The goal of our reserach is to understand fundamentals of electronic structure, molecular magnetism and electron transport at the single molecule scale. We employ low-temperature (1.2 K, 4.5 K) scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultra-high vacuum for our investigations (a scheme of the experimental principle is shown on the left). The large stability and high energy resolution allow us to perform local spectroscopy for the detection of electronic, vibrational, and magnetic properties of single atoms and molecules on surfaces.
Our research program can be divided into four main lines:
1. Electronic properties of molecular nanostructures and thin films:
In this research line we investigate the self-organization of molecular structures on metal surfaces. The goal is to create molecular nanostructures with specific structural and electronic properties. Self-assembled organic charge transfer complexes present a particularly interesting topic. On a metal surface, these systems exhibit an interesting phenomenology, like complex electronic bands at the metal-organic interface and magnetic effects induced by the electronic reorganization and the creation of free radicals, which are stable on a metal surface.
2. Inelastic transport through single molecules:
The excitation of vibrations by tunnelling electrons may lead to an effective heating of a single molecule. This eventually results in thermal decomposition of the molecule. We characterize the power for decomposition in dependence of the electron energy and the lead´s material. Using inelastic tunnelling spectroscopy (IETS) provides information on the preferentially excited molecular vibrations and can also be used as the ultimate chemical fingerprint for the identification of adsorbates. We explore selection rules governing the excitation and detection possibilities of inelastic scattering of tunnel electrons, and its dependence on the substrate material. We further use inelastic effects to manipulate the properties of molecules at surfaces. The excitations eventually lead to chemical transformations, such as cis-trans isomerisation, ring-opening/closing reactions and molecular bi-stabilities. We use STM to detect these processes in molecules on metal surfaces. We also monitor the conductance and forces exerted on the STM tip while controlling external parameters (electric field, bias voltage) and during electron- induced molecular switching (conformational, charge state).
3. Magnetic interactions at surfaces:
The interaction of a localized spin with a metallic substrate leads to the screening by the itinerant conduction electrons. In tunnelling spectroscopy a clear fingerprint of this many-body interaction is given by the Kondo-resonance at zero bias. We detect this feature as a proof of a localized magnetic moment in single molecules. While magnetism is usually associated to transition metal atoms, we could show that charge transfer in a purely organic layer can also lead to stable states with unpaired electron spins. Being localized in an extended π-orbital, the spin state is coupled to molecular vibrations, which we detect by tunnelling spectroscopy. We use different molecular systems to tune the charge transfer properties, the interaction strength with the surface and spin coupling within the layer.
4. Nanoscale magnetism on superconducting surfaces:
An alternative route to the detection of localized magnetic states is the use of superconducting substrates. Magnetic moments interact with the Cooper pairs of a superconductor, which leads to a weaking of their pairing strength and eventually to their suppression. A fingerprint of this interaction can be found as so-called Shiba states within the superconducting energy gap, which we detect by tunnelling spectroscopy. The energy alignment of these sub-gap states gives a direct hint at the interaction strength with the substrate. We resolve the competition of magnetic interactions and superconducting phenomena at the nanoscale.