Christian Wagner

Mechanism of high-resolution STM/AFM imaging with functionalized tips

High resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM) imaging with functionalized tips is well established, but a detailed understanding of the imaging mechanism is still missing. We present a numerical STM/AFM model, which takes into account the relaxation of the probe due to the tip-sample interaction. We demonstrate that the model is able to reproduce very well not only the experimental intra- and intermolecular contrasts, but also their evolution upon tip approach. At close distances, the simulations unveil a significant probe particle relaxation towards local minima of the interaction potential. This effect is responsible for the sharp sub-molecular resolution observed in AFM/STM experiments. In addition, we demonstrate that sharp apparent intermolecular bonds should not be interpreted as true hydrogen bonds, in the sense of representing areas of increased electron density. Instead they represent the ridge between two minima of the potential energy landscape due to neighbouring atoms.

Direct Imaging of Intermolecular Bonds in Scanning Tunneling Microscopy

Local, noncovalent intermolecular interactions in organic monolayers have been directly imaged using scanning tunneling hydrogen microscopy (STHM). Unprecedented spatial resolution directly reveals the relation between the intermolecular interactions, the molecular chemical structure, and the ordering in the film.

Single Molecule and Single Atom Sensors for Atomic Resolution Imaging of Chemically Complex Surfaces

Individual Xe atoms as well as single CO and CH(4) molecules adsorbed at the tip apex of a scanning tunneling microscope (STM) function as microscopic force sensors that change the tunneling current in response to the forces acting from the surface. An STM equipped with any of these sensors is able to image the short-range Pauli repulsion and thus resolve the inner structure of large organic adsorbate molecules. Differences in the performance of the three studied sensors suggest that the sensor functionality can be tailored by tuning the interaction between the sensor particle and the STM tip.

Non-additivity of molecule-surface van der Waals potentials from force measurements.

Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory based. Here we show measurements of the attractive potential between differently sized organic molecules and a metal surface using an atomic force microscope. Our choice of molecules and the large molecule-surface separation cause this attraction to be purely of vdW type. The experiment allows testing the asymptotic vdW force law and its validity range. We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules. Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction.

On the Origin of the Energy Gain in Epitaxial Growth of Molecular Films.

The material properties of organic thin films depend strongly on their order. The different types of epitaxy may complicate the exploration of the large variety of ordered systems and its exploitation in potential electronic devices. In this Letter, we develop a coherent description of the driving force that creates epitaxial systems. We focus on flat-lying organic adsorbates and explain the energy gain in commensurate, point-on-line, and line-on-line epitaxy. We use potential energy maps to visualize our concept and to derive a relation that allows anticipating epitaxial growth from low-energy electron diffraction (LEED) data. A unified description facilitates the identification and interpretation of experimentally observed adsorbate structures, whereas the rationalized expectation from LEED means a considerable speed gain if suitable candidates for organic-organic epitaxy are searched for in a combinatory approach.

Line-on-Line Organic-Organic Heteroepitaxy of Quaterrylene on Hexa-peri-hexabenzocoronene on Au(111)

In a recent paper, we discussed the optical properties of a heterostructure consisting of a highly ordered monolayer of quaterrylene (QT), electronically decoupled from the gold substrate by a predeposited epitaxial monolayer of hexa-peri-hexabenzocoronene (HBC). Here we now present the detailed structural investigation of this organic double-layer system. We show that the structure of the heterosystem can be identified as line-on-line coincidence (lol), a new type of epitaxy discovered by us previously for the system 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on HBC on highly oriented pyrolytic graphite (HOPG). Additionally, we provide evidence on the basis of advanced potential energy calculations that indeed energetic gain drives this lol growth mode.

Modeling intermolecular interactions of physisorbed organic molecules using pair potential calculations.

The understanding and control of epitaxial growth of organic thin films is of crucial importance in order to optimize the performance of future electronic devices. In particular, the start of the submonolayer growth plays an important role since it often determines the structure of the first layer and subsequently of the entire molecular film. We have investigated the structure formation of 3,4,9,10-perylene-tetracarboxylic dianhydride and copper-phthalocyanine molecules on Au(111) using pair-potential calculations based on van der Waals and electrostatic intermolecular interactions. The results are compared with the fundamental lateral structures known from experiment and an excellent agreement was found for these weakly interacting systems. Furthermore, the calculations are even suitable for chemisorptive adsorption as demonstrated for copper-phthalocyanine/Cu(111), if the influence of charge transfer between substrate and molecules is known and the corresponding charge redistribution in the molecules can be estimated. The calculations are of general applicability for molecular adsorbate systems which are dominated by electrostatic and van der Waals interaction.

Epitaxial nanolayers of quaterrylene: Influence of the substrate on the growth of the first and second monolayers

We present a combined low energy electron diffraction and scanning tunneling microscopy study on the growth of the first and second monolayers (MLs) of quaterrylene on single crystalline Au(111). For the first ML, we observe a point-on-line coincident growth of flat lying molecules which agrees well with our potential energy calculations. In contrast, we observe alternating upstanding and flat lying molecules in the second layer. This behavior nicely demonstrates that the substrate has a very strong influence on the structure of the first layer, whereas the second layer is much weakly affected and exhibits properties comparable to the bulk.

Patterning a hydrogen-bonded molecular monolayer with a hand-controlled scanning probe microscope

Summary One of the paramount goals in nanotechnology is molecular-scale functional design, which includes arranging molecules into complex structures at will. The first steps towards this goal were made through the invention of the scanning probe microscope (SPM), which put single-atom and single-molecule manipulation into practice for the first time. Extending the controlled manipulation to larger molecules is expected to multiply the potential of engineered nanostructures. Here we report an enhancement of the SPM technique that makes the manipulation of large molecular adsorbates much more effective. By using a commercial motion tracking system, we couple the movements of an operators hand to the sub-angstrom precise positioning of an SPM tip. Literally moving the tip by hand we write a nanoscale structure in a monolayer of large molecules, thereby showing that our method allows for the successful execution of complex manipulation protocols even when the potential energy surface that governs the interaction behaviour of the manipulated nanoscale object(s) is largely unknown.

The role of surface corrugation and tip oscillation in single-molecule manipulation with a non-contact atomic force microscope

Summary Scanning probe microscopy (SPM) plays an important role in the investigation of molecular adsorption. The possibility to probe the molecule–surface interaction while tuning its strength through SPM tip-induced single-molecule manipulation has particularly promising potential to yield new insights. We recently reported experiments, in which 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules were lifted with a qPlus-sensor and analyzed these experiments by using force-field simulations. Irrespective of the good agreement between the experiment and those simulations, systematic inconsistencies remained that we attribute to effects omitted from the initial model. Here we develop a more realistic simulation of single-molecule manipulation by non-contact AFM that includes the atomic surface corrugation, the tip elasticity, and the tip oscillation amplitude. In short, we simulate a full tip oscillation cycle at each step of the manipulation process and calculate the frequency shift by solving the equation of motion of the tip. The new model correctly reproduces previously unexplained key features of the experiment, and facilitates a better understanding of the mechanics of single-molecular junctions. Our simulations reveal that the surface corrugation adds a positive frequency shift to the measurement that generates an apparent repulsive force. Furthermore, we demonstrate that the scatter observed in the experimental data points is related to the sliding of the molecule across the surface.