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151 records · Page 9Linked to original sources

PubMed2003

In situ electrical measurements and manipulation of B/N-doped C nanotubes in a high-resolution transmission electron microscope.

B/N-doped multiwalled C nanotubes were electrically probed by means of a tungsten needle attached to a piezo-driven stage of a high-resolution transmission electron microscope holder. Two-terminal transport measurements were performed in a 'W needle-nanotube-ground' circuit. The I-V curves were recorded in situ while viewing the nanotubes in the imaging mode of the microscope. This allows us to trace nanotube array morphological changes under applied voltage (up to 50 V). Specific manipulation with nanotube assemblies was found to be possible under applied electrical field: attachment of a tiny nanotube bundle to the W needle and extraction of a given nanotube fragment from an entangled complex bunch were achieved. The electrically-probed B/N-doped C nanotubes exhibited alternating B-rich and C-rich B-C-N domains within tubular layers, as revealed by elemental mapping during energy-filtered TEM (Omega filter). At room temperature the nanostructures displayed resistivity (rho) of approximately 1.8 x 10(-5) omegam and linear I-V curves. The key role of a given contact between the probing needle and a nanotube during electrical measurements was particularly verified.

Electricity↗

PubMed2000

Quantitative intensity measurement of equal thickness fringes in Si and MgO crystal images with an energy-filtering transmission electron microscope using an imaging plate.

Quantitative measurement of intensity profiles of equal thickness fringes has been carried out in Si and MgO crystal images with an energy-filtering transmission electron microscope using an imaging plate. The crystals have a 90 degrees wedge-shape with [110] surfaces for Si and with [100] surfaces for MgO, and are observed under the exact axial incidence of a 200 keV electron beam along the [100] axis for Si and along the [110] axis for MgO. The intensities are measured in bright field and 022 and 040 dark field images for Si, and in bright field and 111, 002, 220, 113, 222, and 004 dark field images for MgO, with and without an energy slit having +/- 5 eV energy width for incident electrons. The intensity profiles obtained from the images are presented as standard experimental data for calculation of electron diffraction intensities. A few simulation programs for high-resolution transmission electron microscopy are checked by comparing the calculated diffraction intensities with the experimental data. The complex potential suitable for matching the data is discussed.

Journal Article↗

PubMed2000

Localization of chromophore absorption signals in TEM with an improved prism-mirror-prism filter.

A corrected prism-mirror-prism electron energy filter with curved entrance and exit faces was designed and incorporated into a Zeiss EM902 transmission electron microscope. The installation of the new filter required little modification to the existing microscope geometry and electronics. The filter had an energy resolution of 1.1 eV over the full image plane (acceptance half angle 10 mradian). The improved energy resolution was applied in studies of the low electron energy loss region that includes molecular orbital excitations or chromophore energy absorptions. Chromophore signal behaviour under electron irradiation was characterized using embedded crystals of hematin and of the dye mercury orange. Images of these crystals confirmed the expected decrease in signal intensity on shifting the selected energy loss from the region of molecular orbital excitations (less than approximately 5 eV) to higher energy losses. Electron irradiation-induced fading of the chromophore signal from hematin and mercury orange yielded similar 1/e dose values of 1.1 x 10(5) e(-) nm(-2) and 1.4 x 10(5) e(-) nm(-2) respectively. In a cellular context, chromophore signals were obtained from energy-filtered images of RIF-1 cell sections containing the photosensitizer chlorin e6 and from sections of BS-C-1 cells with cytoskeletal labelling via FITC-conjugated antibodies. The respective signals were extracted using a dose-dependent method or a shift in selected energy. Chromophore signal distributions were in agreement with fluorescence light microscopic images, but provided detail at higher spatial resolution.

Animals↗

PubMed1991

Contrast in the electron spectroscopic imaging mode of a TEM. IV. Thick specimens imaged by the most-probable energy loss.

When the zero-loss transmission falls below 10(-3) for biological sections of mass-thickness greater than 70 micrograms/cm2, the energy window in the electron spectroscopic imaging (ESI) mode of an energy-filtering electron microscope (EFEM) can be shifted to the most-probable energy loss of the electron energy-loss spectrum. This enables mass-thicknesses up to 150 micrograms/cm2 or thicknesses of 1.5 microns to be examined. Electron energy-loss spectra of thick carbon films calculated by a Fourier method agree with experimental spectra. Measurements of the electron energy-loss spectroscopy and ESI image intensities with an additional platinum film confirm a scattering model for the calculation of the image intensity. This model considers the angular broadening at the most-probable energy loss by introducing an effective illumination aperture of the order of the full-width at half-maximum of the angular distribution.

Fourier Analysis↗

PubMed1992

Zero-loss energy-filtered imaging of frozen-hydrated proteins: model calculations and implications for future developments.

Energy-filtered transmission electron microscopes operating in zero-loss mode are used increasingly to study biological material in frozen-hydrated conditions. The contrast enhancement and improved structural resolution obtainable by this method have been studied using Monte-Carlo model calculations for the scattering processes occurring in such samples. Three models representing typical situations have been analysed, each normalized to minimal beam damage. It is shown that for proteins in thin layers of ice an optimal signal-to-noise ratio is achieved in the 80-120-keV electron energy range. For proteins which have to be embedded in thicker ice layers, a considerably higher acceleration voltage is required. In particular, electron energies above 200 keV would be desirable for electron diffraction work on microcrystals.

Animals↗

PubMed2006

A new experimental procedure to quantify annular dark field images in scanning transmission electron microscopy.

A new procedure to quantify the contrast in annular dark field images recorded without lattice resolution in a scanning transmission electron microscope is proposed. The method relies on the use of an in-column energy filter prior to the annular dark field detector and the acquisition of a series of energy-filtered images as a function of the inner detection angle. When the image contrast of an interface between two materials in such energy-filtered annular dark field images is plotted vs. camera length and extrapolated to zero (i.e. infinite scattering angle), the contrast is shown to behave exactly as predicted by Rutherford's scattering formula (i.e. intensity scales proportional, variantZ2). This can then be used to determine the local chemistry at and the effective chemical widths of interfaces or thin films without any additional spectroscopy method for calibration, provided the global chemical composition is known. As examples, the systems SiGe/Si and InGaAs/Ge are considered in detail.

Journal Article↗

PubMed2004

Growth of polychlorinated-biphenyl-degrading bacteria in the presence of biphenyl and chlorobiphenyls generates oxidative stress and massive accumulation of inorganic polyphosphate.

Inorganic polyphosphate (polyP) plays a significant role in increasing bacterial cell resistance to unfavorable environmental conditions and in regulating different biochemical processes. Using transmission electron microscopy of the polychlorinated biphenyl (PCB)-degrading bacterium Pseudomonas sp. strain B4 grown in defined medium with biphenyl as the sole carbon source, we observed large and abundant electron-dense granules at all stages of growth and following a shift from glucose to biphenyl or chlorobiphenyls. Using energy dispersive X-ray analysis and electron energy loss spectroscopy with an integrated energy-filtered transmission electron microscope, we demonstrated that these granules were mainly composed of phosphate. Using sensitive enzymatic methods to quantify cellular polyP, we confirmed that this polymer accumulates in PCB-degrading bacteria when they grow in the presence of biphenyl and chlorobiphenyls. Concomitant increases in the levels of the general stress protein GroEl and reactive oxygen species were also observed in chlorobiphenyl-grown cells, indicating that these bacteria adjust their physiology with a stress response when they are confronted with compounds that serve as carbon and energy sources and at the same time are chemical stressors.

Biodegradation, Environmental↗