Karina Morgenstern

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.

Self-assembly of 1-nitronaphthalene on Au(111)

Abstract We report low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigations of the organic molecule 1-nitronaphthalene adsorbed on the reconstructed Au(111) surface. Depending on the coverage, molecular structures of distinct dimensionality are observed. After saturation of the step edges with molecules, clusters form on the substrate terraces at coverages up to 0.15 monolayers (ML). The smallest clusters consist of three molecules and are stable at specific surface reconstruction sites only. In contrast, pinwheel-shaped clusters comprised of ten molecules are observed on various inequivalent sites of the reconstructed Au surface. For coverages larger than 0.2xa0ML one-dimensional molecular double chains prevail. The structure of the decamers and the chains is determined by intermolecular electrostatic forces, whereas their position and orientation are affected by the surface reconstruction. At saturation coverage a periodic two-dimensional close-packed arrangement of molecules that is independent of the surface reconstruction occurs simultaneously with one-dimensional structures along the uniaxial reconstruction domains. The structural features are discussed in terms of competing effects of intermolecular forces and the interaction with the substrate.

Local correlation during Ostwald ripening of two-dimensional islands on Ag(111)

Using two-dimensional Ag adatom islands on Ag(111) as a model system, we study the importance of local correlations in diffusion-limited Ostwald ripening. For the coverages studied (0.08, 0.21, and 0.3 ML), we find that the ripening can be surprisingly well described in a nearest neighbour model based on pairwise atom exchange between neighbouring islands. The results reveal the local nature of diffusion-limited Ostwald ripening and demonstrate the limits of the mean-field theories in describing experimental data.

Imaging size-selected silicon clusters with a low-temperature scanning tunneling microscope

Abstract Size-selected Si30 and Si39 clusters produced by a laser vaporization cluster source are deposited on the Ag(111) surface at room temperature and at liquid-nitrogen temperature respectively. Subsequently, the sample is transferred at low temperature (120xa0K) in a separate mobile ultrahigh vacuum chamber (vacuum-suitcase) from the cluster source to a low-temperature scanning tunneling microscope (STM). Soft landing of the supported clusters is indicated by the following observations: (i) atomic-resolution images taken at low bias voltages show transparent Si clusters and an unperturbed Ag(111) substrate; (ii) manipulation experiments on the supported clusters and subsequently taken atomic-resolution images show a defect-free Ag(111) surface. In spite of the fact that the clusters are mass-selected in the gas phase, a statistical analysis of the STM images indicates a finite size-distribution on the support. This finding is attributed to the presence of different isomers and/or cluster orientations on the surface.

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.

Reversed surface corrugation in STM images on Au(111) by field-induced lateral motion of adsorbed molecules

Abstract Submonolayer coverages of the organic molecule 1-nitronaphthalene adsorbed on the reconstructed Au(111) surface were investigated by scanning tunneling microscopy at 50 and 10xa0K. At 50xa0K, upon switching the polarity of the tunneling voltage a peculiar change in the image contrast is observed. At negative sample–tip bias voltage the dislocation lines of the Au(111) surface reconstruction are imaged as protrusions and molecular aggregates appear 0.15xa0nm higher than the substrate. At reversed bias, the interior of the fcc and hcp domains exhibits the same height as the molecular clusters, while the domain walls of the surface reconstruction appear as depressions with a strongly increased corrugation amplitude. At 10xa0K, for the same tunneling parameters, no such contrast reversal is observed. Controlled manipulation experiments at 10xa0K demonstrate that contrast reversal at 50xa0K is due to long-range attractive tip–molecule interactions which assemble small molecular aggregates below the tip apex.

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.

Oxygen molecules on Ag(001): superstructure, binding site and molecular orientation

Abstract Oxygen molecules on Ag(0xa00xa01), adsorbed at about 60 K, are found to form two-dimensional c (2×4) compact islands. We determine binding site and orientation of the molecules within the superstructure by comparing experimental and calculated scanning tunneling images in combination with molecular dynamics simulations. The molecule adsorbs in the thermodynamically stable fourfold hollow site with its molecular axis in the direction of short periodicity of the superstructure. Rehybridization of 1π and 2π orbitals on adsorption is at the origin of the observed image contrast.