F.T. Docherty

Simultaneous multianalyte identification of molecular species involved in terrorism using Raman spectroscopy

Raman spectroscopy is a form of vibrational spectroscopy that is well suited to the molecular identification of a variety of analytes, including both explosives and biological agents. The technique has been gaining more widespread interest due to improvements in instrumentation, sensitivity, and its ease of use, in comparison to other techniques. In this paper, we describe recent advances in Raman spectroscopy with respect to the detection of high-energy explosives and biological materials. In particular, emphasis is placed on the exploitation of enhancement factors that overcome traditional limitations on sensitivity, namely, surface enhancement and resonance enhancement, functionalization of target analytes, and the use of novel lab-on-a-chip technology.

Multiple labelled nanoparticles for bio detection.

Remote nanoparticle detection is required for the development of in situ biological probes. Here we describe the labelling of silver nanoparticles to produce multiply coded particles which can be detected by surface enhanced resonance Raman scattering (SERRS). There is a potential for thousands of codes to be written and read without the need for spatial resolution of components of the code. The use of these systems in bioanlaysis and in situ detection is discussed.

ELNES investigations of the oxygen K-edge in spinels.

The results of a systematic study of the oxygen K-edge electron energy-loss spectroscopy (ELNES) from a series of aluminium- and chromium-containing spinels are presented. Extra fine structure in the region up to 10 eV above the edge onset is observed for the chromium-containing compounds and is assigned to transitions to states created by mixing of oxygen 2p and metal 3d orbitals. The experimental data has been simulated using the multiple scattering code, FEFF8. Good agreement was obtained in the case of magnesium aluminate, but relatively poor agreement was obtained in the case of the chromites. The possible fingerprints in the oxygen K-edge ELNES corresponding to a high degree of inversion the spinel structure and to a tetragonal distortion of the cubic structure are discussed.

SERRS dyes - part 2. Syntheses and evaluation of dyes for multiple labelling for SERRS

The syntheses of a number of azo and azine dyes with various surface attachment groups is described. The dyes use different methods of achieving surface complexing and are evaluated for their suitability as multiple labels for SERRS. The surface complexing agents, 8-hydroxyquinoline, benzotriazole, and pyridine are both shown to form robust layers on the silver surface. The relative intensities of the SERRS signals from each dye were shown to be predictive by considering the molar absorption coefficient at the laser excitation frequency.

Surface-enhanced resonance Raman scattering in optical tweezers using co-axial second harmonic generation

Silica particles were partially coated with silver, and a suitable chromophore, such that they could be simultaneously trapped within an optical tweezers system, and emit a surface-enhanced resonance Raman scattering (SERRS) response. A standard 1064 nm TEM00 mode laser was used to trap the bead whilst a frequency doubling crystal inserted into the beam gave several microwatts of 532 nm co-linear light to excite the SERRS emission. The con fi guration has clear applications in providing apparatus that can simultaneously manipulate a particle whilst obtaining surface sensitive sensory information.

Three-dimensional optical trapping of partially silvered silica microparticles

We demonstrate three-dimensional trapping of micrometer-diameter silica particles, partially coated with silver, within conventional optical tweezers. Although metallic particles are usually repelled from the beam focus by the scattering force, we show that transparent spheres partially coated with silver can be trapped with efficiencies comparable with dielectric particles. The trapping characteristics of these particles are examined as a function of metallic coverage, and the application of these particles to surface-enhanced resonance Raman scattering is investigated.

Effect of short-range magnetic ordering on electron energy-loss spectra in spinels

The energy-loss near-edge structure (ELNES) at the oxygen K edge in two AB/sub 2/O/sub 4/ spinels (A=Mg, B=Al, Cr) is reported. In MgAl/sub 2/O/sub 4/ the experimental data are successfully modelled within the local density approximation framework. In the case of MgCr/sub 2/O/sub 4/ spin polarization, in the form of antiferromagnetic ordering on the Cr sublattice, must be included despite the fact that the measurements were performed at approximately 30 times above T/sub N/. A model in which dynamic short-range antiferromagnetic ordering is present at room temperature is proposed to explain the results of the experiments and calculations.

ELNES as a probe of magnetic order in mixed oxides

A previous study of chromite and ferrite spinels revealed energy-loss near-edge structure (ELNES) in the oxygen K-edge that could not be reproduced in non-spin polarised calculations. Chromite and ferrite spinels typically undergo transitions to long range ordered magnetic structures at temperatures below ~15K. A model in which dynamic magnetic short range order (SRO) persists above the Neel temperature until 100K has been proposed using neutron powder diffraction. In the TEM, the interaction time of the fast electron with the specimen is sufficiently short for dynamic magnetic interactions to influence the observed ELNES at 300K. Here we present new spin polarised calculations performed using the commercially available codes FEFF8.2 and Wien97. The calculated oxygen K-edge ELNES show improved agreement with experiment when magnetic interactions are included in the calculation.

Advanced Nanoanalysis of a Hf-Based High-k Dielectric Stack Prior to Activation

Analytical electron microscopy techniques are used to investigate elemental distributions across a high-k dielectric stack with a metal gate. Electron energy-loss spectroscopy results from a Si(100)/SiO 2 /HfO 2 /TiN/a-Si gate stack confirm the presence of an oxide interfacial phase at the TiN/a-Si interface prior to activation of the stack.

Understanding and preventing beam damage effects in partially processed high-k gate stacks

Electron beam damage effects on a partially and a fully processed HfO2 gate stack on silicon substrates are investigated, and their origins and prevention are discussed. Growth of silica between the silicon and hafnia layers is observed for the partially processed sample but is not seen for the fully processed wafer. Two sources of oxygen are found to react with the substrate to form silica. One is from the glue used in sample preparation. The oxygen from this source can be prevented from diffusing to the substrate by putting a gold barrier layer between the stack and the glue. The other source seems to come from the amorphous HfO2 layer. Using a cooling rod sufficiently slows the diffusion rate so that growth is no longer observed.