Crystallographic orientation assessment by electron backscattered diffraction.

With an angular orientation accuracy of at least 1 , the ability of electron backscattered diffraction (EBSD) to determine and emphasise crystallographic orientation is illustrated. Using the abilities of specially developed software for computing Euler angles derived from the scanned specimen, misorientations are pointed out with acceptable flexibility and graphic output through crystallographic orientation maps or pole figures. This ability is displayed in the particular case of laser cladding of nickel-based superalloy, a process that combines the advantages of a near net-shape manufacturing and a close control of the solidification microstructure (E-LMF: epitaxial laser metal forming).     

Crystallographic analysis of facets using electron backscatter diffraction

Applications of electron backscatter diffraction (EBSD), also known as backscatter Kikuchi diffraction in the scanning electron microscope (SEM) are first and foremost microtexture and grain boundary misorientation analysis on a single polished section in the specimen. A more subtle and revealing approach to analysis of these data is to use EBSD to probe the orientations of planar surfaces, i.e. facets, which bound crystals. These surfaces include: • grain or phase boundaries • fractures • cracks It is of great interest to know the crystallography of such facets since it provides a key to understanding the physical properties of them.
As far as investigation methodology is concerned, surfaces or facets associated with polycrystals are of two types: exposed or unexposed. Exposed facets, such as a fracture surface, can be viewed directly in the SEM, whereas unexposed facets, such as a grain boundary, are usually revealed as an etched trace on a polished surface. Photogrammetric methods can be used to obtain the positional orientation of an exposed facet, and the crystallographic orientation is obtained either directly from the surface or by indirect sectioning. Calibrated sectioning is required to obtain the equivalent parameters for an internal surface. The present paper compares the methods for obtaining and interpreting the crystallography of facets, with illustrations from several materials.     

Scanning electron mirror microscopy

The working principles, instrument designs, and applications of scanning electron mirror microscopy are reviewed. The capability of this technique for direct imaging of topographical surface structures as well as electrostatic and magnetic surface strayfields is demonstrated by a number of examples.     

Scanning transmission electron microscopy

The scanning transmission electron microscope is of quite recent origin, and it is only in the last few years that it has been shown that this instrument is capable of giving the same high resolution as the conventional electron microscope. In this article we examine the conditions necessary for the achievement of high resolution and also the various modes of contrast which can be obtained from this instrument. Finally, we suggest other ways in which the microscope can be used in future investigations.     

Time-resolved cryotransmission electron microscopy

We describe a new technique, time-resolved cryotransmission electron microscopy (TRC-TEM), that can be used to study changes in microstructure occurring during dynamic processes such as phase transitions and chemical reactions. The sample is prepared on an electron microscope grid maintained at a fixed temperature in a controlled atmosphere. The dynamic process is induced on the grid by a change in pH, salt, or reactant concentration by rapid mixing with appropriate solutions. Alternatively, induction is by rapid change of specimen temperature, or by controlled evaporation of a volatile component. We call such procedures on-the-grid processing. The dynamic process is permitted to run for a defined time and then the thin-film specimen is thermally fixed by plunging into liquid ethane at its freezing point, producing a cryotransmission electron microscopy specimen. By repeating this procedure with varying delays between induction and sample fixation, we can observe transient microstructures. We demonstrate the use of TRC-TEM to study the intermediate structures that form during the transitions between Lα, I_II, and H_II liquid crystalline phases in phospholipid systems. We also identify several other possible applications of the technique.     

Cryoprotection in electron microscopy

A set of experiments has been undertaken to resolve questions about the cryoprotection factor afforded by cooling specimens in the electron microscope. Two specimens, an n-alkane (paraffin) and bacterial purple membrane, were prepared and distributed to several laboratories. Radiation damage studies were carried out at these laboratories independently, but following a standard protocol as closely as possible. While it was not possible to make all measurements at all of the laboratories, sufficient data have been obtained to establish a general reproducibility in the overall features of the cryoprotection effect from room temperature to near 4.2 K.     

Comparative light microscopy,scanning electron microscopy and transmission electron microscopy of selected organic walled microfossils

Two simple techniques are described and illustrated. The first is for the study of one specimen by both light microscopy (LM) and scanning electron microscopy (SEM). The second is for the study of one selected specimen by LM, SEM and in ultrathin section by transmission electron microscopy (TEM). Although these techniques were developed for the comparative study of Precambrian organic walled microfossils (OWMs), they could be used for a wide range of other specimens.     

Emission microscopy and related techniques: resolution in photoelectron microscopy, low energy electron microscopy and mirror electron microscopy.

A unified treatment of the resolution of three closely related techniques is presented: emission electron microscopy (particularly photoelectron microscopy, PEM), low energy electron microscopy (LEEM), and mirror electron microscopy (MEM). The resolution calculation is based on the intensity distribution in the image plane for an object of finite size rather than for a point source. The calculations take into account the spherical and chromatic aberrations of the accelerating field and of the objective lens. Intensity distributions for a range of energies in the electron beam are obtained by adding the single-energy distributions weighted according to the energy distribution function. The diffraction error is taken into account separately. A working resolution is calculated that includes the practical requirement for a finite exposure time, and hence a finite non-zero current in the image. The expressions for the aberration coefficients are the same in PEM and LEEM. The calculated aberrations in MEM are somewhat smaller than for PEM and LEEM. The resolution of PEM is calculated to be about 50 A, assuming conventional UV excitation sources, which provide current densities at the specimen of 5 x 10(-5) A/cm2 and emission energies ranging up to 0.5 eV. A resolution of about 70 A has been demonstrated experimentally. The emission current density at the specimen is higher in LEEM and MEM because an electron gun is used in place of a UV source. For a current density of 5 x 10(-4) A/cm2 and the same electron optical parameters as for PEM, the resolution is calculated to be 27 A for LEEM and 21 A for MEM.     

Automated electron tomography with scanning transmission electron microscopy

We report the successful implementation of a fully automated tomographic data collection system in scanning transmission electron microscopy (STEM) mode. Autotracking is carried out by combining mechanical and electronic corrections for specimen movement. Autofocusing is based on contrast difference of a focus series of a small sample area. The focus gradient that exists in normal images due to specimen tilt is effectively removed by using dynamic focusing. An advantage of STEM tomography with dynamic focusing over TEM tomography is its ability to reconstruct large objects with a potentially higher resolution.     

Aspects of dark-field electron microscopy

This paper endeavours to give all the necessary practical information to operate with the contrast-stop method as far as fairly thick objects (from 10 to 100 nm) are concerned; including working diagrams, practical operation out of the stops and also to define the range of application of this method. It is shown that false detail is not to be feared in these rather thick specimens, which is not at variance with the results obtained by Hanszen (1967, 1969) and Thon (1966) as far as very thin specimens are concerned.     

High resolution dark-field electron microscopy

In 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.     

Problem oriented transmission electron microscopy

The relative advantages and disadvantages of the variety of transmission electron microscopical techniques which can be applied to materials science problems are argued by the examination of problems from a range of different fields. The further developments which will be needed before the full potential of some of the more modern approaches can be achieved are also discussed, as are the increasing uses of ‘edge-on’ specimen preparation methods.     

Cathodoluminescence in transmission electron microscopy

We describe a cathodoluminescence spectrometer that is attached to an analytical transmission electron microscope. After a brief consideration of the set‐up and the peculiarities of recording spectra and of mapping defect distributions in panchromatic and monochromatic cathodoluminescence, we discuss two examples of applications. Emphasis is placed on the potential for obtaining novel information about materials and processes on a microscopic and a nanoscopic scale by combining cathodoluminescence with the structural and chemical information for the same site of the specimen. We select an example concerning the role of In distribution in light emission from InGa/GaN quantum wells and a second one concerning the analysis of the initial electron radiation damage of Cu(In,Ga)Se₂ photovoltaic films.     

High-resolution scanning electron microscopy.

The spatial resolution of the scanning electron microscope is limited by at least three factors: the diameter of the electron probe, the size and shape of the beam/specimen interaction volume with the solid for the mode of imaging employed and the Poisson statistics of the detected signal. Any practical consideration of the high-resolution performance of the SEM must therefore also involve a knowledge of the contrast available from the signal producing the image and the radiation sensitivity of the specimen. With state-of-the-art electron optics, resolutions of the order of 1 nm are now possible. The optimum conditions for achieving such performance with the minimum radiation damage to the specimen correspond to beam energies in the range 1-3 keV. Progress beyond this level may be restricted by the delocalization of SE production and ultimate limits to electron-optical performance.     

Freeze-fracture for scanning electron microscopy

Two different freeze-fracture methods are explored for preparation of biological material for scanning electron microscopy. In the simpler method the tissues are first fixed and dehydrated. They are then frozen and fractured, and after thawing, critical-point dried. This method has already been used in a number of studies of animal tissues (heart, liver, kidney). It is applied here to the examination of plant material (leaf mesophyll cells). In the second method tissues, or cells, are first infiltrated with cryoprotectant (dimethylsulphoxide) then frozen and fractured, and not fixed until after thawing. The fixed tissues are finally dehydrated and critical-point dried. This method also has previously been used in the study of animal tissues, and is applied here to carrot protoplasts, chicken erythrocytes, and leaf mesophyll cells.     

Surface crystallography via electron microscopy

The study of atomic structure of surfaces is fundamental to the understanding of electronic, chemical and mechanical properties of surfaces and numerous techniques have been developed to this end. Transmission Electron Microscopy techniques, namely transmission electron imaging (TEM) and diffraction (TED), due to their ability to provide structural information at very high resolutions, have emerged as powerful tools for the study of surface structure. In this article we review the experimental method alongside the various post-processing routines that are necessary to extract vital structural information from experimental data.