Electrochemistry of conductive polymers. 30. Nanoscale measurements of doping distributions and current-voltage characteristics of electrochemically deposited polypyrrole films

Spatial variations in electronic structures and evolutions of band structures of polypyrrole (PPy) have been studied using an atomic force microscope with a conducting tip, a so-called current-sensing atomic force microscope (CS-AFM). PPy films were deposited electrochemically onto gold-on-silicon electrodes, and their doping levels were controlled by successive electrochemical reduction. The topographic and current images of PPy surfaces were obtained simultaneously with nanometer scale spatial resolution using the CS-AFM. Galvanostatically prepared PPy films showed almost uniform current images even when the bias voltage was reduced to as small as 3 mV. High current flowing regions gradually disappeared starting from the top of the globules as the film was reduced progressively, indicating the top of the globules is preferentially reduced. The supporting electrolytes and solvents used during electrochemical preparation of PPy films also affected the doping distribution; the doping can be either nearly uniform or islandlike. The point-contact current-voltage (I-V) characteristics of conducting tip-polymer-Au substrate systems were investigated as a function of the degree of doping, and various I- V curves representing metallic, semiconducting, and insulating states were obtained depending on the doping level of the film.