C.R.M. Grovenor

Gas permeation in silicon-oxide/polymer (SiOx/PET) barrier films: role of the oxide lattice, nano-defects and macro-defects

We propose a model for permeation in oxide coated gas barrier films. The model accounts for diffusion through the amorphous oxide lattice, nano-defects within the lattice, and macro-defects. The presence of nano-defects indicate the oxide layer is more similar to a nano-porous solid (such as zeolite) than silica glass with respect to permeation properties. This explains why the permeability of oxide coated polymers is much greater, and the activation energy of permeation much lower, than values expected for polymers coated with glass. We have used the model to interpret permeability and activation energies measured for the inert gases (He, Ne and Ar) in evaporated SiOx films of varying thickness (13-70 nm) coated on a polymer substrate. Atomic force and scanning electron microscopy were used to study the structure of the oxide layer. Although no defects could be detected by microscopy, the permeation data indicate that macro-defects (>1 nm), nano-defects (0.3-0.4 nm) and the lattice interstices (<0.3 nm) all contribute to the total permeation

Efficient perovskite solar cells by metal ion doping

Realizing the theoretical limiting power conversion efficiency (PCE) in perovskite solar cells requires a better understanding and control over the fundamental loss processes occurring in the bulk of the perovskite layer and at the internal semiconductor interfaces in devices. One of the main challenges is to eliminate the presence of charge recombination centres throughout the film which have been observed to be most densely located at regions near the grain boundaries. Here, we introduce aluminium acetylacetonate to the perovskite precursor solution, which improves the crystal quality by reducing the microstrain in the polycrystalline film. At the same time, we achieve a reduction in the non-radiative recombination rate, a remarkable improvement in the photoluminescence quantum efficiency (PLQE) and a reduction in the electronic disorder deduced from an Urbach energy of only 12.6 meV in complete devices. As a result, we demonstrate a PCE of 19.1% with negligible hysteresis in planar heterojunction solar cells comprising all organic p and n-type charge collection layers. Our work shows that an additional level of control of perovskite thin film quality is possible via impurity cation doping, and further demonstrates the continuing importance of improving the electronic quality of the perovskite absorber and the nature of the heterojunctions to further improve the solar cell performance.

NanoSIMS analysis of arsenic and selenium in cereal grain

*Cereals are an important source of selenium (Se) to humans and many people have inadequate intakes of this essential trace element. Conversely, arsenic (As) is toxic and may accumulate in rice grain at levels that pose a health risk. Knowledge of the localization of selenium and arsenic within the cereal grain will aid understanding of their deposition patterns and the impact of processes such as milling. *High-resolution secondary ion mass spectrometry (NanoSIMS) was used to determine the localization of Se in wheat (Triticum aestivum) and As in rice (Oryza sativa). Combined synchrotron X-ray fluorescence (S-XRF) and NanoSIMS analysis utilized the strengths of both techniques. *Selenium was concentrated in the protein surrounding the starch granules in the starchy endosperm cells and more homogeneously distributed in the aleurone cells but with Se-rich hotspots. Arsenic was concentrated in the subaleurone endosperm cells in association with the protein matrix rather than in the aleurone cells. NanoSIMS indicated that the high intensity of As identified in the S-XRF image was localized in micron-sized hotspots near the ovular vascular trace and nucellar projection. *This is the first study showing subcellular localization in grain samples containing parts per million concentrations of Se and As. There is good quantitative agreement between NanoSIMS and S-XRF.

Characterization of transparent aluminium oxide and indium tin oxide layers on polymer substrates

Abstract The structural, mechanical and gas barrier properties of aluminium oxide and indium tin oxide coatings deposited by DC reactive sputtering on poly(ethylene terephthalate) has been investigated. The oxygen and water vapour transmission rates of the films were measured as a function of temperature in order to assess the effectiveness of the oxides as gas barriers. Both types of composites exhibited good resistance to water vapour transmission. However, the indium tin oxide coating was significantly more effective as a gas barrier than the conventional aluminium oxide. The microstructural integrity of the oxide coatings was examined using scanning electron microscopy and atomic force microscopy. It was found that the sputtered layers featured a relatively low density of defects. The aluminium oxide films featured larger and many more defects than the indium tin oxide composites. This difference, along with a preliminary observation of a difference in the chemical interaction of water vapour with indium tin oxide (ITO) and aluminium oxide (AlOx), is believed to offer an explanation to the observed gas barrier properties. In addition to displaying good gas barrier characteristics, the ITO films were found to have better resistance to mechanical damage than either aluminium oxide and silicon oxide films.

Characterisation of aluminium oxynitride gas barrier films

In the last decade, metal oxide layers deposited on polymer substrates have been utilised as gas barrier films in food packaging as an alternative to the traditional aluminium foil. The resistance of these composite films to gas transmission is controlled predominantly by nano-scale defects created during the fabrication of the oxide layer. The size and density of these defects are believed to be strongly dependent on the intrinsic properties of the metal oxide layer. Changing the chemical composition of these coatings is one possible method to enhance the gas barrier properties of the films. In this work, aluminium oxynitride films, fabricated by reactive magnetron sputtering on Poly (ethylene terephthalate) substrates, have been investigated using a range of analytical techniques including: scanning proton microprobe; atomic force microscopy; scanning electron microscopy; transmission electron microscopy; uni-axial tensile testing; and gas permeation measurements to characterise the gas barrier properties of the film. The structural observations have been correlated with the measurements of the oxygen and water vapour permeation of the composite. Oxygen transmission rates as low as 1 cm3/m2 day·atm and water vapour transmission rates below 0.2 g/m2 day have been measured, and these competitive values can be explained by the relatively low density of defects in the barrier layers.

High Resolution Secondary Ion Mass Spectrometry Reveals the Contrasting Subcellular Distribution of Arsenic and Silicon in Rice Roots

Rice (Oryza sativa) takes up arsenite mainly through the silicic acid transport pathway. Understanding the uptake and sequestration of arsenic (As) into the rice plant is important for developing strategies to reduce As concentration in rice grain. In this study, the cellular and subcellular distributions of As and silicon (Si) in rice roots were investigated using high-pressure freezing, high-resolution secondary ion mass spectrometry, and transmission electron microscopy. Rice plants, both the lsi2 mutant lacking the Si/arsenite efflux transporter Lsi2 and its wild-type cultivar, with or without an iron plaque, were treated with arsenate or arsenite. The formation of iron plaque on the root surface resulted in strong accumulation of As and phosphorous on the epidermis. The lsi2 mutant showed stronger As accumulation in the endodermal vacuoles, where the Lsi2 transporter is located in the plasma membranes, than the wild-type line. As also accumulated in the vacuoles of some xylem parenchyma cells and in some pericycle cells, particularly in the wild-type mature root zone. Vacuolar accumulation of As is associated with sulfur, suggesting that As may be stored as arsenite-phytochelatin complexes. Si was localized in the cell walls of the endodermal cells with little apparent effect of the Lsi2 mutation on its distribution. This study reveals the vacuolar sequestration of As in rice roots and contrasting patterns of As and Si subcellular localization, despite both being transported across the plasma membranes by the same transporters.

A microstructural study of transparent metal oxide gas barrier films

The relationship between the microstructure and the water vapour transmission rates of aluminium oxide and aluminium coatings deposited by magnetron sputtering on polyethylene terephthalate have been investigated. The gas barrier properties of the films have been measured as a function of temperature and a range of techniques used to characterize the coatings including atomic force microscopy, which also provided information on the early growth mechanism. It was found that the Al/PET film showed a better water vapour barrier than the AlOx/PET although the activation energy for water vapour permeation was the same for both. We propose that the interaction of water with the barrier coating plays a significant part in determining the observed gas barrier performance.

Nanoscale imaging reveals laterally expanding antimicrobial pores in lipid bilayers

Antimicrobial peptides are postulated to disrupt microbial phospholipid membranes. The prevailing molecular model is based on the formation of stable or transient pores although the direct observation of the fundamental processes is lacking. By combining rational peptide design with topographical (atomic force microscopy) and chemical (nanoscale secondary ion mass spectrometry) imaging on the same samples, we show that pores formed by antimicrobial peptides in supported lipid bilayers are not necessarily limited to a particular diameter, nor they are transient, but can expand laterally at the nano-to-micrometer scale to the point of complete membrane disintegration. The results offer a mechanistic basis for membrane poration as a generic physicochemical process of cooperative and continuous peptide recruitment in the available phospholipid matrix.

Combined NanoSIMS and synchrotron X-ray fluorescence reveal distinct cellular and subcellular distribution patterns of trace elements in rice tissues

The cellular and subcellular distributions of trace elements can provide important clues to understanding how the elements are transported and stored in plant cells, but mapping their distributions is a challenging task. The distributions of arsenic, iron, zinc, manganese and copper, as well as physiologically related macro-elements, were mapped in the node, internode and leaf sheath of rice (Oryza sativa) using synchrotron X-ray fluorescence (S-XRF) and high-resolution secondary ion mass spectrometry (NanoSIMS). Although copper and silicon generally showed cell wall localization, arsenic, iron and zinc were strongly localized in the vacuoles of specific cell types. Arsenic was highly localized in the companion cell vacuoles of the phloem in all vascular bundles, showing a strong co-localization with sulfur, consistent with As(III)-thiol complexation. Within the node, zinc was localized in the vacuoles of the parenchyma cell bridge bordering the enlarged and diffuse vascular bundles, whereas iron and manganese were localized in the fundamental parenchyma cells, with iron being strongly co-localized with phosphorus in the vacuoles. The highly heterogeneous and contrasting distribution patterns of these elements imply different transport activities and/or storage capacities among different cell types. Sequestration of arsenic in companion cell vacuoles may explain the limited phloem mobility of arsenite.

How cracks in SiOx-coated polyester films affect gas permeation

In this paper theoretical models have been established that can account for the gas transmission through nanocomposite laminates, consisting of an oxide layer of finite permeability containing defects, on a polymer sheet of finite thickness. The defect shapes can either be in the form of long cracks or rectangular holes. The models offer a choice of exact numerical calculations or fast and intuitive analytical approximations. The experimental measurements of oxygen permeation through four different SiOx/poly (ethylene terephthalate) samples that were strained to produce distributions or cracks showed good agreement when compared with predicted results from the approximate analytic model. As a consequence of this observation, a key practical conclusion is that, because of the logarithmic dependence of transmission on the width of a crack, for a given strain it is better to have a small number of large cracks rather than a large number of small cracks