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Abstract: We review the binding and energy level alignment of π-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab-initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.

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Abstract: Organic semiconductors are carbon-based materials with promising features aimed at substituting and/or complementing their inorganic counterparts (e.g. silicon, germanium or gallium arsenide). They can be cheaply mass-produced, they are flexible and their properties can be chemically tuned, all attractive characteristics for the consumer-electronics market. In recent years, remarkable progress has been made and one can already buy smartphones and TVs equipped with organic LEDs. Also, interesting prototypes of roll-up organic solar cells have been presented. Nonetheless, the real breakthrough for these materials is yet to come since fundamental aspects regarding the charge transport through metal electrodes (among others), important for the device circuitry, still limit the overall efficiency. In this context, the energy-level alignment (ELA) between the molecules and the electrode determines the charge injection/extraction energy barriers and, therefore, is responsible for an optimum charge transfer across the interface. A proper rationalization of the ELA requires a full description of the interface properties: electronic, chemical as well as structural, including adsorption distances and molecular distortions. Chemical functionalization of polymers or small molecules by adding side groups with strong electron donor or acceptor behavior has been a way to optimize the ELA. In this work, we employ element-specific techniques such as X-ray photoelectron spectroscopy (XPS) and X-ray standing waves (XSW) to infer how partial nitrogen and fluorine substitution in prototypical and well studied π-conjugated organic molecules affects the geometrical, chemical and electronic properties of these when deposited on metal single-crystal substrates with different reactivities. We show that the combination of high-resolution XPS with XSW is a powerful method to tackle this issue since the adsorption distance and molecular distortion can be readily connected to feature changes in the XP spectra. In particular, we deposit several perylene and pentacene derivatives on different metal and semiconductor surfaces, namely the (111) surface of gold, silver and copper and the polar surfaces (000±1) of zinc oxide (ZnO). Finally, we also show that this method can be extended successfully to study the evolution of the bare ZnO surface under different treatments in real-time. ZnO is a promising transparent inorganic semiconductor that can be engineered easily in different nanostructures. Its polar surfaces represent a conundrum, as the actual composition and conformation of the surface are still a topic of debate. Having extracted the chemical and structural information with a combination of XPS and XSW, we provide new results about the surface and its respective behavior under different treatment conditions.

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Abstract: Organic heterostructures are a central part of a manifold of (opto)electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic–metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic–organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic–metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that of P2O and P4O. We find that, indeed, CuPc forms a bilayer when deposited on a monolayer P4O/Ag(111) but molecular exchange takes place with P2O, as shown by a combination of scanning tunneling microscopy and X-ray standing wave experiments.

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Abstract: We investigated the structural and electronic properties of vacuum sublimed 7,8,15,16-tetraazaterrylene (TAT) thin films on Au(111), Ag(111), and Cu(111) substrates using inverse photoemission spectroscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and the x-ray standing wave (XSW) technique. The LEED reveals a flat adsorption geometry of the monolayer TAT on these three substrates, which is in accordance with the XSW results. The molecules are slightly distorted in monolayers on all three substrates with the nitrogen atoms having smaller averaged bonding distances than the carbon atoms. On Ag(111) and Cu(111), chemisorption with a net electron transfer from the substrate to the adsorbate takes place, as evidenced by UPS and XPS. Combining these results, we gain full insight into the correlation between electronic properties and interface geometry.