Study of thin organic overlayers by local probe microscopies

Abstract

In Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM), the interaction of a very sharp tip with a surface is used to gather information on the latter. In STM, the interaction is measured as an electric current, and in AFM as a force acting on the tip. AFM, which does not require the sample to be electrically conducting, is more general than STM which does, so the study of organic overlayers by STM is limited to very thin layers (thickness comparable to a tunnelling distance i.e. 10 Å), onto conducting substrates. On the other hand, spatial resolution to the atomic scale can hardly be obtained in AFM, and the corresponding mechanism is poorly understood. In all cases, the information obtained is mainly topographic, i.e. crystallographic (at the atomic scale) or morphological (at larger scales). Several examples will be discussed. Monomolecular layers on conducting substrates in air, under vacuum, or in contact with the corresponding bulk liquid, have been studied by STM. Two‐dimensional periodic structures, not found in the bulk crystals, are observed, as well as different kinds of point or line defects. This emphasizes the importance of molecule‐substrate interactions in competition with the usual intermolecular interactions. There are few STM studies of thin continuous polymeric overlayers. Early studies of biological macromolecules have been frustrated by artefacts due to substrate surface defects which mimic the expected polymer images, so AFM is now preferred, at the expense of resolution. AFM is now intensively used for studying the surface morphology of thicker (more than one molecule thick) films of organic molecules or polymers. However, the force exerted by the tip on the surface is often large enough to modify organic surfaces; a more recent variant of AFM, the so‐called ‘‘tapping mode,’’ in which that interaction is reduced, will be presented. A few examples of polymer surface studies using AFM will be discussed. In particular, morphological studies and molecular resolution images (both in the usual ‘‘contact mode’’ and in the tapping mode) of highly oriented polytetra‐fluoroethylene thin films will be presented and discussed, with examples of surface modification by tip scanning.