John Harrington

Plant-driven fungal weathering: Early stages of mineral alteration at the nanometer scale

Plant-driven fungal weathering is a major pathway of soil formation, yet the precise mechanism by which mycorrhiza alter minerals is poorly understood. Here we report the first direct in situ observations of the effects of a soil fungus on the surface of a mineral over which it grew in a controlled experiment. An ectomycorrhizal fungus was grown in symbiosis with a tree seedling so that individual hyphae expanded across the surface of a biotite flake over a period of three months. Ultramicroscopic and spectroscopic analysis of the fungus-biotite interfaces revealed intimate fungal-mineral attachment, biomechanical forcing, altered interlayer spacings, substantial depletion of potassium (~50 nm depth), oxidation of the biotite Fe(II), and the formation of vermiculite and clusters of Fe(III) oxides. Our study demonstrates the biomechanical-chemical alteration interplay at the fungus-biotite interface at the nanometer scale. Specifically, the weathering process is initiated by physical distortion of the lattice structure of biotite within 1 μm of the attached fungal hypha. Only subsequently does the distorted volume become chemically altered through dissolution and oxidation reactions that lead to mineral neoformation.

Characterization of dentine structure in three dimensions using FIB‐SEM

All biological tissues are three dimensional and contain structures that span a range of length scales from nanometres through to hundreds of millimetres. These are not ideally suited to current three‐dimensional characterization techniques such as X‐ray or transmission electron tomography. Such detailed morphological analysis is critical to understanding the structural features relevant to tissue function and designing therapeutic strategies intended to address structural deficiencies encountered in pathological states. We show that use of focused ion beam milling combined with scanning electron microscopy can provide three‐dimensional information at nanometre resolution from biologically relevant volumes of material, in this case dentine.

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO<inf>3</inf>)nBi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf>, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi<inf>5</inf>Fe<inf>0.5</inf>Co<inf>0.5</inf>Ti<inf>3</inf>O<inf>15</inf>, where n = 1 with half the Fe³⁺ sites substituted by Co³⁺ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe<inf>2</inf>O<inf>4</inf>. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M<inf>r</inf> ≈ 76 memu/g which dropped down to almost zero (M<inf>r</inf> ≈ 0.8 memu/g) at 460 °C, far lower than the anticipated for CoFe<inf>2</inf>O<inf>4</inf>.

STEM mode in the SEM: A practical tool for nanotoxicology

Abstract The addition of a transmitted electron detector to a scanning electron microscope (SEM) allows the recording of bright and dark field scanning transmission electron microscope (STEM) images and the corresponding in-lens secondary electron images from the same region of a thin sample. These combined imaging techniques have been applied here to the analysis of ultrathin sections of cells exposed in vitro to nanomaterials for toxicology investigation. Electron microscopy in general permits the exact nature of the interaction of nanomaterials and cells to be elucidated, and in addition the use of STEM mode in the SEM enables the easy identification and exclusion of artefacts produced by ultramicrotome sectioning. The imaging and analysis obtained by using the STEM mode in the SEM configuration from three different nanomaterial systems of importance (iron oxide nanoparticles, single-walled carbon nanotubes and cadmium selenide quantum dots) indicate that it is a simple, practical and cost-effective tool for nanotoxicological research.

Active glass–polymer superlattice structure for photonic integration

We propose an all-laser processing approach allowing controlled growth of organic-inorganic superlattice structures of rare-earth ion doped tellurium-oxide-based glass and optically transparent polydimethyl siloxane (PDMS) polymer; the purpose of which is to illustrate the structural and thermal compatibility of chemically dissimilar materials at the nanometer scale. Superlattice films with interlayer thicknesses as low as 2 nm were grown using pulsed laser deposition (PLD) at low temperatures (100 °C). Planar waveguides were successfully patterned by femtosecond-laser micro-machining for light propagation and efficient Er(3+)-ion amplified spontaneous emission (ASE). The proposed approach to achieve polymer-glass integration will allow the fabrication of efficient and durable polymer optical amplifiers and lossless photonic devices. The all-laser processing approach, discussed further in this paper, permits the growth of films of a multitude of chemically complex and dissimilar materials for a range of optical, thermal, mechanical and biological functions, which otherwise are impossible to integrate via conventional materials processing techniques.

LASER TRANSFER PROCESSING AND THE INTEGRATION OF FERROELECTRIC FILMS

ABSTRACT Thin films of barium strontium titanate (BST), lanthanum modified bismuth titanate (LBT) and lead zirconate titanate (PZT) were fabricated by sol-gel methods onto sapphire substrates. They were transferred to a second substrate by using UV eximer laser radiation to delaminate the films from the fabrication substrate. Scanning electron microscopy revealed melting at the interfacial region of the LBT and PZT films, thus providing the delamination mechanism, but melting was not observed in the BST films. Only the PZT film, after laser transfer, retained its ferroelectric properties, with remnant polarisation of ∼30 μC/cm2, and a high coercive field of 150 kV/cm.

Exploring backscattered imaging in low voltage FE-SEM

Contrast levels in backscattered SEM images were investigated, utilising stage deceleration for low voltage imaging and also electron energy filtering. Image contrast variations are explained via use of Monte Carlo simulations which can predict the optimum accelerating and filter voltages for imaging complex sample mixtures.

STEM mode in the SEM for the analysis of cellular sections prepared by ultramicrotome sectioning

The use of the dual imaging capabilities of a scanning electron microscope fitted with a transmitted electron detector is highlighted in the analysis of samples with importance in the field of nanotoxicology. Cellular uptake of nanomaterials is often examined by transmission electron microscopy of thin sections prepared by ultramicrotome sectioning. Examination by SEM allows for the detection of artefacts caused by sample preparation (eg. nanomaterial pull-out) and the complementary STEM mode permits study of the interaction between nanomaterials and cells. Thin sections of two nanomaterials of importance in nanotoxicology (cadmium selenide quantum dots and single walled carbon nanotubes) are examined using STEM mode in the SEM.

TEM Characterization of a Magnetic Tunnel Junction

TEM characterization of a CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) has been performed. A TEM cross section was prepared using focused ion beam (FIB) milling techniques. However, the high energy Ga+ beam causes sample damage and induces the redeposition of the sputtered materials on the section surface. Complementary investigation of the crystal structure of the active trilayer in the MTJ was performed by depositing films directly onto a TEM holey carbon film. The TEM imaging, selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) analysis were employed to study the nanostructure. The MgO layer is found to be incompletely crystalline with randomly oriented MgO crystallites. The CoFeB layer is amorphous and is homogenously deposited.