Dark Field Electron Microscopy for Observing the Atoms in Molecules and Crystals

Abstract

The present author and his coleagues showed the arrangement of Th-atoms in long chain molecule (BTCA, benzentetracarboxylic acid) and crystals (ThO2) can be observed in the dark field images of conventional 100 kV electron microscope. They observed the bright spots which are arranged with the expected spacings in the molecules and crystals and have expected image diameters ( ∼ 4 Å) as shown in Fig.1. Since their observation have been carried out by using a microscope with the spherical aberration constant Cs=0.15 cm, which seems to be rather large for obtaining the resolved images of atoms with detectable contrast in the brightfield image, it is desired that certain theoretical interpretation will give the reason why the resolved image can be obtained in the dark field images.Using a theory which is originally developed by Scherzer, dark field images of single atoms and chain of atoms in long chain molecules and crystals have been calculated.