ESR study on potassium doped pentacene

Abstract

Doping of potassium into pentacene (PEN) causes a drastic transformation from an insulator to a conductor, with conductivity increasing 10 8 times to 0.3 S cm −1 at room temperature. The properties of potassium doped PEN were investigated over a wide temperature range for the first time. ESR was measured at 9.3 GHz and 7.7 MHz over the total temperature region of 300 K to 0.05 K. Two components of ESR spectra were observed, of which the broader signal seemed to be the intrinsic result of doping, while the sharper signal appeared to be due to pinning by contamination. Abrupt behavior was observed at 0.2 K, with the ESR linewidth and intensity decreased rapidly below 0.2 K. From the magnitude of change it appeared that the broad signal underwent some transition and, in fact, disappeared somewhere above the lowest temperature. This is the first time that such behavior has been observed for this kind of alkali doped molecular material.