Heather M. Yates

Photocatalytic antimicrobial activity of thin surface films of TiO2, CuO and TiO2/CuO dual layers on Escherichia coli and bacteriophage T4

TiO2-coated surfaces are increasingly studied for their ability to inactivate microorganisms. The activity of glass coated with thin films of TiO2, CuO and hybrid CuO/TiO2 prepared by atmospheric Chemical Vapour Deposition (Ap-CVD) and TiO2 prepared by a sol–gel process was investigated using the inactivation of bacteriophage T4 as a model for inactivation of viruses. The chemical oxidising activity was also determined by measuring stearic acid oxidation. The results showed that the rate of inactivation of bacteriophage T4 increased with increasing chemical oxidising activity with the maximum rate obtained on highly active sol–gel preparations. However, these were delicate and easily damaged unlike the Ap-CVD coatings. Inactivation rates were highest on CuO and CuO/TiO2 which had the lowest chemical oxidising activities. The inactivation of T4 was higher than that of Escherichia coli on low activity surfaces. The combination of photocatalysis and toxicity of copper acted synergistically to inactivate bacteriophage T4 and retained some self-cleaning activity. The presence of phosphate ions slowed inactivation but NaCl had no effect. The results show that TiO2/CuO coated surfaces are highly antiviral and may have applications in the food and healthcare industries.

Enhancement of the photonic gap of opal-based three-dimensional gratings

The “semimetallic” photonic band gap formed in a synthetic opal has been increased by depositing a layer (InP or TiO2) with high refractive index on the inner surface of opal voids. Reflectance spectra of the composites (nanolayers assembled within grating voids) are correlated with both the photonic structure of the opal and electronic structure of the semiconductor.

Opal-based photonic crystal with double photonic bandgap structure

The interior surfaces of one part of a piece of artificial opal have been coated with GaP so that the remaining part of the opal crystal remains empty, thus forming a photonic heterostructure. Two Bragg resonances have been observed in the optical transmission and reflectance spectra. These two resonances were found to behave differently with changes in the polarization of the incident light and the angle of propagation of the light with respect to the (111) planes of opal. Depolarization of the light was observed to occur most effectively at frequencies within the stop-bands, apparently due to the re-coupling of the propagating electromagnetic wave to a different system of eigenmodes when it crosses the interface separating two parts of the double photonic crystal.

Novel quantum confined structures via atmospheric pressure MOCVD growth in asbestos and opals

Porous host materials of opal and asbestos have been infilled with InP by atmospheric pressure MOCVD. These were characterised by a variety of techniques including PL at 4 and 300 K. The PL gave clear evidence of quantum confinement with the InP bound exciton blue shifted by at least 0.2 eV. OpalInP gave the additional characteristic of controlling the stop-band of the host. These all represent a new method of producing easily contactable quantum device structures.

Optical properties of self-assembled arrays of InP quantum wires confined in nanotubes of chrysotile asbestos

Three-dimensional arrays of structurally confined InP wire-like nanostructures were grown in channels (nanotubes) of a chrysotile asbestos matrix by metalorganic chemical vapor deposition. The formation of the InP compound was confirmed by absorption spectroscopy, X-ray diffraction and Raman scattering. It is shown that the density of states around the band edge increases with the InP loading of the matrix. Photoluminescence spectra of the asbestos filled in with InP consist mainly of two bands: a high energy band which is interpreted to be associated with charge transfer from InP to defect states of the asbestos and a low energy band which is associated with energy relaxation in the InP deposit itself. We show that the optical properties of this material are dominated by the size and dimensionality of the pore system of the matrix for heavy loading and by the semiconductor-to-matrix interface for light loading of the matrix with InP.

Biocidal Silver and Silver/Titania Composite Films Grown by Chemical Vapour Deposition

This paper describes the growth and testing of highly active biocidal films based on photocatalytically active films of , grown by thermal CVD, functionally and structurally modified by deposition of nanostructured silver via a novel flame assisted combination CVD process. The resulting composite films are shown to be highly durable, highly photocatalytically active and are also shown to possess strong antibacterial behaviour. The deposition control, arising from the described approach, offers the potential to control the film nanostructure, which is proposed to be crucial in determining the photo and bioactivity of the combined film structure, and the transparency of the composite films. Furthermore, we show that the resultant films are active to a range of organisms, including Gram-negative and Gram-positive bacteria, and viruses. The very high-biocidal activity is above that expected from the concentrations of silver present, and this is discussed in terms of nanostructure of the titania/silver surface. These properties are especially significant when combined with the well-known durability of CVD deposited thin films, offering new opportunities for enhanced application in areas where biocidal surface functionality is sought.

Atmospheric pressure MOCVD growth of crystalline InP in opals

Atmospheric pressure metal-organic chemical-vapour deposition has been used to infill the voids within synthetic opals with InP in an attempt to modify the natural photonic behaviour of these materials. The process has been optimised to increase the semiconductor loading. By increasing the extent of InP infill within the voids, which in turn increases the extent of refractive index contrast between the silica spheres and the void, it is possible to modify the opal photonic band gap in a systematic manner. The InP grown was shown to be crystalline and of high quality by use of Raman spectroscopy, X-ray diffraction, and atomic force microscopy.

Photoactive Thin Silver Films by Atmospheric Pressure CVD

We report the visible and UV activity of thin silver films. The films are grown using a CVD process employing aqueous-based silver precursors, flame-assisted chemical vapour deposition. This approach overcomes many of the previously encountered limitations to silver deposition by employing an atmospheric pressure process, low-cost and low-toxicity precursors. The resultant films are assessed for activity using stearic acid destruction as a model compound. We also report on the addition of titania to these silver films to increase the potential functionality. This activity is also demonstrated, where the films appear largely transparent to the eye, further widening the potential application of this work. It is speculated that the nanoparticulate nature, of the CVD silver, is crucial in determining photoactivity.

Optimum performance solar cells using atmospheric pressure chemical vapour deposition deposited TCOs

High performance transparent conducting oxides (TCOs), have significance for optimising PV performance. We have developed an advanced atmospheric pressure chemical vapour deposition process, by applying fast experimentation and using a combinatorial chemistry approach to aid the studies. The deposited films were characterised for crystallinity, morphology (roughness), haze and resistivity to aid optimisation of material suitable for solar cells. Optical measurements on these samples showed low absorption losses, less than 1% around 500 nm for one pass, which is much lower than those of industrially available TCOs. Selected samples were then used in manufacture of single junction a-Si : H solar cells, which showed high initial solar energy conversion efficiencies up to 9.3% and high short circuit current densities of 15 mA/cm². Compared with (commercially available) TCO CVD coated glasses, these TCO coatings show excellent performance resulting in a high quantum efficiency yield for a-Si : H solar cells.

Progression towards high efficiency perovskite solar cells via optimisation of the front electrode and blocking layer

The effects of a fluorine doped tin oxide (FTO) electrode, titanium dioxide (TiO2−x) blocking layer (BL) and perovskite (methyl ammonium lead triiodide) preparation on the overall properties of the photovoltaic cells have been studied. The FTO electrode was deposited by atmospheric pressure chemical vapour deposition (APCVD) and the hole blocking layer by spin coating, atomic layer deposition (ALD) or sputtering. We have shown the importance of obtaining uniform thin films of FTO, with low sheet resistance to aid the formation of pin hole free uniform TiO2−x blocking layers and hence well adhered, perovskite layers. The optimal BL thickness was 20 nm, while thicker films gave decreased shunt resistance and thinner a greater number of pin holes through the layers. We also showed that the conformal nature of ALD and magnetron sputtering, along with their increased uniformity control over spin coating again improved cell efficiency. The main improvement comes for the smaller Roc, attributed to an improved electrical transport through particularly the sputtered TiO2−x blocking layer. After identifying the optimised parameters, all the properties were combined to fabricate large solar cells (1 cm2) yielding power conversion efficiencies beyond 16%.