Structural and Vibrational Properties of CuPc/Ag(111) Ultrathin Films

The initial stages of copper­(II)–phthalocyanine (CuPc) thin film growth on Ag(111) have been investigated using Fourier transform infrared absorption spectroscopy (FT-IRAS), spot profile analysis low-energy electron diffraction (SPA-LEED), and thermal desorption spectroscopy (TDS). Starting at (sub)­monolayer coverages up to 5 monolayers (ML), a number of ordered overlayers are found. Vibrational spectroscopy shows characteristic spectroscopic signatures for the individual submonolayer phases as well as for the bilayer, trilayer, and multilayers. Highly asymmetric line shapes of the in-plane vibrational modes of submonolayer CuPc provide unequivocal evidence for interfacial dynamical charge transfer between the metal electronic states and CuPc molecular orbitals, indicative for a partially filled LUMO at the Fermi energy as well as the prevalence of severe nonadiabaticity in the electron–vibron coupling. Growth of the second and third CuPc layers proceeds in a layer-by-layer fashion (Frank van der Merwe growth). Higher layers deposited at 300 K, on the other hand, transform into 3D crystallites on top of the CuPc trilayer upon annealing. For CuPc/Ag(111) monolayers thermal desorption spectra reveal intact CuPc desorption for coverages above 0.9 ML. At lower coverages an alternative reaction path involving partial dissociation of the CuPc molecules is found, with ¹/₄ Pc as the desorbing stable product species. In this study IR absorption spectra have been obtained at an exceptionally high spectral resolution of 0.5 cm^–1, which allows a spectral discrimination of molecular species with unprecedented detail. Specifically, CuPc mono-, bi-, and trilayers as well as the bulk-like crystalline phase of CuPc have been discriminated based on clearly resolved, closely spaced vibrational bands.