17-21 July 2017
Santiago de Compostela, Facultade de Química
Europe/Madrid timezone

Chiral Organic Molecules for Molecular Spintronics Applications

17 Jul 2017, 18:20
Aula Química Analítica (Facultad Química (USC))

Aula Química Analítica

Facultad Química (USC)

Magnetic Materials and Applications (CEMAG) Magnetic Materials and Applications (CEMAG)


Dr. Juan José de Miguel (Universidad Autónoma de Madrid)


Chiral molecules are fascinating objects lying behind some deep, still unexplained puzzles of Nature such as the fundamental asymmetry found in living beings, which only utilize molecules with a specific helicity, called enantiomers. The relationship between chirality and magnetism has long puzzled researchers. Recently, layers of purely organic chiral molecules have been shown to scatter electrons differently depending on their spin [1] or to induce the appearance of strong spin polarization in initially unpolarized electron currents [2], opening up the possibility to use these materials as spin filters or to control enantioselective reactions by means of spin polarization. In this work we report on an experimental study of different spin-related phenomena observed in layers of chiral molecules adsorbed on single-crystal metal surfaces. Monolayer-thick films of chiral 1,2-diphenyl-1,2-ethanediol (DPED) have been studied by means of spin-polarized, angle-resolved photoemission (SPARUPS) at the MAX-lab synchrotron in Lund, Sweden [3]. This molecule has two chiral centers located at the two carbon atoms of the ethane chain and presents two chiral enantiomers which are designated according to their conformation and optical activity as (R,R)-(+)-DPED and (S,S)-(–)-DPED. The molecular layers were adsorbed in ultra-high vacuum on top of ferromagnetic Co films epitaxially grown on Cu(100) whose in-plane spin polarization served as an in situ reference. The photoelectons emitted through adsorbed layers of both chiral enantiomers display a clear spin polarization at room temperature, independent of their binding energy. Nevertheless, as Figure 1 shows, the spins point along different directions in space: in-plane for (R,R)-(+)-DPED and out-of-plane for (S,S)-(–)-DPED, which makes the DPED molecule a prototype system to study enantioselective spin filtering. These results will be compared to other recent experiments carried out on a similar molecule, 1,2-diphenylethylenediamine, adsorbed on Cu(100), highlighting the importance of the substrate and the charge transfer between this matter and the adsorbate. One of the main characteristics of chiral molecules is their optical activity, that has long been known and studied for the visible and UV wavelengths but not so much in the x ray range. Enantio-pure ultrathin films of DPED adsorbed on Cu(100) at 100 K have also been studied at MAXlab using circularly polarized x rays absorption (XAS) at the carbon K edge. XAS excites element-specific core electrons to empty levels in the ground state thus probing the molecule's electronic configuration. Pairs of spectra with circularly polarized x rays of opposite helicity were acquired and compared by taking their intensity ratio. Several clearly dichroic features have been observed (see Figure 2) and assigned to transitions into empty C–C and C–H π-type molecular orbitals; as expected, these dichroic ratios show opposite signs for the two enantiomers studied. However, the magnitude of the asymmetry measured is much larger than expected for the natural circular dichroism associated to the chiral configuration of the molecule. Preliminary analysis of these data indicates that the charge transfer between the Cu(100) substrate and the DPED molecules upon adsorption might be enantio-selective and strongly polarized in orbital moment. The results described above suggest that this type of compounds may be useful for molecular spintronics applications.

Primary author

Dr. Juan José de Miguel (Universidad Autónoma de Madrid)


Prof. Dimitri Arvanitis (Uppsala University) Mr. Francisco Jesús Luque (Universidad Autónoma de Madrid) Dr. Iwona A. Kowalik (Institute of Physics, Polish Academy of Sciences) Dr. Miguel Ángel Niño (IMDEA-Nanociencia)

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