Photoelectron spectroscopic studies of ultra-thin CuPc layers on a Si(111)-(√3 × √3)R30°-B surface

Abstract

The adsorption of copper phthalocyanine (CuPc) molecules on Si(111)-(√3 × √3)R30°-B surface is investigated at room temperature under ultra-high vacuum. Crystallographic, chemical and electronic properties of the interface are investigated by low energy electron diffraction (LEED), ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and X-ray photoemission diffraction (XPD). LEED and XPD results shed light on the growth mechanism of CuPc on this substrate. At one monolayer coverage the growth mode was characterized by the formation of crystalline 3D nanoislands. The molecular packing deduced from this study appears very close to the one of the bulk CuPc α phase. The 3D islands are formed by molecules aligned in a standing manner. XPS core level spectra of the substrate reveal that there is no discernible chemical interaction between molecules and substrate. However there is charge transfer from molecules to the substrate. During the growth, the work function (WF) was found to decrease from 4.50eV for the clean substrate to 3.70eV for the highest coverage (30 monolayers). Within a thickness of two monolayers deposition, an interface dipole of 0.50eV was found. A substrate band bending of 0.25eV was deduced over all the range of exposure. UPS spectra indicate the existence of a band bending of the highest occupied molecular orbital (HOMO) of 0.30eV. The changes in the work function, in the Fermi level position and in the onset of the molecular HOMO state have been used to determine the energy level alignment at the interface.