Measurement of the binding energies of the organic-metal perylene-teracarboxylic-dianhydride/Au111 bonds by molecular manipulation using an atomic force microscope.

Based on single molecule manipulation experiments in a combined scanning tunneling microscope/frequency modulated atomic force microscope, we quantify the individual binding energy contributions to an organic-metal bond experimentally. The method allows the determination of contributions from, e.g., local chemical bonds, metal-molecule hybridization, and van der Waals interactions, as well as the total adsorption energy.

Imaging Pauli repulsion in scanning tunneling microscopy.

A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single D2 molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultrahigh geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-distance spectroscopy and corresponding density functional calculations the mechanism of the sensor and transducer is identified. It probes the short-range Pauli repulsion and converts this signal into variations of the junction conductance.

Site-specific polarization screening in organic thin films.

Systematic local spectroscopy of the affinity levels, by means of a scanning tunneling microscope, in highly ordered molecular semiconductor films of tetracene reveals strong energy level shifts by up to approximately 1.0 eV from molecule to molecule. This final state effect can be traced back to the site specificity of the polarization energy in organic materials with complex unit cells, caused by a combination of different molecular environments, the intrinsically anisotropic molecular polarizability, and the influence of the substrate.

Role of intermolecular interactions on the electronic and geometric structure of a large pi-conjugated molecule adsorbed on a metal surface.

The organic semiconductor molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) exhibits two adsorption states on the Ag(111) surface: one in a metastable disordered phase, prepared at low temperatures, the other in the long-range ordered monolayer phase obtained at room temperature. Notably, the two states differ substantial in their vertical bonding distances, intramolecular distortions, and electronic structures. The difference is explained by intermolecular interactions, which are particularly relevant for the long-range ordered phase, and which hence require attention.

Kondo effect by controlled cleavage of a single-molecule contact.

Conductance measurements of a molecular wire, contacted between an epitaxial molecule-metal bond and the tip of a scanning tunnelling microscope, are reported. Controlled retraction of the tip gradually de-hybridizes the molecule from the metal substrate. This tunes the wire into the Kondo regime in which the renormalized molecular transport orbital serves as a spin impurity at half-filling and the Kondo resonance opens up an additional transport channel. Numerical renormalization group simulations suggest this type of behaviour to be generic for a common class of metal-molecule bonds. The results demonstrate a new approach to single-molecule experiments with atomic-scale contact control and prepare the way for the ab initio simulation of many-body transport through single-molecule junctions.

Free-electron-like dispersion in an organic monolayer film on a metal substrate.

Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively, particularly the degree of electron delocalization that can be achieved. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47m(e), where m(e) is the bare electron mass. This dispersion is far stronger than expected for the organic film alone, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.

Structure and bonding of the multifunctional amino acid L-DOPA on Au(110).

In investigations of the proteins which are responsible for the surface adhesion of the blue mussel Mytilus edulis, an unusually frequent appearance of the otherwise rare amino acid 3-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA) has been observed. This amino acid is thought to play a major role in the mechanism of mussel adhesion. Here we report a detailed structural and spectroscopic investigation of the interface between L-DOPA and a single-crystalline Au(110) model surface, with the aim of understanding fundamentals about the surface bonding of this amino acid and its role in mussel adhesion. Molecular layers are deposited by organic molecular beam deposition (OMBD) in an ultrahigh-vacuum environment. The following experimental techniques have been applied: ex situ Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). Vibrational spectra of isolated L-DOPA molecules and the zwitterionic bulk have been calculated using density functional theory (DFT). The predicted modes are assigned to observed spectra, allowing conclusions regarding the molecule-substrate and molecule-molecule interactions at the L-DOPA/Au(110) interface. We find that zwitterionic L-DOPA forms a monochiral, one-domain commensurate monolayer on Au(110), with the catechol rings on top of [110] gold rows, oriented parallel to the surface. The (2 x 1)-Au(110) surface reconstruction is not lifted. The carboxylate group is found in a bidentate or bridging configuration, the amino group is tilted out of the surface plane, and the hydroxyl groups do not dehydrogenate on Au(110). Similar to the case for the bulk, molecules form dimers on Au(110). However, the number of hydrogen bridge bonds between L-DOPA molecules is reduced as compared to the bulk. Thicker layers which are deposited onto the commensurate interface do not order in the bulk structure. In conclusion, our study shows that the aromatic ring system of L-DOPA functions as a surface anchor. Since it is also known that the hydroxyl groups support cross-link reactions between L-DOPA residues in the mussel glue protein, we can conclude that the catechol ring supports surface adhesion of mussel proteins via two independent functions.