Universität zu Köln

Safari, Mohammad Reza (2025). Spin selectivity of chiral molecules on surfaces. PhD thesis, Universität zu Köln.

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Abstract

The purpose of this thesis is to investigate the spin selectivity of chiral molecules adsorbed onmagneticsubstrates, withtheaimofimprovingourunderstandingofthecomplexinter- actions between chirality and magnetism. This research centers on the chirality-induced spin selectivity (CISS) effect, an emerging phenomenon that has captured the interest of the scientific community due to its potential applications in spintronics, efficient quantum computing, enantioseparation, and selective chemical processes. Since the initial identi- fication of the CISS effect, extensive research on various molecules and substrates has yielded significant outcomes. Observations have been made that electrons transmitted through chiral molecules at room temperature exhibit spin polarizations exceeding sev- eral tens of percent. The findings also include the enantiospecific adsorption of chiral molecules on perpendicularly magnetized ferromagnetic substrates. This area of research, which explores both theoretical and experimental aspects of the CISS effect, remains a topic of scientific interest. Despite substantial experimental evi- dencesupportingCISS,acomprehensivetheoreticalunderstandingremainselusive. Present theoretical approaches often fail to bridge the gap between the magnitudes of experimen- tal results and theoretical predictions, such as spin polarization values or enantiospecific adsorption energies. A significant challenge in bridging experimental and theoretical studies stems from the complexity of real-world experiments. Often, these involve molecular ensembles and yield data that reflect averages of various configurations, such as adsorption sites and pathways of electric current. Additionally, experimental conditions may necessitate modifications likeprotectivecoatingstopreventoxidation(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 research project 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 and theoretical 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 substrates like 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 surfaces through sublimation under UHV conditions. Themoleculesremainstructurallyintactafterdeposition, withtheproximalphenanthrene group aligned parallel to the substrate surface. The high-resolution STM topography data 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: molecules undergo 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 principal aspects. 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 these molecules. In the experiment using a racemic mixture, a detailed statistical analysis of over 740 molecules across 110 islands revealed an enantiomeric excess ratio of 0.7. This ratio, 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 not related to differences in adsorption energy. This finding was further supported by sub- sequent experiments with more than 2100 molecules on 225 islands, using enantiopure molecules, demonstrating a consistent picture of enantioselectivity. The results of our experiment were then compared with state-of-the-art density functional theory (DFT) calculations performed by our colleagues at the Peter Grünberg Institute (PGI-1), Jülich Research Center. Interestingly, these advanced spin-resolved ab initio simulations showed no significant differences in enantio-dependent chemisorption ener- gies. This discrepancy between the experimental results and the simulations, along with further experimental findings that molecular mobility decreases significantly when reach- ingthechemisorbedstateonthecobaltislands, ledustohypothesizethatenantioselection primarily occurs during an earlier, physisorbed state. These observations suggest that van der Waals interactions, which are critical for molecular magnetochiral processes, should also 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 deposited on two distinct types of ferromagnetic bilayer substrates: racemic mixtures are deposited on Co/Cu(111), while enantiopure molecules are deposited on Fe/W(110). This experi- mentalapproachenablesthedirectmeasurementoftunnellingcurrentsthroughindividual molecules under precisely controlled conditions. In this setup, the magnetization direction of either the STM tip or the substrate can be systematically reversed, and the chirality of the molecules can be selectively chosen, especially in experiments involving racemic mixtures. This methodology enables accurate assessments of magnetochiral conductance asymmetry (MChA) by comparing the tunneling current measured through two different enantiomers depositedonthesamemagneticdomain. Additionally, itallowsfortheevaluationofenan- tiospecific magnetic conductance asymmetry (EMA) by comparing the tunneling current measured 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 islands reaching as high as 50%. This detailed investigation also allows us to effectively exclude ensemble effects and electron-phonon coupling as primary contributing factors, thereby taking a significant step forward in clarifying the underlying mechanisms influencing spin- selective transport through chiral molecules.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Safari, Mohammad Reza mrezasfr@gmail.com UNSPECIFIED UNSPECIFIED
Contributors:	Contribution Name Email Author Safari, Mohammad Reza mrezasfr@gmail.com
URN:	urn:nbn:de:hbz:38-751084
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Chirality, spin, chiral molecules, single molecule level English
Date of oral exam:	6 December 2024
Referee:	Name Academic Title Bürgler, Daniel E. PD Dr. Michely, Thomas Prof. Dr. Zacharias, Helmut Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75108