Flat lying physisorbates on surfaces can be expected to be lifted by heating. The details of this lattice expansion depend on the competition between two mechanisms. On the one hand, the interaction potential of each atom with the substrate is anharmonic and constitutes the reason for lifting. On the other hand, the internal bending stiffness of the planar adsorbate reduces its tendency to lift with temperature. Here we show that this competition can be investigated in a simplified molecular-dynamics simulation which considers only the vertical motion of the atoms of the adsorbate in respective model potentials. For two prototypical systems, i.e., graphene and 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111), we observe lifting by 0.04 angstrom (graphene) and 0.06 angstrom (NTCDA) between zero and room temperature. For the latter system, the impact of the temperature-dependent geometrical and vibrational properties on the occurring interface state (IS) is discussed. By introducing a local electron-phonon interaction model, we combine the indirect dependence of the IS on the average binding distance with direct coupling to the vibrational modes and obtain good agreement with the experimental results.

The interface state (IS) between a noble metal and an organic layer shifts to lower energy by similar to 100 meV when heated from zero to room temperature. Here, we show that this shift can only be explained including the energy renormalization caused by a quantum-mechanical electron-phonon interaction (EPI) between molecular vibrations and the IS. We investigate the EPI by a simplified model on the bases of ab initio data. By combining EPI with a classical thermal lifting effect of the molecular layer, we explain the recently published temperaturedependent experimental data for the NTCDA/Ag(111) system.

We revisit the question of kekulene’s aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene’s highest occupied molecular orbital by photoelectron tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C–C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.

We employ density functional theory (DFT) to analyze the dispersion of the electronic state that exists at the commensurate interface between a monolayer of 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) and the Ag(111) surface. First, we present and verify a hydrogen-termination approach which allows a meaningful DFT description of the interface state with relatively thin silver slabs. Complemented with a projection technique which maps the interface electronic structure onto the original Ag(111) Shockley state, the DFT calculations enable us to analyze the evolution of the dispersion of the NTCDA/Ag(111) interface state when changing of the molecular coverage. Our calculations yield a difference between the interface state energy and the Shockley state energy that scales linearly with coverage. Furthermore, they predict a pronounced anisotropy of the dispersion of the interface state at long wavelengths which also depends linearly on the molecular coverage. The dispersion anisotropy is fully confirmed by our Fourier transform (FT) scanning tunneling spectroscopy (STS) experiments performed on a relaxed phase NTCDA/Ag(111) monolayer. Using feature detection STS (FD-STS), we moreover measure a band gap in the interface state band structure at the Brillouin zone boundary which indicates Bragg scattering of the interface state electrons in the periodic potential of the molecular layer. We thus observe an influence of the molecular layer on the interface state both at long (DFT, STS) and short wavelengths (STS).