Das Innere einzelner Moleküle mittels Kelvin-Probe- und Kraftfeld-Spektroskopie untersuchen

• Antragsteller: Professor Dr. Jascha Repp
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie ; Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
• Förderung: Förderung von 2014 bis 2017
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 249365894

• Erstellungsjahr: 2017

Zusammenfassung der Projektergebnisse

In our project we wanted to make use of tip functionalization in atomic force microscopy (AFM) – that was relatively novel at that time – to study (i) molecular interactions with intramolecular spatial resolution and (ii) the polarity of bonds and their response to local fields. To account for some experimental issues, with respect to (i) we concentrated on the possibility to obtain three-dimensional geometrical information for individual adsorbed molecules from AFM data with functionalized tips, rather than just focusing on chemical sensitivity. In this context, we analyzed the geometric structure of nonplanar molecules via two methods: For the first one, we recorded time-demanding full 3D datasets. In the second case, we use AFM data recorded in planes perpendicular to the sample surface to identify subtle conformational details of tetraphenylporphyrine molecules. This novel data acquisition method not only allows for unambiguous species identification but also for a quantitative determination of vertical movement of a single atom within an individual molecule upon tip induced conformational switching. We investigated the regioselectivity of chemical reactions by thermal annealing of a heteroaromatic precursor. To do so, we had to identify the different reaction sites within the molecules, and hence, the position of different types of atoms (carbon versus nitrogen) inside the molecules, which is at the core of research goal (i). We could identify the preferentially activated atomic site of the precursor molecule as well as shed new light onto the role of substrate metal atoms during covalent C–C bond formation. We investigated molecules with polar bonds and identified bias induced relaxations as a systematic limitation of KPFS at close tip-sample spacing. This failure of KPFS is easily identified in maps of residuals of the parabolic fit of the experimental data. To overcome this limitation of KPFS, we introduced a novel AFM-based spectroscopy scheme. This novel method allows disentangling electrostatic from other signal contributions. In addition, experiments at close tip-sample spacing become possible, which finally allowed us to record charge related data with contrast along individual polar bonds. As a side aspect, we benchmarked STM-based work-function determination from maps of the current-distance decay constant against established KPFS experiments. We showed that the former is not suitable for a characterization of local dipole distributions on submolecular length scales. The preliminary results on the sample’s polarizability on submolecular length scales are very promising, but require further experiments and theory calculations prior to publication.

Projektbezogene Publikationen (Auswahl)

• ‘Characterization of a Surface Reaction by Means of Atomic Force Microscopy’, J. Am. Chem. Soc. 137, 7424 (2015)
  F. Albrecht, N. Pavlicek, C. Herranz-Lancho, M. Ruben, and J. Repp
  
• ‘Local tunneling decay length and Kelvin probe force spectroscopy’, Phys. Rev. B 92, 235443 (2015)
  F. Albrecht, M. Fleischmann, M. Scheer, L. Gross, and J. Repp
  
• ‘Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy’, Phys. Rev. Lett. 115, 076101 (2015)
  F. Albrecht, J. Repp, M. Fleischmann, M. Scheer, M. Ondrácek, and P. Jelínek
  
• ’Visualisierung der Polarität chemischer Bindungen’, Physik in unserer Zeit 46, 266 (2015)
  J. Repp, F. Albrecht, M. Fleischmann, and M. Scheer
  
• ‘Control of Reactivity and Regioselectivity for On-Surface Dehydrogenative Aryl-Aryl Bond Formation’, J. Am. Chem. Soc. 138, 5585 (2016)
  N. Kocić, X. Liu, S. Chen, S. Decurtins, O. Krejčí, P. Jelínek, J. Repp, and S. X. Liu
  
• ‘Direct Identification and Determination of Conformational Response in Adsorbed Individual Nonplanar Molecular Species Using Noncontact Atomic Force Microscopy’, Nano Lett. 16, 7703 (2016)
  F. Albrecht, F. Bischoff, W. Auwärter, J. V. Barth, and J. Repp