The relative intensities of electron diffraction maxima in mixed rare-earth oxides

The relative intensities of electron diffraction maxima from yttrium aluminium perovskite were analysed in terms of the atomic scattering contribution to the structure factors and in terms of double diffraction. It has been shown that double diffraction influences the visible intensity between diffraction spots in electron diffraction patterns to a greater extent than does the magnitude of the structure factor. A direct consequence of this is that the intensity of diffraction maxima can increase when a specimen is tilted off an exact zone axis.     

Is precession electron diffraction kinematical? Part II: A practical method to determine the optimum precession angle

A series of experiments was undertaken to investigate the kinematical nature of precession electron diffraction data and to gauge the optimum precession angle for a particular system. Kinematically forbidden reflections in silicon were used to show how a large precession angle is needed to minimise multi-beam conditions for specific reflections and so reduce the contribution from dynamical diffraction. Small precession angles were shown to be detrimental to the kinematical nature of some low-order reflections. By varying precession angles, precession electron diffraction data for erbium pyrogermanate were used to investigate the effect of dynamical diffraction on the output from structure solution algorithms. A good correlation was noted between the precession angle at which the rate of change of relative intensities is small and the angle at which the recovered structure factor phases matched the theoretical kinematical structure factor phases.     

Structure solution with three-dimensional sets of precessed electron diffraction intensities

The use of the precession technique for obtaining three-dimensional (3D) sets of electron diffraction intensities suitable for structure solution is discussed. The minerals uvarovite and ?kermanite have been used as testing structures. The electron diffraction data sets obtained on these samples retain an acceptable linear relation with calculated structure factor amplitudes. The quality of these data is suitable to solve both structures using direct methods opening the possibility to use 3D precession ED data for solving unknown mineral structures.     

Refinement of the 200 structure factor for GaAs using parallel and convergent beam electron nanodiffraction data

We present a new method to measure structure factors from electron spot diffraction patterns recorded under almost parallel illumination in transmission electron microscopes. Bloch wave refinement routines have been developed to refine the crystal thickness, its orientation and structure factors by comparison of experimentally recorded and calculated intensities. Our method requires a modicum of computational effort, making it suitable for contemporary personal computers. Frozen lattice and Bloch wave simulations of GaAs diffraction patterns are used to derive optimised experimental conditions. Systematic errors are estimated from the application of the method to simulated diffraction patterns and rules for the recognition of physically reasonable initial refinement conditions are derived. The method is applied to the measurement of the 200 structure factor for GaAs. We found that the influence of inelastically scattered electrons is negligible. Additionally, we measured the 200 structure factor from zero loss filtered two-dimensional convergent beam electron diffraction patterns. The precision of both methods is found to be comparable and the results agree well with each other. A deviation of more than 20% from isolated atom scattering data is observed, whereas close agreement is found with structure factors obtained from density functional theory [A. Rosenauer, M. Schowalter, F. Glas, D. Lamoen, Phys. Rev. B 72 (2005), 085326-1], which account for the redistribution of electrons due to chemical bonding via modified atomic scattering amplitudes.     

Structure analysis by diffraction of amorphous zones created by Ni ion implantation into pure Al

The implantation of Ni ions into pure Al leads to the formation of approximately 10 nm amorphous zones (AZ) which induce diffuse rings in the diffraction pattern in addition to the diffraction spots of the crystal. Measurements by energy dispersive spectrometry and electron energy loss spectrometry attributed to the amorphous zones an average Ni concentration of 25 at%. The exact structure of these AZ is still unknown. The structure is characterized here by both the total and partial radial distribution functions (RDF). Structure factor deduced from experiments is compared to calculated one. For this purpose, molecular dynamic (MD) simulations are used to model the AZ structure. The RDF are determined using this structure and analytical calculation of the diffraction pattern is achieved. Simulations of the diffraction pattern of the simulated MD sample using both a kinematic and a dynamic approach are achieved to refine the analytical procedure used on the experimental diffraction patterns. It appears that the amorphous structure is well reproduced by the MD simulations. Analytical calculation reveals the presence of a well-established chemical order in the amorphous material.     

Crystallographic image processing approach to crystal structure determination

It is shown that the crystallographic image-processing technique based on the weak-phase object approximation and on the combination of high-resolution electron microscopy and electron diffraction is applicable to crystal structure determination. The technique consists of two stages: image deconvolution and phase extension. In the first stage an image taken at an arbitrary defocus condition can be transformed into the structure image with the resolution limited by the resolution of the electron microscope. In the second stage the image resolution is enhanced to the diffraction resolution limit so that most unoverlapped atoms can be resolved individually in the final image. Although the experimental diffraction intensities are available for the image deconvolution, they must be corrected for the phase extension. The proposed empirical method of electron diffraction intensity correction seems effective for obtaining a set of corrected diffraction intensities which are approximately equal to square structure factors.
When the crystal structure under examination belongs to a known typical type, it is easy to propose the structure model by referring to the deconvoluted image which reveals the low-resolution structure, and the high-resolution structure can also be determined by image simulation.     

Direct electron crystallographic determination of zeolite zonal structures

The prospect for improving the success of ab initio zeolite structure investigations with electron diffraction data is evaluated. First of all, the quality of intensities obtained by precession electron diffraction at small hollow cone illumination angles is evaluated for seven representative materials: ITQ-1, ITQ-7, ITQ-29, ZSM-5, ZSM-10, mordenite, and MCM-68. It is clear that, for most examples, an appreciable fraction of a secondary scattering perturbation is removed by precession at small angles. In one case, ZSM-10, it can also be argued that precession diffraction produces a dramatically improved 'kinematical' data set. There seems to no real support for application of a Lorentz correction to these data and there is no reason to expect for any of these samples that a two-beam dynamical scattering relationship between structure factor amplitude and observed intensity should be valid. Removal of secondary scattering by the precession mode appears to facilitate ab initio structure analysis. Most zeolite structures investigated could be solved by maximum entropy and likelihood phasing via error-correcting codes when precession data were used. Examples include the projected structure of mordenite that could not be determined from selected area data alone. One anomaly is the case of ZSM-5, where the best structure determination in projection is made from selected area diffraction data. In a control study, the zonal structure of SSZ-48 could be determined from selected area diffraction data by either maximum entropy and likelihood or traditional direct methods. While the maximum entropy and likelihood approach enjoys some advantages over traditional direct methods (non-dependence on predicted phase invariant sums), some effort must be made to improve the figures of merit used to identify potential structure solutions.     

Is precession electron diffraction kinematical? Part I:: "Phase-scrambling" multislice simulations

Results from multislice simulations are presented which demonstrate that diffracted intensities obtained using precession electron diffraction are less sensitive to the phases of structure factors compared to electron diffraction intensities recorded without precession. Since kinematical diffraction intensities depend only on the moduli of the structure factors, this result supports previous research indicating that the application of precession leads to electron diffraction intensities becoming more kinematical in nature.     

The effect of inelastic scattering on crystal structure refinement from electron diffraction patterns recorded under almost parallel illumination

Recently a number of crystal structures were determined using electron diffraction data with an almost parallel electron beam. In many cases no energy filtering was applied. On the other hand, the contrast in convergent beam electron diffraction patterns is greatly improved by energy filtering of the electrons. To investigate whether energy filtering will improve the accuracy of the structure analysis from diffraction data recorded under an almost parallel beam condition, we recorded diffraction patterns of the [100] zone of YBa(2)Cu(3)O(7) using unfiltered electrons, zero-loss electrons and plasmon-loss electrons, respectively. Subsequently, the structure is refined based on these different electron diffraction datasets, using the program MSLS (Acta Crystallogr. A 54 (1998) 91). The results show that the obtained atomic positions are not significantly different for the chosen filter conditions. Even with amorphous carbon deposited on the specimen, which will cause a significant increase (>40 times) of energy-loss electrons, the structure refinement led to the same atomic positions.     

Visualization and electron diffraction on cryosections of stratum corneum: a utopia?

The skin acts as an effective barrier to protect the body against penetration of substances from the environment and against desiccation. The main barrier function resides in the stratum corneum, and more specifically in the intercellular lipid domains. Several techniques have been used to elucidate the local lipid crystal arrangements in these domains, but they either needed an extensive pretreatment of the skin with the risk of damaging the native structure, or were not suited to obtain local structure information as bulk quantities of stratum corneum were required. In this paper a method of performing local structure analysis (electron diffraction) on cryo-fixed specimens is described. Therefore a cold chain procedure was used to obtain cryosections of stratum corneum. On these sections visualization and electron diffraction at low temperature were carried out.
Using a so-called tape sandwich method, cryosections were prepared in which corneocytes and lipid matrix could easily be distinguished. Moreover, detailed cellular components such as desmosomes and intracellular lipid domains were observed. However, probably due to the limited amount of intercellular lipids in the stratum corneum, electron diffraction on cryosections did not result in diffraction patterns that were undoubtedly originating from the intercellular lipids. In the electron diffraction patterns of a skin lipid model system reflections were present that were indicative of hexagonal and orthorhombic sublattices. The d-spacings of these reflections were similar to the spacings of the high-intensity reflections of the X-ray diffraction pattern of the same mixture. This showed agreement between a bulk and a local technique, X-ray and electron diffraction.     

Accurate stigmating of a high voltage electron microscope

To reach the 0·2 nm point-to-point resolution possible with some high voltage electron microscopes, the astigmatism of the objective lens must be compensated to within 5 nm. Due to a number of factors the resolution of the image seen on the viewing screen of the high voltage microscope is, however, quite poor and does not permit compensation of such accuracy. We describe a technique for evaluating and correcting the astigmatism that starts from a recorded micrograph of a thin amorphous specimen. The astigmatism is determined from the optical diffraction pattern using a variation of the Thon method. This variation avoids any direct measurement of the radii of the contrast transfer zones, and is extremely rapid and convenient. Adjusting the stigmator coil currents, calibrated in terms of their stigmating power, for zero astigmatism completes the correction in less than 10 min after the recording of the micrograph.     

Characterization of polymer blends and block copolymers by conventional and low voltage SEM

Methods for characterizing block copolymers and polymer blends have been developed. Results from unstained and ruthenium tetroxide-stained samples obtained by scanning electron microscopy at various acceleration voltage and by transmission electron microscopy are presented. The contrast in secondary images between components in stained polymer blends, where one component is preferentially stained, is maximized at higher acceleration voltage (10–25 keV). For measurement of particle size and shape, this is the preferred operating condition. To obtain high-resolution images showing surface topography and fine structure, such as 20 nm domains in block copolymers, low-voltage (<5 keV) imaging is preferred. Observation of the 20 nm domain structure in hydrogenated styrenebutadiene-styrene shows that the spatial resolution now possible by scanning electron microscopy is comparable to that obtained by the traditional method of transmission electron microscopy.     

Electron diffraction of synthetic polymers: The model compound approach to polymer structure

X-ray fiber diffraction is a technique most often used to establish the structure of chemically regular crystalline polymers. However, this technique has many shortcomings. Some of them are: ambiguities regarding the choice of the space group, limited intensity data, often large diffraction spots, and overlapping intensities. Microsingle crystals of synthetic polymers, when they can be produced, happen to have dimensions that are well suited to being studied by electron diffraction. Electron diffraction data from microsingle crystals often complement the X-ray fiber data. Strictly speaking, one does not solve the crystal structure of a polymer in the conventional way but rather one chooses from among many potentially acceptable models the one which fits the X-ray and/or the electron diffraction data best. The various steps of model building, its placing within the unit cell, and structure refinement will be discussed.     

Electron diffraction and diffraction contrast imaging of thin organic films

The applications of electron diffraction and diffraction contrast electron microscopy with which to study the structure and dynamics of organic thin films are discussed. The techniques of making thin film deposits on substrates and of forming free-standing thin films over holes on the substrate are described. Selected area electron diffraction and diffraction contrast imaging techniques for thin film studies are elaborated, and examples are given. Methods to reduce radiation damage and environmental protection of the thin film specimen are outlined. The interpretation of electron diffraction and imaging data is given for three cases: (1) The effects of film tilting and molecular tilting (with respect to the film plane) are examined. (2) The detection of phase transition is illustrated. (3) The use of labels to mark film domains is shown together with the measurement of dynamic movement.     

Electron diffraction analysis and techniques for studying polypeptides and other polymers

A review of electron diffraction studies of polypeptide structures is given together with a discussion of the methods for obtaining diffraction patterns without serious specimen damage by electrons. Electron diffraction studies of poly-β-benzyl-L-aspartate show that diffraction intensities can be used quantitatively to determine polypeptide structures and the advantages of these studies in observing high angle meridional reflections are shown up by the observation of a novel structure in the unheated left handed helical form of the polypeptide. A large catalogue of techniques for preparing orientated films of polypeptides suitable for electron diffraction investigations makes the technique a powerful one for studying these structures. Polypeptides are shown to be useful for studying electron beam damage in protein-like materials.     

Molecular microscopy of polydiacetylene monolayers

High resolution electron microscopy, selected-area electron diffraction (SAD), and digital recording of electron diffraction intensities have been used to study the structure properties and electron beam sensitivity of pristine and osmium-tetroxide-treated monolayer (Langmuir-Blodgett) films of the polydiacetylene, 10, 12-nonacosadiynoic acid. Electron diffraction and computed b-c projections of the Patterson function of self-supporting polydiacetylene monolayers indicated that the packing symmetry is unaffected by the osmate ester reaction. These results are combined with published data and used to support the contention that the deleterious secondary processes which limit attainable resolution of organic polymers can be minimized by heavy-metal “staining”.     

A charge coupled device camera with electron decelerator for intermediate voltage electron microscopy

Electron microscopists are increasingly turning to intermediate voltage electron microscopes (IVEMs) operating at 300-400 kV for a wide range of studies. They are also increasingly taking advantage of slow-scan charge coupled device (CCD) cameras, which have become widely used on electron microscopes. Under some conditions, CCDs provide an improvement in data quality over photographic film, as well as the many advantages of direct digital readout. However, CCD performance is seriously degraded on IVEMs compared to the more conventional 100 kV microscopes. In order to increase the efficiency and quality of data recording on IVEMs, we have developed a CCD camera system in which the electrons are decelerated to below 100 kV before impacting the camera, resulting in greatly improved performance in both signal quality and resolution compared to other CCDs used in electron microscopy. These improvements will allow high-quality image and diffraction data to be collected directly with the CCD, enabling improvements in data collection for applications including high-resolution electron crystallography, single particle reconstruction of protein structures, tomographic studies of cell ultrastructure, and remote microscope operation. This approach will enable us to use even larger format CCD chips that are being developed with smaller pixels.     

Progress toward structure determination

Convergent-beam electron diffraction provides more precise measurements of diffracted intensity than the traditional method of selected-area diffraction. The intensity is recorded at well-defined beam directions for each reflection in the pattern within disks defined by the incident cone of rays. Measurements relating to structure factors or parameters can be arranged in different ways: intensities at the zone axis position; Kossel line profiles or integrated intensities across Kossel lines; conditions for vanishing contrast at a Kossel line (e.g., critical voltage); separation between Kossel line segments at intersections. Examples of application to refinement of structure parameters (zone axis intensities) and structure factor determination (Kossel line methods) are given. The relation of these magnitudes to theory is discussed, especially for the Kossel line methods. These are described in terms of effective Fourier potentials or gaps at the Bloch-wave dispersion surface. Use of the methods for refinement of structure parameters and structure factors is reviewed with special attention to recent developments. This is seen along two lines: (1) extended scope for the more accurate methods in order to cover larger unit cells and (2) better precision in measurements of intensities.     

In situ electron backscattered diffraction of individual GaAs nanowires

We suggest and demonstrate that electron backscattered diffraction, a scanning electron microscope-based technique, can be used for non-destructive structural and morphological characterization of statistically significant number of nanowires in situ on their growth substrate. We obtain morphological, crystal phase, and crystal orientation information of individual GaAs nanowires in situ on the growth substrate GaAs(111) B. Our results, verified using transmission electron microscopy and selected area electron diffraction analyses of the same set of wires, indicate that most wires possess a wurtzite structure with a high density of thin structural defects aligned normal to the wire growth axis, while others grow defect-free with a zincblende structure. The demonstrated approach is general, applicable to other material systems, and is expected to provide important insights into the role of substrate structure on nanowire structure on nanowire crystallinity and growth orientation.