Effects of the confined synthesis on conjugated polymer transport properties

Previous studies on conjugated polymer nanotubules, prepared by the template method, established that improvements in the molecular and supermolecular structure are responsible for the increase in the room temperature conductivity when the diameter decreases. Our study clearly indicates that a more complex situation can result from the confined synthesis of conjugated polymers in nanopores of a template. Nanowires of poly(3,4-ethylenedioxythiophene), a polythiophene derivative, have been prepared by electrochemical synthesis in polycarbonate templates with diameters ranging between 35 and 200 urn. The electrical transport study (down to 1.5 K) shows that the smaller the nanowire diameter, the stronger the insulating behavior. The temperature dependence of the sample resistance ranges between two extremes, a nonthermally activated regime for the PEDOT film close to the metal-insulator transition and a 2D Mott-variable range hopping-like regime for 35-nm nanowires. It is partly related to the doping level, deduced from X-ray photoelectron spectroscopy (XPS) measurements, which is reduced by about a factor of 2 for 35-nm nanowires, in comparison with the film. But this reduced doping level cannot account for the whole electrical effect. Another important result is the evidence of a decrease in the spin/charge ratio when the nanowire diameter goes from 100 to 50 nm as shown by electron spin resonance (ESR). It suggests that the polaron/bipolaron population ratio is modified. This result will also have important consequences for the transport mechanism.