G. De Stasio

X-ray absorption spectroscopy of silicates for in situ, sub-micrometer mineral identification

Abstract We present X-ray absorption near-edge structure (XANES) spectroscopy of 11 silicate and aluminosilicate minerals and two glasses at the SiK and SiL2,3, and OK edges. The similar nearestneighbor environments lead to similar spectral lineshapes at each edge, but the fine-structure differences allow individual and groups of structurally similar minerals to be distinguished. By combining spectra and their first energy derivative from three absorption edges, we show that every mineral studied is distinguishable with XANES. This background work, combined with X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM), allows non-destructive in situ, sub-micrometer (to 35 nm) X-ray analysis of materials, including silicate inclusions, which has not been possible previously. Images and spectra from a 7 μm × 3.5 μm quartz inclusion in zircon are presented as a test of this novel technique in geology.

Spectromicroscopy of tribological films from engine oil additives. Part I. Films from ZDDP's

Antiwear films formed from pure neutral alkyl‐ and aryl‐ZDDPs, and a commercial ZDDP, have been studied with high resolution synchrotron‐based photoemission spectromicroscopy with a new instrument, MEPHISTO. Good P L‐edge XANES spectra have been taken on areas between 12 and 400 μm2, and good images of phosphates and ZDDP have been obtained at ∼1 μm resolution on both smooth and rough steel. These spectra, and corresponding images, show immediately that both the chemistry and the morphology of the alkyl and aryl films are very different. The alkyl film contains a range of smaller and larger protective polyphosphate pads from a few to ∼25 μm2 in area. We have shown that the chemistry of small and large pads are different. The large pads contain very long chain polyphosphate; while the smaller pads contain short chain polyphosphate. The aryl films contain ortho‐ or pyro‐phosphates, are much thinner and more uniform, with obviously more streaking from initial wear, and no obvious protective pad formation. Antiwear films generated from the commercial ZDDP, rubbed in base oil, show that the long chain polyphosphate is converted to ortho‐ or pyro‐phosphate, but the amount and distribution of phosphate does not change noticeably. The antiwear films are remarkably stable physically.

Spatial Distribution of the Chemical Species Generated Under Rubbing from ZDDP and Dispersed Potassium Triborate

The partial replacement of ZDDP by dispersed potassium triborate in engine oil was studied. X-ray Absorption Spectroscopy at the phosphorus, sulphur and boron edges was used to identify the chemical species in tribochemical films on the macro-scale. PhotoElecton Emission Microscopy (PEEM) was performed to study the distribution of the chemical species on the micron level. It was found that tribofilms are homogeneous at the 1-micron scale. It was established that zinc is associated only with sulphides and not with phosphates, and phosphates and borate have the same distribution. The cation associated with phosphates is most likely to be potassium from the borate additive. The wear scar width measured on the tribofilms and XANES analysis indicated that dispersed potassium triborate associated with ZDDP has a positive effect on wear prevention.

Uv-ozone ashing of cells and tissues for spatially resolved trace element analysis

UV/ozone ashing of thin tissue sections and cell cultures is a simple technique to enhance relative elemental concentrations, while maintaining their spatial location at the sub-micron level. This approach may enhance the capability of spatially resolved analysis techniques to detect the distribution of trace elements in biological matrices. We present results from light microscopy and x-ray spectromicroscopy studies of tissues and cells demonstrating that the micro-structure is very well conserved. We show the signal enhancement resulting from the removal of carbon, which allows otherwise undetectable gadolinium to be mapped in cancer tissue for a novel neutron capture therapy.

X-Ray Secondary-Emission Microscopy (Xsem) of Neurons

We present the first X-ray secondary (photoelectron) emission microscopy (XSEM) pictures and video microimages of an uncoated and unstained neuron specimen. This novel kind of synchrotron radiation microscopy is suitable for local chemical analysis with a lateral resolution in the micron range. We explored the details of the neuron system, demonstrated chemical contrast by scanning the photon energy, studied in real time the photoelectron emitting properties of the specimens components, and made preliminary tests of the radiation damage. These results significantly enhance the potential role of photoemission techniques in the life sciences and specifically in neurobiology.

Synchrotron spectromicroscopy in medicine and biology

The ability to couple high-resolution specimen imaging with chemical state analysis allows problems in biology and medicine to be approached in a novel and powerful way. Element specific chemical state analysis can be performed at tunable synchrotron X-ray sources through core level absorption spectroscopy, and a number of methods have been developed to routinely use this technique at the microscopic scale relevant to cellular phenomena. A diverse range of biological specimens has been studied in the MEPHISTO spectromicroscope, and some results are discussed here. The examples are used to illustrate the ability of spectromicroscopy to obtain certain specific results, such as semi-quantitative element detection, and chemical state elucidation or mapping. Essential practical considerations are also emphasized, including sample preparation, the sensitivity of absorption spectroscopy, interferences between elements, and the use of complementary sample analyses.

NATIVE AND MODIFIED UNCOATED NEURONS OBSERVED BY ATOMIC FORCE MICROSCOPY

Dried, fixed uncoated neuron granule cells and their neurites have been imaged in air by an atomic force microscope (AFM) in the repulsive regime of contact mode. Neurons have also been observed after swelling with glutamate or decapping. Modifications induced by glutamate resulted in a drastic reduction in the cell body height (from 1.5 to 2.0 μm for the untreated ones down to 0.7–1.0 μm) and in an increased corrugation of the granule cell membrane. In the decapped neurons AFM revealed decreasing cell body height for an increasing amount of removed material during the decapping procedure. These results demonstrate that AFM is the ideal technique to observe cell corrugation and height modifications during cell treatments of neurobiological interest.

XANES microspectroscopy of biominerals with photoconductive charge compensation

Specimen charging under X-ray illumination is a well known phenomenon that can seriously obstruct the analysis of insulating samples. Synchrotron X-PEEM spectromicroscopy can reach a lateral resolution of 20 nm, 1-2 orders of magnitude larger than electron microscopies, but has the added capacity to probe oxidation state through total yield X-ray absorption near edge structure (XANES) spectroscopy. This capability may be compromised, however, if specimen charging restricts electron emission, as was encountered in the study of silicified bacteria from an Icelandic hot spring microbial mat. Bacteria living in an environment containing a high concentration of dissolved silica provide nucleation sites for amorphous silicate precipitation, a process which may lead to the preservation of the cellular structure, i.e. fossilization. TEM studies of bacteria in progressive stages of mineralization showed that mineral formation was initiated in the extracellular sheath, reaching the cell interior after death. Spectromicroscopy at the Si L-edge of sectioned mineralized bacteria encountered major charging difficulties, which were relieved by simultaneously illuminating the specimen with 325 nm HeCd laser light during the analysis. The low energy light excites mobile free electrons below the work function threshold, which can offset surface positive charge. This approach allowed spectroscopy to be performed from microscopic areas, and may be applicable to a wider range of insulating samples

Scanning photoemission microscopy on MAXIMUM reaches 0.1 micron resolution

We present the first results from the upgraded version of the scanning photoemission spectromicroscope MAXIMUM, bared on synchrotron undulator fight and on a multilayer-coated Schwarzschild objective. The upgrade involved nearly all parts of the instrument, notably the beamline and the electron analysis system. Micro-images of Fresnel zone plates and of metal test patterns on semiconductor substrates reached a new record in lateral resolution, well beyond 0.1 micron. The first spectromicroscopy tests were also successfully performed on the new instrument, with analysis of f and d core levels in different systems.

The Chemical and Morphological Properties of Boron–Carbon Alloys Grown by Plasma-Enhanced Chemical Vapour Deposition

The stoichiometry and morphology of boron–carbon alloy thin films grown by plasma-enhanced chemical vapour deposition can be significantly modified by varying the deposition rate. Films grown at a rate of 5.5 nm min−1 are characterized by an amorphous-like matrix with carbon-rich and dome-like inclusions. Films grown at a deposition rate of 33 nm min−1 are found to be much more homogeneous and free of carbon-rich and dome-like inclusions. An excitation at 191.7 eV in the B 1s absorption spectrum has been associated with amorphous growth. The relative intensities of the π* and σ* excitations across the C 1s absorption edge of these boron-carbon alloys indicate that carbon bonding is predominantly through sp3 hybridization, while boron bonding is a mix of sp2 and sp3 hybridization.