Two-dimensional growth of dendritic islands of NTCDA on Cu(001) studied in real time.

The success of future organic electronic devices distinctively depends on the electronic and geometric properties of thin organic films. Although obviously these properties are strongly influenced by the growth mechanisms, real time growth studies are relatively rare since not many experimental techniques exist that allow in situ studies in ultra high vacuum. In this context, we investigated the prototypical system 1,4,5,8-naphtalene-tetracarboxylic-dianhydride (NTCDA) on Cu(001). We used low-energy electron microscopy (LEEM) for the real-time growth study, and a variety of other techniques for investigating the geometric and electronic structure. While for similar model systems well known and well characterized growth modi occur (e.g., compact, well ordered islands or disordered, gas-like layers), for NTCDA/Cu(001) we observe the growth of dendrite-like, fractal structures. The dendritic structures arise from a strongly preferred one-dimensional growth mode forming a long-range ordered network of thin molecular chains spanning over the entire surface already at small coverages. Later in the growth process, the voids in the network structure are incrementally filled. These results are very unexpected for such a simple adsorbate system consisting of well investigated components, the properties of which were believed to be already well understood. We explain this unexpected behavior by a dendritic growth model that is supported by energetic arguments based on pair-potential calculations. These calculations give reason for the experimentally observed growth of one-dimensional structures, and therefore represent the key to a semi-quantitative understanding of this dendritic growth mode.

Momentum microscopy on the micrometer scale: photoemission micro-tomography applied to single molecular domains.

Photoemission tomography (PT) is a newly developed method for analyzing angular resolved photoemission data. In combination with momentum microscopy it allows for a comprehensive investigation of the electronic structure of (in particular) metal-organic interfaces as they occur in organic electronic devices. The most interesting aspect in this context is the band alignment, the control of which is indispensable for designing devices. Since PT is based on characteristic photoemission patterns that are used as fingerprints, the method works well as long as these patterns are uniquely representing the specific molecular orbital they are originating from. But this limiting factor is often not fulfilled for systems exhibiting many differently oriented molecules, as they may occur on highly symmetric substrate surfaces. Here we show that this limitation can be lifted by recording the photoemission data in a momentum microscope and limiting the probed surface area to only a few micrometers squared, since this corresponds to a typical domain size for many systems. We demonstrate this by recording data from a single domain of the archetypal adsorbate system 1,4,5,8-naphthalenetetracarboxylic dianhydride on Cu(0 0 1). This proof of principle experiment paves the way for establishing the photoemission [Formula: see text]-tomography method as an ideal tool for investigating the electronic structure of metal-organic interfaces with so far unraveled clarity and unambiguity.

Fully Atomistic Understanding of the Electronic and Optical Properties of a Prototypical Doped Charge-Transfer Interface.

The current study generates profound atomistic insights into doping-induced changes of the optical and electronic properties of the prototypical PTCDA/Ag(111) interface. For doping K atoms are used, as KxPTCDA/Ag(111) has the distinct advantage of forming well-defined stoichiometric phases. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combine state-of-the-art density-functional theory calculations with optical differential reflectance data, photoelectron spectra, and X-ray standing wave measurements. In combination with the full structural characterization of the KxPTCDA/Ag(111) interface by low-energy electron diffraction and scanning tunneling microscopy experiments (ACS Nano 2016, 10, 2365-2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal-organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications. The combination of the employed complementary techniques allows us to understand the peculiarities of the optical spectra of K2PTCDA/Ag(111) and their counterintuitive similarity to those of neutral PTCDA layers. They also clearly describe the transition from a metallic character of the (pristine) adsorbed PTCDA layer on Ag(111) to a semiconducting state upon doping, which is the opposite of the effect (degenerate) doping usually has on semiconducting materials. All experimental and theoretical efforts also unanimously reveal a reduced electronic coupling between the adsorbate and the substrate, which goes hand in hand with an increasing adsorption distance of the PTCDA molecules caused by a bending of their carboxylic oxygens away from the substrate and toward the potassium atoms.

Controlling the growth of multiple ordered heteromolecular phases by utilizing intermolecular repulsion.

Metal/organic interfaces and their structural, electronic, spintronic and thermodynamic properties have been investigated intensively, aiming to improve and develop future electronic devices. In this context, heteromolecular phases add new design opportunities simply by combining different molecules. However, controlling the desired phases in such complex systems is a challenging task. Here, we report an effective way of steering the growth of a bimolecular system composed of adsorbate species with opposite intermolecular interactions-repulsive and attractive, respectively. The repulsive species forms a two-dimensional lattice gas, the density of which controls which crystalline phases are stable. Critical gas phase densities determine the constant-area phase diagram that describes our experimental observations, including eutectic regions with three coexisting phases. We anticipate the general validity of this type of phase diagram for binary systems containing two-dimensional gas phases, and also show that the density of the gas phase allows engineering of the interface structure.

Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds.

Hybridization-related modifications of the first metal layer of a metal-organic interface are difficult to access experimentally and have been largely neglected so far. Here, we study the influence of specific chemical bonds (as formed by the organic molecules CuPc and PTCDA) on a Pb-Ag surface alloy. We find that delocalized van der Waals or weak chemical π-type bonds are not strong enough to alter the alloy, while localized σ-type bonds lead to a vertical displacement of the Pb surface atoms and to changes in the alloy's surface band structure. Our results provide an exciting platform for tuning the Rashba-type spin texture of surface alloys using organic molecules.

Unexpected interplay of bonding height and energy level alignment at heteromolecular hybrid interfaces.

Although geometric and electronic properties of any physical or chemical system are always mutually coupled by the rules of quantum mechanics, counterintuitive coincidences between the two are sometimes observed. The coadsorption of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and copper-II-phthalocyanine on Ag(111) represents such a case, since geometric and electronic structures appear to be decoupled: one molecule moves away from the substrate while its electronic structure indicates a stronger chemical interaction, and vice versa for the other. Our comprehensive experimental and ab-initio theoretical study reveals that, mediated by the metal surface, both species mutually amplify their charge-donating and -accepting characters, respectively. This resolves the apparent paradox, and demonstrates with exceptional clarity how geometric and electronic bonding parameters are intertwined at metal-organic interfaces.

Focal-series reconstruction in low-energy electron microscopy.

In low-energy electron microscopy (LEEM) we commonly encounter images which, beside amplitude contrast, also show signatures of phase contrast. The images are usually interpreted by following the evolution of the contrast during the experiment, and assigning gray levels to morphological changes. Through reconstruction of the exit wave, two aspects of LEEM can be addressed: (1) the resolution can be improved by exploiting the full information limit of the microscope and (2) electron phase shifts which contribute to the image contrast can be extracted. In this article, linear exit wave reconstruction from a through-focal series of LEEM images is demonstrated. As a model system we utilize a heteromolecular monolayer consisting of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and Cu-II-Phthalocyanine, adsorbed on a Ag(111) surface.

Commensurate registry and chemisorption at a hetero-organic interface.

We present evidence for a partly chemisorptive bonding between single monolayers of copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) that are stacked on Ag(111). A commensurate registry between the two molecular layers and the substrate, i.e., a common crystallographic lattice for CuPc and PTCDA films as well as for the Ag(111) surface, indicates that the growth of the upper layer is dominated by the structure of the lower. Photoemission spectroscopy clearly reveals a gradual filling of the lowest unoccupied molecular orbital of PTCDA due to CuPc adsorption, which proves the chemisorptive character.

Modeling intermolecular interactions of physisorbed organic molecules using pair potential calculations.

The understanding and control of epitaxial growth of organic thin films is of crucial importance in order to optimize the performance of future electronic devices. In particular, the start of the submonolayer growth plays an important role since it often determines the structure of the first layer and subsequently of the entire molecular film. We have investigated the structure formation of 3,4,9,10-perylene-tetracarboxylic dianhydride and copper-phthalocyanine molecules on Au(111) using pair-potential calculations based on van der Waals and electrostatic intermolecular interactions. The results are compared with the fundamental lateral structures known from experiment and an excellent agreement was found for these weakly interacting systems. Furthermore, the calculations are even suitable for chemisorptive adsorption as demonstrated for copper-phthalocyanine/Cu(111), if the influence of charge transfer between substrate and molecules is known and the corresponding charge redistribution in the molecules can be estimated. The calculations are of general applicability for molecular adsorbate systems which are dominated by electrostatic and van der Waals interaction.

Ensemble modeling of very small ZnO nanoparticles.

The detailed structural characterization of nanoparticles is a very important issue since it enables a precise understanding of their electronic, optical and magnetic properties. Here we introduce a new method for modeling the structure of very small particles by means of powder X-ray diffraction. Using thioglycerol-capped ZnO nanoparticles with a diameter of less than 3 nm as an example we demonstrate that our ensemble modeling method is superior to standard XRD methods like, e.g., Rietveld refinement. Besides fundamental properties (size, anisotropic shape and atomic structure) more sophisticated properties like imperfections in the lattice, a size distribution as well as strain and relaxation effects in the particles and-in particular-at their surface (surface relaxation effects) can be obtained. Ensemble properties, i.e., distributions of the particle size and other properties, can also be investigated which makes this method superior to imaging techniques like (high resolution) transmission electron microscopy or atomic force microscopy, in particular for very small nanoparticles. For the particles under study an excellent agreement of calculated and experimental X-ray diffraction patterns could be obtained with an ensemble of anisotropic polyhedral particles of three dominant sizes, wurtzite structure and a significant relaxation of Zn atoms close to the surface.