High Hole-Mobility Molecular Layer Made from Strong Electron Acceptor Molecules with Metal Adatoms

The electronic structure of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-TCNQ (F₄TCNQ) monolayers on Au(111) has been investigated by means of angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation. In contrast to the physisorbed TCNQ/Au(111) interface, the high-resolution core-level photoemission spectra and the low-energy electron diffraction at the F₄TCNQ/Au(111) interface show evidence for the strong charge transfer (CT) from Au to F₄TCNQ and for the Au atom segregation from the underlying Au(111) surface, suggesting a possible origin of the spontaneous formation of the two-dimensional F₄TCNQ-Au network. The ARPES experiment reveals a low hole-injection barrier and large band dispersion in the CT-induced topmost valence level of the F₄TCNQ-Au network with 260 meV bandwidth due to the adatom-mediated intermolecular interaction. These results indicate that strong electron acceptor molecules with metal adatoms can form high hole-mobility molecular layers by controlling the molecule-metal ordered structure and their CT interaction.