Enantiospecific Adsorption on a Ferromagnetic Surface at the Single-Molecule Scale
Daniel E. Bürgler and Mohammad Reza Safari
Peter Grünberg Institute, Electronic Properties (PGI-6), Forschungszentrum Jülich, Germany
Chirality-induced spin selectivity (CISS), which results from the interaction of electron spin and handedness of chiral molecules, has attracted much attention in recent years and is expected to lead to a chirality-based quantum leap in quantum sciences . CISS manifests itself in the generation of spin polarization when electrons pass through chiral molecules  and enantiospecific adsorption of chiral molecules on perpendicularly magnetized ferromagnetic surfaces . Therefore, surface-adsorbed chiral molecules represent a promising approach for potential applications in spintronics and enantioselective chemistry.
Here, I report detailed growth studies of chiral heptahelicene (H) molecules on Co bilayer nanoislands on Cu(111) by low-temperature spin-polarized scanning tunneling microscopy (STM) in ultra-high vacuum (UHV)  and give unequivocal evidence for enantiospecific adsorption at the single-molecule scale . The sublimed molecules remain intact and adsorb at RT at specific sites with the proximal phenanthrene group aligned parallel to the surface. This enables determination of the handedness of each molecule (represented by the helix pitch vector h, see Figure) from topographic STM images , while simultaneously measured spin-polarized dI/dV maps reveal the magnetization direction M of the underlying Co nanoislands. Analysis of more than 700 molecules on 110 islands yields the enantiomeric adsorption imbalance R for H on Co, i.e., the ratio of the number of enantiomers with h parallel versus antiparallel to M. R = 0.69±0.05 corresponds via a Boltzmann factor to an enantiospecific energy difference of DE = (9±2) meV. I will discuss the interpretation of DE in the context of precursormediated chemisorption, where enantiospecific van der Waals interactions may play a decisive role. Our well-defined surface science approach in UHV excludes external influences, such as undesired coadsorption of other species or the need for a gold capping layer to prevent oxidation, and moreover shows that enantiospecificity is due to single-molecule properties rather than ensemble or cooperative effects. The model system H on Co islands should be well suited for theoretical analysis and modeling that will shed light on the microscopic origin of enantiospecific adsorption on ferromagnetic surfaces.
Figure: (P)-H and (M)-H enantiomers (green and red helix pitch vector h, respectively) are adsorbed on perpendicularly magnetized Co nanoislands (yellow and blue magnetization vector M). Statistical analysis yields an adsorption imbalance R between h parallel to M versus h antiparallel to M.
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