Titanylphthalocyanine Films on Ag(111): An Epitaxial Metal/Organic Heterosystem with an Exceptional Smooth Surface

The fabrication of smooth organic semiconductor films with homogeneous thickness is of key importance for the improvement of organic electronic devices and realization of well-defined molecular heterostructures. Although many π-conjugated molecular materials form highly ordered monolayers on (single) crystalline metal substrates, further deposition typically obeys a Stranski–Krastanov growth and results in considerable layer roughness. Here, we examine the evolution of titanylphthalocyanine (TiOPc) films on Ag(111) for thicknesses ranging from an initial seed layer to thick multilayers (200 nm) by combining scanning probe microscopy [scanning tunneling microscopy (STM) and atomic force microscopy (AFM)] with X-ray diffraction and synchrotron-based photoelectron spectroscopy [X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption spectroscopy (NEXAFS)]. While the crystallinity of the TiOPc films increases with substrate temperature during growth, even at temperatures close to the onset of desorption, extended and molecularly flat islands are formed, which cover more than 80% of the substrate area resulting in exceptional smooth layers. The crystalline TiOPc films exclusively exhibit the phase I polymorph where molecular planes are oriented nearly parallel to the substrate surface and adopt an alternating (up/down) stacking of their titanyl groups forming stable bilayer units. Similar to the first bilayer, TiOPc multilayers are also epitaxially aligned with respect to the substrate. STM and AFM data further show that the macroscopic film roughness is essentially due to rotational domain boundaries in the seed layer which are not overgrown and result in characteristic macroscopic trenches. The high crystalline ordering and exceptional smoothness make this metal/organic heterosystem particularly suitable as a molecular spacer layer and allows for thickness-dependent studies of optoelectronic excitations and their dynamics.