Towards predictive modelling of near-edge structures in electron energy loss spectra of AlN based ternary alloys

Although electron energy loss near edge structure analysis provides a tool for experimentally probing unoccupied density of states, a detailed comparison with simulations is necessary in order to understand the origin of individual peaks. This paper presents a density functional theory based technique for predicting the N K-edge for ternary (quasi-binary) nitrogen alloys by adopting a core hole approach, a methodology that has been successful for binary nitride compounds. It is demonstrated that using the spectra of binary compounds for optimising the core hole charge ($0.35\,\mathrm{e}$ for cubic Ti$_{1-x}$Al$_x$N and $0.45\,\mathrm{e}$ for wurtzite Al$_x$Ga$_{1-x}$N), the predicted spectra evolutions of the ternary alloys agree well with the experiments. The spectral features are subsequently discussed in terms of the electronic structure and bonding of the alloys.Comment: 11 pages, 9 figures, 1 tabl

Research in universities, independent institutes and government departments : Eleven contributions to a discussion

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69026/2/10.1177_053901846700600421.pd

Geomorphological evidence of carbonate build-up demise on equatorial margins: A case study from offshore northwest Australia

The demise of Miocene carbonate build-ups in the Browse Basin, Northwest Australia, has been explained as relating to geological and oceanographic processes. These include accelerated tectonic subsidence driven by subduction, ocean cooling following the mid-Miocene climate optimum, nutrient excess, poisoning by sediment drifts and local erosion driven by current winnowing, occurring discretely or simultaneously. Here, we critically assess the evidence for these different mechanisms using a combination of high-resolution 3-D seismic data, regional 2-D seismic profiles, and numerical stratigraphic forward modelling. Seismic interpretation and numerical modelling found that the proposed uniform subsidence rate of 125 m/Ma between 16.5 Ma and 5.33 Ma for the northern Browse Basin (Belde et al., 2017), when combined with the published estimate of eustatic sea level in Miller et al. (2005), was insufficient to drown the Miocene carbonate sequence and generate the geomorphological changes (barrier reef to isolated carbonate build-ups) observed on seismic data. Instead, a subsidence profile comprising pulses of rapid and slow subsidence is required. Significantly, our results suggest that subsidence rates exceeded 400 m/Ma in the northern Browse Basin, and that parts of the basin record the accumulation of sediment drifts. These sediment drifts are interpreted to have buried some carbonate build-ups, while suspended sediment reduced light transmissivity, inhibiting carbonate production. Thus, we postulate that current activity and excess nutrient supply are key, but often overlooked, oceanographic processes that lead to the demise of carbonate build-ups

Managing dose-, damage- and data-rates in multi-frame spectrum-imaging

As an instrument, the scanning transmission electron microscope is unique in being able to simultaneously explore both local structural and chemical variations in materials at the atomic scale. This is made possible as both types of data are acquired serially, originating simultaneously from sample interactions with a sharply focused electron probe. Unfortunately, such scanned data can be distorted by environmental factors, though recently fast-scanned multi-frame imaging approaches have been shown to mitigate these effects. Here, we demonstrate the same approach but optimized for spectroscopic data; we offer some perspectives on the new potential of multi-frame spectrum-imaging (MFSI) and show how dose-sharing approaches can reduce sample damage, improve crystallographic fidelity, increase data signal-to-noise, or maximize usable field of view. Further, we discuss the potential issue of excessive data-rates in MFSI, and demonstrate a file-compression approach to significantly reduce data storage and transmission burdens

Catalyst preparation for CMOS-compatible silicon nanowire synthesis

Metallic contamination was key to the discovery of semiconductor nanowires, but today it stands in the way of their adoption by the semiconductor industry. This is because many of the metallic catalysts required for nanowire growth are not compatible with standard CMOS (complementary metal oxide semiconductor) fabrication processes. Nanowire synthesis with those metals which are CMOS compatible, such as aluminium and copper, necessitate temperatures higher than 450 C, which is the maximum temperature allowed in CMOS processing. Here, we demonstrate that the synthesis temperature of silicon nanowires using copper based catalysts is limited by catalyst preparation. We show that the appropriate catalyst can be produced by chemical means at temperatures as low as 400 C. This is achieved by oxidizing the catalyst precursor, contradicting the accepted wisdom that oxygen prevents metal-catalyzed nanowire growth. By simultaneously solving material compatibility and temperature issues, this catalyst synthesis could represent an important step towards real-world applications of semiconductor nanowires.Comment: Supplementary video can be downloaded on Nature Nanotechnology websit

Investigating the role of microbes in mineral weathering: Nanometre-scale characterisation of the cell-mineral interface using FIB and TEM

Focused ion beam (FIB) sample preparation in combination with subsequent transmission electron microscopy (TEM) analysis are powerful tools for nanometre-scale examination of the cell-mineral interface in bio-geological samples. In this study, we used FIB-TEM to investigate the interaction between a cyanobacterium (Hassallia byssoidea) and a common sheet silicate mineral (biotite) following a laboratory-based bioweathering, incubation experiment. We discuss the FIB preparation of cross-sections of the cell mineral interface for TEM investigation. We also establish an electron fluence threshold (at 200. keV) in biotite for the transition from scanning (S)TEM electron beam induced contamination build up on the surface of biotite thin sections to mass loss, or hole-drilling within the sections. Working below this threshold fluence nanometre-scale structural and elemental information has been obtained from biotite directly underneath cyanobacterial cells incubated on the biotite for 3 months. No physical alteration of the biotite was detected by TEM imaging and diffraction with little or no elemental alteration detected by STEM-energy dispersive X-ray (EDX) elemental line-scanning or by energy filtered TEM (EF-TEM) jump ratio elemental mapping. As such we present evidence that the cyanobacterial strain of H. byssoidea did not cause any measurable alteration of biotite, within the resolution limits of the analysis techniques used, after 3 months of incubation on its surface

Z-contrast cryo-electron tomography probes shell porosity in multi-shell nanocomposites

International audienceStructures of nanoparticles are becoming increasingly complex as they keep acquiring multi-functionality. These nanoparticles often adopt a core–shell structure and comprise different types of materials. Their outer shell forms the interface between the particle core and the environment. and its porosity governs any material exchange. Current techniques for measuring porosity exist for microscopic samples but techniques adapted to nanoscale samples are wanting. We probed indirectly the porosity of an outer silica shell grown over silica nanospheres by means of metal-chelating agents located in the interior of the nanospheres. Using a high-tilt cryo-holder, we performed electron tomography with a high-angle annular dark field detector to determine the 3D structure of these multi-shell nanocomposites. Contamination was significantly reduced at 104 K. Tomographic reconstructions revealed the distribution of functional domains binding metal ions that had penetrated the porous silica shell around the core particle

Late Triassic dinosaur tracks from Penarth, south Wales

International audienceAbstract Evidence of Late Triassic large tetrapods from the UK is rare. Here, we describe a track-bearing surface located on the shoreline near Penarth, south Wales, United Kingdom. The total exposed surface is c. 50 m long and c. 2 m wide, and is split into northern and southern sections by a small fault. We interpret these impressions as tracks, rather than abiogenic sedimentary structures, because of the possession of marked displacement rims and their relationship to each other with regularly spaced impressions forming putative trackways. The impressions are large (up to c. 50 cm in length), but poorly preserved, and retain little information about track-maker anatomy. We discuss alternative, plausible, abiotic mechanisms that might have been responsible for the formation of these features, but reject them in favour of these impressions being tetrapod tracks. We propose that the site is an additional occurrence of the ichnotaxon Eosauropus , representing a sauropodomorph trackmaker, thereby adding a useful new datum to their sparse Late Triassic record in the UK. We also used historical photogrammetry to digitally map the extent of site erosion during 2009–2020. More than 1 m of the surface exposure has been lost over this 11-year period, and the few tracks present in both models show significant smoothing, breakage and loss of detail. These tracks are an important datapoint for Late Triassic palaeontology in the UK, even if they cannot be confidently assigned to a specific trackmaker. The documented loss of the bedding surface highlights the transient and vulnerable nature of our fossil resources, particularly in coastal settings, and the need to gather data as quickly and effectively as possible