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| Book/Dissertation / PhD Thesis | FZJ-2025-01689 |
2025
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-810-0
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Please use a persistent id in citations: doi:10.34734/FZJ-2025-01689
Abstract: The purpose of this thesis is to investigate the spin selectivity of chiral molecules adsorbedonmagneticsubstrates, withtheaimofimprovingourunderstandingofthecomplexinter-actions between chirality and magnetism. This research centers on the chirality-inducedspin selectivity (CISS) effect, an emerging phenomenon that has captured the interest ofthe scientific community due to its potential applications in spintronics, efficient quantumcomputing, enantioseparation, and selective chemical processes. Since the initial identi-fication of the CISS effect, extensive research on various molecules and substrates hasyielded significant outcomes. Observations have been made that electrons transmittedthrough chiral molecules at room temperature exhibit spin polarizations exceeding sev-eral tens of percent. The findings also include the enantiospecific adsorption of chiralmolecules on perpendicularly magnetized ferromagnetic substrates.This area of research, which explores both theoretical and experimental aspects of theCISS effect, remains a topic of scientific interest. Despite substantial experimental evi-dencesupportingCISS,acomprehensivetheoreticalunderstandingremainselusive. Presenttheoretical approaches often fail to bridge the gap between the magnitudes of experimen-tal results and theoretical predictions, such as spin polarization values or enantiospecificadsorption energies.A significant challenge in bridging experimental and theoretical studies stems from thecomplexity of real-world experiments. Often, these involve molecular ensembles and yielddata that reflect averages of various configurations, such as adsorption sites and pathwaysof electric current. Additionally, experimental conditions may necessitate modificationslikeprotectivecoatingstopreventoxidation(e.g., Aucoatingonferromagneticsubstrates)and can introduce other variables such as water from ambient humidity. Meanwhile, the-oretical models typically overlook these complexities. In response, this PhD researchproject is designed to thoroughly investigate the CISS effect under ultra-high vacuum(UHV) conditions, with precise control over geometric configurations at the atomic scale.This approach could facilitate a closer alignment between experimental observations andtheoretical calculations. This thesis investigates chiral heptahelicene (7[H]) molecules adsorbed on various single-crystalline substrates, ranging from the noble metal Cu(111) to more reactive substrateslike ferromagnetic Co bilayer nanoislands on Cu(111) and Fe bilayers on W(110). Low-temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy(SP-STS) are employed to examine molecules that have been deposited on these surfacesthrough sublimation under UHV conditions.Themoleculesremainstructurallyintactafterdeposition, withtheproximalphenanthrenegroup aligned parallel to the substrate surface. The high-resolution STM topographydata provide a precise method for accurately determining the chirality of individual hep-tahelicene molecules on these crystalline substrates. Additionally, detailed SP-STS mea-surements from individual molecules indicate distinct adsorption properties: moleculesundergo physisorption on Cu(111) and chemisorption on Co and Fe bilayers. Notably,both Co and Fe bilayers maintain their ferromagnetic properties and exhibit out-of-plane(OOP) magnetization.Building on our preliminary observations, our investigation delved deeper into the in-teractions between chiral molecules and magnetic substrates, focusing on two principalaspects.First, we explored the enantiospecific adsorption of [7]H molecules onto OOP magne-tized cobalt nanoislands, using both racemic and enantiopure samples. Our investiga-tions revealed a magnetization-dependent enantiomeric excess upon deposition of thesemolecules. In the experiment using a racemic mixture, a detailed statistical analysis ofover 740 molecules across 110 islands revealed an enantiomeric excess ratio of 0.7. Thisratio, when expressed by a Boltzmann factor, corresponds to an energy difference of ap-proximately 10 ±2 meV. Further experiments revealed that this energy difference is notrelated to differences in adsorption energy. This finding was further supported by sub-sequent experiments with more than 2100 molecules on 225 islands, using enantiopuremolecules, demonstrating a consistent picture of enantioselectivity.The results of our experiment were then compared with state-of-the-art density functionaltheory (DFT) calculations performed by our colleagues at the Peter Grünberg Institute(PGI-1), Jülich Research Center. Interestingly, these advanced spin-resolved ab initiosimulations showed no significant differences in enantio-dependent chemisorption ener-gies. This discrepancy between the experimental results and the simulations, along withfurther experimental findings that molecular mobility decreases significantly when reach-ingthechemisorbedstateonthecobaltislands, ledustohypothesizethatenantioselectionprimarily occurs during an earlier, physisorbed state. These observations suggest that van der Waals interactions, which are critical for molecular magnetochiral processes, shouldalso take spin fluctuations into account.Secondly, spin-selective electron transport through chiral [7]H molecules at a low temper-ature of 5K is investigated using a spin-sensitive STM tip. These molecules are depositedon two distinct types of ferromagnetic bilayer substrates: racemic mixtures are depositedon Co/Cu(111), while enantiopure molecules are deposited on Fe/W(110). This experi-mentalapproachenablesthedirectmeasurementoftunnellingcurrentsthroughindividualmolecules under precisely controlled conditions. In this setup, the magnetization directionof either the STM tip or the substrate can be systematically reversed, and the chiralityof the molecules can be selectively chosen, especially in experiments involving racemicmixtures.This methodology enables accurate assessments of magnetochiral conductance asymmetry(MChA) by comparing the tunneling current measured through two different enantiomersdepositedonthesamemagneticdomain. Additionally, itallowsfortheevaluationofenan-tiospecific magnetic conductance asymmetry (EMA) by comparing the tunneling currentmeasured through molecules of the same handedness on two magnetic domains with op-posing OOP magnetization. As a result, there is significant conductance asymmetry for[7]H molecules across both types of magnetic substrates, with EMA values on Co islandsreaching as high as 50%. This detailed investigation also allows us to effectively excludeensemble effects and electron-phonon coupling as primary contributing factors, therebytaking a significant step forward in clarifying the underlying mechanisms influencing spin-selective transport through chiral molecules.
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