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

Abstract

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.

Formation of Oriented Bilayer Motif: Vanadyl Phthalocyanine on Ag(100)

“…We found that such odd-layer islands were relatively rare and small compared to 2 and 4 ML regions, suggesting a strong preference for the bilayer motif. This alternating O-up/O-down order should result in a net cancellation of dipole moments, consistent with the bulk crystal phases of VOPc, and previous STM studies of other nonplanar MPcs. ,,,, …”

“…This alternating O-up/ O-down order should result in a net cancellation of dipole moments, consistent with the bulk crystal phases of VOPc, 41 and previous STM studies of other nonplanar MPcs. 15,29,30,42,43 Compared to similar molecular systems, we find that the overall morphology in Figure 3 is more uniform. For example, deposition of >1 ML VOPc on either Ag(111) or Au(111) results in the formation of islands that are tilted with respect to the surface in addition to flat-lying films.…”

Infrared spectroscopic monitoring of solid-state processes

“…Despite these difficulties, RAIRS is particularly useful for the characterization of systems with low surface area, as well as to investigate adsorption of gas/solid and liquid/solid interfaces, 232,233 by providing accurate information on intermolecular interactions and additionally the bonding geometry of adsorbates. 234–243 Actually, the pioneering application of RAIRS was to investigate the adsorption of carbon monoxide on metal films. 244,245 The application of polarization modulation is also frequently used to improve the signal-to-noise ratio and to discriminate between adsorbed and free surrounding molecules.…”

Orienting dilute thin films of non-planar spin-1/2 vanadyl–phthalocyanine complexes

“…In monolayer and few-layer heterostructures non-planar phthalocyanine molecules are most often oriented flat with their macrocycles parallel to the substrate plane. [16][17][18][19][20] Upon increasing the thickness and on non-metallic substrates the molecules tend to assume a 'standing-up' geometry. [21][22][23] In order to grow films with a flatter orientation of the molecular macrocycles it has been shown that suitable templating layers can be applied for this purpose.…”