The Metal/Organic Interface - Binding, Adsorption Geometry, and Electronic Structure

Metal/organic interfaces as they appear between electrodes and organic semiconductors in organic electronics decisively determine device properties of transistors, light emitting diodes, or photovoltaic cells. The interactions within the organic semiconductor and between organic adsorbate and metallic substrate lead to characteristic properties of the particular interface. These properties, namely the binding strength, the adsorption geometry, and the electronic structure, have been studied with comprehensive surface sensitive experimental methods like high-resolution electron energy-loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The use of single crystal metal surfaces as substrates and self-assembling small organic molecules as adsorbates lead to insights into structure-property relationships that will contribute to the further development of materials and devices. The first part of this work investigates the bonding strength between metal substrates and organic adsorbates. With the quantification of binding energies of simple aromatic molecules on coinage metal surfaces by means of TPD, this part enters questions of basic surface science. Besides the delivery of benchmarks of unrivalled accuracy for the further development of computational methods to model binding properties of adsorbate-covered surfaces the focus of this part also lays on the first investigation of the extraordinary coverage dependency of the binding energy of such systems. The second part is about the self-assembly of small-molecule organic semiconductors on metal surfaces, and how this arrangement is influenced by the molecular structure. This part covers the elucidation of adsorption geometries of N-heteropolycyclic aromatic molecules on the Au(111) surface by means of vibrational HREELS. Moreover, electronic HREELS enabled us to get insight into the electronic structure of these interfaces. To maximize the interaction between metal bands and the pi-system of the adsorbate the planar molecules prefer a planar adsorption geometry. This presetting of a flat geometry works subsequently as a template for further layers which leads to a growth mechanism and therefore film structure significantly different from that of the bulk crystal. The last part of this work studies the influence of organic adsorbate films on collective electronic properties of the metal surface with angle-resolved HREELS. Characteristic collective excitations of a two-dimensional electron gas present on the pristine gold surface are strongly influenced in their properties by adsorbate layers, e.g., they show a strongly enhanced intensity and a varied dispersion relation

Correlation of vibrational excitations and electronic structure with submolecular resolution

The detection of vibrational excitations of individual molecules on surfaces by scanning tunneling spectroscopy does not obey strict selection rules but rather propensity rules. The experimental verification of these excitations is challenging because it requires the independent variation of specific parameters, such as the electronic structure, while keeping the vibrational modes the same. Here, we make use of the versatile self-assembled structures of Fe-tetra-pyridyl-porphyrin molecules on a Au(111) surface. These molecules exhibit different energy-level alignments of the frontier molecular orbitals, thus allowing the correlation of the electronic structure and detection of vibrations. We identify up to seven vibrational modes in the tunneling spectra of the molecules in some of the arrangements, whereas we observe none in other structures. We find that the presence of vibrational excitations and their distribution along the molecule correlate with the observation of energetically low-lying molecular states. This correlation allows the explanation of the different numbers of vibrational signatures for molecules embedded within different structures as well as the bias asymmetry of the vibrational intensities within an individual molecule. Our observations are in agreement with the resonant enhancement of vibrations by the virtual excitation of electronic states

Binding energies of benzene on coinage metal surfaces: Equal stability on different metals

Interfaces between organic molecules and inorganic solids adapt a prominent role in fundamental science, catalysis, molecular sensors, and molecular electronics. The molecular adsorption geometry, which is dictated by the strength of lateral and vertical interactions, determines the electronic structure of the molecule/substrate system. In this study, we investigate the binding properties of benzene on the noble metal surfaces Au(111), Ag(111), and Cu(111), respectively, using temperature-programmed desorption and first-principles calculations that account for non-locality of both electronic exchange and correlation effects. In the monolayer regime, we observed for all three systems a decrease of the binding energy with increasing coverage due to repulsive adsorbate/adsorbate interactions. Although the electronic properties of the noble metal surfaces are rather different, the binding strength of benzene on these surfaces is equal within the experimental error (accuracy of 0.05 eV), in excellent agreement with our calculations. This points toward the existence of a universal trend for the binding energy of aromatic molecules resulting from a subtle balance between Pauli repulsion and many-body van der Waals attraction

Author Correction: Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00314-