Matthias Böhringer

Highly ordered structures and submolecular scanning tunnelling microscopy contrast of PTCDA and DM-PBDCI monolayers on Ag(111) and Ag(110)

Abstract Highly ordered monolayers of two perylene derivatives (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DM-PBDCI)) were prepared by vacuum sublimation on Ag(111) and Ag(110) surfaces. Using scanning tunnelling microscopy (STM) and low energy electron diffraction, the nucleation, growth, step site interaction, and superstructures were investigated and compared. All adsorbate structures are commensurate, with the exception of DM-PBDCI on Ag(111). A similar, but not identical, submolecular STM resolution was obtained for both molecules and on both surfaces. The local distribution shows ten dominant intensity maxima per molecule and corresponds to that of the lowest unoccupied orbital. From these results we draw conclusions concerning the tunnelling mechanism, the adsorbate–substrate, and the adsorbate–adsorbate interactions, which are discussed in relation to the formerly published results on the electronic structure of PTCDA/Ag(111).

Separation of a Racemic Mixture of Two‐Dimensional Molecular Clusters by Scanning Tunneling Microscopy

Adsorption of sub-monolayer amounts of 1-nitronaphthalene (NN) onto Au(111) leads to the aggregation of NN decamers, which exhibit two-dimensional chirality and represent a racemic mixture. In analogy to Pasteurs experiment of 1848 a scanning tunneling microscope can be used to discriminate and separate the enantiomers on a molecular scale.

Trennung eines racemischen Gemisches zweidimensionaler molekularer Cluster mit dem Rastertunnelmikroskop

Die Adsorption von Submonolagen von 1-Nitronaphthalin (NN) auf Au(111) fuhrt zur Aggregation von NN-Decameren, die eine zweidimensionale Chiralitat aufweisen und als racemisches Gemisch anfallen. In Analogie zu Pasteurs Experiment von 1848 werden mit einem Rastertunnelmikroskop auf molekularer Ebene die Enantiomere unterschieden und getrennt.

Adsorption site determination of PTCDA on Ag(110) by manipulation of adatoms

Abstract Simultaneous imaging of organic molecules and an underlying metallic substrate lattice is often hampered by tip-induced molecular motion, even at low temperatures. In the case of perylene-tetracarboxylic acid-dianhydride (PTCDA) on Ag(110), this problem is circumvented by using tip-induced motion of a coadsorbed Ag atom to obtain convenient markers of the substrate lattice. A model for the registry of PTCDA on Ag(110) is deduced.

Two-dimensional self-assembly of magic supramolecular clusters

We report low-temperature scanning tunnelling microscope (STM) observations of magic two-dimensional supramolecular clusters formed after low-coverage deposition of 1-nitronaphthalene (NN) molecules on a reconstructed Au(111) substrate. The clusters that consist predominantly of ten NN molecules are chiral and form a racemic mixture on the surface. STM manipulation of the clusters shows that their internal structure does not depend on the atomistic details of the substrate. Manipulation of individual molecules within the clusters hints at utility for nanoscale devices. The enantiomers of these supramolecular clusters are discriminated and separated in a molecular-scale analogue to Pasteurs experiment.

Two-dimensional self-assembly of supramolecular structures

We briefly review recent low temperature scanning tunneling microscopy (STM) investigations performed in our laboratory1–5 on the self-assembly of the dipolar organic molecule 1-nitronaphthalene (NN) adsorbed on the reconstructed Au(111) surface. NN becomes chiral upon planar adsorption on the gold surface. We observe several coverage-driven structural transformations which are associated with simultaneous changes in the enantiomeric composition of the self-assembled molecular structures. At low coverages almost exclusively decamers with an 8:2 ratio of the enantiomers are formed. In a medium coverage range enantiopure one-dimensional molecular double chains prevail on the surface. Subsequently, molecules with opposite handedness are admixed until at monolayer coverage racemic one- and two-dimensional structures coexist. Modeling shows that hydrogen bonding causes the observed self-assembly. A subtle interplay between the electrostatic interactions among the molecules and their interaction with the reconstructed metal surface is the origin of the observed coverage-driven chiral phase transition in two dimensions.