are saturated anticoagulant action becomes lethal, whereas quail are able to survive and extensively metabolize the compound.

The percutaneous fate of the rodenticide flocoumafen in the rat: role of non-biliary intestinal excretion

1. Appreciable penetration of radioacticity occurred through rat skin following percutaneous administration of 14C-flocoumafen. At 7 days after dosing 12% of the administered radioactivity remained at the site of application, while 25% was located in the liver as unchanged flocoumafen. 2. Excretion of flocoumafen metabolites via the urine accounted for 10% dose over the 7 day experiment, this is some 30-fold greater than that seen after a single oral dose. 3. Unchanged flocoumafen comprised the major product detected in faeces. Biliary elimination was a very minor route of excretion and did not account for all of the unmodified flocoumafen present in faeces. 4. Considerable amounts of unchanged flocoumafen found associated with the contents of the large intestine after intraperitoneal administration to rats fitted with biliary fistulae indicates that, in the intact rat, flocoumafen enters the intestine by a non-biliary intestinal excretion mechanism.

Elimination and accumulation of the rodenticide flocoumafen in rats following repeated oral administration

1. Following multiple oral administration of 14C-flocoumafen to rats at 0.02 and 0.1 mg/kg per week, appreciable cellular accumulation was seen in the liver. 2. Residues in the liver increased with dose throughout the duration of the experiment (14 weeks) at the low dose, but reached a plateau after 4 weeks at the high dose. The major component was unchanged flocoumafen together with a minor polar metabolite seen also in faeces. 3. The data suggest the presence in rat liver of a saturable high-affinity binding site for flocoumafen and a second binding site of lower affinity. 4. Lethal anticoagulant action occurs only when the binding sites have become saturated. 5. A range of haematological and clinical chemistry measurements failed to predict the onset of anticoagulant toxicity seen in the high dose treatment group. 6. Flocoumafen was not extensively metabolised; at the low dose, approximately 30% of the cumulative administered dose was eliminated in the faeces within 3 days of each dosing, mainly as unchanged rodenticide. At the high dose, this value ranged from 18% after the first dose to 59% after the tenth dose. 7. Two more polar metabolites and a lipophilic compound were minor products in faeces. Amounts of the polar products increased with cumulative dosage received. The urinary route of elimination was a very minor one (less than 1.6%) at both doses.

2,4-Difluoroaniline and 4-fluoroaniline exposure: monitoring by methaemoglobin and urine analyses

Two possible methods for monitoring exposure to 2,4-difluoroaniline and 4-fluoroaniline have been investigated: measurement of methaemoglobin content in blood and measurement of urinary metabolites. Experiments using rats dosed by the oral route as a model system show that measurement of methaemoglobin content provides a very rapid and simple monitoring method, but is not very sensitive. Measurement of the ortho-hydroxy metabolites of the two compounds, as their benzoxazole derivatives, provides a much more sensitive, but complicated technique. Details of both methods are presented.