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Abstract: Organic conjugated molecules have become one of the fundamental materials in the semiconductor community in recent years. Devices made by organic semiconductor (OSC) materials, like organic light emitting diodes (OLED), organic photovoltaic cells (OPV) and organic field effect transistors (OFET), exhibit comparable efficiencies compared with that of inorganic counterparts, moreover with superior properties in terms of light weight, mechanical flexibility and low cost, showing excellent application potential in the commercial market. To further promote the OSC device efficiency, a comprehensive understanding of the heterojunction interfaces comprised by different OSC materials from crystal structure to electronic structure is rather essential, as these inevitable interfaces play a crucial role in the charge carrier transportation process. In order to obtain the information of the structural property (molecular adsorption height and stacking arrangement) and electronic structure of heterostructures, X-ray standing wave (XSW), low-energy electron diffraction (LEED), high-resolution X-ray photoelectron spectroscopy (HR-XPS) and ultraviolet photoelectron spectroscopy (UPS) are employed in this work. Coinage metals are commonly used as electrodes at applied devices due to their favourable conductivity. For prototypical studies here, atomic clean-coinage metal crystals are used, as their surfaces are flat and ordered, meanwhile the surface chemical activity can change from inert [Au (111)] to active [Cu (111)], which can further affect the coupling strength with the subsequent deposited OSC molecules. In the beginning of this work, copper-hexadecafluorophthalocyanine (F16CuPc)-derived bilayers, with intermediate layers of 5,7,12,14-pentacenetetrone (P4O) and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI), are built on Au(111) to explore the influence of the organic-metal interaction strength. It has been found that the bilayers are well formed and the F16CuPc exhibits an inverted intramolecular distortion compared to its monolayer structure. Secondly, a donor-acceptor (D-A) counterpart, pentacene-perfluoropentacene (PEN-PFP), is taking to further study the bilayer formation on the same substrate. It has been proven, however, that the molecular mixture occurs despite of the weakly interacting substrate. Finally, a more complicated heterostructure, the trilayer, has been employed on Ag(111), consisting of a zero-net-dipole titanyl-phthalocyanine (TiOPc) bilayer and then a third organic molecular layer (F16CuPc, P4O) adsorbs on top of it. We found that none of the intramolecular distortion or molecular exchange have been observed, implying that an ideal organic-organic interface was formed. By utilizing the powerful XSW, HR-XPS and UPS techniques, we have accessed the electronic and structural information of several heterostructures on the coinage metal substrates, which could inspire or promote more researches on the OSC-based fundamental and application field.

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Abstract: We studied the structural and electronic properties of 2,3,9,10-tetrafluoropentacene (F4PEN) on Ag(111) via X-ray standing waves (XSW), low-energy electron diffraction (LEED) as well as ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). XSW revealed that the adsorption distances of F4PEN in (sub)monolayers on Ag(111) were 3.00 Å for carbon atoms and 3.05 Å for fluorine atoms. The F4PEN monolayer was essentially lying on Ag(111), and multilayers adopted π-stacking. Our study shed light not only on the F4PEN–Ag(111) interface but also on the fundamental adsorption behavior of fluorinated pentacene derivatives on metals in the context of interface energetics and growth mode.

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Abstract: Heteromolecular bilayers of π-conjugated organic molecules (COM) on metals, considered as model systems for more complex thin film heterostructures, are investigated with respect to their structural and electronic properties. By exploring the influence of the organic-metal interaction strength in bilayer systems, we determine the molecular arrangement in the physisorptive regime for copper-hexadecafluorophthalocyanine (F16CuPc) on Au(111) with intermediate layers of 5,7,12,14-pentacenetetrone (P4O) and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI). Using the X-ray standing wave (XSW) technique to distinguish the different molecular layers, we show that these two bilayers are ordered following their deposition sequence. Surprisingly, F16CuPc as the second layer within the heterostructures exhibits an inverted intramolecular distortion compared to its monolayer structure.

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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 present a comprehensive study of the complex interface between perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) and the (111) surfaces of the three coinage metals. The specific structural, electronic, and chemical properties of the interface rendered by the different substrate reactivities are monitored with low-energy electron diffraction (LEED), x-ray standing waves (XSW), and ultraviolet and x-ray photelectron spectroscopy (UPS and XPS). In particular, the balance between molecule-substrate and molecule-molecule interactions is considered when interpreting the core-level spectra of the different interfaces. By presenting additional adsorption distances of the unsubstituted perylene, we show that the molecular functionalization via end groups with acceptor character facilitates the charge transfer from the substrate but it is not directly responsible for the associated short adsorption distances, demonstrating that this frequently assumed correlation is not necessarily correct.