Basel Universität

Functional Nano-sized Materials

Taking a Look into the “Eye” of Majoranas

Cartoon of the Majorana bound state located at the ends of Fe chains on Pb(110).

Majorana bound states in one-dimensional nanowires have attracted wide attention in recent years due to their potential use as qubits for topological quantum computation based on braiding. Scientists at the University of Basel have assembled mono-atomic iron chains on the surface of superconducting lead Pb(110) and combined for the first time scanning tunneling microscopy and atomic force microscopy to spatially characterize the chains down to the atomic scale. The Majorana bound states appear as a zero-energy mode in the STM and as a pronounced halo in the AFM measurement at 5K, disappear above the superconducting state of lead and are well-localized at the chain ends (25 nm) with two localization lengths and wave function oscillations as predicted by theory. These observations thus lend strong support to the existence of Majorana bound states in hybrid Fe/Pb systems and provide strong motivation to use them as qubits for quantum computing devices.

see our publication in NPJ Quantum Info as well as press release.

Chain-like structure in Ni40Ta60 metallic glass

Movie composed of 15 STM images of the Ni40Ta60 metallic glass.

The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. We used scanning tunneling microscopy with large tunneling resistance of 6GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility (see movie), which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

see our publication in Scientific Reports

Nanodiamonds: Photo-excitation of nitrogen vacancy centers.

STM derivative imag of the nanodiamond layer adsorbed on graphite. Nitrogen-vacancy defect centers (NV) contained in nanodiamonds (NDs) are a promising candidate for quantum information processing or single photon sources. However, their detection with traditional optical techniques becomes challenging when probing high NV densities at the nanometer scale. In this project, we combined scanning probe techniques to characterize the structural and electronic properties of bucky-diamonds with sizes below 10 nm adsorbed on graphite. We employed force- and current- spectroscopic techniques under controlled irradiation to reveal the charge state of photo-excited NV centers contained in the ND core. Our approach detects in principle NV at the nanometer scale where conventional techniques are limited. This work has been conducted in collaboration with Dr. V. Pichot and Prof. Dr. D. Spitzer who synthesized the nanodiamond particle by detonation technique (Institut franco-allemand de Recherches de Saint-Louis-ISL).

See our publication in Nanoletters and App. Phys. Lett.
Highlighted in unibasel news, or at , and CNRS website

CaF2 Nanoparticles

We explore the adhesion properties of CaF2 nanoparticles adsorbed on mica and on polished teeth enamel in liquid. Calcium fluoride is of high interest in saliva chemistry and in the context of reducing acid dissolution of teeth, better known as caries. CaF2 nanoparticles have been shown to have the ability for reducing caries by acting as a potential source of fluoride preventing teeth demineralization. Amplitude-modulation atomic force microscopy (AM-AFM), also called as tapping mode AFM, was used to manipulate the pre- synthesized nanoparticles. In this dynamic technique, a cantilever is excited in resonance where for imaging the tip touches the surface at the bottom of each oscillation cycle. By recording the phase lag of the excitation signal with respect to the vibrating tip, phase images are generated which are closely related to energy dissipation maps. We recorded the phase lag at the point the particles were manipulated to calculate the energy dissipated mainly used to push the particle apart. This enabled us to examine the substrate dependent retention properties CaF2 nanoparticles which vary in size, shape and surface morphology. We show that the same CaF2 nanoparticles show up to three times higher retention when adsorbed on polished teeth enamel than on mica. This indicates that the chemical affinity of CaF2 nanoparticles with the teeth enamel is stronger compared to mica. We also show that particles which have ordered surface structures adhere stronger than chaotic ones. With this technique we will be able to propose optimized CaF2 nanoparticles with enhanced adhesion strength possibly used as additives in dental care products reducing caries.

Publications related to this research topic:

Single-molecule manipulation experiments to explore friction and adhesion
R. Pawlak, S. Kawai, T. Meier, T. Glatzel, A. Baratoff, E. Meyer
J. Phys. D : App. Phys., (50), (2017), 113003 , pdf.
Single Molecule Tribology: Force Microscopy Manipulation of a Porphyrin Derivative on a Copper Surface
R. Pawlak, W. Ouyang, A. E. Filippov, L. Kalikhman-Razvozov, S. Kawai, T. Glatzel, E. Gnecco, A. Baratoff , Q.-S. Zheng, O. Hod, M. Urbakh, E. Meyer
ACS Nano, 10, (2016), 713-722, pdf.
Single-Molecule Experiments to Explore Friction And Adhesion (Invited)
R. Pawlak , S. Kawai, A. Baratoff, W.G. Ouyang, T. Meier, E. Gnecco, T. Glatzel, A. Filippov, A. Baratoff, M. Urbakh, E. Meyer
AVS conference, 2015-10-22, San José, (US).
Surface chemistry, substrate electronic structure, and surface-bound water effect on surface potential of detonation nanodiamonds
S. Stehlik , Th. Glatzel, V. Pichot, R. Pawlak, E. Meyer, D. Spitzer, and B. Rezek
9th International Conference on New Diamond and Nano Carbons, 2015-05-27, Shizuoka, (Japan), Abstract (PDF).
Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy
R. Pawlak, L. Marot, A. Sadeghi, S. Kawai, Th. Glatzel, P. Reimann, S. Goedecker, H.-J. Güntherodt, E. Meyer
Sci. Rep., 5, (2015), 13143, pdf.
Local Detection of Nitrogen-Vacancy Centers in a Nanodiamond Monolayer
R. Pawlak , Th. Glatzel, V. Pichot, L. Schmidlin, S. Kawai,S. Fremy, D. Spitzer, E. Meyer
ncAFM2014, 2014-08-01, Tsukuba, (Japan).
Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy
R. Pawlak , L. Marot, A. Sadeghi, S. Kawai, T. Glatzel, P. Reimann, S. Goedecker, H.-J. Güntherodt, E. Meyer
eMRS, 2014-07-15, Warsow, (Poland).
Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy
R. Pawlak , L. Marot, A. Sadeghi, S. Kawai, T. Glatzel, P. Reimann, S. Goedecker, H.-J. Güntherodt, E. Meyer
NanoManipulation Workshop, 2014-06-18, Mulhouse, (France).
Exploring the retention properties of CaF2 nanoparticlesas possible additives for dental care application with tapping-modeatomic force microscope in liquid
M. Wasem, J. Köser, S. Hess, E. Gnecco, and E. Meyer
Beilstein Journal of Nanotechnology, 5, (2014), 36–43, pdf.
Local Detection of Nitrogen-Vacancy Centers in a Nanodiamond Monolayer
R. Pawlak, Th. Glatzel, V. Pichot, L. Schmidlin, S. Kawai,S. Fremy, D. Spitzer, E. Meyer
Nano Lett., 13, (2013), 5803-5807, pdf.
Two-dimensional nanodiamond monolayers deposited by combined ultracentrifugation and electrophoresis techniques
L. Schmidlin, V. Pichot, S. Josset, R. Pawlak, T. Glatzel, S. Kawai, E. Meyer, and D. Spitzer
Appl. Phys. Lett., 101, (2012), 253111, pdf.
Orientation dependent molecular friction on organic layer compound crystals
G. Fessler, I. Zimmermann, Th. Glatzel, E. Gnecco, P. Steiner, R. Roth, T. D. Keene, S.-X. Liu, S. Decurtins, and E. Meyer
Appl. Phys. Lett., 98, (8), (2011), 083119, pdf.
Atomic-scale stick slip
R. Bennewitz, E. Meyer, M. Bammerlin, T. Gyalog
in: Fundamentals of Tribology and Bridging the Gap Between t, (2001), Kluwer Academic Publ.
Atomic friction studies on well defined surfaces
R. Bennewitz, E. Gnecco, T. Gyalog, and E. Meyer
Tribology Letters, 10, (2001), 51.
Molecular dynamics study of scanning force microscopy on self-assembled monolayers
Bonner T, Baratoff A
SURFACE SCIENCE, 127, (1), (1997), 1082-1086.
MD-Simulations of scanning force microscopy on self-assembled monolayers
T. Bonner, A. Baratoff, H.-J. Güntherodt
HELVETICA PHYSICA ACTA, 68, (2), (1995), 199-200.