LI-7 NMR-STUDY OF LI-DOPED POLYACENIC SEMICONDUCTOR (PAS) MATERIALS

The polyacenic semiconductor (PAS) material prepared from phenol resin at relatively low temperature (680 degrees C) can be doped by a much larger amount of lithium atoms up to the C2Li state compared with grap hite (C6Li state). To understand the Li storage mechanism as well as t he electronic structure, Li-7 nuclear magnetic resonance (NMR) measure ments have been performed for the Li-doped PAS materials. In the initi al doping stage a broad signal was observed at 0 ppm, while the succes sive doping causes a Knight shift to 9 ppm. It is suggested from this Knight shift that the Li nucleus slightly undergoes the Fermi contact interaction with conduction electrons delocalizing through carbon pi-a tomic orbitals. From the spin-lattice relaxation rate (1/T)(1) it is f ound that the Li nucleus in the PAS material loses its mobility by the surrounding Li nuclei with the proceeding doping and that the Li nucl eus is more loosely trapped than that in the C6Li state of graphite. ( C) 1997 Elsevier Science S.A.