Evolution of TiOPc Films on Au(111): from Seed Layer to Crystalline Multilayers

Since optoelectronic properties of organic semiconductor films are closely linked to their structural properties (such as molecular orientation, crystalline phase and orientation, degree of crystallinity, etc.), a microscopic understanding of heterogrowth behavior is of utmost importance to control and improve such films for device applications. For the case of titanyl phthalocyanine (TiOPc), a widely used molecular semiconductor for organic optoelectronics, the initial film growth on metal substrates has already been studied extensively by means of scanning tunneling microscopy (STM), whereas discussion of the molecular arrangement in seed layers and the subsequent film growth includes some controversy. Here, we combined scanning probe microscopy [(STM) and atomic force microscopy (AFM)] with X-ray diffraction (XRD) and reflection absorption infrared spectroscopy (RAIRS) to shed some light on this debate and analyzed the evolution of TiOPc films on Au(111) ranging from initial seed layers up to thick multilayers (50 nm) aiming at preparing long-range ordered films and identifying their structure. We demonstrate that long-range ordered films are formed at 450 K, where molecules reveal an all oxygen-up orientation in the monolayer and an oxygen-down geometry in the second layer, while ruling out phases comprising inclined molecules based on corresponding RAIRS data. Such bilayers are stabilized by the oppositely oriented axial dipole moments forming stable building blocks that cause a subsequent bilayer-wise growth. These yield very smooth multilayers which exclusively crystallize in the phase I polymorph and hence parallel the growth scenario observed before for Ag(111) substrates. Films prepared at room temperature exhibit reduced ordering and the appearance of several metastable phases, none of them involving inclined molecules. The present study emphasizes the importance of using complementary techniques to obtain a comprehensive picture of organic film growth, while an analysis based solely on imaging techniques can sometimes lead to incorrect conclusions.