HOMO-band fine structure of OTi- and Pb-phthalocyanine ultrathin films: effects of the electric dipole layer

Abstract

Ultraviolet photoelectron spectra were measured for titanyl- and lead-phthalocyanine ultrathin films prepared on graphite in order to study effects of the electric dipole layer on the organic energy levels. Each of these molecules has an electric dipole perpendicular to the molecular plane, and hence a well-defined electric dipole layer could be intentionally prepared by using oriented monolayer of these molecules. For as-grown films, the observed highest occupied molecular orbital (HOMO) band consists of many peaks that could be assigned to different molecular orientations/aggregations. For well-oriented monolayer films obtained by annealing the as-grown films, we observed a very sharp HOMO band with two satellites for both molecules as for copper phthalocyanine. Difference of binding energy of HOMO bands between the oriented monolayer and the doublelayer in which molecular electric dipoles are cancelled was found to agree with the vacuum level shift for both molecules, leading to important conclusions that (1) the molecular energy level with respect to the substrate Fermi level is changed when the molecule is in the dipole layer and (2) the binding-energy shift corresponds with the vacuum level shift.