Molecular image resolution in electron microscopy

Abstract

In order to determine the ultimate molecular resolution attainable with a conventional electron microscope, the direct observation of hexadecachloro-Cu-phthalocyanine molecules was attempted. Since phthalocyanine derivatives are known to form crystalline films with columns of parallel stacks of planar molecules, the specimens were prepared by epitaxial growth on KCl cleavage face through vacuum evaporation so that the column axis was directed almost normal to the thin-film surface holding an orientation suitable for the observation. The molecular orientation was determined by Patterson synthesis based on the laser optical transform of the electron diffraction pattern obtained from the individual crystallites placed on the microgrid mesh. The direct observation was carried out with the 100-kV electron beam incident on the specimen along the column axis. The crosslike images arrayed in a centered rectangular net were clearly resolved, well representing the molecular shape of phthalocyanine with the configuration like a four-leaf clover. The effect of the spherical aberration and the defocus value for the objective lens are also discussed in relation to the contrast inversion of the total images. It was also found that the chlorinated Cu-phthalocyanine is 40 times more resistant to the electron radiation damage than the ordinary phthalocyanines.