THE ROLE OF REDOX CHEMISTRY IN SCANNING-TUNNELING-MICROSCOPY IMAGING OF ELECTROACTIVE FILMS

The dependence of tunneling current I on bias voltage V-b in scanning tunneling microscopy (STM) measurements of multilayer electroactive films is interpreted in terms of a redox mechanism involving electron transfer reactions at the STM tip\film and electrode\film interfaces. Analysis suggests that a symmetrical I-V-b response is anticipated when the molecular film is sufficiently thick (ca. 2-3 monolayers) that the electron-transfer steps at the STM tip and electrode can be considered as discrete steps in the current-carrying processes. A macroscopic two-electrode thin-layer electrochemical cell is used to mimic the response of an electrode\redox film\STM tip junction. Steady-state symmetrical I-V responses are obtained for the thin-layer cell, even when the solution initially contains only one half of an electroactive redox couple. Tunneling spectroscopic measurements, using a STM, of dry films of protoporphyrin(IX)Fe(III)Cl deposited on highly oriented pyrolytic graphite electrodes are interpreted in terms of the proposed redox mechanism.