Electrochemistry of conductive polymers. 33. Electrical and optical properties of electrochemically deposited poly(3-methylthiophene) films employing current-sensing atomic force microscopy and reflectance spectroscopy

Abstract

Electrical and optical properties of poly(3-methylthiophene) (P3MeT) films have been studied by the current-sensing atomic force microscopic (CS-AFM) and near-normal incidence reflectance spectroscopic (NNIRS) techniques. The P3MeT films were electrochemically deposited onto gold-on-silicon electrodes and their doping levels were controlled by a series of electrochemical reductions. At each doping stage, a two-dimensional current map, as well as a number of current-voltage (I-V) curves at selected locations, was obtained with nanometer-scale spatial resolution by use of the current-sensing microscope, and the corresponding absorption spectrum was obtained by the NNIRS technique. The current flowing through the P3MeT film decayed exponentially and the I-V curves changed from metallic to semiconducting behavior as the film dedoping progressed. Rectifying behaviors were observed from the P3MeT film when its highly doped form was reduced to an appropriate level; these behaviors appear to result from the unique contacts formed between the polymer film and the tip at a particular doping level. To compare the effects of substituents on the electronic states, thiophene and 3-hexylthiophene were also polymerized under the same conditions as 3-methylthiophene. The results were significantly different from those shown by P3MeT, indicating that their doping behavior was different from that shown by P3MeT.