Layered transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2), are currently in the focus of interest due to their novel electronic properties. The adsorption of molecules is a promising way to tune the electronic structure of TMDCs. We study interface properties between MoS2 and differently fluorinated iron phthalocyanines (FePcF x , x = 0, 4, 16) using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), angle-resolved photoelectron spectroscopy (ARPES), and X-ray absorption spectroscopy (XAS). A key parameter for the charge transfer is the ionization potential of FePcF x . A distinct electron transfer from a molecule to a substrate is observed for FePc and FePcF4. From energy-momentum ARPES maps, we suppose that the substrate and FePc-related states hybridize at the interface. This study demonstrates that a controlled tuning of the electronic structure of MoS2 by electron donors is possible, driven by the ionization potential difference between the substrate and the adsorbate.

The initial growth and interfacial electronic structure of perfluorinated iron phthalocyanine (FePcF 16 ) on Cu(111) has been studied using X-ray photoelectron spectroscopy (XPS) and polarization dependent X-ray absorption spectroscopy (XAS). The planar molecules are oriented preferred flat lying on the substrate surface during the growth of the first layers while the tilt angle is increased in thicker films. A clear interaction at the interface is observed, involving both the central metal ion and the macrocycle. At monolayer coverages, the Fe2p spectrum shows an interface signal at 707.1 eV, while the C-N component of the C1s spectrum is distinctly shifted with respect to the thicker films. In addition, the nitrogen atom is involved in the complex interaction (including charge transfer), best visible in the change of the shape in the π Ã resonance of N K edge spectra recorded from molecules at the interface.

Electronic interface properties and the initial growth of hexa-peri-hexabenzocoronene with a borazine core (BN-HBC) on Au(111) have been studied by using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). A weak, but non-negligible, interaction between BN-HBC and Au(111) was found at the interface. Both hexa-peri-hexabenzocoronene (HBC) and BN-HBC molecules form well-defined monolayers. The different contrast in STM images of HBC and BN-HBC at different tunneling voltages with submolecular resolution can be ascribed to differences in the local density of states (LDOS). At positive and negative tunneling voltages, STM images reproduce the distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) as determined by density functional theory (DFT) calculations very well.

The electronic structure of the central iron ion of perfluorinated iron phthalocyanine (FePcF 16 ) in thin films has been studied on Cu(111) and Ag(111) using polarization dependent X-ray absorption spectroscopy (XAS). The data are compared to FePc on Ag(111). Ligand field parameters have been computed, and multiplet calculations (CTM4XAS) were carried out to simulate XAS spectra. The planar molecules are preferentially oriented lying flat on the substrate surface during the growth of the 1−4 nm thick films. A clear polarization dependence of the Fe L edge absorption spectra is observed, arising from transitions into orbitals with in-plane and out-of-plane character. The shape of the spectra for three to four monolayers of FePcF 16 on Cu(111) is comparable to that of the thin films of FePc on Ag(111). However, a drastic change of the XAS peak shape is observed for thicker FePcF 16 films on both Ag(111) and Cu(111), although the molecular orientation is very similar to coverages consisting of a few monolayers. Since in both cases the film thickness is distinctly beyond the monolayer regime, interface interactions can be ruled out as a possible origin of this behavior. Rather, the different XAS peak shapes seem to indicate that the multiplicity may depend on the detailed arrangement of the FePcF 16 molecules. The large flexibility of the ground state of Fe could be of high interest for spintronic applications.

Larger acenes, such as hexacene, are increasingly considered as promising materials for applications in optoelectronic devices. We studied electronic interface properties and the molecular orientation of hexacene on Cu(110)−(2 × 1)O using X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy. Interactions between hexacene and the substrate are weak. The detailed investigation of the orientation greatly benefits from a combination of polarization-dependent XAS and angle-resolved photoemission. The angular dependence of valence band features indicates that the molecules grow highly oriented with their long axes parallel to the oxygen rows of the Cu(110)−(2 × 1)O substrate, whereas XAS reveals that the short axis of the molecule is distinctly tilted with respect to the substrate surface. This orientation is maintained up to a film thickness of at least 16 nm, indicating that Cu(110)−(2 × 1)O acts as a template for the hexacene film growth.