| Department of Physics
University of Basel
| Klingelbergstr. 82
CH-4056 Basel, Switzerland
| Office: +41 (0)61 207 3725
Lab: +41 (0)61 207 3797
These past years as a student or researcher, I worked on ultra-high vacuum (UHV) apparatus to charaterize surfaces at the atomic scale. I developped strong skills in scanning tunneling microsope (STM) and atomic force microscopy (AFM) performed in cryogenic conditions (5K). Atomic force microscopy based on a tuning fork sensor in the qPlus configuration operated in the non-contact mode using a phase lock loop (PLL), together with typical STM characterizations (topographic imaging in the constant-current mode or scanning tunneling spectroscopy (STS)).
Part of my work consists in preparing samples in such atomically cleaned conditions (typically Cu(111), large band-gap semiconductors (TiO2, SrTio3) or superconductors (NbSe2, Pb(110), Pb(111))). Molecule and atoms are evaporated from a quartz crucible installed in the preparation chamber on substrates kept at temperatures from 10 K to 1000 K.
One of my main research task is the acquisition of highly resolved AFM images with functionalized tips enabling unprecedented resolution of molecule structures. Such AFM experiments is possible thank to the small amplitudes of tuning fork sensors operated at low temperature. In such conditions, a single carbon monoxide (CO) molecule can be attached to the the very end of the AFM tip, which drastically enhances the image contrast by probing Pauli repulsive force between tip and sample (Gross, Science 325, 1110 (2009)). Such real-space investigations can adress various on-surface chemical reactions allowing an unambiguous visualization of the atom/molecule organizations, chemical reaction and their biproducts.
Beside imaging surfaces, I acquired in the group of Prof. E. Meyer a long-standing experience in exploring the most advanced force and current spectroscopic methods, to give new insights into the mechanical, structural and electronic properties of single molecules at the atomic scale.
Scanning tunneling spectroscopy (STS) acquired using the standard lock-in technique helps determining the local electronic density of states (LDOS) of various samples down to the atomic scale. While inevestigating single molecules, this technique enables the determination of the Highest Occupied Molecular Orbitals (HOMO) and Lowest Unoccupied Molecular Orbitals (LUMO).
Force spectroscopy, on the other hand, is obtained by measuring interaction forces betwen tip and sample in the non-contact mode. This is done by detecting the frequency shift variations, Δf, compared to the resonance frequency of the oscillator. Over the past years, I particularly focused on building three-dimensional force field above single-molecules using atom-tracked positioning. Based on either Δf(z) or Δf(V) spectroscopic curves, this technique is now routinely employed to determine the mechanical properties of molecules as well as their charge redistribution at the sub-molecular scale. See review here.
Pulling, sliding, stretching a single molecule, repositionning single atoms to build atomic structures on a surface... Single-molecule manipulation experiments can be achieved with an STM/AFM at low temperature enabling the investigation of the mechanical counter-reactions of a single molecule under a stress or en electrical field, and this well-below the effect of thermal fluctuations.
I devote quite some efforts to build up such experiments, understand them and use them to engineer matter down to the atomic scale.
Figure 3: Movie composed of 28 STM images showing the displacement at 4.5 K of a single molecule on Au(111). The translation is propelled by an electrical current injected from the tip.