Lead Phthalocyanine\non Graphitic Surfaces: A Case\nStudy of the Determination of Epitaxial Relations

Heteroepitaxy of\nmolecular adsorbates on crystalline\nsubstrates\nis used to assess fundamental aspects of layer formation and interaction\nacross the interface. Depositing organic molecules on graphene, for\ninstance, is often stimulated by the desire to noncovalently tune\nits electronic properties. Here, we study monolayers and bilayers\nof the near-infrared absorber lead phthalocyanine (PbPc) by means\nof scanning tunneling microscopy and spectroscopy (STM, STS), low-energy\nelectron diffraction (LEED), and optical differential reflectance\nspectroscopy (DRS). In an earlier report [Nhung Nguyen, T. T. et al. <i>Phys. Rev. B</i> <b>2021</b>, <i>103</i>, L201408],\nit was claimed that PbPc would form a monolayer structure on highly\noriented pyrolytic graphite (HOPG) with adsorbate lattice vectors\ndiffering by as much as 10% from those on monolayer graphene. Yet,\nfrom our combined LEED and STM analysis on single-crystal graphite\nand few-layer graphene on SiC(0001), we find that the lattice parameters\ndiffer no more than 0.7% among the different substrates. For PbPc\nbilayers the structural and spectroscopical properties are found to\nbe caused by physical dimerization, i.e., the formation of face-to-face\nstacked molecules. The electronic interaction with graphite and graphene,\nhowever, is interpreted to be rather weak as attested by the PbPc\nmonomer behavior observed for the first monolayer.