Infrared and raman spectroscopies of ethylenedithiodialkylthiotetrathiafulvalene langmuir-blodgett films doped with iodine vapor

Abstract

Conducting LANGMUIR-BLODGETT films (LB) present a great deal of interest for molecular electronic devices and chemical sensors. We reported that 1:1 mixture of a fatty acid with ethylenedithiodioctadecylthio-tetrathiafulvalene give LB films which become conducting after iodine exposure (1). For a better understanding of the iodine doping mechanism, we have analysed the infrared spectra evolution of films versus time. In a first step a new absorption line is observed at 1333 cm −1 and attributed to a charged dimeric form, while the films are still insulating. After a couple of hours at room temperature, the intensity of the 1333 cm −1 line decreases to zero and a new broad charge-transfer band associated to an intense absorption centered at 1250 cm −1 of vibronic origin is detected. The final state corresponds to the conducting phase already described (1). Raman spectroscopy on the latter phase indicates the formation of the I 3 - species which are ordered in chains. A mechanism of the doping process is proposed from which the film stoichiometry is derived.