Pa Warburton

ZnO tetrapod nanocrystals

ZnO has received considerable attention because of its unique optical, piezoelectric, and magnetic properties. It also readily self-assembles into a family of nanocrystalline structures. We review the current status of research into ZnO tetrapod nanocrystals. These crystals consist of a ZnO core in the zinc blende structure from which four ZnO arms in the wurtzite structure radiate. The arms are cylinders of hexagonal cross section, with each arm of equal length and diameter. Possible applications in optoelectronics, photovoltaics, spintronics, and piezoelectricity are discussed.

ZnO tetrapod Schottky photodiodes

The fabrication of an ultraviolet photodiode employing a single ZnO tetrapod nanocrystal is reported. This diode structure is prepared by depositing W and Pt electrodes to form Ohmic and Schottky contacts, respectively. Dark current-voltage measurements show rectifying behavior. The properties of the metal-semiconductor interface are studied with above and below band gap illumination. It is found that with increasing UV excitation the device converts from a rectifying to an Ohmic behavior. This effect is attributed to a flattening of the energy bands due to the migration of photogenerated carriers within the space charge region at the metal-semiconductor interface.

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.

Mobility Enhancement by Sb-mediated Minimisation of Stacking Fault Density in InAs Nanowires Grown on Silicon

We report the growth of InAs(1-x)Sb(x) nanowires (0 ≤ x ≤ 0.15) grown by catalyst-free molecular beam epitaxy on silicon (111) substrates. We observed a sharp decrease of stacking fault density in the InAs(1-x)Sb(x) nanowire crystal structure with increasing antimony content. This decrease leads to a significant increase in the field-effect mobility, this being more than three times greater at room temperature for InAs0.85Sb0.15 nanowires than InAs nanowires.

Plasmon-Enhanced Sub-Wavelength Laser Ablation: Plasmonic Nanojets

In response to the incident lights electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.

Zinc Oxide Nanostructures and High Electron Mobility Nanocomposite Thin Film Transistors

This paper reports on the synthesis of zinc oxide (ZnO) nanostructures and examines the performance of nanocomposite thin-film transistors (TFTs) fabricated using ZnO dispersed in both n- and p-type polymer host matrices. The ZnO nanostructures considered here comprise nanowires and tetrapods and were synthesized using vapor phase deposition techniques involving the carbothermal reduction of solid-phase zinc-containing compounds. Measurement results of nanocomposite TFTs based on dispersion of ZnO nanorods in an n-type organic semiconductor ([6, 6]-phenyl-C61-butyric acid methyl ester) show electron field-effect mobilities in the range 0.3-0.6 cm2 V-1s-1, representing an approximate enhancement by as much as a factor of 40 from the pristine state. The on/off current ratio of the nanocomposite TFTs approach 106 at saturation with off-currents on the order of 10 pA. The results presented here, although preliminary, show a highly promising enhancement for realization of high-performance solution-processable n-type organic TFTs.

Tunability of the superconductivity of tungsten films grown by focused-ion-beam direct writing

We have grown tungsten-containing films by focused-ion-beam (FIB)-induced chemical vapor deposition. The films lie close to the metal-insulator transition with an electrical conductivity which changes by less than 5% between room temperature and 7 K. The superconducting transition temperature Tc of the films can be controlled between 5.0 and 6.2 K by varying the ion-beam deposition current. The Tc can be correlated with how far the films are from the metal-insulator transition, showing a nonmonotonic dependence, which is well described by the heuristic model of [Osofsky et al., Phys. Rev. Lett. 87, 197004 (2001)]. Our results suggest that FIB direct-writing of W composites might be a potential approach to fabricate mask-free superconducting devices as well as to explore the role of reduced dimensionality on superconductivity.

Synthesis and characterisation of zinc oxide tetrapod nanocrystals

Zinc oxide is an important group II-VI semiconductor material with optical properties that permit stable emission at room temperature. We report on the synthesis of highly uniform nanocrystalline ZnO tetrapod (ZnO-T) nanostructures through a modified chemical vapour transport process. These self assembled nanocrystals are characterised by four cylindrical arms with a hexagonal facet all of which are joined at a tetrahedral core. Studies are carried out on ZnO tetrapods using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), photoluminescence spectroscopy (PLS) and Raman measurements. We find a simple technique to quench visible emission found in ZnO tetrapods as grown. We also observe Raman active modes suggesting that nitrogen is incorporated within our samples.

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.

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.