Fachbereich Physik


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Research

Current and past research projects focus on the fabrication and manipulation of materials at the single molecular level in insulating environments. Using scanning probe methods, especially high-resolution atomic force microscopy, we study molecular and atomic adsorption on surfaces of insulating materials or on thin insulating films, where the electronic properties of the adsorbates are rather unaffected by the underlying substrate:
 

Charge state modification

Electron transfer is the key process in solar energy conversion, in molecular electronic devices, or in OLEDs to name but a few. With scanning probe microscopy, we study electron transfer with high spatial resolution, and especially investigate charging, discharging, and the charge distribution using single molecules and small molecular assemblies. [Rahe et al., Nano Letters 16, 911 (2016)]

 

 

Kelvin Probe Force Microscopy

Investigating charged systems requires a specialised "tool" sensitive to charges. Kelvin Probe Force Microscopy operated under ultra-high vacuum as an extension of frequency-modulated atomic force microscopy allows to map charge distributions with high spatial resolution. Current projects focus especially on the physical interpretation of the main measurement signal in Kelvin Probe Force Microscopy, ΔΦ, when imaging molecule-on-insulator systems. [Neff and Rahe, Phys. Rev. B 91, 052424 (2015); Söngen et al., J. Appl. Phys. 119, 025304 (2016)] 

Quantitative force measurements

The core capability of frequency-modulated (also known as 'dynamic' or 'non-contact') atomic force microscopy is to quantitatively measure forces with nN down to pN resolution, enabling the exciting possibility to investigate chemical or physical interactions at the atomic scale. Current projects include the application of this technique to a wide variety of systems – especially including surface-supported molecular assemblies – as well as further optimisation of the measurement protocols, the tip preparation, the careful determination of system parameters and the usage of suitable force recovery algorithms. [Kuhn and Rahe, Phys. Rev. B. 89, 235417 (2014); Sweetman et al., Nature Commun. 7, 10621 (2016)]

Atom tracking and drift compensation

Thermal drift – the result of minute temperature changes within a scanning probe microscope – is a well-known nuisance, and especially hinders a reliable absolute tip positioning. It especially forbids long-term data acquisition in constant-height mode as well as the acquisition of dense force volume data. An elegant solution is to "lock" the scanning probe tip to one surface site and to track the virtual movement, a technique known as atom tracking. We continuously extend and optimise a flexible atom-tracking system to enable new data acquistion strategies. [Rahe et al., Rev. Sci. Instrum. 82, 063704 (2011)]

Properties of the calcite (1 0 1̅ 4) surface

Calcite is the most abundant simple salt in nature, and is best known within the field of biominerals where it combines with an organic phase to materials with outstanding material properties and amazing elegancy. The (1 0 1̅ 4) surface is the most stable natural cleavage plane and proved itself in a number of studies as an extremely promising substrate for insulator-supported molecular self-assembly. With a focus on this application, we studied the complex contrast formation as well as apparent reconstructions of this surface from the atomic-scale force interactions using atomic force microscopy under ultra-high vacuum conditions. [Kuhn et al., Phys. Rev. B. 90, 195405 (2014); Rahe et al., J. Phys. Cond. Matter 24, 084006 (2012)]

Insulator-supported molecular assembly

Molecular self-assembly offers an exciting route for the parallel creation of new materials. Here, especially the calcite(1 0 1̅ 4) surface was identified as a most suitable substrate for studying molecular self-assembly on insulators [Rahe et al., Phys. Chem. Chem. Phys. 14, 6537 (2012)]. Besides the presentation of numerous structures of varying complexity which are stable at room temperature – i.e. slim molecular rows [Rahe et al., J. Phys. Chem. C 114, 1547 (2010)] or hydrogen-bonded network structures [Rahe et al., small 8, 2969 (2012)] – the possibility to anchor molecules to dielectric substrates [Rahe et al., Adv. Mater. 25, 3948 (2013)] lead to exciting experiments, such as the covalent linking of molecules in an on-surface synthesis step [Lindner et al., Angew. Chem. Ind. Ed. 53, 7952 (2014); Kittelmann et al., ACS Nano 5, 8420 (2011)].