Transition-Metal Phthalocyanines on Transition-Metal Oxides: Iron and Cobalt Phthalocyanine on Epitaxial MnO and TiO_x Films

Abstract

The interaction at interfaces between transition-metal phthalocyanines and ultrathin transition-metal oxide films is studied by means of photoemission (PES) and X-ray absorption spectroscopy (XAS). Our results are compared to the recently investigated system CoPc on MnO. A flat-lying adsorption geometry of iron and cobalt phthalocyanines (FePc and CoPc) on the different oxide substrates was observed: ultrathin epitaxially grown MnO films and ultrathin TiO x films. A charge transfer from the MnO, in particular, to the Fe atom of the FePc molecule is observed by both PES and XAS. X-ray absorption spectra of the N K-edge of FePc do not hint at a nitrogen involvement in the interaction process. As a consequence of the charge transfer, a shift of the Fermi level of the semiconducting MnO films is observed, which is visible as a shift of MnO related core levels to lower binding energy. In contrast, CoPc and FePc deposited on TiO x show no hints for a charge transfer, although the flat-lying adsorption geometry allows in principle a maximum interaction between the π-system and the substrate. Increased surface roughness (compared to MnO) and an oxygen termination of the surface of the TiO x films is considered to suppress a possible strong interaction between the organic molecules and the substrate.

FePc and FePcF16 on Rutile TiO2(110) and (100): Influence of the Substrate Preparation on the Interaction Strength

“…In agreement with the degree of orientation obtained from N-K XAS spectra, the anisotropy is stronger for 0.3 nm compared to 0.6 nm. Most important, the monolayer spectra does not exhibit additional or shifted features as observed for strongly interacting systems, such as FePc on Ag(111) [25] or FePc on MnO [27], in good agreement with the discussion of Fe 2p PES spectra above. Such strong interactions can include a hybridization of Fe and substrate-related states and a redistribution of electrons at the Fe atom [13,25,27].…”

“…Most important, the monolayer spectra does not exhibit additional or shifted features as observed for strongly interacting systems, such as FePc on Ag(111) [25] or FePc on MnO [27], in good agreement with the discussion of Fe 2p PES spectra above. Such strong interactions can include a hybridization of Fe and substrate-related states and a redistribution of electrons at the Fe atom [13,25,27]. However, the feature denoted "A 1 " for the 0.3 nm spectrum at grazing incidence seems to exhibit a distinctly lower relative intensity compared to the reference spectra (FePc on Ag(111)), which cannot be understood alone by a slightly different molecular orientation (obtained from N-K XAS spectra) or by the different grazing incidence angle (20 • and 10 • with respect to the substrate surface in Figure 9a,b, respectively).…”

“…Black and red curves in Figure 4 correspond to FePc deposited on rutile TiO 2 (100) prepared in the presence and absence of O 2 partial pressure during the last annealing step. The shapes of the spectra for the thickest films are typical for (iron) phthalocyanines and in excellent agreement to the literature [25][26][27][28][29][30]. The Fe 2p spectra show an intensity maximum at a binding energy (BE) of 708.8 eV and exhibit the typical multiplet structure for Fe in FePc (e.g., [25][26][27]).…”

“…Therefore, Fe 2p photoemission spectra do not hint to an interaction between FePc and rutile TiO 2 (100) involving the Fe ion. The result reminds to FePc on TiO x epitaxially grown on platinum(111) [27], although the detailed structure of substrate surfaces in both cases is different.…”

“…In Figure 8, we compare the intensity ratio between features A and C of the N-K absorption spectra of FePc and CoPc on several substrates as a function of the film thickness. The data were taken from [13,25,27,42,43]. Like in Figure 7, we analyzed spectra recorded at grazing incidence, since for most systems at this geometry, the N 1s-π * transitions are strongest due to the preferred flat lying molecular orientation on the substrate surfaces.…”