| AVS 60th International Symposium and Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS-ThM |
| Session: | Chirality & Enantioselectivity on Surfaces |
| Presenter: | B. Goehler, University of Muenster, Germany |
| Authors: | B. Goehler, University of Muenster, Germany M. Kettner, University of Muenster, Germany T.Z. Markus, Weizmann Institute of Science, Israel R. Naaman, Weizmann Institute of Science, Israel H. Zacharias, University of Muenster, Germany |
| Correspondent: | Click to Email |
Since more than a decade the field of spintronics deals with the manipulation of the electron spin to facilitate electronic operations. For such devices spin-dependent electron transfer processes (namely spin filters) are needed, which are usually realized using either magnetic materials or systems containing heavy atoms to make use of spin-orbit coupling. Here we present spin-selective electron transmission through organic molecules with helical symmetry of quite high filtering efficiencies.
The work is based on “electron dichroism”, an effect that describes different interactions of longitudinally spin-polarized electrons with chiral molecules. Experiments in the mid-90s showed that spin-polarized electron beams, guided through vapor of chiral molecules, are attenuated differently, depending on the longitudinal spin polarization of the electrons and the enantiomer of the molecules.
Studying the transmission of low-energy photoelectrons through ordered self-assembled monolayers of chiral molecules on gold, an intensity dichroism was observed between the excitation with circularly polarized light of opposite helicity, which has been interpreted as a spin-dependent transmission through these ordered layers. We extend these studies by directly measuring the electron spin polarization using a calibrated Mott detector [1]. The observed spin selectivity at room temperature is extremely high as compared to other known spin filters. A systematic study on DNA monolayers shows that the spin filtration efficiency depends on the length and organization of the adsorbed molecules: single-stranded DNA molecules, which form a rather floppy instead of an ordered layer, show almost no spin-filtering effect. Recently, the spin transmission studies have been extended to bacteriorhodopsin membrane protein physisorbed on gold and aluminum surfaces. Spin polarization measurements yield up 15% spin polarization for transmitted electron ensembles with a minor dependency on the preparation scheme. Results obtained using an Al substrate yield that the high spin-orbit coupling of the Au substrate used in former experiments does not influence the effect. Because of the low atomic numbers of the constituents (P, C, H) of the organic molecules adsorbed on the surface, spin-orbit-coupling is not sufficient to explain the observation of the quite large spin-specific interaction. Even though very recently different models were published aiming to rationalize the observed effect, it is not understood so far.
[1] Science, 331, 894 (2011)