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 regioregular D1-A-D2-A terpolymer PDTSTTBDT incorporating dithieno[3,2-b:2′,3′-d]silole (DTS, D1) and benzo[1,2-b:4,5-b]dithiophene (BDT, D2) units with perfectly controlled thieno[3,4-b]thiophene (TT, A) orientation was synthesized for the first time. The thermal, optical, and electrochemical properties of the regioregular PDTSTTBDT were characterized and compared with the random PDTSTTBDT without structural regioregularity. The regioregular PDTSTTBDT showed ideal optical bandgap (1.45 eV), lower lying HOMO energy level, and higher degree of crystallinity compared to the random PDTSTTBDT. Moreover, it exhibited excellent solubility in nonhalogenated solvents as well as halogenated solvents. The inverted bulk-heterojunction polymer solar cells (PSCs) based on the regioregular PDTSTTBDT and o-xylene process solvent showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTSTTBDT-based PSCs. It was found that the remarkable enhancement of photovoltaic performance in regioregular PDTSTTBDT-based PSCs is mainly due to improved light absorption, effective polymer ordering, and high charge carrier mobility.
Glassy carbon has been prepared in the shape of disk and fibre by direct pyrolysis of a phenolic resin. Carbonization studies indicate that the unique structure of the final glassy carbon is a direct consequence of the production of very stable aromatic ribbon molecules by the coalescence of phenolic polymer chains at an early stage of pyrolysis. It is shown that molecular orientation induced in the initial polymer before pyrolysis is 'memorized’ to some extent after carbonization. Molecular orientation imposed in this type of carbon is not an intrinsic structural feature, but a physical characteristic which can be varied by the formation process or by extension at high temperatures; there is no essential structural difference apart from preferred orientation between polymeric units or microfibrils in well-oriented carbon fibres and isotropic glassy carbon. High resolution electron microscopy confirms this directly. We thus identify a new class of ‘polymeric carbons’, that consist of intertwined microfibrils comprising stacks of narrow graphitic ribbons. The fibrils are held together with covalent interfibrillar links of strength lower than that in the ribbons themselves. A ribbon structure has been proposed previously by Ruland (1971) for the specific case of high modulus carbon fibre. The structure is elaborated and extended here to cover all polymeric carbons and the steps in its development during carbonization are decisively detailed.
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