Energy-filtered Electron Research Articles

Novel unconventional inorganic nanowires: fabrication, structural analysis and electrical property evaluation

The authors' endeavors in recent years to synthesize novel inorganic semiconducting nanowires, to analyze their atomic structures using state-of-the-art electron microscopy facilities and techniques, and to evaluate electrical properties are highlighted. Along with a general survey of nanostructures prepared and thoroughly characterized in the laboratory, particular emphasis is placed on analysis of non-standard nanowires in the Si-ZnS and B-C-N inorganic systems. Si-ZnS biaxial nanowire heterojunctions are found to be n-type semiconductors, due primarily to a lower resistance Si-path. Fractal-like B-C-N nanofibers display high-resistivity I-V curves arising from enrichment of the nanostructure periphery by the insulating BN-rich phase, as revealed by spatially-resolved chemical mapping during energy-filtering electron microscopy.

High resolution quantitative two-dimensional dopant mapping using energy-filtered secondary electron imaging

We have quantified two-dimensional dopant profiles in Si with high accuracy using a commercial field emission gun scanning electron microscope with added secondary electron energy-filtering capabilities. Quantification was achieved for dopant concentrations of 2.8×1017–7.5×1019cm−3 with an accuracy of ±8.5%. Using energy filtering, we have measured the surface potential difference ΔΦs across a 3nm wide p-doped Si layer with respect to the n-doped substrate of a test structure to be 0.72±0.15V. Spatial resolution limits are discussed and are shown to be limited by the secondary electron escape depth. A technique for the rapid collection of calibrated energy-filtered images is also proposed.

Analysis of Nano-scale Strain Near Shallow Trench Isolation Structures by Energy-filtered Convergent Beam Electron Diffraction

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Electron-beam radial distribution analysis of irradiation-induced amorphous SiC

Advanced electron microscopy techniques have been employed to examine atomistic structures of ion-beam-induced amorphous silicon carbide (SiC). Single crystals of 4H-SiC were irradiated at a cryogenic temperature (120K) with 300keV Xe ions to a fluence of 1015cm−2. A continuous amorphous layer formed on the topmost layer of the SiC substrate was characterized by energy-filtering transmission electron microscopy in combination with imaging plate techniques. Atomic pair-distribution functions obtained by a quantitative analysis of energy-filtered electron diffraction patterns revealed that amorphous SiC networks contain heteronuclear Si–C bonds, as well as homonuclear Si–Si and C–C bonds, within the first coordination shell. The effects of inelastically-scattered electrons on atomic pair-distribution functions were discussed.

Analysis of Nano-scale Stress in Strained Silicon Materials and Microelectronics Devices by Energy-filtered Convergent Beam Electron Diffraction

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Cationic hydrous thorium dioxide colloids – a useful tool for staining negatively charged surface matrices of bacteria for use in energy-filtered transmission electron microscopy

BackgroundSynthesis of cationic hydrous thorium dioxide colloids (ca. 1.0 to 1.7 nm) has been originally described by Müller [22] and Groot [11] and these have been used by Groot to stain acidic glucosaminoglycans for ultrastructure research of different tissues by conventional transmission electron microscopy.ResultsSynthesis of colloidal thorium dioxide has been modified and its use as a suitable stain of acidic mucopolysaccharides and other anionic biopolymers from bacteria, either as whole mount preparations or as preembedment labels, is described. The differences in stain behavior relative to commonly used rutheniumred-lysine and Alcian Blue™ electron dense acidic stains has been investigated and its use is exemplified for Pseudomonas aeruginosa adjacent cell wall biopolymers. For the first time thorificated biopolymers, i.e. bacterial outer cell wall layers, have been analysed at the ultrastructural level with electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI), leading to excellent contrast and signal strength for these extracellular biopolymers.ConclusionApplication of cationic hydrous ThO2 colloids for tracing acidic groups of the bacterial surface and/or EPS has been shown to be rather effective by transmission electron microscopy. Because of its high electron density and its good diffusibility it stains and outlines electro-negative charges within these biopolymers. In combination with ESI, based on integrated energy-filtered electron microscopy (EFTEM) Th-densities and thus negative charge densities can be discriminated from other elemental densities, especially in environmental samples, such as biofilms.

Measuring Electrostatic Potential Profiles across Amorphous Intergranular Films by Electron Diffraction

Amorphous 1-2-nm-wide intergranular films in ceramics dictate many of their properties. The detailed investigation of structure and chemistry of these films pushes the limits of today's transmission electron microscopy. We report on the reconstruction of the one-dimensional potential profile across the film from an experimentally acquired tilt series of energy-filtered electron diffraction patterns. Along with the potential profile, the specimen thickness, film orientation with respect to the grain lattice and specimen surface, and the absolute specimen orientation with respect to the laboratory frame of reference are retrieved.

Energy-Filtered Electron Holography

Abstract When electrons pass through a material, they can pass through without losing energy such as elastically scattered electrons or they can lose or gain energy by inelastically scattering with the material's electrons. The elastically scattered electrons have been used in the simulations of lattice images, which are used to help determine the atomic structure of materials. Inelastically scattered electrons were ignored in the simulations because it was believed that they did not have the required coherence to interfere with themselves and they contributed only to the background intensity. Recently though, a great deal of interest has been generated in knowing whether the inelastically scattered electrons can also contribute to the lattice images in order to help explain the Stobbs factor [1], where the contrast in the lattice images is often three or more times less than the theory predicts. The Stobbs factor makes it difficult, if not impossible, to establish quantitative values to high-resolution lattice images.

Recent Advances in Electron Cryotomography and their Application to Imaging Purified Protein Complexes, Isolated Organelles, and Small Cells

A new 300kV, FEG, helium-cooled, energy-filtered G2 Polara electron cryoTEM was recently installed at Caltech. Using this instrument, we have now extensively tested the potential advantages of liquid helium cooling and the inclusion of two orthogonal tilt-series in the context of electron cryotomography. These results and the application of the technology to several specific biological problems are reported.

Radial Distribution Function Analyses of Amorphous Carbon Films Containing Silicon and Hydrogen by Energy-Filtered Diffraction and EXELFS

Short-range order in amorphous materials is most commonly characterized with the use of radial distribution functions (RDFs). Two analytical electron microscopy methods were used in this study to measure RDFs from amorphous carbon films containing different levels of silicon and hydrogen (Si-aC:H): energy-filtered convergent-beam electron diffraction (EFCBED) and extended electron energy-loss fine structure (EXELFS) analyses. The films were deposited in an industrial-scale system onto a thin adhesive titanium layer on silicon substrates by reactive sputtering of carbon with a feed gas of tetramethyl silane (TMS) and argon, to produce a series of films with different Si and H contents (Si/C = 0, 0.04, 0.10, and 0.18).

Discrimination of B–C–N nanotubes through energy-filtering electron microscopy

Various multi-walled nanotubes in the B–C–N system are thoroughly investigated using a JEOL-3100FEF high-resolution field emission transmission electron microscope operating at 300 kV and equipped with an in-column built Omega filter. Spatially-resolved B, C and N elemental maps of the nanotubes are constructed. It is realized that a wide variety of tubular arrays composed of B, C and N atoms may exist in the system. Sandwich-like BN-rich and C-rich alternating tubular shells, graphitic C layers inside and outside of pure BN shells induced either by surface contamination, or electron beam irradiation, separation of C-rich and BN-rich tubes and/or BN particles within tubular bunches may take place. One should carefully take these effects into account while analyzing nanotube physical properties, e.g., electrical or optical, rather than simply rely on electron energy loss spectra typically collected from B, C and N containing nanostructures as a whole. Striking dependence of an individual nanotube electrical conductivity on tubular shell chemistry is demonstrated using I– V curve recording in an atomic force microscope.

Energy-filtered electron-diffracted beam holography

A method of energy-filtered electron holography is described where any two electron-diffracted beams can be interfered using an electron biprism. A Gatan image filter is used to select the energy loss of the electrons produced in the holograms. Gallium arsenide is used as the TEM specimen. This method of microscopy confirms that fringes extending beyond a limiting aperture were due to inelastically scattered electrons and specifically electrons scattered from the bulk plasmon. The degree of coherence of the zero-loss and energy-loss electrons were high and measured to be ∼0.3, which was maintained even for the high energy-loss electrons up to 100 eV. Future systematic studies using this method should help understand the Stobbs factor and contribute to the development of quantitative high-resolution electron microscopy.

Characterization of Ambient Airborne Particles by Energy-Filtered Transmission Electron Microscopy

This article demonstrates that energy-filtered trairsmission electron microscop, (EFTEM) can be a valuable tool for the identification and analysis of nanometer-sized airborne particles. Ambient airborne particulate matter (PM) samples collected at Lexington, Kentucky were investigated using EFTEEM in combination with electron energy-loss spectroscopy (EELS), energg-dispersive Xray spectroscopy (EDS), selected area electron diffraction(SAED), and high-resolution transmission electron microscopy (HARTEM). EFTEM provides a fast and convenient way to map the distribution of carbonaceous aggregates, which cnstitute a significant fraction of the samples. Sulfur-bearing particles are common in the submicron inorganic fraction. Sulfur elemental maps were obtained from some sulfur-beariang particles, and the oxidation state of sulfur was revealed to be primarily S(VI) as sulfate from the electron energy-loss near-edge fine structures of the sulfur L-edge. Other types of inorganic particles, such as silicon-bearin...

Lattice parameter determination of mullite by energy-filtered needle-texture electron diffraction pattern

The thermal transformations of pyrophyllite to mullite by heating were re-examined using mainly energy-filtering transmission electron microscopy and, for the first time, the texture electron diffraction pattern of the mullite was completely interpreted. Through a temperature range in which pyrophyllite dehydroxylate maintained a long-range order with a fluctuation of approximately 1% in d-spacings of (100) and (010) planes at 1000 degrees C, without prominent exothermic feature, pyrophyllite dehydroxylate was gradually decomposed and transformed into mullite through topotaxy. Pyrophyllite dehydroxylate did not collapse completely until 1100 degrees C, which promoted the rapid growth of mullite in random orientation at 1200 degrees C and the crystallization of amorphous silica to cristobalite at 1300 degrees C. The mullite needles, having their c-axis (texture axis) parallel to the elongation direction, lined up along the b(*)-axis of the pyrophyllite dehydroxylate in the needle-texture electron diffraction patterns. The mullite needles had monoclinic symmetry with lattice parameters of 7.27 A (a), 7.75 A (b), 2.90 A (c), 90 degrees (alpha), 90 degrees (beta) and 88.41 degrees (gamma), which, because of the structural affiliation to the parent pyrophyllite dehydroxylate, differ to the orthorhombic 3/2-mullite.

Development of imaging energy analyzer using multipole Wien filter

We discussed a new design of a Wien filter energy analyzer for an energy-filtered X-ray photoemission electron microscopy system. We have demonstrated that the second-order aberration and the third-order aperture aberration can be corrected by the multipole Wien filter by adjusting multipole components of electric and magnetic fields up to octupole components. The three-dimensional charge simulation method indicated that 12 electrodes and magnetic poles can effectively reproduce these ideal electric and magnetic fields.

Observation of Element-Specific Energy-Filtered X-Ray Photoemission Electron Microscopy Images of Au on Ta Using a Wien Filter Type Energy Analyzer

We have shown the possibility of a Wien filter as an energy analyzer for the energy-filtered X-ray photoemission electron microscopy (EXPEEM) by obtaining images of Au islands on a Ta substrate using an inner shell photoelectron excited by an X-ray with a photon energy of 2300 eV. When the photoelectron with kinetic energy of nearly 0 eV was selected, brighter Au islands were observed. When the kinetic energies of photoelectrons passing the Wien filter are set at those of Au 3d5/2 and Ta 3p3/2 photoelectrons, the Au and Ta regions appeared brighter, respectively, indicating that the chemical imaging was successful using a high energy X-ray and a Wien filter energy analyzer.

Recent progress in energy-filtered high energy X-ray photoemission electron microscopy using a Wien filter type energy analyzer

Energy-filtered X-ray photoemission electron microscopy (EXPEEM) is a microscopy technique which has the potential to provide surface chemical mapping during surface chemical processes on the nanometer scale. We studied the possibilities of EXPEEM using a Wien filter type energy analyzer in the high energy X-ray region above 1000eV. We have successfully observed the EXPEEM images of Au islands on a Ta sheet using Au 3d5/2 and Ta 3d5/2 photoelectron peaks which were excited by 2380eV X-rays emitted from an undulator (BL2A) at Photon Factory. Our recent efforts to improve the sensitivity of the Wien filter energy analyzer will also be discussed.

AssessingThermodynamic Properties of Amorphous Nanostructures by Energy-Filtered Electron Diffraction

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Radial distribution function analyses of amorphous carbon thin films containing various levels of silicon and hydrogen

Short-range order in amorphous carbon thin films containing silicon and hydrogen (Si-aC:H) was characterized using radial distribution functions (RDFs). RDFs describe the average number of neighbors located a distance r away from an arbitrary atom in the sample. Two methods in the transmission electron microscope (TEM) were used in this study to obtain RDFs: energy-filtered convergent-beam electron diffraction (EFCBED) and extended electron energy-loss fine structure (EXELFS) analyses. Four Si-aC:H film samples were prepared in cross section for TEM using a focused ion beam system. Both EFCBED and EXELFS techniques were used to study the same samples. RDF interpretation included estimation and comparison of near-neighbor distances, average bond angles, and coordination numbers as a function of sample composition. Key RDF characteristics were correlated to thin film material properties.

Three-dimensional distributions of elements in biological samples by energy-filtered electron tomography

By combining electron tomography with energy-filtered electron microscopy, we have shown the feasibility of determining the three-dimensional distributions of phosphorus in biological specimens. Thin sections of the nematode, Caenorhabditis elegans were prepared by high-pressure freezing, freeze-substitution and plastic embedding. Images were recorded at energy losses above and below the phosphorus L 2,3 edge using a post-column imaging filter operating at a beam energy of 120 keV. The unstained specimens exhibited minimal contrast in bright-field images. After it was determined that the specimen was sufficiently thin to allow two-window ratio imaging of phosphorus, pairs of pre-edge and post-edge images were acquired in series over a tilt range of ±55° at 5° increments for two orthogonal tilt axes. The projected phosphorus distributions were aligned using the pre-edge images that contained inelastic contrast from colloidal gold particles deposited on the specimen surface. A reconstruction and surface rendering of the phosphorus distribution clearly revealed features 15–20 nm in diameter, which were identified as ribosomes distributed along the stacked membranes of endoplasmic reticulum and in the cytoplasm. The sensitivity of the technique was estimated at <35 phosphorus atoms per voxel based on the known total ribosomal phosphorus content of approximately 7000 atoms. Although a high electron dose of approximately 10 7 e/nm 2 was required to record two-axis tilt series, specimens were sufficiently stable to allow image alignment and tomographic reconstruction.