Observation of charge transfer states of F4-TCNQ on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface

We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F4-TCNQ is 5.24 eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface to the vacuum level is 4.1 eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F4-TCNQ, characteristic peaks are observed at 1.1 and 2.5 eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F4-TCNQ. The work function change is increased up to +1.3 eV as a function of F4-TCNQ coverage. These results support the occurrence of charge transfer into F4-TCNQ on the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface.