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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: One of the most important functionalities of the atomically thin insulator hexagonal boron nitride (hBN) is its ability to chemically and electronically decouple functional materials from highly reactive surfaces. It is therefore of utmost importance to uncover its structural properties on surfaces on an atomic and mesoscopic length scale. In this paper, we quantify the relative coverages of structurally different domains of a hBN layer on the Ni(111) surface using low-energy electron microscopy and the normal incidence x-ray standing wave technique. We find that hBN nucleates on defect sites of the Ni(111) surface and predominantly grows in two epitaxial domains that are rotated by 60 ∘ with respect to each other. The two domains reveal identical adsorption heights, indicating a similar chemical interaction strength with the Ni(111) surface. The different azimuthal orientations of these domains originate from different adsorption sites of N and B. We demonstrate that the majority ( ≈ 70 % ) of hBN domains exhibit a ( N , B ) = ( top , fcc ) adsorption site configuration while the minority ( ≈ 30 % ) show a ( N , B ) = ( top , hcp ) configuration. Our study hence underlines the crucial role of the atomic adsorption configuration in the mesoscopic domain structures of in situ fabricated two-dimensional materials on highly reactive surfaces.

Open Access

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Abstract: Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch px ± ipy-derived wave functions.

Open Access

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Abstract: We assess the crucial role of tetrapyrrole flexibility in the CO ligation to distinct Ru‐porphyrins supported on an atomistically well‐defined Ag(111) substrate. Our systematic real space visualisation and manipulation experiments with scanning tunnelling microscopy directly probe the ligation, while bond‐resolving atomic force microscopy and X‐ray standing waves measurements characterise the geometry, X‐ray and ultraviolet photoelectron spectroscopy the electronic structure, and temperature programmed desorption the binding strength. Density functional theory calculations provide additional insight into the functional interface. We unambiguously demonstrate that the substituents regulate the interfacial conformational adaptability, either promoting or obstructing the uptake of axial CO adducts.

Open Access

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Abstract: As crucial element in organic opto-electronic devices, heterostructures are of pivotal importance. In this context, a comprehensive study of the properties on a simplified model system of a donor–acceptor (D–A) bilayer structure is presented, using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and normal-incidence X-ray standing wave (NIXSW) measurements. Pentacene (PEN) as donor and perfluoropentacene (PFP) as acceptor material are chosen to produce bilayer structures on Au(111) and Cu(111) by sequential monolayer deposition of the two materials. By comparing the adsorption behavior of PEN/PFP bilayers on such weakly and strongly interacting substrates, it is found that: (i) the adsorption distance of the first layer (PEN or PFP) indicates physisorption on Au(111), (ii) the characteristics of the bilayer structure on Au(111) are (almost) independent of the deposition sequence, and hence, (iii) in both cases a mixed bilayer is formed on the Au substrate. This is in striking contrast to PFP/PEN bilayers on Cu(111), where strong chemisorption pins PEN molecules to the metal surface and no intermixing is induced by subsequent PFP deposition. The results illustrate the strong tendency of PEN and PFP molecules to mix, which has important implications for the fabrication of PEN/PFP heterojunctions.