Proton exchange mechanism of synthesizing CdS quantum dots in nafion

Nanocrystals of CdS are synthesized in the proton exchange membrane nafion in different sizes in the range 1.6 to 6 nm. To understand the process lead ing to the formation of these quantum dots, we have probed the proton excha nge by ac conductance measurements in the frequency range 100 Hz to 13 MHz. Nafion shows good electrical conductivity due to proton transport probably via the Grothus mechanism. Incorporation of cadmium ions by replacement of the hydrogen ions in the sulphonic acid group resulted in a large decrease in conductance indicating the reduction of the mobile carrier density. The conductivity plots all show strong frequency dependence with higher conduc tance towards the higher frequencies where a near-flat frequency response i s seen. After the formation of CdS clusters, there is a partial recovery of conductance corresponding to the reinstatement of the protonic carriers on the side groups. The conductivity of the nafion films embedded with the se miconductor quantum dots exhibits a size-dependence with the highest conduc tivity obtained for the largest clusters. These findings lend clear experim ental evidence for the model of synthesis of quantum dots in nafion by the exchange mechanism.