Über unbesetzte elektronische Zustände und die Dynamik des Ladungstransfers an der TiOPc/Ag(111)-Grenzfläche und an PTCDA/TiOPc-Heteroschichten

The present thesis investigated the polar titanylphthalocyanine (TiOPc) on Ag(111) with a focus on the influence of the metal/organic interface on electronic structure and charge transfer. The band structure of the unoccupied first image state of 1 ML TiOPc/Ag(111) exhibits a pronounced increase in binding energy and band gap compared to the non-polar copper phthalocyanine (CuPc). An electrostatic model was constructed that explains these observations. A positive charge at the metal center surrounded by a negative charge on the molecular backbone models the phthalocyanines. Furthermore, an additional positive and negative charge form the intramolecular dipole of TiOPc. The intramolecular dipole of TiOPc is directly responsible for the increase in binding energy. The increase in band gap, on the other hand, is caused by a nonlinear superposition of the intramolecular charge distribution and the dipole. The observed effects can be generalized and are expected to occur on a variety of electronic states in the presence of periodically arranged dipoles. An unoccupied Shockley-type metal/organic interface state (IS) was found on 1 ML and 2 ML TiOPc/Ag(111) at an energy of 0.23 eV and 0.33 eV, respectively. Time-resolved 2PPE measurements were performed to clarify the influence of this interface state on charge transfer dynamics at metal-molecule contacts. In addition, TiOPc was combined with PTCDA. TiOPc and PTCDA form a donor-acceptor pair with disjoint optical gaps. A tuneable pump pulse ranging from 1.6 to 2.5 eV allowed a selective excitation of the molecular species. A strong resonant enhancement of the IS intensity was observed for pump photon energies matching the optical gaps of the molecules in the first four layers. Time-resolved measurements showed a delayed population of the IS in resonance. The resonances result from an optical excitation of excitons in the organic layers followed by an efficient ultrafast electron transfer into the IS. The transfer time critically depends on the molecule-metal distance and the energetic alignment of the involved levels. It grows from 18 fs for electrons form the second layer of 2 ML TiOPc/Ag(111) up to 159 fs for electrons from the third and fourth layer of 2 ML PTCDA/2 ML TiOPc/Ag(111). No efficient IS-related transfer could be observed any longer for electrons from the fifth and sixth organic layer in 2 ML PTCDA/4 ML TiOPc/Ag(111). In conclusion, the results highlight the capability of interface states to mediate charge transfer at metal/organic contacts.