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.     

High resolution in-focus Lorentz electron microscopy

The Foucault in-focus method for viewing magnetic domains in a conventional electron microscope has been modified. The main feature of our modification is to introduce an aperture below the intermediate lens and to use it for stopping out one part of the split central spot instead of the objective aperture. This arrangement substantially reduces the axial astigmatism due to the objective aperture and makes it possible to reach the point resolution 5–6 nm in both light and dark domains. The image resolution is limited by the small image magnification achieved using the three-stage optical system of a Tesla BS 413 microscope.     

High resolution electron microscopy of carbon structure

An investigation is made of the inherent performance of a high-resolution transmission electron microscope applied to the study of developing graphitic-sheet structures in heat-treated, coal-tar pitch carbons. Image detail is shown to be highly dependant on instrumental defocus. It is not obvious which form will be assumed by artefacts in these images; consequently, anomalous features are illustrated by reference to a specific electron-optical case and a corresponding light-optical analogy. Despite the difficulties associated with locating an optimum level of focus, optical diffractometry confirms that adequate conditions of microscopy are attainable on a routine basis. In a quantitative analysis of morphology in coal-tar pitch carbons, the technique reveals the cause of incipient, thermally-induced, molecular distortions which evidently result in a regression in the improvement of order otherwise developing with progressive heat-treatment.     

High resolution electron microscopy of some carbonaceous materials

The image contrast of extremely thin films of amorphous carbonaceous materials has been studied experimentally using thin supporting films of single crystal graphite. The use of thin specimens reduces the ‘overlap’ effect in imaging amorphous materials. Image contrast of their images was therefore interpreted by the ‘projected potential approximation’. Single sheets of graphitic carbon crystal were imaged with c perpendicular to the beam and their image details were compared with those of the amorphous carbon films. This comparison demonstrated that amorphous carbon films prepared by evaporation contained microcrystalline regions having graphitic structure.     

High resolution imaging by 1 MV electron microscopy

High voltage electron microscopy is well suited to the high resolution imaging of local structural changes in a material. New information at atomic resolution can be obtained on a broad range of material problems. Investigations of the structure of dislocations and planar faults in metallic and covalent crystals is presented.     

High resolution cryo electron microscopy of specifically stained specimens

At very low temperatures organic specimens with attached metal atoms can be irradiated with a relatively high electron fluence without losing the characteristic features of their architecture. This is demonstrated on three examples: a metal-organic molecule, a membrane-like biological layer, and a three-dimensional polymeric copper complex.     

High resolution transmission electron microscopy of Sm-Co based permanent magnets

Interface microstructures between rhombohedral Sm₂Co₁₇ and hexagonal SmCo₅ phases in a sintered permanent magnet of Sm(Co_0.675Fe_0.175Ni0.05Cu_0.1)₇ have been investigated by edge-on observations with the many-beam imaging technique. In the specimen annealed at 800°C for 4 h which shows a maximum magnetic coercivity, the interface boundaries were coherent accompanying the lattice distortion; the habit planes were parallel to the {101}_2:17 and {111}_1:5 planes with the orientation relationship of (0001)_2:17//(0001)_1:5 and [1010]_2:17//[1120]_1:5. In the specimen annealed for 150 h which shows a lower coercivity, the interface boundaries were characterized with regular alignments of interfacial edge dislocations. The observed interface structures were discussed in connection with the change in magnetic properties.     

High resolution scanning electron microscopy at the subcellular level*

Recently developed scanning electron microscopes provide sufficient resolution to allow useful observation of subcellular biological objects. Preparation methods for such objects need not be limited to the traditional coating and mounting procedures. Many methods developed for transmission electron microscopy are immediately adaptable to scanning electron microscopy. We show that a number of techniques are available to the microscopist which yield adequate contrast and high resolution. As examples we show skeletal muscle myofibrils dispersed to reveal thick filaments, uncoated on a thin carbon film; a tropomyosin tactoid, negatively stained with uranyl acetate; oncornavirus, conventionally coated; and T4 bacteriophage on an aluminium substrate.     

High resolution shadowing for electron microscopy by sputter deposition.

The sputtering process by an ion beam well collimated and highly accelerated provides a valuable means of high resolution shadowing, replication of a fine object by a combination of pre-shadowing and deposition as well as a preparation of supporting films. High resolution shadowing and films with grains smaller than 1 nm can be obtained by argon ion-sputtering targets of tungsten and tungsten/tantalum alloy. The resolution of carbon replicas pre-shadowed with tungsten/tantalum is determined from the radius of curvature of replicated magnesium oxide crystal corners.     

High resolution electron microscopy of interfaces in chlorinated phthalocyanine molecular crystals

Molecular images of chlorinated copper phthalocyanine have shown the arrangement of molecules at the interface between two crystals. Mismatch does not cause distortion or recrystallization for low index junctions but for more irregular interfaces strain causes alteration of the molecular tilt angle and accommodation of the mismatch by the formation of edge dislocations whose Burger's vectors are parallel to the interface.     

Achievement of atomic resolution electron microscopy

Atomic-level details are easily resolved in the latest generation of intermediate voltage electron microscopes, but structural information on the same scale can only be extracted under certain specific conditions. Some understanding of imaging theory, as well as an awareness of correct operating conditions, is required for reliable image interpretation. Several representative examples are chosen to illustrate the possibilities for atomic-resolution imaging of materials, and perspectives and outlook for the technique are briefly discussed.     

High resolution electron microscopy of partial dislocations in the Laves phase structure

A Mg-base Laves phase was investigated by high resolution electron microscopy (HREM). Linear defects found at terminations of stacking faults were classified into three groups. The first is a partial dislocation at a termination of a stacking fault, the second is a superposed partial dislocation which is defined as a defect produced by a superposition of terminations of two or more stacking faults lying on neighbouring layers, and the third is a combined linear defect which consists of a characteristic combination of terminations of stacking faults. In the last case the total stacking fault vector becomes equal to the translation vector in the basal plane, so that the defect needs no relaxation of the lattice. The Burgers vectors of the partial dislocations were estimated with the aid of modified Burgers circuits.     

Investigations of the structure of amorphous and partially crystalline metallic alloys by high resolution electron microscopy

A number of amorphous and partially crystalline palladium-silicon alloys have been examined by high resolution transmission electron microscopy at 500 kV. With the directly interpretable resolution extending beyond the first peak in the structure factor at 0?23 nm, details of the local microstructure at the atomic level are visible. Images of small metallic particles show a well-defined pattern of fringes over local regions. In some instances, especially in partially-ordered alloys, neighbouring or overlapping fringe patterns appear to be orientationally-related in a similar manner to fringe systems seen in symmetrically multiply-twinned particles. The significance of this type of structural examination for amorphous metals is discussed.     

Kodirex for high resolution electron microscopy.

Measurement of the important performance parameters shows that Kodak Kodirex film is more suitable than conventional ones for dark-field transmission electron microscopy of molecules at very high mignification. Results are cited for 120 kV; but the relation ship is valid up to 3 MV.     

Problems of interpretation in high resolution electron microscopy

Current assumptions in wave aberration theory and specimen scattering theory are reviewed. More quantitative image simulations would be valuable as well as use of a wider range of imaging techniques, particularly STEM. The severe difficulties of high resolution three-dimensional reconstruction are described and illustrated.     

Selective contrast in electron microscopy of crystalline cytochrome oxidase

We have used various techniques for preparation of specimens for electron microscopy in order to selectively contrast different regions of vesicle crystals of cytochrome c oxidase dimers. The results are consistent with a dimer composed of two y-shaped monomers [Fuller et al., J. Mol. Biol. 134 (1979) 305] aligned along one pair of arms with the other pair of arms approximately 70 A apart. The four arms of the monomers lie within and perpendicular to the lipid bilayer in which the dimer is embedded, and the arms protrude approximately 25 A from the lipid bilayer on the matrix side of the membrane. The cytoplasmic side domains of the two monomers split away from one another forming a large cleft in the dimer. Monovalent antibodies (Fab fragments) to subunit IV appear to bind to the two monomer arms which are closely apposed across the two-fold axis of the dimer.     

High resolution microanalysis of aluminium solid solution alloys using combined electron microscopy and energy analysis

The technique of combined electron microscopy and energy analysis is described, together with the main energy loss process that is used to analyse composition. It is shown that the spatial resolution of the technique is ～ 14 nm which is ten times better than any other technique available at present. The composition resolution in aluminium alloys is < 1 at.%, the exact value depends upon which alloy system is used. Applications of the technique to the measurement of fine scale concentration gradients are described for Al-Cu, Al-Mg and Al-Zn-Mg alloys.