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@PHDTHESIS{Safari:1038874,
author = {Safari, Mohammad Reza},
title = {{S}pin selectivity of chiral molecules on surfaces},
volume = {108},
school = {Köln},
type = {Dissertation},
address = {Jülich},
publisher = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
reportid = {FZJ-2025-01689},
isbn = {978-3-95806-810-0},
series = {Schriften des Forschungszentrums Jülich Reihe Information
/ Information},
pages = {xiv, 165},
year = {2025},
note = {Dissertation, Köln, 2024},
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.},
cin = {PGI-6},
cid = {I:(DE-Juel1)PGI-6-20110106},
pnm = {5213 - Quantum Nanoscience (POF4-521)},
pid = {G:(DE-HGF)POF4-5213},
typ = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
doi = {10.34734/FZJ-2025-01689},
url = {https://juser.fz-juelich.de/record/1038874},
}
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