Electric Field Assisted Effects on Molecular Orientation and Surface Morphology of Thin Titanyl(IV)phthalocyanine Films

Abstract

Herein we present electric field assisted effects on the molecular orientation, the polymorphism, and the surface morphology of thin titanyl(IV)phthalocyanine (TiOPc) films. The ability of electric fields to affect the thin film structure of polar molecules is demonstrated using titanyl(IV)phthalocyanine as a model compound exhibiting both a permanent and an induced electric dipole moment. Thin films of TiOPc prepared by organic molecular beam deposition (OMBD) in the absence and in the presence of an electric field during the thin film growth are characterized using polarization dependent Raman spectroscopy and atomic force microscopy (AFM). The distinctive pattern of vibrational modes of the phthalocyanine skeleton indicates different molecular orientations in these thin films: Raman spectra of regions where an electric field is present during thin film growth reveal a preferential molecular orientation with an inclination angle of the molecular plane (pseudoplanar macrocycle) with respect to the substrate plane of nearly 90 degrees. Contrary to that, in regions where the electric field was absent, the molecules adopt predominantly a configuration with a smaller tilt angle (approximately 60 degrees). In addition, an electric field assisted change is apparent in AFM images: A large amount of well-formed steplike crystallites lying parallel to the substrate is observed when no electric field was present, whereas in the case when an electric field was applied during thin film growth the crystallites exhibit a tilt with respect to the substrate plane.