Photoelectron fine structures of uppermost valence band for well-characterized ClAl-phthalocyanine ultrathin film: UPS and MAES study

Metastable atom electron spectroscopy was used to characterize monolayer formation of chloroaluminum phthalocyanine (ClAlPc) prepared on graphite. For as-grown film, molecules form island structure of staggered doublelayers on the substrate. By annealing the film, molecules diffuse to form a uniform monolayer where all the molecules are oriented flat with Cl atom directed to the vacuum. After the confirmation of the oriented monolayer formation, high-resolution ultraviolet photoelectron spectra were measured to study effects of the molecular orientation on the energy levels. ClAlPc has an electric dipole perpendicular to the molecular plane, hence a well-defined electric dipole layer could be intentionally prepared by using the oriented monolayer. Difference of binding energies of HOMO bands between the oriented monolayer and the doublelayer was found to agree with the vacuum level shift, leading to a conclusion that the molecular energy level with respect to the substrate Fermi level is changed when the molecule is in the dipole-layer field.