Dark-field Electron Micrographs Research Articles

Ag2Al plates in Al–Ag alloys

Abstract The late stages of precipitation in aluminum-rich Al –Ag alloys with silver contents from 3 at.% to 22 at.% were studied with transmission electron microscopy. Shapes, sizes, and arrangements of plates of the stable hexagonal Ag2Al phase forming on {111} planes of the f.c.c. matrix were analysed. The high-angle annular dark-field contrast in scanning transmission electron microscopy is calibrated. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag2Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on. For samples with silver contents above 12 at.% a parallel lamellar arrangement of fine γ-plates and Al-rich matrix is found after extended heat treatments.

Dislocation structure and microstrain evolution during spinodal decomposition of reactive magnetron sputtered heteroepixatial c-(Ti0.37,Al0.63)N/c-TiN films grown on MgO(001) and (111) substrates

Heteroepitaxial c-(Ti0.37,Al0.63)N thin films were grown on MgO(001) and MgO(111) substrates using reactive magnetron sputtering. High resolution high-angle annular dark-field scanning transmission electron micrographs show coherency between the film and the substrate. In the as-deposited state, x-ray diffraction reciprocal space maps show a strained epitaxial film. Corresponding geometric phase analysis (GPA) deformation maps show a high stress in the film. At elevated temperature (900 °C), the films decompose to form iso-structural coherent c-AlN- and c-TiN-rich domains, elongated along the elastically soft <100> directions. GPA analysis reveals that the c-TiN domains accommodate more dislocations than the c-AlN domains. This is because of the stronger directionality of the covalent bonds in c-AlN compared with c-TiN, making it more favorable for the dislocations to accumulate in c-TiN. The defect structure and strain generation in c-(Ti,Al)N during spinodal decomposition is affected by the chemical bonding state and elastic properties of the segregated domains.

Dark-field transmission electron microscopy and the Debye-Waller factor of graphene.

Graphene's structure bears on both the material's electronic properties and fundamental questions about long range order in two-dimensional crystals. We present an analytic calculation of selected area electron diffraction from multi-layer graphene and compare it with data from samples prepared by chemical vapor deposition and mechanical exfoliation. A single layer scatters only 0.5% of the incident electrons, so this kinematical calculation can be considered reliable for five or fewer layers. Dark-field transmission electron micrographs of multi-layer graphene illustrate how knowledge of the diffraction peak intensities can be applied for rapid mapping of thickness, stacking, and grain boundaries. The diffraction peak intensities also depend on the mean-square displacement of atoms from their ideal lattice locations, which is parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a suspended monolayer of exfoliated graphene and find a result consistent with an estimate based on the Debye model. For laboratory-scale graphene samples, finite size effects are sufficient to stabilize the graphene lattice against melting, indicating that ripples in the third dimension are not necessary.

Size distribution of BaF 2 nanocrystallites in transparent glass ceramics

In glasses with the composition 1.9 Na 2O–15 K 2O–7.5 Al 2O 3–69.6 SiO 2–6 BaF 2 (in mol.%), BaF 2 nanocrystalline precipitates are formed upon heat treatment. Using dark-field and bright-field transmission electron micrographs, crystallite size distributions are obtained for samples crystallized at various temperatures. According to the “tomato-salad problem”, the size distributions are corrected and then compared to various theories of grain growth taking into account coarsening of the crystallites during heat treatment. The experimental crystallite size distributions show for smaller mean crystallite sizes a more symmetric shape in comparison to the theories of Lifshitz–Slyozov–Wagner (LSW) or Brailsford and Wynblatt (B&W). With increasing mean crystallite sizes to about 18 nm at higher heat-treatment temperatures, the full width at half maximum of the observed distributions decreases and becomes even narrower than the LSW function. These findings indicate that in the investigated nano glass ceramics no coarsening by Ostwald ripening or coalescence occurs. This is explained by the formation of a diffusion barrier around each nanocrystallite which limits the size of the crystallites and hence results in such a narrow and uniform crystallite size distribution.

Ag2Al plates in Al–Ag alloys

Abstract The late stages of precipitation in aluminum-rich Al–Ag alloys with silver contents from 3at.% to 22at.% were studied with transmission electron microscopy. Shapes, sizes, and arrangements of plates of the stable hexagonal Ag2Al phase forming on {111} planes of the f.c.c. matrix were analysed. The high-angle annular dark-field contrast in scanning transmission electron microscopy is calibrated. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag2Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on. For samples with silver contents above 12at.% a parallel lamellar arrangement of fine -plates and Al-rich matrix is found after extended heat treatments.

Synthesis and characterization of nano-sized nickel(II), copper(I) and zinc(II) oxide nanoparticles

Ultrafine, equiaxed and monodisperse oxide particles with an average grain diameter in the range of 1–10 nm have been prepared by a two-step chemical approach: the chemical reduction of metallic salts by activated sodium hydride in tetrahydrofuran solvent, followed by oxidation of the metallic species with small amounts of O 2–N 2 gas. Such particles are easily, quantitatively and reproducibly prepared and are stable on storage. The average crystallite sizes and the agglomeration of particles were estimated from dark-field transmission electron micrographs. The nature of the chemical bonding was studied by electron-energy-loss spectroscopy and structural information were obtained using selected area electron diffraction patterns.

Electron microscopical investigation of as cast AZ91D alloy

A detailed examination of a magnesium alloy, AZ91D, in as cast condition was carried out by using electron microscopy techniques. Particles of Al17Mg12, known as β and Al8Mn5 precipitates have been identified by electron diffraction. It was found that Al8Mn5 particles adopted coarse angular shapes, while β phase existed in several distinct morphologies, namely grain boundary blocky particles, lamella of the colonies of discontinuous precipitation, irregular ones in the α constituent of the cellular nodules, and two types of intragranular continuous precipitates, i.e. plates and plaques. The crystallographic relationships between the αand β constituents of the discontinuous precipitation, and between the intragranular β and the α matrix have also been determined. It was observed that the morphologies and the crystallographic relationship observed are identical to the one that was reported in the literature for an aged structure. Based on the dark field electron micrographs a model has been proposed to explain the lengthening of plates of β. Basic morphological differences between the structures of the ingot and aged AZ91 are discussed.

Experimental Methods and Data Analysis for Fluctuation Microscopy

AbstractWe have developed a new electron microscopy technique called fluctuation microscopy which is sensitive to medium-range order in disordered materials. The technique relies on quantitative statistical analysis of low-resolution dark-field electron micrographs. Extracting useful information from such micrographs involves correcting for the effects of the imaging system, incoherent image contrast caused by large scale structure in the sample, and the effects of the foil thickness.

Segregated AIP phase in a near-rapidly solidified Al-5Si-1Ti alloy

During transition electron microscopy (TEM) studies of microstructures of a near-rapidly solidified (strip casting) AI-5Si-ITi (wt %) alloy, AlP phase was observed near the crotch end of the crown form (tree-like) Si phase, with superlattice diffraction patterns in electron diffraction and fine domains on the TEM image. The formation of the AlP phase was caused by the chemical segregation of the purity P. A series of studies of aluminium phosphide have been reported on its atomic arrangement, electron structure and metamorphism in aluminium alloys [1-6]. Wentore [2] suggested a hexagonal structures for AlP while Wang and Spinar [1] and Addamiano [5] found AlP to be of the zinc-blende type (B3). TEM analyses of AlP phase in aluminium alloys have not been demonstrated before. In the present work, AI-5Si-ITi strips with a thickness of 250/xm were prepared by DC casting at a cooling rate of 103 Ks -1. The strips were mechanically worn and jet-electropolished, and then ionpolished for examination in a JEM 2000-FX transmission electron microscope. Fig. 1 indicates that there were two phases distributed on the dentritic structures of the A1 matrix: the larger crown form Si phase (marked A) located on the dentritic boundaries; the smaller AlP phase (marked B) near the crotch end of the Si phase. These were defined by electron diffraction patterns, as shown in Fig. 2 and Fig. 3a. A sequence of diffraction patterns of Si and AlP phases with the unit stereographic triangle were obtained on tilting from [1 i 1], [001], [101] to [1 1 1]. The diffraction patterns of the Si phase could be indexed to be of diamond type with a cell parameter of 0.5404 nm; those of the AlP phase could be indexed as facecentred cubic (B3 type) with a cell parameter of 0.5428 nm, in agreement with Wang and Spinar (a = 0.5467 nm) [1] and Addamiano (a = 0.5451) [5]. The unit cell of B3-ordered superlattices is shown in Fig. 3b. Fig. 4a and b shows the (1 1 1) bright-field (BF) and dark-field (DF) electron micrographs while Fig. 5a and b shows those of (1 f 1). These all reveal the presence of extremely fine B3 domains. The presence of the extremely fine domains indicates that the ordering reaction occurred during quenching or near-rapid solidification. However, no Ti-enriched phases were found in the alloy, which indicates that Ti has been trapped in the A1 matrix at the cooling rate of 103 Ks -1. In this instance, Ti was more easily dissolved in the AI matrix than Si and P. This probably resulted from the strong interaction between Ti and vacancies in the A1 saturation [7]; therefore, the dissolution of other elements in the A1 matrix was restrained. In addition, since its crystal structure including the cell parameter was very near that of Si, the AlP is usually used as a nucleus of Si in conventional A1-Si alloy castings [6]. In this work, no evidence was found showing that the AlP helped Si to nucleate, for example, no constant orientation relationship was found between AlP and Si phases. One reason was that the P content in the alloy was too low, the other was that the rapid solidification has a strong ~ effect on the relative kinetics of the formation of the competing solid phases.

Three-dimensional reconstruction from dark-field electron micrographs of macromolecules at random unknown angles

Electron microscopy produces 2D images of 3D objects with resolutions generally from about 2-5 nm for stained or shadowed specimens, to as good as 0.3-0.5 nm for unstained specimens using bright field or dark field techniques. Many groups have worked on methods that attempt to recover the 3D information that is lost in the 2D representation. We have built on and extended previous techniques, and report here the development and application of a robust, unbiased quaternion-based alignment procedure to facilitate 3D reconstruction of molecules imaged at random unknown orientations. The approach is demonstrated using dark field electron micrographs of the Klenow fragment of DNA polymerase.Many procedures for 3D structure determination using electron microscopy have been reported. For fairly simple, small proteins it has been possible to derive a correct high resolution 3D structure by folding the known amino acid chain using dark field electron micrographs as geometric constraintse.g.. However, in general the reconstruction of a 3D density distribution requires knowledge of the relative orientation of different projection images of the object. To determine such orientations a variety of techniques have been employed, frequently adapted to the specimen of interest.

Synthesis and characterization of fine grain diamond films

A fine grain diamond film has been developed by microwave plasma assisted chemical vapor deposition. Various analytical techniques, including Rutherford backscattering, proton recoil analysis, Raman spectroscopy, and X-ray diffraction, were utilized to characterize the diamond films. The grain size of the film was determined from bright and dark field electron micrographs, and found to be 200–1000 Å. The films exhibited good optical transmission between 2.5 and 10 μm, with a calculated absorption coefficient of 490 cm−1. The friction coefficients of this film were found to be 0.035 and 0.030 at dry nitrogen and humid air environments, respectively, and the films had low wear rates.

Structural investigation of reactively sputtered boron nitride films

Abstract The microstructures of boron nitride films deposited on stainless steel substrates by an r.f. sputtering technique have been investigated by scanning and transmission electron microscopy and X-ray and electron diffraction. They were found to consist of a mixture of approximately 10 nm diameter grains of cubic structure and smaller, about 1 nm, crystallites of hexagonal structure. The distributions of crystallite sizes were determined directly from dark field electron micrographs. The nominal crystallite diameters derived from X-ray diffraction line broadening measurements were found to be in good agreement with the electron microscopy observations for the hexagonal but not for the cubic phase. This discrepancy is apparently due to the presence of fine twinning in the cubic grains.

Microstructure of phospholipid monolayers studied by dark field electron and fluorescence microscopy

The preparation method of horizontal transfer onto hydrophobic carbon substrates was used to investigate pure dipalmitoyl phosphatidylcholine (DPPC) monolayers at various surface pressures by high contrast dark field electron microscopy. A microporous structure of a continuous monolayer containing lipid free regions of about 0.1 ∼ 2 μm in diameter was found. The lipid-free regions on the substrate, which decrease in size with growing surface pressure, are related to parts of the phospholipid monolayer in the liquid phase at the air/water interface. The pressure dependence of the density of these liquid phase regions is described by a simple two-phase model for the pressure range from the liquid-expanded/liquid-condensed (LE/LC) phase transition up to high pressures. For evaluating the horizontal transfer properties, DPPC monolayers containing ∼ 1 mol% fluorescent dye molecules were investigated in the same way and by fluorescence microscopy. The structure of these mixed monolayers consisted of small solid phase domains with diameters of about 0.02 ∼ 5 μm arranged in periodic patterns at a submicroscopic scale in a way similar to that known from fluorescence observations (1, 2, 4). Evidence is provided that different contents of impurities may be responsible for the formation of the observed structures of the pure and mixed system. In addition, from high magnification dark field electron micrographs the submicrometer sizes of solid phase domains in DPPC monolayers were estimated for the first time.

Influence of defects on the incommensurate modulation in irradiated Ba2NaNb5O15.

Defect-induced properties of barium sodium niobate (Ba{sub 2}NaNb{sub 5}O{sub 15}) have been studied as a function of electron irradiation dose. Birefringence measurements at low doses indicate that the lock-in transition temperature, originally at about 280 {degree}C, decreases linearly with increasing dose. Complementary irradiations {ital in} {ital situ} in a high-voltage electron microscope show that the incommensurate phase can be stabilized down to room temperature. Simultaneously the satellite diffraction spots broaden and the lock-in transition becomes diffuse. The modification of the hysteresis of the birefringence, observed already at the lowest doses, indicates a progressive extension of the stability range of the 2{ital q} modulated phase to lower temperatures as the defect concentration increases. A similar conclusion can be drawn from the satellite reflection dark-field electron micrographs that show, once the incommensurate phase is stabilized at room temperature, the doubly modulated texture characteristic of the 2{ital q} modulated phase. Low doses of irradiation do not change qualitatively the configuration of the residual discommensurations in the lock-in phase nor the temperature dependence of the incommensurability. Accordingly, already in the as-grown samples the defects dominate the pinning of the incommensurate modulation and the intrinsic properties of this incommensurate system are not clearly observable.

Atomic ordering in GaAsP

CuPt type ordering, which consists of a monolayer compositional modulation along one of the 4 〈111〉 directions in the lattice, was studied using transmission electron microscopy for GaAs1−xPx with values of x extending from 0.25 to 0.85. The samples were grown by organometallic vapor phase epitaxy on nominal (001) GaAs substrates that were misoriented by varying amounts in three directions. No CuPt type ordering was observed for GaAs1−xPx with x ≤0.35, while ordering was found to occur for 0.4≤x≤0.85. The direction of substrate misorientation has a major effect on the determination of which of the four possible CuPt variants are formed for 0.4≤x≤0.85. Two variants, with ordering on the (1̄11) and (11̄1) planes, appear for epilayers grown on substrates oriented exactly on the (001) plane and for substrates misoriented by 6° towards the [110] direction. Only one variant, with ordering on the (1̄11) plane, appears for epilayers grown on substrates misoriented by 6° towards [1̄10]. These ordering-induced spots observed in transmission electron diffraction (TED) patterns for GaAsP occur only for the [110] cross section. From TED studies of GaInP grown on similar substrates, we conclude that the CuPt variants in GaAsP are exactly the same as for GaInP. Further evidence supporting this conclusion was obtained by growing first a layer of GaInP followed by a layer of GaAsP. High-resolution dark field electron micrographs show domains of the same variants in both layers. A mechanism describing the formation of the specific ordered variant for both GaAsP and GaInP is proposed. From studies of ordering in a strain-layer superlattice, the strain due to lattice mismatch was found to play no significant role in the propagation of ordered domains. Microtwins, also generated due to lattice mismatch, can act as domain boundaries and prevent the propagation of the ordered domains.

Branched-chain alpha-keto acid dehydrogenase complex from bovine kidney: radial distribution of mass determined from dark-field electron micrographs.

Scanning transmission electron microscopy (STEM) was used to determine the radial distribution of mass within the bovine kidney branched-chain alpha-keto acid dehydrogenase complex (E1-E2) and its core enzyme, dihydrolipoamide acyltransferase (E2). The particle mass of E2 measured by STEM is (1.19 +/- 0.02) x 10(6). Assuming 24 subunits per E2 core, this value corresponds to a subunit molecular weight of (4.96 +/- 0.08) x 10(4), which agrees well with the subunit molecular weight estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 5.2 x 10(4) (Pettit et al., 1978) and that deduced from the gene sequence, 46,518 (Griffin et al., 1988). Thus, the STEM data reaffirms the 24-subunit model for this E2. Previous studies indicated that the E2 subunits contain an extended, outer lipoyl-bearing domain connected by a trypsin-sensitive segment to a compact, inner catalytic domain. The assemblage of 24 inner domains comprises a cubelike inner core. The quantity and spatial distribution of mass determined from STEM images for the E2 inner core are consistent with this model. The lipoyl-bearing domains are shown to occupy a zone defined by radii of 80-130 A over which the lipoyl moiety may range. This zone overlaps the positions of the 24 branched-chain alpha-keto acid dehydrogenase (E1) molecules, which apparently are located on the of the cubelike inner core.

Transmission electron microscopic study of the ordered structure in GaInP/GaAs epitaxially grown by metalorganic chemical vapor deposition

GaInP epitaxial crystals grown on (001) GaAs at 660–700 °C by metalorganic chemical vapor deposition are examined by transmission electron microscopy. The computer-processed image of the high-resolution electron micrograph clearly reveals a lamellate-ordered domain structure of two variants of {111} superlattices, which is also investigated using cross-section and plan-view dark field electron micrographs. The spikes of well-defined superspots in the diffraction pattern were found to originate from the shape of the domains. The investigation of GaInP grown with different Zn concentrations showed that the disordering occurs as a result of a decrease in the density rather than the size of the domain.

Oxygen stoichiometry and structure of YBa 2Cu 3O 7− δ and their relation to superconductivity

Thermogravimetric curves of the superconducting samples (0.0 ≤ δ 〈0.5) of YBa 2Cu 3O 7− δ are shown to be characteristically different from those of the non-superconducting samples ( δ⋍0.5–1.0). The variation of T c (from resistivity measurements) with δ confirms for a change from chain-super-conductivity to plane-superconductivity. Bands found in bright or dark field electron micrographs are shown to arise for different orientations of the [CuO 2] ∞ planes, causing oxygen enrichment in the boundaries. A new defect with missing Y-rows is found in the images of Y 1−xBa 2Cu 3O 7.

High resolution analysis of paired helical filaments in Alzheimer's disease

AbstractResults of this comparative study indicated that electron microscopy of supra‐ultrathin sections provided the best resolution, revealing the subunit structures, globules (32 Å ± 4), and longitudinal bars (47 Å ± 6) of paired helical filaments (PHF). A three‐dimensional model of PHF substructure is therefore presented. The image of brightfield electron micrographs of isolated PHF positively stained with 2% phosphotungstic acid (PTA) also provided high resolution and revealed the presence of the beaded structures of these heavily stained filaments in the crossing‐over region of PHF. Noisy background of the negatively stained preparation of isolated PHF substantially reduced the resolution. Severe flattened morphology of this preparation further complicated the interpretation of image analysis. Darkfield electron micrographs offered high‐image contrast with low noise in the background clearly demonstrating the ropelike twisting configuration of PHF, but did not reveal the substructures of PHF. The tilting (±45°) analysis of these PHF prepared by uranyl acetate staining and rotary‐shadowing and unidirectional shadowing methods with platinum showed that these heavy metals were deposited on the top filaments (away from the carbon film on the grid) in the crossing‐over region of PHF.

Automatic selection of molecular images from dark field electron micrographs

A method is described which can be used objectively to select putative molecular images from dark field electron micrographs of unstained molecules. The only characteristic of the molecule required for automatic selection is an estimate of molecular weight. Structures are selected from micrographs by a series of steps including: low pass filtering, edge detection and mass determination. The procedure is shown to be reliable for images with signal-to-noise ratios of at least 4.0. Moreover, the method is insensitive to both the shape and the number of molecules in the image. Five different molecules with molecular weights between MW 330,000 and MW 4 000 are successfully selected from low dose STEM and high dose tilt beam dark field electron micrographs.