J. Ivanco

Oxygen induced molecular reorientation on aluminum

Para-sexiphenyl films grow on Al(111) at room temperature such that the molecules are oriented with their molecular axes parallel to the Al surface. Here we demonstrate that the presence of oxygen on the Al substrate prior to deposition leads to a reorientation of the molecules in which their axes are nearly perpendicular to the Al surface. This reorientation also leads to a difference in the electronic structure and an improvement in the band alignment of 1.0eV. The contribution of substrate order to molecular orientation in organic films is discussed.

Oxygen as a surfactant for Al contact metallization of organic layers

Valence band and spatially resolved x-ray photoemission studies of the Al growth on sexiphenyl films, in both an ultrahigh vacuum (UHV) and in a partial pressure of oxygen, are reported. We show that in an UHV, even for very high coverages, the Al balls up on the organic film and is discontinuous. In contrast, for growth in an oxygen partial pressure, similar to that in standard high-vacuum systems used in organic device production, very thin continuous conducting wetting layers are formed. We suggest that the oxygen acts like a surfactant that allows the high surface free-energy metal to wet low surface free-energy organic films.

Critical evaluation of band bending determination in organic films from photoemission measurements

Electronic properties of conjugated films, namely, the band bending as derived from photoemission spectroscopy, are critically discussed. The study demonstrates that conclusions on the presence of the band bending deduced from the shift of electronic energy levels with the organic film thickness may be erroneous if the analysis does not consider the evolution of the film’s work function. The work function change—besides that being induced by the interfacial dipole—may occur due to the change in the molecular orientation from the lying down toward upright, as it often occurs with the increased thickness of molecular films.