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Interfaces between molecules and crystalline phases

We investigate the structural and electronic properties of interfaces between molecules and crystalline phases, because these interfaces form an integral part of almost any molecular nanostructure in which quantum functionalities may be exploited—after all, the substrate on which the nanostructure is fabricated is usually a single-crystalline material.

When designing functional molecular structures for quantum nanoscience, the molecule-substrate interface plays a key role, because it may influence functionality as profoundly as the interactions within the molecular structures themselves. We investigate molecule-substrate interfaces with a wide range of spectroscopic, microscopic and diffraction-based approaches. For example, with our aberration-corrected electron spectromicroscope we study the growth kinetics of molecular films. This not only discloses fundamentals about the molecule-substrate interaction in relation to intermolecular interactions, but may also reveal avenues to desired structures that could form the basis for further fabrication steps. Because molecular nanostructures for quantum nanoscience often contain several molecular species, we also study the growth of mixed layers.

Compared to inorganic (e.g., metal-on-metal) epitaxy, where the building blocks are point-like single atoms, organic molecules add dimensionality, anisotropy and conformation that result in complex growth mechanisms. Phases consisting of two or more molecular species add even more design opportunities, but at the expense of adding further complexity. In this context, we have discovered and explained a novel and generic way of steering the growth [1]. It relies on opposite intermolecular interactions between the adsorbate species: molecules of one type attract each other to form two-dimensional islands, while the second molecular species shows repulsive interactions, yielding a so-called lattice gas of adsorbed molecules. By general thermodynamics considerations, this situation leads to a rich phase diagram that can be used for steering the growth of a desired interface structure. The mechanism is expected to be of general validity (News and views: Opposite interaction matters). The interactions between the molecular species which are crucial for growth control in this scheme are determined by charge transfer with the substrate [2, 3, 4, 5, 6] -- this can be engineered by the specific choice of molecules. Results from photoemission tomography and normal incidence X-ray standing waves help to select suitable molecules.

Structural phase diagram of heteromolecular layersStructural phase diagram of heteromolecular layers formed by PTCDA and CuPc in the submonolayer regime.