Associate Professor Vicki Keast

The corrosion of silver nanoparticles (AgNPs) on exposure to ambient air was studied using imaging and analysis in the scanning transmission electron microscope (STEM). Secondary particles are formed on exposure to ambient air, and these are more numerous and more widely distributed as the relative humidity increases. Energy-dispersive X-ray analysis (EDS) confirms that the particles contain Ag and S. Electron energy loss spectra (EELS) in the valence part of the spectrum (< ~ 50¿eV) identify the corrosion product as Ag2S on comparison with spectra from reference compounds. The EELS measurements also allow for a direct visualisation of the shift in the energy of the surface plasmon peak that occurs when the corrosion product is in contact with the particle. The experiments confirm that advanced electron microscopy methods have an important role in investigating corrosion of nanoparticulate systems.

The tungsten bronzes (MxWO3) are non-stoichiometric metal oxides which have attracted interest for their potential applications in plasmonics. Although the cubic structures of the sodium tungsten bronzes (NaxWO3) have been extensively studied, reports into the tetragonal-II structures (TII-) of any tungsten bronze are comparatively rare. In this work, TII-NaxWO3 and TII-KxWO3 were prepared by a furnace-assisted method, and characterised by X-ray and neutron powder diffraction, selected-area electron diffraction and electron energy-loss spectroscopy (EELS). A structural determination of TII-NaxWO3 was performed in the space group I4/m, in a 2×2×2 supercell of the P4/mbm TII-KxWO3 structure. Density functional theory is then used to calculate the valence electronic structure and optical properties to support the EELS measurements. Similar to the cubic structures, the conduction band of TII- is composed of hybridised O 2p and W 5d states, which is filled by electrons donated from the inserted Na or K. The O sites which are in-plane with the W sites are found to have highly localised bands, resulting in low-energy interband transitions in the a/b direction of the unit cell. In contrast, purely free-electron behaviour is observed in the c-direction below 2 eV. High-quality plasmon resonances are thus only observed in the c-direction, with energy and quality similar to those of the cubic structures. These anisotropic optical properties make TII-NaxWO3 and TII-KxWO3 interesting materials for further study as potential plasmonic materials.

Sodium tungsten bronze (NaxWO3) is a promising alternative plasmonic material to nanoparticulate gold due to its strong plasmonic resonances in both the visible and near-infrared (NIR) regions. Additional benefits include its simple production either as a bulk or a nanoparticle material at a relatively low cost. In this work, plasmonic NaxWO3nanoparticles were introduced and mixed into the nanoparticulate zinc oxide electron transport layer of a water processed poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) nanoparticle (NP) based organic photovoltaic device (NP-OPV). The power conversion efficiency of NP-OPV devices with NaxWO3NPs added was found to improve by around 35% compared to the control devices, attributed to improved light absorption, resulting in an enhanced short circuit current and fill factor.

The effect of aluminium on the optical properties of AlxAu1-x (0 < x < 0.12) substitutional solid solutions is determined. Reflectance spectra and ellipsometric data of a range of compositions are measured and the dielectric functions extracted. Al additions flatten the reflection edge and increase optical loss in the Drude region. The implications of these changes for selected technological applications are investigated. Aluminium is found to be a very effective bleaching agent for Au and could have application for the formulation of improved 'white gold' jewellery alloys. However, due to its limited solubility in Au a-phase, other alloying additions would still be required to make a commercially acceptable 'white gold'. In contrast, the increased optical loss brought by the Al makes the alloys less suitable than pure Au for plasmonic applications or for spectrally selective coatings.

Excessive corrosion of silver nanoparticles is a significant impediment to their use in a variety of potential applications in the biosensing, plasmonic and antimicrobial fields. Here we examine the environmental degradation of triangular silver nanoparticles (AgNP) in laboratory air. In the early stages of corrosion, transmission electron microscopy shows that dissolution of the single-crystal, triangular, AgNP (side lengths 50¿120¿nm) is observed with the accompanying formation of smaller, polycrystalline Ag particles nearby. The new particles are then observed to corrode to Ag2S and after 21¿days nearly full corrosion has occurred, but some with minor Ag inclusions remaining. In contrast, a bulk Ag sheet, studied in cross section, showed an adherent corrosion layer of only around 20¿50¿nm in thickness after over a decade of being exposed to ambient air. The results have implications for antibacterial properties and ecotoxicology of AgNP during corrosion as the dissolution and reformation of Ag particles during corrosion will likely be accompanied by the release of Ag+ ions.

Equiatomic (Au,Cu) solid solution orders below 658 K to form a tetragonal AuCu (I) phase with significant changes in physical properties and the crystal structure. The effect of ordering on the dielectric function of the material is controversial however, with inconsistent results reported in the literature. Since the nature of any localized surface plasmon resonance (LSPR) in the nanostructures is very sensitive to the dielectric function, this uncertainty hinders the use of AuCu in plasmonic devices or structures. Therefore, we re-examine the question using a combination of measurements and computations. We find that no significant change in the dielectric function occurs when this material becomes ordered, at least over the range of photon energies relevant to LSPRs. The likely properties of LSPRs in plasmonic devices made of AuCu are analyzed. Use of the alloy offers some advantages over pure Cu, however pure Au would still be the superior option in most situations.

The color of Cu-Zn brasses range from the red of copper through bright yellow to grey-silver as the Zn content increases. Here we examine the mechanism by which these color changes occur. The optical properties of this set of alloys has been calculated using density functional theory (DFT) and compared to experimental spectroscopy measurements. The optical response of the low Zn content a-brasses is shown to have a distinctly different origin to that in the higher content ß', ¿ and e-brasses. The response of ß'-brass is unique in that it is strongly influenced by an overdamped plasmon excitation and this alloy will also have a strong surface plasmon response.

Dark-mode plasmon resonances can be excited by positioning a suitable nano-antenna above a nanostructure to couple a planar incident wave-front into a virtual point source. We explore this phenomenon using a prototypical nanostructure consisting of a silver nanotriangle into which a hole has been drilled and a rod-like nano-antenna of variable aspect ratio. Using numerical simulations, we establish the behavior of the basic drilled nanotriangle under plane wave illumination and electron beam irradiation to provide a baseline, and then add the nano-antenna to investigate the stimulation of additional dark-mode plasmon resonances. The introduction of a suitably tuned nano-antenna provides a new and general means of exciting dark-mode resonances using plane wave light. The resulting system exhibits a very rich variety of radiant and sub-radiant resonance modes.

Time-dependent density functional theory (TDDFT) was used to calculate the optical absorption spectra of gold clusters of 20-171 atoms. The spectra for the smallest clusters agree with previous results, and the spectra for the largest clusters show features consistent with classical Mie theory. The systematic exploration of particles of sizes within these two extremes has allowed the trends linking optical absorption spectra and particle size and symmetry to be identified. A transition from molecular-like spectra to a more classical response is observed. © 2014 American Chemical Society.

the sp3 (diamond like bonding) fraction and surface properties are the important determining factors among all structural and compositional characteristics which control the biocompatibility of diamond-like carbon (DLC) coating. Previous reports in this area have not adequately correlated these factors with coating optimization for specific biomedical applications. The present work is aimed at studying the effect of the structural and compositional properties on the biocompatibity of both hydrogenated and unhydrogenated tetrahedral DLC films produced by commercial deposition methods. Electron energy loss spectroscopy (EELS) and X-Ray photoelectron spectroscopy (XPS) were used for structural and compositional characterization. The human bone derived cells (HBDC) 3 day cell-culture showed good attachment to the DLC surfaces but no significant difference in morphology on different types of DLC. Blood protein adsorption study showed an increased albumin to fibrinogen ratio as the sp 3 fraction increases. © 2006 IEEE.

Electron energy-loss spectroscopy (EELS) is widely used to study the composition and electronic structure of materials at the nanometre scale. The low-loss region of the EEL spectrum (< ~50 eV) is of particular interest for the study of optical properties. Examples of the applications of EELS to measure the optical properties, and, in particular, of individual nanoparticles will be presented. Comparison to calculations of optical properties and EEL spectra using density functional theory will be given.

The quality of GaN epilayers was studied as a function of buffer layer thickness and annealing time. By increasing the buffer layer thickness and annealing time, the structural quality, mobility and photoluminescence of the epilayers were significantly improved.

Electron energy loss spectroscopy and density functional theory have been used to show that there is a covalent component to the bonding in NiAl, CoAl and FeAl, between the transition metal atom and Al. There is no charge transfer and no ionic component to the bonding in NiAl and probably not in CoAl and FeAl. The bonding in non-stoichiometric NiAl is studied. Preliminary results are given for a £3 boundary in NiAl. © 1999 Materials Research Society.