A test object and criteria for high resolution electron microscopy

Abstract

The desirability and requirements for a specimen capable of testing the resolving power and other image characteristics of an electron microscope are discussed in detail. In this discussion, the underlying diffraction phenomena are particularly utilized. A partially graphitized carbon black is shown to satisfy the requirements extremely well and constitutes an easily prepared specimen for conducting tests of image quality in the molecular size range. The structure of the test object is known in detail with the result that readily interpretable phase contrast images are obtained. Micrographs illustrating the use of the 3.4/~ (002) spacing for magnification calibration, astigmatism and asymmetry check as well as resolving power are exhibited. The second order c-spacing of 1.7 ,~ is occasionally found in an image. The micrographs shown herein were taken with two different electron microscopes by different operators obtaining the same structural detail in the images. It is concluded that the carbon black test object offers the best possibilities for evaluating image performance of any specimen yet suggested. Introductory discussionThe rapidly increasing interest in electron microscopy of structures in the molecular size range poses several problems among which specimen preparation, contrast interpretation and performance testing of instruments are paramount. This paper is concerned with the last mentioned subject since the primary requirement is presently a knowledge of the performance and ability of the microscope itself.Although discussions and theoretical treatments of resolution and resolving power have continued for many years, there is still no general agreement on a test object for quickly evaluating image quality. Lattice images exhibiting interplanar spacings down to about 1.2 A~ have been obtained by means of optimum tilting of the illumination to minimize spherical and chromatic aberration. The resolving power so obtained is not isotropic, however, and there has been a tendency to de-emphasize these results as not indicative of the highly desirable 'point-to-point' resolution. The criterion of point-to-point or isotropic resolving power requires well aligned, axial illumination along with compensation of objective lens asymmetry and astigmatism. Yada & Hibi (1966) have demonstrated lattice image spacings of 1.81 A, using axial illumination. This is point-to-point resolving power even though the contrast mechanism be phase rather than diffraction contrast. Major factors in phase contrast are source stability and coherence, the latter being greatly improved by the use of pointed filaments as earlier shown by Hibi (1962). Since the contrast to be realized in the molecular domain of structure is predominantly phase contrast (Thon, 1966), the coherence of the source is highly important. Two methods of estimating resolving power that have been in use for some time are the edge contour width suggested by Haine (1961) and the finely divided evaporated metal specimens. The experience of a number of workers seems...

A molecular model for carbon black primary particles with internal nanoporosity

“…The larger shell units (d = 3.7 nm) have a well-defined concentric structure compared to that of the small ones (d = 2.7 nm). This effect has also been observed in the experiments, where heat treatment is often applied to partially graphitize carbon blacks [32]. The growth and parallel alignment occurs with graphite layers near the carbon surface, eventually leading to the formation of graphite outer shell [27,49].…”

“…interlayer spacing is set at 0.4 nm according to the largest distance derived from XRD [26,32]. The simulation results show that about 50% of the graphite layers are those do not belong to any microcrystallite.…”

“…The widely accepted model of carbon black proposed by Heidenreich et al is a spherical carbon particle with the concentric organization of small graphite sheets from the surface to the center [32]. The concentric arrangement is more pronounced near the outer surface than the inner cores, leading to the formation of amorphous carbon in the core region.…”

“…The level of crystalline growth differs from particle to particle, and the stacking variation of poorly developed graphitic layers is responsible for the broad distribution of interlayer spacings measured by XRD. Models of carbon blacks have been proposed by many early studies with a tendency toward the concentric alignment of graphite layers roughly parallel to the external surface of carbon particles [30][31][32]. This tendency is most pronounced for crystallites located near the carbon surface, and gradually diminishes toward the center of carbon particles.…”

“…In addition to these major peaks, one may note that the base line of RDF rises from about 0.32 nm. This is attributed to the carbon atoms located in different graphite layers since the maximum interlayer spacing of graphite units in carbon blacks is around 0.4 nm [26,32,51]. To explicitly examine the interlayer connectivity of graphite units, the RDF is calculated for only the shell units coordinated by their centers drawn in Fig.…”