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 electronic structure of the central Fe ion of iron phthalocyanine (FePc) and perfluorinated iron phthalocyanine (FePcF 16 ) in thin films is investigated by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), supported by photoemission. Both molecules grow in a flat-lying adsorption geometry in thin films. Fe L-edge X-ray absorption spectra of FePc and FePcF 16 exhibit minor, but distinct, differences of the shape, indicating a different electronic structure. A significantly stronger XMCD signal and thus larger magnetic moments were observed for FePc compared to FePcF 16 at low temperatures (15 K). Multiplet calculations have been used to simulate the XA and XMCD spectra and give detailed insight into the electronic structure of Fe in FePc and FePcF 16 . We suppose that the electronic structure crucially depends on the detailed arrangement of FePc and FePcF 16 molecules in thin films.
The properties of low band gap polymers in devices such as solar cells are strongly influenced by their morphology and ability of self-organization in thin films and interface properties. We study the influence of alkyl and alkoxy side chain position for four conjugated, alternating oligothiophene-benzothiadiazole copolymers on the molecular orientation in thin films and electronic interface properties using photoemission, X-ray absorption spectroscopy (XAS) at the sulfur K edge, and polarization modulation-infrared reflection–absorption spectroscopy (PMIRRAS). The interface charge transfer (ICT) model is used to explain interface properties of the polymers on substrates with different work functions. We find that the position of the side chains has a significant influence on the orientation and thus on self-organization properties of the polymers in thin films, whereas the electronic structure is less affected. The preferred molecular orientation is further affected by annealing, leading to a higher degree of ordering. Results from complementary methods with different surface sensitivities (XAS in total electron yield and fluorescence mode and PMIRRAS) are discussed.
Side chains play an important role in the photo-oxidation process of low band gap (LBG) polymers. For example, it has been shown that their photostability can be increased by the introduction of aromatic-oxy-alkyl links. We studied the photostability of prototypical LBG polymers with alkyl and oxyalkyl side chains during irradiation with white light (AM 1.5 conditions) in dry air using UV/vis and IR spectroscopy. Though its degradation kinetics were distinctly affected by the presence or absence of oxygen in the structure of the side chains, in particular cases, the stability was more affected by the presence of linear or branched side chains. Moreover, we showed that the exact position of the alkyl/oxyalkyl side chain at the polymer backbone could be crucial. Although minor effects of chemical modifications on the electronic parameters (ionization potential and gap) were observed, the molecular orientation, determined by polarization modulation-infrared reflection-absorption spectroscopy (PMIRRAS), could be affected. The aggregation and crystallinity of these polymers may distinctly affect their stability.
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