The Observation of Thick Specimens by High Voltage Electron Microscopy. Experiment with Molybdenite Films at 50-500 kV

Abstract

This study was designed to investigate how much the maximum thickness of crystalline specimens which was adequate for transmission observation could be increased with the accelerating voltage. Molybdenite films of 0.5–7.0 microns thickness were used for specimens. Dislocations in films of about 0.5, 1, 2 and 3 microns were found to be observable under routine irradiating conditions at 50, 100, 200 and 500 kV, respectively. With focused irradiation the thicknesses were about 30% larger, although the image quality was low. Dislocation images were generally obtained when the effect of the so-called anomalous transmission, which disappears for very thick specimens, was apparent in the corresponding diffraction pattern. The theoretical background for images of thick specimens is discussed. Reference is also made to preliminary results obtained with silicon. No conclusion has been drawn on the voltage dependence of the maximum thickness of specimens adequate for transmission observation.