High-resolution Dark-field Electron Microscopy Research Articles

Homologous DNA Targeting with RecA Protein-coated Short DNA Probes and Electron Microscope Mapping on Linear Duplex Molecules

We demonstrate that RecA protein-coated, short single-stranded DNA probes paired with a specific homologous DNA sequence in a linear duplex target molecule and accurately targeted the selected DNA sequence. RecA protein-coated complementary ssDNA probes were reacted with linear duplexes, and the homologously paired molecules were observed by electron microscopy. The sites of interaction between the RecA protein-coated DNA probes and the uncoated duplex DNA targets were directly visible on individual target DNA molecules by high-resolution darkfield electron microscopy, without chemical fixation or sample shadowing. The efficiency and specificity of pairing were verified with 446 and 222 base single-stranded DNA probes that shared no homology with one another, and several linear duplex target DNAs with their respective probe homology sites at different locations with respect to the ends of the double-stranded DNA molecules. Measurements of the position of RecA protein-coated probes paired to individual target molecules, observed at high magnification, showed that DNA probes specifically paired at their corresponding homologous target sequences. This RecA protein-mediated DNA mapping method allows homologous sequence positioning and gene mapping on individual double stranded DNA molecules. Targeting reactions in which two different probe/target sites were 900 bases apart on a single duplex target molecule allowed both sites to be mapped in the same targeting reaction; although targets displaying both probes simultaneously were seen much less frequently than expected. The possible torsional or mechanistic constraints related to these reactions are briefly discussed.

Crystallography and a model of the α → ω phase transformation in zirconium

The omega phase in pure Zr formed at 300 K under high pressure of 6.0 G Pa was examined using high resolution dark-field electron microscopy and selected area diffraction. Pressurized Zr has a two phase structure consisting of small elongated ellipsoidal ω particles of d ∼- 40–50 A ̊ and l = 100–120 A ̊ which are precipitated in an α matrix. There is a strong resemblance between morphologies of omega originated in either α or β. The principally new orientation relationship between α and ω phase is established. There are three possible orientations (variants) of ω in an α matrix. The basal (0001) plane of α is parallel to {12̄10} ∞ and the [0001] ∞ direction coincides with everyone of the three directions of α: [ 2 ̄ 110 ∞(ω 1, [ 1 ̄ 1 ̄ 20] ∞(ω 2) and [ 1 ̄ 2 1 ̄ 0] ∞(ω 3) with deviations ~ ±5–7°. The atomistic model explaining the data obtained is proposed. The α → ω transformation is regarded as a displacive type. According to the model, every three neighbouring close-packed atomic rows [12̄10] x are displaced in a basal plane (0001) x on a distance a x 4 along [12̄10] x direction and the next three along the opposite direction [1̄21̄0] x and so on. Such displacements accompanied by small shuffles form {12̄10} ∞ planes. The common traits in α → ω and β → ω transformations are also discussed together with possible mechanism of the appearance of linear defects along closepacked rows necessary for displacive transformations in Zr (Ti) and its alloys.

Characterization of small platinum particles supported on graphite by electron microscopy

Characterization of small platinum particles in the Pt/graphite catalyst is done using high-resolution dark-field electron microscopy and single-particle electron diffraction. Two types of hexagonal-shaped particles were found. Both are shown to correspond to a three-dimensional cubo-octahedral shape. The first type has a (111) face in contact with the (0001) face of the graphite surface. The second type has a (110) face parallel to the (0001) face of the support surface. The faceted Pt particles are bounded by (100) and (111) crystallographic planes, which provide specific sites for a catalytic reaction.

On the utility of dark-field electron microscopy in the determination of the degree of deformation in chrysotile asbestos: An environmental research application

The degree of microcrystalline deformation in fibers of chrysotile asbestos may be distinguished using high resolution dark-field electron microscopy. This is demonstrated by comparing underformed chrysotile with Union Internationale Contre le Cancer (UICC) standard reference samples. The UICC samples are shown to be partially deformed as a result of milling in the mixing process. Samples of used and unused brake shoe lining dust were examined using this technique; it is shown that chrysotile asbestos in various stages of deformation—relatively undeformed to heavily deformed—survives in automobile brake drum dust. Such dark-field images can serve to identify the source of asbestos found in environmental pollution samples.

The direct imaging of a linear defect using diffuse scattering in Zr-Nb b.c.c. solid solutions

Abstract High-resolution dark-field electron microscopy, using diffuse scattering, has been employed in order to directly image a linear ω-like defect in Zr-Nb b.c.c. solid solutions. Phase-contrast calculations have been made of the defect image using the atomic structure of the defect which had previously been determined by a detailed analysis of the diffuse scattering distribution. Dark-field images formed using two diffuse ω reflections, show rows of white dots along the [111] direction, which are similar in appearance to the calculated images for the defect model. Quantitative measurements of the image intensity indicate that the observed images are from both single and superimposed linear defects. By a careful comparison of the fine structure of the calculated and observed images it is possible to confirm certain details of the atomic structure of the defect. The good agreement between the calculated and observed images from the standpoint of image size, [111] directionality, intensity and fine deta...

High-resolution dark-field electron microscopy of small metal particles

A technique to study the shape of small metal particles (20 to 200 A in diameter) is described. This consists in high-resolution dark-field electron microscopy using weak beams under diffraction conditions in which a large number of beams are excited. Many-beam dynamical electron diffraction theory is used to explain the effect. Examples of application of this technique in the study of growth of thin films are presented. It is found that epitaxial gold particles growing on a NaCl substrate have square and rounded pyramids shapes which are very often truncated. On decrit une technique pour etudier la forme de petites particules de metal (20 a 200 A en diametre). Cette technique est celle de la microscopie electronique en champ noir de haute resolution en utilisant des faisceaux a faible intensite sous des conditions de diffraction, dans la quelle un grande nombre de faisceaux sont excites. La theorie dynamique des faisceaux est utilisee pour expliquer cet effet. On presente des examples sur l'application de cette technique en l'etude de l'agrandissement des pellicules minces. On trouve que des particules d'or du type epitaxial que croissent sur un substrat de NaCl ont une forme pyramidal carree et arrondie, cettes formes sont tres frequement tronquees.

The Electron Metallography of Antiphase Domain Growth in FeCo–2%V

High-resolution dark-field electron microscopy has been used to image antiphase domain boundaries (APB) in a commercial equiatomic FeCo alloy containing 2 wt.-%V. Direct measurement of antiphase domain size was carried out as a function of annealing time in the range 773–823 K. The domain size increased as a linear function of the square root of annealing time. The activation energy for domain coarsening was found to be 178 kJ/mol, which is lower than earlier estimates made using X-ray techniques.

High-resolution dark-field electron microscopy. I. Useful approximations

Simple expressions are derived for the form of the intensity distributions in high-resolution dark-field electron micrographs of thin specimens for which the phase-object approximation may be applied. For cases in which the phase change of the electron wave can be assumed to be small it is shown that there is an optimum defocus value which gives the best, interpretable resolution. For dark-field images obtained either with a central beam stop or with a tilted incident beam, the image intensity is shown to depend on the square of the deviation of the projected potential from an average value. The further complications resulting when the phase change cannot be assumed small are discussed and illustrated by means of a simple example.

High-resolution dark-field electron microscopy. II. Short-range order in crystals

The simple theory for the intensity distributions in high-resolution dark-field electron microscope images of thin specimens, derived from the phase-object approximation, is applied to discussion of images obtained from the diffuse scattering in electron diffraction patterns from crystals having short-range ordering of atoms on the lattice sites. It is shown that interpretation of the intensity distribution in terms of images of 'microdomains' is not usually justified. It is necessary to take into account the statistics of superposition of defects, possible dynamical diffraction effects and the fact that both positive and negative deviations from the average value of projected potential may give intensity maxima. Examples used to illustrate these points include a hypothetical case of a disordered CuAu crystal and some experimental observations of disordered LiFeO2 showing a resolution of details on an atomic scale.

High resolution dark-field electron microscopy.

SUMMARYIn the last few years some promising images of biological specimens have been obtained using the high contrast and resolution of dark‐field electron microscopy. However, important problems of image interpretation and difficulties in specimen preparation limit at the present time, the usefulness of this mode of image formation. The destruction of the sample by the electron beam, is of utmost importance. Some possibilities of partly overcoming it are discussed.

The structure and decomposition of Zr and Ti b.c.c. solid solutions

The literature concerning the decomposition of Zr and Ti alloys is reviewed with particular emphasis on the b.c.c. ( β)→athermal omega ( ω) transformation and the structure of Zr and Ti b.c.c. solid solutions. Illustrative examples are taken principally from the ZrNb and TiNb systems. High resolution dark-field electron microscopy, diffraction, and ultrasonic techniques have been used to follow the β→ ω phase transformation in Zr and Ti alloys. The ω morphology was shown to consist of 10–15 Å diameter particles arranged in 〈111〉 rows. As well, clusters of rows were presented in low solute content alloys and the cluster size decreased with increasing solute content. The observed morphology can explain the presence of {111} planes of diffuse intensity in reciprocal space and the change in appearance of the ω reflections with increasing solute content. The mechanism of the transformation is discussed in terms of instability or metastability of the b.c.c. structure relative to a 2 3 〈111〉 longitudinal displacement wave. This mechanism can be thought of as involving the 〈111〉 displacement of 2 neighboring close-packed rows of atoms relative to each other. Single crystal elastic constant measurements on ZrNb alloys show that C 44 decreases in the vicinity of the β→ ω transformation. This is attributed to a softening of the b.c.c. structure which is a precursor to the actual transformation. Alloys that are known to be well above the M s temperature for the β→ ω transformation, and hence must be single phase b.c.c. solid solutions, exhibit diffuse ω reflections. It is suggested that the diffuse intensity is due to the presence of a 〈111〉 linear fault (or disorder) in the b.c.c. phase. The atom movements associated with this fault are similar to the displacements required for the β→ ω transformation.

The formation of the ω phase in Ti-Nb alloys

The as-quenched and the aged form of theω phase in Ti-Nb alloys were examined using high-resolution dark field electron microscopy and diffraction. The as-quenchedω morphology was shown to be based upon rows of equiaxed particles which are 12 to 20A in diameter and spaced 20 to 22A apart. After short aging times the row length increased, and eventually the isolated rows were replaced by clusters of rows. With continued aging the clusters increased in size and evolved into an ellipsoidal shape. The as-quenched Ti-18 at. pct Nb alloy exhibited sharpω reflections and {111} planes of diffuse intensity in reciprocal space. As the niobium content of the alloy increased theω reflections broadened, and in alloys containing 34 at. pct Nb or more, the 0001 and 0002ω reflections were shifted toward each other along an 〈0001〉 direction. The forbidden 10–10 and 20–20ω reflections were shown to be real in aged alloys, indicating that the aged form of theω phase is ordered.

A study of the domain structure of carbon black by both high-resolution dark-field electron microscopy and X-ray diffraction

High-resolution dark-field electron microscopy was used to observe the domain structure in IRB-2 carbon black. The domains are equiaxed with a dia. of 16±0.5 A ̊ . Most of them have their layerplanes tangent to the surface of the surface of the particle, but occasional domains are observed that are not oriented in this manner. The same carbon black has also been studied by X-ray diffraction techniques. The measured value of the domain size normal to the layer-planes is 16.5 Å; this is in excellent agreement with the electron-microscopic observations. However, the measured values of the layer-plane size varies from 30 Å to 43 Å, depending on the method used to analyze the data.

A New High-resolution Dark-field Electron Microscope Technique

IT is now generally recognized that high-resolution dark-field electron microscopy is essential for a comprehensive study of inperfections in crystalline materials. However, commercial electron microscopes have not been designed in the past with this technique in mind, and consequently the amount of dark-field work that has been carried out to date is limited and has only been possible as a result of a considerable amount of perseverance on the part of the operator. Moreover, the operation has taken so long that injurious contamination of the specimen has occurred. A technique has now been developed here in co-operation with Associated Electrical Industries, Ltd., which allows a suitably modified E.M.6 electron microscope to be used with equal facility for high-resolution dark-field or light-field work.