Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence from Infrared Spectroscopy

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection-absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F₆-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.