What is it about?
Recent research on the so-called chirality-induced spin selectivity (CISS) effect has demonstrated that chiral materials (those that appear in two forms that are mirror images of each other) interact differently with electrons depending on the orientation of the latters’ spin moment. This work makes use of XPS, an electron spectroscopy technique with chemical sensitivity, to study the diffusion and growth of homochiral molecules deposited under ultra-high vacuum conditions on two types of substrates: one, non-magnetic copper, and the other a ferromagnetic film of Co with a different population of up and down spins within the surface plane. The observation of differences in the growth kinetics and the ability to cover the spin-polarized Co surface between the two chiral forms, or enantiomers, of the same molecule unequivocally reveals an interaction that is sensitive to the relative orientation between the molecular configuration and the alignment of the spins in the substrate.
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Why is it important?
Chirality is a very general property of both objects and phenomena, with implications in many different areas of science. In Physics for instance it is related to the concept of parity conservation and its violation in certain processes of radioactive disintegration. In Condensed Matter Physics this concept is receiving increasing attention due to its far-reaching implications related to topological properties or unconventional superconductivity, among others. One of the most suggestive manifestations of chiral asymmetry is found in living beings, which only admit one of the two possible chiral forms of certain types of molecules: left-handed (L-type) aminoacids and right-handed (D-type) sugars. The origin of this preference is not known, but it has a profound impact in the pharmacological, agricultural or livestock industries because the wrong enantiomer can have harmful effects on the organisms. The demonstration of enantioselective interaction between the molecules and the spin-polarized substrates can lead to more efficient methods to separate the useful enantiomer from the noxious one, and also eventually to the achievement of asymmetric heterogeneous catalysts capable of producing only the desired form of any particular molecule, with the subsequent reduction of cost and waste.
Perspectives
While the existence of some deep relationship between molecular chirality and spin polarization (the CISS effect) seems to be well established by now, the profound origin of this selectivity and its relationship with the atomic configuration of the molecules is not yet well understood. Beyond the direct demonstration of the different kinetics that chiral molecules display when diffusing over a spin-polarized substrate, this research suggests a possible method to systematically investigate the response of molecules with diverse configurations in contact with substrates magnetized along different directions (parallel, perpendicular to the surface) and shed light on the relationship between the molecular structure and the preferred orientation directions of the electronic spin.
Juan José de Miguel
Universidad Autonoma de Madrid
Read the Original
This page is a summary of: Enantiosensitive growth dynamics of chiral molecules on ferromagnetic substrates and the origin of the CISS effect, The Journal of Chemical Physics, September 2023, American Institute of Physics,
DOI: 10.1063/5.0160011.
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