Observation of nanometer ordering in multilayer films of pyromellitic dianhydride–oxydianiline using the scanning tunneling microscope and ultrahigh vacuum

Abstract

We have obtained topographic images and spatially resolved spectroscopic information of uncoated pyromellitic dianhydride–oxydianiline (PMDA-ODA) films. We find that prolonged exposure to vacuum is a key to successful imaging of this nominally insulating film surface. It is postulated that water desorption from the film is responsible. This mechanism may apply to many other organic materials. Topographic results indicate that there is a fair amount of ordering in this polymer, at least for dip-coated films. Finally, we see that PMDA-ODA has low-density states very close to the pseudo-Fermi energy, in contrast to the wide band gap predicted by ultraviolet photoelectron spectroscopy results. This provides an explanation for the anomalous conductivity seen in these thin films.