High magnetic field influence on the molecular orientation and the morphology of iron phthalocyanine thin films

Abstract

Molecular orientation and stacking mode are commonly considered to have vital influence on the optoelectronic performances of organic semiconductor devices via changing the dynamics of charge carriers transferring among the molecules. Highly ordered and homogeneous stacking would allow a fast band transfer mechanism in the phase domain. Therefore the controls of the molecular orientation and the stacking behavior are of great significance for optimizing the device natures. In this work, the modification and control of iron phthalocyanine (FePc) molecular orientation on Si(111) are accomplished with the aid of high steady magnetic field at room temperature. The FePc films are grown in situ by organic molecular beam deposition on the Si(111) substrates under a high magnetic field strength of 8.5 T. The Si(111) substrates are preserved at room temperature and are kept perpendicular to the magnetic field. The influences of magnetic field on the molecular orientations and the morphologies of FePc thin films are investigated by X-ray diffraction, angle dependent near edge X-ray absorption fine structure (NEXAFS), Raman spectroscopy and atomic force microscopy (AFM). In the presence of the external magnetic field, the deposited FePc films each show a higher crystallinity and slightly closer packing in (002) plane than those without magnetic field. The AFM images verifies more ordered and uniform morphologies of the FePc films grown in the magnetic field. NEXAFS and Raman results both reveale a standing-up configuration of FePc molecules on the Si(111) substrate surface. The average tilting angle of the molecules changes from 63.6 to 67.1 when 8.5 T magnetic field is employed. The results demonstrate that the external high magnetic field distinctly enhances the orientation order of FePc molecules on Si(111) surface due to the magnetic-magnetic interactions between the magnetic field and the molecular magnetic moment. This work also demonstrates that external magnetic field is an efficient means to regulate the orientation and stacking behavior of magnetic molecules, which may open a new way to optimize the performances of the organic semiconductor devices.