Imaging layers in thin-film molecular devices by transmission electron microscopy, using milling by focused ion beams and deposition on NaCl and Si

The performance of molecule-based thin-film devices such as organic light-emitting diodes, photovoltaic cells, and thin-film transistors depends on the electronic properties of the individual molecular components, as well as on their association to form complex morphologies. Transmission electron microscopy (TEM) can be used to image the morphologies and help reveal how the devices work and can be improved. We have examined the suitability of various ways to prepare samples of thin molecular films for imaging by TEM. Specifically, we have used focused ion beams to mill cross sections of complete devices that have been glued together with epoxy adhesives. In addition, thin films of the type used as active layers in molecule-based devices can be deposited on disks of NaCl, which can then be dissolved in water to release free-standing films that can be imaged by TEM, without loss of nanostructural details. Films of this type can also be deposited on Si wafers, which can then be fractured to expose sections of film that overhang edges of fragments and can be imaged conveniently by TEM. This allows TEM to be used as a quick method for screening samples and monitoring the purification of active materials.