High-resolution Dark-field Electron Research Articles

Compositional accuracy in atom probe tomography analyses performed on III-N light emitting diodes

Laser-assisted atom probe tomography (APT) and high-resolution dark-field electron holography (HR-DFEH) were performed to investigate the composition of a polar [0001] GaN/AlxGa1 − xN/InyGa1 − yN light emitting diode. In particular, the III-site fraction of both AlxGa1 − xN and InyGa1 − yN alloys was studied adopting a comparative approach. HR-DFEH allows mapping the projected strain with a subnanometer spatial resolution which is used for the calculation of the two-dimensional alloy composition distribution. APT provides three-dimensional alloys composition distribution with a nanometer spatial resolution. However, here we reveal that important inaccuracies affect local composition measurements. A Ga-poor composition is obtained in high DC-electric field regions. Moreover, such inaccuracies may be locally enhanced where the [0001] pole intersects the surface of the analyzed specimen, leading to a lower fraction of Ga measured. III-site fractions closer to the nominal values were measured at low field conditions. Ga loss is thought to be due to preferential DC field induced evaporation of Ga ions between laser pulses. This is explained in terms of formation of a metallic layer on the tip surface during APT analysis, where weak Ga-Ga bonds are formed, promoting the loss of Ga at high field conditions.

High Resolution ADF-STEM Imaging Application for Organic Crystals

High-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging was applied for structural observations of organic crystals so as to achieve quantitative atomically resolved images. In particular, a low angle annular dark-field method (LAADF-STEM) was found to be highly advantageous for radiation sensitive organic materials, because stronger thermal diffuse scatterings can be utilized for imaging. So called quantitative Z-contrast images of organic molecules at atomic resolution were demonstrated in several organic crystals.

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.

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.

The atomic scale structure and the electronic properties of a graded composition semiconductor layer

We report the results of a study on the relationship between the structural and electronic properties of a graded layer of undoped AlxGa1−xAs, where x was specified to increase linearly from 0 to 0.2 over 50 nm. The layer was grown by molecular-beam epitaxy between layers of doped GaAs. A high-resolution dark-field transmission electron microscopy imaging technique of combining information from 002 and 002̄ reflections of AlxGa1−xAs allowed us to image the graded region with near atomic resolution. Differences between the predicted and measured diode performance of the graded layer are discussed in the light of our results.

A model for the three-dimensional structure of glucagon

A literature search on the structural aspects of glucagon in dilute aqueous solution has been undertaken. We have found that a compact, well-defined structure must exist and propose a model for that structure. In doing so, care was taken to distinguish between the raw data themselves and the interpretations drawn from them, and to bring about a model consistent with as much of the data as possible. The model building was performed on Corey-Pauling-Koltun (CPK) space-filling models using secondary structure prediction rules, experimental data such as fluorescence quenching, circular dichroism, NMR and high resolution dark field electron microscopy, and was guided by a hierarchy of intramolecular interactions which places hydrophobic bonding first and hydrogen bonding second. This last criterion places a strict requirement on the model-building to maximize contacts among complementary hydrophobic surfaces; this means that no empty spaces are allowed inside the folded molecule. The resultant model is consistent with all the relevant data. Furthermore, as demanded by any structure building exercise, the model suggests structure-function relationships. One of the predictions drawn from the structure—the binding of guanosine-5′-triphosphate (GTP)—has been confirmed by a preliminary experiment (reported elsewhere). Another aspect of the structure suggests a subtle mechanism for allostery.

Transmission electron microscopic study of single crystals of Fe 7C 3

The formation of microcrystals of Fe 7C 3 from the disproportionation of CO on Fe at 500°C is described. Selected area electron diffraction confirms that the structure is orthorhombic, in agreement with the work of Fruchart et al. Analysis of a structural fault present in these crystals - imaged using high resolution dark field electron microscopy - leads to a plausible explanation for the variety of structures previously reported for Fe 7C 3.

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.

The initial stages of ordering in alloys studied by electron diffraction and high resolution microscopy

SUMMARYThe combined use of selected area electron diffraction and high resolution electron microscopy can provide important information about the microstructure of materials. Both techniques have been applied to study the ordering behaviour in a number of alloys. The ordering process is not always straightforward and mostly intermediate long period modulated structures are formed. Using dark field high resolution electron microscopy the ordering sequence as well as the structure of the intermediate phases have been established for Au4Mn and Au5Mn2. For Ni4Mo the high resolution observations provide new information about the ordering mechanism in its very earliest stages.

Visualization of thermally denatured nucleosomes

Electron images of biological specimens composed of periodic structures are generally spurious. The reason is that unit structures may superimpose along the direction of view, and a “see through” image is formed especially from negatively stained and unstained objects. Methods to detail features of such objects from two dimensional images included optical and computer processing or combination of optical Fourier transformation and filtering. However, rotational superimposition may be employed when objects have simple symmetry.We described here the use of stroboscope illumination photography (stroboscopy) to demonstrate the distribution of globular units along nucleoprotein fibers of thermally denatured mononucleosomes. The information was further used to predict the structure of intact nucleosomes as seen from bright field (BF) and dark field (DF) electron micrographs.We previously obtained coSnsitometric scans from high resolution DF electron micrographs of partially relaxed unstained nucleosomes isolated from nuclease (EC. 3.1.4.7) digested goose erythrocyte nuclei and showed the presence of high density, periodic, contiguous globular regions (3-4 nm) along the DMA moiety.

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.

Inelastic Scattering in Electron Microscopy

The effort to image biological molecules by high resolution (2-4 Å) dark field electron microscopy has stimulated interest in those factors Which influence image contrast. It is known that elastically scattered electrons can be used to obtain high resolution information about a specimen. On the other hand, most inelastically scattered electrons cannot contribute any high resolution information about the specimen since they are the result of a nonlocalized interaction of the incident electrons with the electrons in the specimen. Moreover, in the conventional transmission electron microscope (CTEM) without a chromatic aberration corrector or an energy filter between the specimen and recording plane, inelastically scattered electrons blur the image, due to the chromatic aberration of the objective lens. This has particular importance in biological electron microscopy, since the ratio of total inelastic to elastic scattering for carbon is 1.6.

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.

Origin of Bright Spots in High Resolution Dark Field Electron Microscopy

SMALL bright spots (0.3–0.7 nm) have been observed in the dark field images of evaporated carbon films1 but their origin has not been explained. Here I suggest that these bright spots are present in the dark field image at an over-focus (a higher objective lens current) of 300 nm±50 nm relative to the bright field focus (the finest granulation in the phase structure).