Method for the cross-sectional examination of molecular semiconductor/metal film junctions using transmission electron microscopy

Abstract

Junctions between thin films of metals and molecular semiconductors are of current interest for their uses in electrochemistry and in photovoltaic devices. Characteristics such as interdiffusion, interfacial structure, and pinhole formation are important for various applications. A method has been developed for microstructural characterization of these junctions by the examination of thin cross sections using transmission electron microscopy (TEM). This method incorporates preparation techniques that are commonly used for both inorganic and biological samples. The thin-film system is vapor deposited onto an epoxy medium, then a second epoxy layer is added, so that one obtains a ’’sandwich’’ arrangement, in which the thin-film system is wholly contained within the rigid epoxy casting. A typical system is zinc phthalocyanine (ZnPc) placed between two gold or silver layers. The whole sandwich casting is then cross sectioned by ultramicrotomy. Results concerning the interfacial morphology, obtained by TEM examination of the thin sections, seem to indicate a sharp interface with no bulk diffusion of one material into the other. However, both the metal and the ZnPc seem to exhibit significant surface mobility during deposition when the substrate is held at room temperature. Pinholes formed in the ZnPc were filled with the metal and formed shorts between the two metal layers. This phenomenon is explained and interpreted.