Strong K-induced changes in perylene-tetracarboxylic-dianhydride films on Ag(110) studied by HREELS and LEED

Using high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) we have studied the strong structural, electronic, and vibrational changes occurring in K-"doped" films of perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(110) as a function of K concentration and annealing temperature. Two major phase transitions were observed in the vibrational and electronic excitations as well as in the LEED patterns. At low K-coverages (similar to0.5 K/PTCDA) mainly structural changes are observed. Enormous changes occur for K-concentrations of 1 to 2 K atoms per PTCDA molecule which are tentatively attributed to the formation of a charge transfer complex leading to a semiconductor-metal transition. This state is characterized by a drastic intensity increase of vibrational modes at 392, 532, 1042, 1210 and 1540 cm(-1) and two low-energy electronic excitations at 0.6 and 0.9 eV, which are tentatively assigned to K,(PTCDA) and K(PTCDA) complexes. The second transition, perhaps a metal-semiconductor transition, occurs at higher K concentration and/or annealing temperature and is correlated with the abrupt disappearance of the prominent spectroscopic features related to the charge transfer state. (C) 2001 Elsevier Science B.V. All rights reserved.