Atomic-Scale Science on Insulating Films

Workgroup Repp, Faculty of Physics, University of Regensburg


Crystallization of a Two-Dimensional Hydrogen-Bonded Molecular Assembly

L.L. Patera, X. Liu, N. Mosso, S. Decurtins, S.-X. Liu and J. Repp

We investigated the structures of a two-dimensional hydrogen-bonded film, both in the kinetically trapped amorphous state and in the thermodynamically stable crystalline phase. The sub-molecular resolution provided by nc-AFM allows resolving the changes in the bonding motifs upon transition to the crystalline state. These results reveal how the crystallization governs the length scale of the network order for non-flexible molecular species without affecting the local bonding schemes.

Angewandte Chemie International Edition 56, 10786 (2017).

Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging

T. Cocker, D. Peller, P. Yu, J. Repp, and R. Huber

In a close collaboration with Prof. Rupert Huber and his team we succeeded in combining sub-molecular STM imaging and subcycle lightwave electronics. In a low-temperature STM, an intense phase-locked half-cycle THz pulse was tightly focused onto the tunneling junction. The THz field acted as a quasi-instantaneous transient bias for the tunnelling junction and the resulting time-averaged tunnelling current was measured. It was possible to record a femtosecond snapshot image of an individual molecular orbital, with a time resolution of 115 fs, determined with an autocorrelation method. By correlating two successive tunnelling events, the coherent THz vibrations of the single molecule were directly tracked in time with sub-angstrom precision.

Nature 539, 263 (2016).

Charge-State-Dependent Diffusion of Individual Gold Adatoms on Ionic Thin NaCl Films

J. Repp, W. Steurer, I. Scivetti, M. Persson, L. Gross, and G. Meyer

Metal atoms on insulating films can exhibit several different charge states, which can be controlled on the level of individual atoms. As an atomís electron shell is decisive for almost all of its physical and chemical properties, a control of the charge state offers the prospect of controlling adsorbates in many other aspects.

We demonstrated that the diffusion behavior of individual gold adatoms on ultra-thin NaCl films depends strongly on its charge state and can therefore be controlled by means of STM-based manipulation.

Physical Review Letters 117, 146102 (2016).

Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy

Florian Albrecht, Jascha Repp, Martin Fleischmann, Manfred Scheer, Martin Ondráček, and Pavel Jelínek

Kelvin Probe Force Spectroscopy (KPFS) is an established technique based on atomic force microscopy (AFM) to determine local fluctuations of a sampleís work function. Such fluctuations can be attributed to local charges or dipoles at the sample surface. Triggered by the leap in resolution in low-temperature AFM with functionalized tips [Gross et al. Science 325, 1110 (2009)], KPFS was recently applied to study individual molecules exhibiting a dipole moment with sub-molecular resolution.

In our work we apply KPFS to successfully map out the polar nature of bonds inside individual molecules. In addition, we clarify the origin of certain artifacts in KPFS that inevitably occur at very close tip-sample distances. We introduce a new method to determine the charge distributions with sub-molecular resolution, and we benchmark this method for various systems. Thereby we demonstrate that this new method allows for resolving intramolecular charge distributions with unprecedented resolution - a regime for which conventional KPFS is shown to fail.

Physical Review Letters 115, 076101 (2015).

See also: Synopsis in Physics, Physics world, Chemistry World, COSMOS

Fixing the Energy Scale in Scanning Tunneling Microscopy on Semiconductor Surfaces

Gerhard Münnich, Andrea Donarini, Martin Wenderoth, and Jascha Repp

In cross-sectional scanning tunneling microscopy (X-STM), the presence of the microscope's tip can severely influence experimental spectra due to so-called tip-induced band bending (TIBB), an effect that obscures the energy scale in the tunneling junction. In this context, experimental data of shallow acceptors buried in gallium arsenide have remained controversial for almost two decades. In our study, we combine X-STM with Kelvin probe force spectroscopy performed simultaneously in the same setup with the very same tip. This combination of tools allows us to experimentally recover the zero point of the energy scale, and thereby to solve a long-standing problem in STM experiments on semiconductors.

From the experimentally recovered energy scale we see that conductance spectra can involve a cross-over between the different transport regimes, as observed here for the specific case of shallow acceptors buried in gallium arsenide. Such cross-over can be modelled using a many-body Anderson-Hubbard description.

Physical Review Letters 111, 216802 (2013).

Symmetry Dependence of Vibration-Assisted Tunneling

Niko Pavliček, Ingmar Swart, Judith Niedenführ, Gerhard Meyer, and Jascha Repp

Our findings provide a simple model to understand and potentially engineer the efficiency of charge transport in future molecular and organic electronic devices.

In organic semiconductors charge transport is due to electron hopping between adjacent molecules. To facilitate low-dissipation future devices it is necessary to minimize the excitation probability of vibrations when an electron hops from one site to the next. In the common models this excitation probability is given by the geometric relaxations taking place inside a given molecule and does not depend on how the electron enters. In this paper, we show in fact that dissipation is highly dependent on the injection point of electrons. We investigated the excitation probability of vibrations in individual pentacene molecules for electron hopping by low-temperature scanning tunneling spectroscopy. We are able to prove that the local symmetry of initial and final electron wave function determines the dissipation and provide a simple model to understand the observed behavior. Based on this novel understanding low-dissipation materials may be engineered in the future.

Physical Review Letters 110, 136101 (2013).

Controlling the orbital sequence in individual Cu-Phthalocyanine molecules

Christof Uhlmann, Ingmar Swart, and Jascha Repp

We investigated a molecular switch that is based on a novel concept: controlling the energetic order of molecular orbitals. This is fundamentally different from previously proposed mechanisms for molecular switching such as isomerization, charge-, or spin-state manipulation. The switching takes place in negatively charged Copper(II)phthalocyanine molecules on NaCl/Cu(100). In these, a Jahn-Teller distortion lifts the degeneracy of the lowest unoccupied molecular orbitals. By placing Au and Ag-atoms close to the molecule, the energetic order of these two levels can be switched in a bistable fashion. Hence, such a molecular switch can be actuated by subtle changes in the local vicinity of a molecule, i.e. without directly injecting charge carrier into the switch itself.

Nano Lett. 13, 777 (2013).

Two configurations of DBTH molecules on a NaCl(100) surface Atomic force microscopy reveals bistable configurations of dibenzo[a,h]thianthrene and their interconversion pathway

Niko Pavliček, Benoit Fleury, Mathias Neu, Judith Niedenführ, Coral Herranz-Lancho, Mario Ruben, and Jascha Repp

We have visualized a difference in the chemical structure of two molecular configurations of dibenzo[a,h]thianthrene (DBTH) molecules by means of atomic force microscopy.

Recently, Gross et al. (Science, 2009) have demonstrated the role of tip-functionalization in atomic force microscopy by resolving the chemical structure of pentacene molecules. Shortly after, this technique was used to identify the structure of an organic molecule. In this paper, we have investigated DBTH molecules adsorbed on NaCl layers in a combined scanning tunneling and atomic force microscopy (STM/AFM) study. DBTH molecules are derivates of the butterfly-shaped thianthrene molecule, in which the wings are extended by one benzene ring. By inelastic current excitations in STM mode, we can switch the wings between pointing up or down. The positions of the wings, however, could only be revealed in the AFM mode. The atomically resolved AFM images also reveal that the butterfly is flapping its wings as opposed to turning upside down.

Physical Review Letters 108, 086101 (2012).

Video of 4NCuPc trimer formation on NaCl/Cu(111)
Direct links: MP4 format | Ogg format | WebM format

Controlled Lateral Manipulation of Molecules on Insulating films

Ingmar Swart, Tobias Sonnleitner, Judith Niedenführ, and Jascha Repp

It has been demonstrated that the tip of a scanning tunneling microscope/atomic force microscope can be used to build nanostructures with atomic scale position on metallic and semiconductor surfaces. In contrast, controlled lateral manipulation on insulators had not been reported. We demonstrated that molecules adsorbed on ultrathin insulating films can be laterally manipulated in a controlled way by injecting inelastically tunneling electrons at well-defined positions in a molecule. This technique was found to be applicable to several different molecules.

Nano Lett. 12, 1070 (2012).

Molecular Symmetry Governs Surface Diffusion

Tobias Sonnleitner, Ingmar Swart, Niko Pavliček, Andreas Pöllmann, and Jascha Repp

In many areas of chemistry and physics the symmetry of an object or process plays a decisive role. One prominent example is the selection rules governing optical transitions. We investigated the influence of molecular symmetry on the surface potential landscape. For this purpose, the nonthermal diffusion induced by inelastic excitations of a molecule, for which four symmetry distinct isomers exist, was studied. The observed nonthermal diffusion was found to be qualitatively different for all four symmetry distinct isomers. This demonstrates that adsorbate symmetry plays an important role in determining the surface potential landscape.

Phys. Rev. Lett. 107, 186103 (2011).

Controlling the charge state of single molecules: visualizing changes in the tunneling barrier with submolecular resolution

Ingmar Swart, Tobias Sonnleitner, and Jascha Repp

The tip of a scanning tunneling microscope was used to controllably add and remove an additional electron to and from a single molecule. It is shown that charge-state control of individual molecules adsorbed on surfaces can be obtained by choosing a substrate system with an appropriate workfunction. By subtracting images of molecules in the neutral and anionic charge state, information regarding the spatial distribution of the additional charge was obtained. These difference images show marked intramolecular contrast.

Nano Letters 11, 1580 (2011).

Imaging Bond Formation Between Gold and Pentacene on an Insulating Surface

Jascha Repp, Gerhard Meyer, Sami Paavilainen, Fredrik E. Olsson, Mats Persson

A covalent bond between an individual pentacene molecule and a gold atom is formed by means of single-molecule chemistry inside a scanning tunneling microscope junction. The bond formation is reversible, and different structural isomers can be produced. The single-molecule synthesis is done on ultrathin insulating films that electronically isolate the reactants and products from their environment. Direct imaging of the orbital hybridization upon bond formation provides insight into the energetic shifts and occupation of the molecular resonances.

Science 312, 1196 (2006).

Scanning Tunneling Microscopy Imaging of Individual Molecular Orbitals

Jascha Repp, Gerhard Meyer, Sladjana M. Stojkovic, Andre Gourdon, Christian Joachim

Ultrathin insulating NaCl films have been employed to decouple individual pentacene molecules electronically from the metallic substrate. This allows the inherent electronic structure of the free molecule to be preserved and studied by means of low-temperature scanning-tunneling microscopy. Thereby direct images of the unperturbed molecular orbitals of the individual pentacene molecules are obtained. Elastic scattering quantum chemistry calculations substantiate the experimental findings.

Phys. Rev. Lett. 94, 026803 (2005).

Controlling the Charge State of Individual Gold Adatoms

Jascha Repp, Gerhard Meyer, Fredrik E. Olsson, Mats Persson

The nature and control of individual metal atoms on insulators are of great importance in emerging atomic-scale technologies. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, that are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip.

Science 305, 493 (2004).