Probing Deformation and Revealing Microstructural Mechanisms with Cross-Correlation-Based, High-Resolution Electron Backscatter Diffraction

High-resolution, cross-correlation-based, electron backscatter diffraction (HR-EBSD) is an emerging technique capable of measuring elastic strains, lattice rotations, and defect populations in crystalline materials. Here we briefly review development of the technique and the fundamental method. Application of HR-EBSD to metallic samples is illustrated with three examples: nickel-superalloy matrix/carbide interactions during cyclic deformation; interaction of a slip band and grain boundary; and patterning of stress and dislocation storage in deformed copper. These three examples highlight the ability of HR-EBSD to deliver new science by revealing new insights into the fundamental nature of deformation, as well as validating existing models. Application of the technique is now commonplace, and emergence of the technique is opening it up to the wider materials science community to tackle grand challenges.     

Validation and application of adsorption breakthrough models for the chemical filters used in the make-up air unit (MAU) of a cleanroom

The dynamic adsorption capacity calculated from the breakthrough curves progressively decreased with the increases in the face velocity, suggesting that the effect of intraparticle diffusion and possibly the rate of adsorption as the rate-limiting mechanism were increasingly more profound for a chemical filter-type adsorber configuration. The Yoon–Nelson model generally matched well with the experimental breakthrough curved for breakthrough fraction less than 50%. However, the proportionality constant in the Yoon–Nelson model needed modification through the method from which the mass transfer coefficient (k_v) in the Wheeler–Jonas equation is determined. Subsequently, a series of breakthrough curves for the hypothetical toluene concentrations and face velocities simulating realistic operating conditions was generated, and their validity was verified against the adsorption capacity predicted by the Dubinin–Radushkevich isotherm equation. The useful life of a chemical filter could henceforth be estimated with confidence using the breakthrough curves predicted by the modified Yoon–Nelson model.     

Large-area nanopatterned graphene for ultrasensitive gas sensing

Chemical vapor deposited （CVD） graphene is nanopatterned using a spherical block copolymer etch mask. The use of spherical rather than cylindrical block copolymers allows homogeneous patterning of cm-scale areas without any substrate surface treatment. Raman spectroscopy was used to study the con- trolled generation of point defects in the graphene lattice with increasing etching time, confirming that alongside the nanomesh patterning, the nanopatterned CVD graphene presents a high defect density between the mesh holes. The nanopatterned samples showed sensitivities for NO2 of more than one order of magnitude higher than for non-patterned graphene. NO2 concentrations as low as 300 ppt were detected with an ultimate detection limit of tens of ppt. This is the smallest value reported so far for non-UV illuminated graphene chemiresistive NO2 gas sensors. The dramatic improvement in the gas sensitivity is believed to be due to the high adsorption site density, thanks to the combination of edge sites and point defect sites. This work opens the possibility of large area fabrication of nanopatterned graphene with extremely high densities of adsorption sites for sensing applications.     

Low energy ion scattering: surface preparation and analysis of Cu(In,Ga)Se2 for photovoltaic applications

Cu(In,Ga)Se₂ (CIGS) single‐crystal epitaxial films have been analyzed by low energy ion scattering, which is sensitive to exactly the outermost surface atomic layer, to determine the surface chemistry as a function of preparation conditions. The samples were grown by a hybrid sputtering and evaporation method on cation (A) or anion (B) terminated (111) GaAs substrates and had smooth surfaces. The samples were exposed to excited atomic oxygen or hydrogen beams or were sputtered with 500 eV Ar⁺ ions. Atomic O* treatment resulted in an otherwise clean, oxidized surface including all film constituents. Atomic H* resulted in strong enhancement of the surface Ga population, probably due to a preexisting Ga native oxide in the outermost atomic layer. Sputtering produced a clean surface that was closest to the bulk composition of the film as measured by energy‐dispersive spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.     

Low-dose aberration corrected cryo-electron microscopy of organic specimens

Spherical aberration (C(s)) correction in the transmission electron microscope has enabled sub-angstrom resolution imaging of inorganic materials. To achieve similar resolution for radiation-sensitive organic materials requires the microscope to be operated under hybrid conditions: low electron dose illumination of the specimen at liquid nitrogen temperature and low defocus values. Initial images from standard inorganic and organic test specimens have indicated that under these conditions C(s)-correction can provide a significant improvement in resolution (to less than 0.16nm) for direct imaging of organic samples.     

Analytical strategies for identifying drug metabolites

With the dramatic increase in the number of new chemical entities (NCEs) arising from combinatorial chemistry and modern high-throughput bioassays, novel bioanalytical techniques are required for the rapid determination of the metabolic stability and metabolites of these NCEs. Knowledge of the metabolic site(s) of the NCEs in early drug discovery is essential for selecting compounds with favorable pharmacokinetic credentials and aiding medicinal chemists in modifying metabolic "soft spots". In development, elucidation of biotransformation pathways of a drug candidate by identifying its circulatory and excretory metabolites is vitally important to understand its physiological effects. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) have played an invaluable role in the structural characterization and quantification of drug metabolites. Indeed, liquid chromatography (LC) coupled with atmospheric pressure ionization (API) MS has now become the most powerful tool for the rapid detection, structure elucidation, and quantification of drug-derived material within various biological fluids. Often, however, MS alone is insufficient to identify the exact position of oxidation, to differentiate isomers, or to provide the precise structure of unusual and/or unstable metabolites. In addition, an excess of endogenous material in biological samples often suppress the ionization of drug-related material complicating metabolite identification by MS. In these cases, multiple analytical and wet chemistry techniques, such as LC-NMR, enzymatic hydrolysis, chemical derivatization, and hydrogen/deuterium-exchange (H/D-exchange) combined with MS are used to characterize the novel and isomeric metabolites of drug candidates. This review describes sample preparation and introduction strategies to minimize ion suppression by biological matrices for metabolite identification studies, the application of various LC-tandem MS (LC-MS/MS) techniques for the rapid quantification and identification of drug metabolites, and future trends in this field.     

Cation segregation in Nb16W18O94 using high angle annular dark field scanning transmission electron microscopy and image processing

We report the characterization of the complex oxide Nb₁₆W₁₈O₉₄ using high angle annular dark field imaging at 200 kV in a scanning transmission electron microscope. The results of this study suggest that the W and Nb cations are not uniformly distributed among the cation columns projected along [001] but that there is preferential segregation of the heavier species to certain column sites. In order to analyse the experimental data obtained, an image processing methodology has been developed which may also find application in locating specific motifs within a generally distorted image field.     

Simulation of thermal diffuse scattering including a detailed phonon dispersion curve

Thermal vibration of the atoms in a crystal give rise to a diffuse background in the diffraction pattern (in between the normal allowed Bragg reflections). The Einstein model for phonon vibrations in a crystal leads to Gaussian statistics for the phonons. However, the Einstein model ignores the possibility of correlation between the atoms. An accurate model of the phonon dispersion curves for silicon is used to generate a set of more accurate random atomic displacements. These displacements are used in a multislice-style simulation to gauge the validity of the Einstein approximation. The phonon dispersion curve yields a small additional oscillatory structure in the thermal diffuse scattering (TDS) pattern. This does not produce significant changes in the annular dark field scanning transmission electron microscope (ADF-STEM) image signal, but could have a large impact on convergent beam measurements of bond charges.