Chirality Meets Quantum Mechanics
Chirality or enantiomorphism describes the geometrical property of an object that it cannot be superimposed to its mirror image by any combination of rotations and translations. Chirality is omnipresent in nature and plays a major role for organic life. Enantioselectivity of reactions and interactions has always been considered as a purely geometrical effect.
Only a few years ago, an interaction between the handedness of chiral molecules and the quantum mechanical electron spin was reported, which is nowadays referred to as the chirality-induced spin selectivity (CISS). CISS is manifested by the observations that (i) photoemission or conduction electrons become spin-polarized when passing through a layer of enantiopure molecules and (ii) the adsorption of chiral molecules on magnetized surfaces is enantio-selective. The first effect allows the realization of purely organic and molecule-sized sources of spin-polarized currents that reach several 10% polarization at room temperature without relying on ferromagnetic materials. The second effect promises new and highly efficient methods for the separation or selection of enantiomers, which are technologically extremely important but have been very laborious until now. Any technology that takes advantage of controlling spin currents, including quantum information technology and spin chemistry, is likely to benefit from CISS.
By now, CISS effects are experimentally well established and have been observed for various classes of chiral molecules and with a wide range of measurement techniques. Surprisingly, there is as yet no clear theoretical understanding of the CISS effects. A natural starting point to explain an interaction between spin and the chiral structure in real space is spin-orbit coupling (SOC). Indeed, models based on conventional SOC in organic molecules predict CISS, but the effect sizes are far too small compared to experiments.
Obviously, a deeper understanding of the CISS effects in various molecules and experimental situations requires detailed knowledge of the respective electronic structure and its impact on spin-dependent transport and photoemission or on the enantiospecific interaction with ferromagnetic surfaces. This is where the research at PGI-6 comes in.
Spin-dependent scanning tunnelling microscopy and spectroscopy (SP-STM/STS) offer the possibility to simultaneously characterize the molecule-substrate configuration on the atomic scale and to access the spin-resolved electronic structure near the Fermi edge via spin-dependent (tunnelling) transport. We apply SP-STM/STS under ultra-high-vacuum conditions and at low temperature (4 K) to individual model molecules and self-assembled monolayers thereof, both deposited onto well-defined, crystalline non-magnetic (Cu, Au) and ferromagnetic (Fe, Co) surfaces. These extremely well-defined conditions will help to bridge the gap between theoretical models and experiments on CISS reported to date.
We study molecules with a helical framework, such as helicenes. Preliminary results on heptahelicene (H) indicate that this molecule can be sublimed ultra-high vacuum (UHV) onto highly reactive Co bilayer nanoislands on Cu(111) and Fe bilayers on W(110), which both are ferromagnetic and magnetized out-of-plane even in the presence of H molecules. The molecules remain intact and adsorb in a flat configuration with specific in-plane orientations, indicating unique adsorption sites for both substrates. Furthermore, we are able to determine the handedness of individual molecules chemisorbed on both surfaces The demonstrated deposition control and STM/STS imaging capabilities for heptahelicene on Co/Cu(111) and Fe/W(110) substrate systems lay the foundation for studying the CISS effect in ultra-high vacuum and on the microscopic level of single molecules in controlled atomic configurations.