Lattice mismatch and spectroscopy of buckybowls on Ag(111)

We investigated the adsorption of corannulene (C20H10) on the Ag(111) surface by experimental and simulated scanning tunneling microscopy (STM) and X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Structural optimizations of the adsorbed molecules were performed by density functional theory (DFT) and spectra evaluated within the transition potential approach. Corannulene is adsorbed in a bowl-up orientation displaying a very high mobility (diffusing, tilting, and spinning) at room temperature. At the monolayer saturation coverage, molecules order into a close compact phase with an average intermolecular spacing of ~10.5±0.5Å. The lattice mismatch drives a long wavelength structural modulation of the molecular rows, which however couldn't be identified with a specific superlattice periodicity. We show that both the structural and spectroscopic properties are intermediate between those predicted for a simple on-hexagon geometry and a on-pentagon one, which can be accounted for by calculating a three-fold (~8.6Å spacing) and a four-fold (~11.5Å) phase, respectively. We suggest that molecules smoothly change their equilibrium configuration along the observed long wavelength modulation of the molecular rows by varying their tilt and azimuth in between the geometric constraints calculated for molecules in the three-fold and four-fold phases. The good agreement in the peak sequence and energetic position between the experimental and calculated NEXAFS resonances allows us to interpret the measurements by virtue of the numerical analysis at the level of individual transitions with sigma and pi symmetry. In analyzing the spectra, we demonstrate that the curved aromatic backbone introduces a non-trivial dependence of the NEXAFS spectral intensities on the adsorption angle beyond the usual Stohr’s result.