Evaluating focused ion beam induced damage in soft materials.

Focused ion beam (FIB) microscopy uses Ga(+) ions to remove material from a sample for a variety of imaging and preparation techniques. While considerable work has examined the effects of FIB exposure on a number of materials, optimized FIB conditions for use with softer polymeric materials are yet to be determined. In this report we use phase contrast AFM to measure local changes in the elastic modulus of polycarbonate surfaces parallel to a sectioning FIB at varying beam energies. We show that polycarbonate surfaces exposed to lower FIB energies appear stiffer than the bulk material whereas surfaces exposed to the higher beam energies of up to 25keV are more representative of the bulk material. Energy dispersive spectroscopy (EDS) indicates that the polymer surfaces become stiffer because of Ga(+) implantation from the FIB. Our experimental observations are supported by computer simulations showing an increase in the residual Ga(+) concentration near-surface at lower FIB energies. A high energy FIB is therefore shown to be less invasive, producing a surface more representative of the bulk material, than using low energy FIB when sectioning polymers.

The application of STEM and in situ controlled dehydration to bacterial systems using ESEM.

Transmission imaging with an environmental scanning electron microscope (ESEM) (Wet STEM) is a recent development in the field of electron microscopy, combining the simple preparation inherent to ESEM work with an alternate form of contrast available through a STEM detector. Because the technique is relatively new, there is little information available on how best to apply this technique and which samples it is best suited for. This work is a description of the sample preparation and microscopy employed by the authors for imaging bacteria with Wet STEM (scanning transmission electron microscopy). Three different bacterial samples will be presented in this study: first, used as a model system, is Escherichia coli for which the contrast mechanisms of STEM are demonstrated along with the visual effects of a dehydration-induced collapse. This collapse, although clearly in some sense artifactual, is thought to lead to structurally meaningful morphological information. Second, Wet STEM is applied to two distinct bacterial systems to demonstrate the novel types of information accessible by this approach: the plastic-producing Cupriavidus necator along with wild-type and ΔmreC knockout mutants of Salmonella enterica serovar Typhimurium. Cupriavidus necator is shown to exhibit clear internal differences between bacteria with and without plastic granules, while the ΔmreC mutant of S. Typhimurium has an internal morphology distinct from that of the wild type.

Ultra-high resolution nano-characterisation and analysis using advanced S/TEM.

As the frontiers of nanotechnology are expanded ever further, so too we must push back the boundaries of imaging and analysis. The need for tools that can deliver new, ultra-high resolution information is driving the development of electron microscopy and spectroscopy to the extremes of performance. For example, aberration-corrected TEM (transmission electron microscopy) and STEM (scanning transmission electron microscopy) gives us the ability to work at sub-angstrom length-scales. Combine this capability with an unprecedented electron beam energy resolution, and spectroscopy at the atomic level revealing knowledge about inter-atomic bonding becomes a fact. This enables full characterization of chemical composition, electronic structure and mechanical properties. In addition, there is scope for capturing time-resolved structural transformations with sub-nanometer detail, enabling us to directly observe and understand the dynamics of a range of chemical processes in situ.

New methods for the study and fabrication of nano-structured materials using FIB SEM.

Techniques for characterisation and methods for fabrication at the nanoscale are becoming more powerful, giving new insights into the spatial relationships between nanostructures and greater control over their development. A case in point is the application of state-of-the-art focused ion beam technology (FIB), in combination with high-performance scanning electron microscopy (SEM), to generate cross-sections into bulk material and create a sequential image series. These two-dimensional images can then be correlated and rendered into a three-dimensional representation. In addition, site-specific, ultra-thin lamellar specimens can be made for observation in the transmission electron microscope (TEM) or scanning transmission electron microscope (STEM), with the further advantage that FIB cutting through hard-soft interfaces poses fewer difficulties compared to ultramicrotomy. Another big impact of FIB SEM on nanotechnology is the ability to use either ions or electrons to perform advanced nanolithography, via etching or chemical vapour deposition. In all cases, numerous parameters must be considered in order to achieve high quality results, particularly where stringent critical dimensions are required or when dealing with challenges such as electrically insulating and/or soft materials. We have developed strategies to address these issues, enabling results across a wide range of nanotechnology applications.

The crystallization kinetics and morphology of nitric acid trihydrate.

Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.

Recent advances in electron imaging, image interpretation and applications: environmental scanning electron microscopy.

One of the latest developments in electron microscopy is the environmental scanning electron microscope (ESEM), which enables soft, moist and/or electrically insulating materials to be viewed without pre-treatment, unlike conventional scanning electron microscopy, in which specimens must be solid, dry and usually electrically conductive. Such an advance has significant implications for studies of the 'native' surfaces of specimens including rocks and minerals, polymers, biological tissues and cells, food and pharmaceutical products, precious artefacts and forensic material, for example. Previous types of electron microscopes made scientists think carefully about the physics of electron-beam interactions with specimens and, hence, the interpretation of images. We now face additional factors influencing the emission and detection of electron signals, unique to the imaging of specimens in the partial vacuum of an ESEM. Just as importantly, we must consider the thermodynamic and kinetic stability of specimens, as appropriate, and explore the possibilities for new applications, particularly those of a dynamic nature. This paper briefly describes some of the issues involved and reviews the current state of understanding.