Artifacts from manganese reduction in rock samples prepared by focused ion beam (FIB) slicing for X-ray microspectroscopic analysis

Abstract. Manganese (Mn)-rich natural rock coatings, so-called rock varnishes, are discussed controversially regarding their genesis. Biogenic and abiogenic mechanisms, as well as a combination of both, have been proposed to be responsible for the Mn oxidation and deposition process. We conducted scanning transmission X-ray microscopy - near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) measurements to examine the abundance and spatial distribution of the different oxidation states of Mn within these nano- to micrometer thick crusts. Such microanalytical measurements of thin and hard rock crusts require sample preparation with minimal contamination risk. Focused ion beam (FIB) slicing, a well-established technique in geosciences, was used in this study to obtain 100–200 nm thin slices of the samples for X-ray transmission spectroscopy. However, even though this preparation is suitable to investigate element distributions and structures in rock samples, we observed that, using standard parameters, modifications of the Mn oxidation states occur in the surfaces of the FIB slices. Based on our results, the preparation technique likely causes the reduction of Mn4+ to Mn2+/3+. We draw attention to this issue, since FIB slicing, SEM imaging, and other preparation and visualization techniques operating in the keV range are well-established in geosciences, but researchers are often unaware of the potential for reduction of Mn and possibly other elements in the samples’ surface layers

Understanding the interaction between energetic ions and freestanding graphene towards practical 2D perforation

We report experimentally and theoretically the behavior of freestanding graphene subject to bombardment of energetic ions, investigating the ability of large-scale patterning of freestanding graphene with nanometer sized features by focused ion beam technology. A precise control over the He+ and Ga+ irradiation offered by focused ion beam techniques enables to investigate the interaction of the energetic particles and graphene suspended with no support and allows determining sputter yields of the 2D lattice. We find strong dependency of the 2D sputter yield on the species and kinetic energy of the incident ion beams. Freestanding graphene shows material semi-transparency to He+ at high energies (10-30 keV) allowing the passage of >97% He+ particles without creating destructive lattice vacancy. Large Ga+ ions (5-30 keV), in contrast, collide far more often with the graphene lattice to impart significantly higher sputter yield of ~50%. Binary collision theory applied to monolayer and few-layer graphene can successfully elucidate this collision mechanism, in great agreement with experiments. Raman spectroscopy analysis corroborates the passage of a large fraction of He+ ions across graphene without much damaging the lattice whereas several colliding ions create single vacancy defects. Physical understanding of the interaction between energetic particles and suspended graphene can practically lead to reproducible and efficient pattern generation of unprecedentedly small features on 2D materials by design, manifested by our perforation of sub-5-nm pore arrays. This capability of nanometer scale precision patterning of freestanding 2D lattices shows practical applicability of the focused ion beam technology to 2D material processing for device fabrication and integration.Comment: 31 pages of main text (with 4 figures) plus 4 pages of supporting information (with 2 figures). Original article submitted to a journal for consideration for publicatio

Multiscale correlative tomography: an investigation of creep cavitation in 316 stainless steel

Creep cavitation in an ex-service nuclear steam header Type 316 stainless steel sample is investigated through a multiscale tomography workflow spanning eight orders of magnitude, combining X-ray computed tomography (CT), plasma focused ion beam (FIB) scanning electron microscope (SEM) imaging and scanning transmission electron microscope (STEM) tomography. Guided by microscale X-ray CT, nanoscale X-ray CT is used to investigate the size and morphology of cavities at a triple point of grain boundaries. In order to understand the factors affecting the extent of cavitation, the orientation and crystallographic misorientation of each boundary is characterised using electron backscatter diffraction (EBSD). Additionally, in order to better understand boundary phase growth, the chemistry of a single boundary and its associated secondary phase precipitates is probed through STEM energy dispersive X-ray (EDX) tomography. The difference in cavitation of the three grain boundaries investigated suggests that the orientation of grain boundaries with respect to the direction of principal stress is important in the promotion of cavity formation

Focused Ion Beam Recovery of Hypervelocity Impact Residue in Experimental Craters on Metallic Foils

The Stardust sample return capsule will return to Earth in January 2006 with primitive debris collected from Comet 81P/Wild-2 during the fly-by encounter in 2004. In addition to the cometary particles embedded in low-density silica aerogel, there will be microcraters preserved in the Al foils (1100 series; 100 micrometers thick) that are wrapped around the sample tray assembly. Soda lime spheres (approximately 49 m in diameter) have been accelerated with a light-gas-gun into flight-grade Al foils at 6.35 km s(sup -1) to simulate the potential capture of cometary debris. The preserved crater penetrations have been analyzed using scanning electron microscopy (SEM) and x-ray energy dispersive spectroscopy (EDX) to locate and characterize remnants of the projectile material remaining within the craters. In addition, ion beam induced secondary electron imaging has proven particularly useful in identifying areas within the craters that contain residue material. Finally, high-precision focused ion beam (FIB) milling has been used to isolate and then extract an individual melt residue droplet from the interior wall of an impact penetration. This enabled further detailed elemental characterization, free from the background contamination of the Al foil substrate. The ability to recover pure melt residues using FIB will significantly extend the interpretations of the residue chemistry preserved in the Al foils returned by Stardust

Fossil biomass preserved as graphitic carbon in a late paleoproterozoic banded iron formation metamorphosed at more than 550°C

Metamorphism is thought to destroy microfossils, partly through devolatilization and graphitization of biogenic organic matter. However, the extent to which there is a loss of molecular, elemental and isotope signatures from biomass during high-temperature metamorphism is not clearly established. We report on graphitic structures inside and coating apatite grains from the c. 1850 Ma Michigamme silicate banded iron formation from Michigan, metamorphosed above 550°C. Traces of N, S, O, H, Ca and Fe are preserved in this graphitic carbon and X-ray spectra show traces of aliphatic groups. Graphitic carbon has an expanded lattice around 3.6 Å, forms microscopic concentrically-layered and radiating polygonal flakes and has homogeneous δ13C values around −22‰, identical to bulk analyses. Graphitic carbon inside apatite is associated with nanometre-size ammoniated phyllosilicate. Precursors of these metamorphic minerals and graphitic carbon originated from ferruginous clayrich sediments with biomass. We conclude that graphite coatings and inclusions in apatite grains indicate fluid remobilization during amphibolite-facies metamorphism of precursor biomass. This new evidence fills in observational gaps of metamorphosed biomass into graphite and supports the existence of biosignatures in the highly metamorphosed iron formation from the Eoarchean Akilia Association, which dates from the beginning of the sedimentary rock record

Site specific characterisation of hydrocracking catalysts using nanoanalytical electron microscopy

During use, carbonaceous material or ‘coke’ can deposit on catalysts resulting in decreased activity and lifetime. In this thesis, the results of investigations into the structure and distribution of coke, on hydrocracking catalysts, are reported. The material consists of zeolite Y, alumina binder as well as tungsten and nickel sulfide. An extensive investigation regarding the preparation of the catalysts for electron microscopy was carried out. It was established that microtoming produced specimen damage and hence regions of porosity, zeolite and alumina binder were difficult to identify. Single beam and dual beam focused ion beam (FIB) milling produced intact specimens and the spatial distribution of the catalysts was maintained, although thinner specimens were obtained using the latter technique. Energy-dispersive X-ray (EDX) mapping identified gallium and platinum as artefacts in specimens that had been prepared by a single beam FIB system. In addition, argon ion beam milling was used and this technique produced large regions of thin material. Energy-filtered transmission electron microscopy (EFTEM) was employed to reveal the distribution of carbon in the catalyst. Carbon was identified on alumina binder, zeolite grains and meso-/macro-pores, although the distribution of carbon was generally not uniform as it is determined by the density and strength of acid sites, geometry of pores and the proximity of metal sulfide crystallites. All of these factors, especially pores size and shape, vary in the catalysts. Coke is thought to consist of polyaromatic hydrocarbons (PAHs). Electron energy-loss spectroscopy (EELS), of selected PAH standards, was performed to obtain the electron energy-loss near edge structure (ELNES) of carbon. In addition, the ELNES of four PAHs was modelled using multiple scatter calculations. EELS of the catalysts revealed that PAHs are present on zeolitic components but ELNES was not identified on the alumina binder. This is possibly because alumina contains larger pores than zeolite Y; therefore larger molecules can diffuse into the alumina structure, which increases the chemical variety of the coke species as the molecules are not sterically impeded

Site-Specific Plan-view (S)TEM Sample Preparation from Thin Films using a Dual-Beam FIB-SEM

Plan-view transmission electron microscopy (TEM) samples are key to understand the atomic structure and associated properties of materials along their growth orientation, especially for thin films that are stain-engineered onto different substrates for property tuning. In this work, we present a method to prepare high-quality plan-view samples for analytical STEM study from thin-films using a dual-beam focused ion beam scanning electron microscope (FIB-SEM) system. The samples were prepared from thin films of perovskite oxides and metal oxides ranging from 20-80 nm thicknesses, grown on different substrates using molecular beam epitaxy. A site-specific sample preparation from the area of interest is described, which includes sample attachment and thinning techniques to minimize damage to the final TEM samples. While optimized for the thin film-like geometry, this method can be extended to other site-specific plan-view samples from bulk materials. Aberration-corrected scanning (S)TEM was used to access the quality of the thin film in each sample. This enabled direct visualization of line defects in perovskite BaSnO3 and Ir particle formation and texturing in IrO2 films

Nanowires fabricated by Focused Ion Beam

This thesis reports research on nanowires fabricated by FIB lithography with experiments to understand their mechanical, electrical and hydrodynamic properties. Au nanowires fabricated on Si$_3$N$_4$ membranes with width below 50nm exhibit liquid like instabilities and below $\sim$20nm the instabilities grow destroying the nanowires due to the Rayleigh- Plateau instability. Stability is better in the case for Si substrates than for the insulators Si0$_2$ and Si$_3$N$_4$. A series of 4-terminal resistance measurements were carried out on a "platinum" nanowire grown by FIB-induced decomposition of an organometallic precursor. Such nanowires are found to be a two phase percolating system, containing up to 70% by volume carbon. They have unexpected temperature behaviour which is explained using a percolation model with Kirkpatrick conduction in the presence of temperature induced strain. Au nanowire bridges of very small diameter were probed using AFM to investigate their deformation and fracture strength. Below a diameter $\sim$50nm, the mechanical properties are consistent with liquid-like behaviour. After reaching the fracture, the gold molecules from the bridge retract towards the fixed ends; rebinding of the gold causing reforming of the nanowire bridge can occur. FIB fabrication was also used to form a thermal bimorph MEMS cantilever which was investigated by AFM during actuation