@phdthesis{Voroshnin2023, author = {Voroshnin, Vladimir}, title = {Control over spin and electronic structure of MoS₂ monolayer via interactions with substrates}, doi = {10.25932/publishup-59070}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-590709}, school = {Universit{\"a}t Potsdam}, pages = {viii, 125}, year = {2023}, abstract = {The molybdenum disulfide (MoS2) monolayer is a semiconductor with a direct bandgap while it is a robust and affordable material. It is a candidate for applications in optoelectronics and field-effect transistors. MoS2 features a strong spin-orbit coupling which makes its spin structure promising for acquiring the Kane-Mele topological concept with corresponding applications in spintronics and valleytronics. From the optical point of view, the MoS2 monolayer features two valleys in the regions of K and K' points. These valleys are differentiated by opposite spins and a related valley-selective circular dichroism. In this study we aim to manipulate the MoS2 monolayer spin structure in the vicinity of the K and K' points to explore the possibility of getting control over the optical and electronic properties. We focus on two different substrates to demonstrate two distinct routes: a gold substrate to introduce a Rashba effect and a graphene/cobalt substrate to introduce a magnetic proximity effect in MoS2. The Rashba effect is proportional to the out-of-plane projection of the electric field gradient. Such a strong change of the electric field occurs at the surfaces of a high atomic number materials and effectively influence conduction electrons as an in-plane magnetic field. A molybdenum and a sulfur are relatively light atoms, thus, similar to many other 2D materials, intrinsic Rashba effect in MoS2 monolayer is vanishing small. However, proximity of a high atomic number substrate may enhance Rashba effect in a 2D material as it was demonstrated for graphene previously. Another way to modify the spin structure is to apply an external magnetic field of high magnitude (several Tesla), and cause a Zeeman splitting, the conduction electrons. However, a similar effect can be reached via magnetic proximity which allows us to reduce external magnetic fields significantly or even to zero. The graphene on cobalt interface is ferromagnetic and stable for MoS2 monolayer synthesis. Cobalt is not the strongest magnet; therefore, stronger magnets may lead to more significant results. Nowadays most experimental studies on the dichalcogenides (MoS2 included) are performed on encapsulated heterostructures that are produced by mechanical exfoliation. While mechanical exfoliation (or scotch-tape method) allows to produce a huge variety of structures, the shape and the size of the samples as well as distance between layers in heterostructures are impossible to control reproducibly. In our study we used molecular beam epitaxy (MBE) methods to synthesise both MoS2/Au(111) and MoS2/graphene/Co systems. We chose to use MBE, as it is a scalable and reproducible approach, so later industry may adapt it and take over. We used graphene/cobalt instead of just a cobalt substrate because direct contact of MoS2\ monolayer and a metallic substrate may lead to photoluminescence (PL) quenching in the metallic substrate. Graphene and hexagonal boron nitride monolayer are considered building blocks of a new generation of electronics also commonly used as encapsulating materials for PL studies. Moreover graphene is proved to be a suitable substrate for the MBE growth of transitional metal dichalcogenides (TMDCs). In chapter 1, we start with an introduction to TMDCs. Then we focus on MoS2 monolayer state of the art research in the fields of application scenario; synthesis approaches; electronic, spin, and optical properties; and interactions with magnetic fields and magnetic materials. We briefly touch the basics of magnetism in solids and move on to discuss various magnetic exchange interactions and magnetic proximity effect. Then we describe MoS2 optical properties in more detail. We start from basic exciton physics and its manifestation in the MoS2 monolayer. We consider optical selection rules in the MoS2 monolayer and such properties as chirality, spin-valley locking, and coexistence of bright and dark excitons. Chapter 2 contains an overview of the employed surface science methods: angle-integrated, angle-resolved, and spin-resolved photoemission; low energy electron diffraction and scanning tunneling microscopy. In chapter 3, we describe MoS2 monolayer synthesis details for two substrates: gold monocrystal with (111) surface and graphene on cobalt thin film with Co(111) surface orientation. The synthesis descriptions are followed by a detailed characterisation of the obtained structures: fingerprints of MoS2 monolayer formation; MoS2 monolayer symmetry and its relation to the substrate below; characterisation of MoS2 monolayer coverage, domain distribution, sizes and shapes, and moire structures. In chapter~4, we start our discussion with MoS2/Au(111) electronic and spin structure. Combining density functional theory computations (DFT) and spin-resolved photoemission studies, we demonstrate that the MoS2 monolayer band structure features an in-plane Rashba spin splitting. This confirms the possibility of MoS2 monolayer spin structure manipulation via a substrate. Then we investigate the influence of a magnetic proximity in the MoS2/graphene/Co system on the MoS2 monolayer spin structure. We focus our investigation on MoS2 high symmetry points: G and K. First, using spin-resolved measurements, we confirm that electronic states are spin-split at the G point via a magnetic proximity effect. Second, combining spin-resolved measurements and DFT computations for MoS2 monolayer in the K point region, we demonstrate the appearance of a small in-plane spin polarisation in the valence band top and predict a full in-plane spin polarisation for the conduction band bottom. We move forward discussing how these findings are related to the MoS2 monolayer optical properties, in particular the possibility of dark exciton observation. Additionally, we speculate on the control of the MoS2 valley energy via magnetic proximity from cobalt. As graphene is spatially buffering the MoS2 monolayer from the Co thin film, we speculate on the role of graphene in the magnetic proximity transfer by replacing graphene with vacuum and other 2D materials in our computations. We finish our discussion by investigating the K-doped MoS2/graphene/Co system and the influence of this doping on the electronic and spin structure as well as on the magnetic proximity effect. In summary, using a scalable MBE approach we synthesised MoS2/Au(111) and MoS2/graphene/Co systems. We found a Rashba effect taking place in MoS2/Au(111) which proves that the MoS2 monolayer in-plane spin structure can be modified. In MoS2/graphene/Co the in-plane magnetic proximity effect indeed takes place which rises the possibility of fine tuning the MoS2 optical properties via manipulation of the the substrate magnetisation.}, language = {en} } @article{PatelNoackVacogneetal.2019, author = {Patel, Dhananjay I. and Noack, Sebastian and Vacogne, Charlotte D. and Schlaad, Helmut and Bahr, Stephan and Dietrich, Paul and Meyer, Michael and Thissen, Andreas and Linford, Matthew R.}, title = {Poly(L-lactic acid), by near-ambient pressure XPS}, series = {Surface Science Spectra}, volume = {26}, journal = {Surface Science Spectra}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {1055-5269}, doi = {10.1116/1.5110309}, pages = {8}, year = {2019}, abstract = {Near ambient pressure - x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at 2500Pa or higher. With NAP-XPS, one can analyze moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission we show C 1s, O 1s, and survey NAP-XPS spectra from poly(L-lactic acid). The C 1s and O 1s envelopes were fit with three and two Gaussian-Lorentzian sum functions, respectively. Water vapor (800Pa) was used as the residual gas for charge compensation, which was confirmed by the sharp peak at 535.0 eV in the O 1s narrow scan. The uniqueness plot corresponding to the C 1s fit shows that the fit parameters had statistical significance. C 1s and O 1s spectra of PLLA damaged by exposure to x-rays for ca. 1 hour are also included. Published by the AVS.}, language = {en} } @article{MadaanRomriellTuscanoetal.2015, author = {Madaan, Nitesh and Romriell, Naomi and Tuscano, Joshua and Schlaad, Helmut and Linford, Matthew R.}, title = {Introduction of thiol moieties, including their thiol-ene reactions and air oxidation, onto polyelectrolyte multilayer substrates}, series = {Journal of colloid and interface science}, volume = {459}, journal = {Journal of colloid and interface science}, publisher = {Elsevier}, address = {San Diego}, issn = {0021-9797}, doi = {10.1016/j.jcis.2015.08.017}, pages = {199 -- 205}, year = {2015}, language = {en} } @article{KocSchardtNolteetal.2020, author = {Koc, Julian and Schardt, Lisa and Nolte, Kim and Beyer, Cindy and Eckhard, Till and Schwiderowski, Philipp and Clarke, Jessica L. and Finlay, John A. and Clare, Anthony S. and Muhler, Martin and Laschewsky, Andr{\´e} and Rosenhahn, Axel}, title = {Effect of dipole orientation in mixed, charge-equilibrated self-assembled monolayers on protein adsorption and marine biofouling}, series = {ACS applied materials \& interfaces}, volume = {12}, journal = {ACS applied materials \& interfaces}, number = {45}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.0c11580}, pages = {50953 -- 50961}, year = {2020}, abstract = {While zwitterionic interfaces are known for their excellent low-fouling properties, the underlying molecular principles are still under debate. In particular, the role of the zwitterion orientation at the interface has been discussed recently. For elucidation of the effect of this parameter, self-assembled monolayers (SAMs) on gold were prepared from stoichiometric mixtures of oppositely charged alkyl thiols bearing either a quaternary ammonium or a carboxylate moiety. The alkyl chain length of the cationic component (11-mercaptoundecyl)-N,N,N-trimethylammonium, which controls the distance of the positively charged end group from the substrate's surface, was kept constant. In contrast, the anionic component and, correspondingly, the distance of the negatively charged carboxylate groups from the surface was varied by changing the alkyl chain length in the thiol molecules from 7 (8-mercaptooctanoic acid) to 11 (12-mercaptododecanoic acid) to 15 (16-mercaptohexadecanoic acid). In this way, the charge neutrality of the coating was maintained, but the charged groups exposed at the interface to water were varied, and thus, the orientation of the dipoles in the SAMs was altered. In model biofouling studies, protein adsorption, diatom accumulation, and the settlement of zoospores were all affected by the altered charge distribution. This demonstrates the importance of the dipole orientation in mixed-charged SAMs for their inertness to nonspecific protein adsorption and the accumulation of marine organisms. Overall, biofouling was lowest when both the anionic and the cationic groups were placed at the same distance from the substrate's surface.}, language = {en} } @article{JainWheelerEssetal.2019, author = {Jain, Varun and Wheeler, Joshua J. and Ess, Daniel H. and Noack, Sebastian and Vacogne, Charlotte D. and Schlaad, Helmut and Bahr, Stephan and Dietrich, Paul and Meyer, Michael and Thissen, Andreas and Linford, Matthew R.}, title = {Poly(gamma-benzyl l-glutamate), by near-ambient pressure XPS}, series = {Surface science spectra : SSS : an international journal \& database devoted to archiving spectra from surfaces \& interfaces}, volume = {26}, journal = {Surface science spectra : SSS : an international journal \& database devoted to archiving spectra from surfaces \& interfaces}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {1055-5269}, doi = {10.1116/1.5109121}, pages = {10}, year = {2019}, abstract = {Near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i. e., at greater than 2500 Pa. In this study, poly(.- benzyl L- glutamate) (PBLG) with a molar mass of 11.3 kg/mol was analyzed by NAP-XPS; here, we show the survey, C 1s, N 1s, and O 1s narrow scans of PBLG. The C 1s peak envelope was fitted in three different ways, to five, six, or seven synthetic peaks. In each fit, there was also a shake-up signal. The O 1s narrow scan was well fit with three peaks: CZO and CvO in a 1:2 ratio from the polymer, and a higher energy signal from water vapor. Hartree-Fock orbital energies of a model monomer served as a guide to an additional fit of the C 1s envelope.}, language = {en} } @phdthesis{Ehlert2016, author = {Ehlert, Christopher}, title = {Simulationen von R{\"o}ntgenabsorptionsprozessen zur Charakterisierung von Systemen in kondensierter Phase}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-104844}, school = {Universit{\"a}t Potsdam}, pages = {142}, year = {2016}, abstract = {Die vorgelegte Dissertation pr{\"a}sentiert wissenschaftliche Ergebnisse, die in der Zeit vom Dezember 2012 bis August 2016, erarbeitet wurden. Der zentrale Inhalt der Arbeit ist die Simulation von R{\"o}ntgenabsorptionsprozessen von verschiedenen Systemen in kondensierter Phase. Genauer gesagt, werden Nahkantenabsorptions- (NEXAFS) sowie R{\"o}ntgenphotoelektronenspektren (XPS) berechnet. In beiden F{\"a}llen wird ein R{\"o}ntgenphoton von einem molekularen System absorbiert. Aufgrund der hohen Photonenenergie wird ein stark gebundenes kernnahes Elektron angeregt. Bei der XPS gelangt dieses mit einer zu messenden kinetischen Energie in Kontinuumszust{\"a}nde. In Abh{\"a}ngigkeit der eingestrahlten Photonenenergie und der kinetischen Energie des austreten Elektrons, kann die Bindungsenergie berechnet werden, welche die zentrale Gr{\"o}ße der XPS ist. Im Falle der NEXAFS-Spektroskopie wird das kernnahe Elektron in unbesetzte gebundene Zust{\"a}nde angeregt. Die zentrale Gr{\"o}ße ist die Absorption als Funktion der eingestrahlten Photonenenergie. Das erste Kapitel meiner Arbeit er{\"o}rtert detailliert die experimentellen Methoden sowie die daraus gewonnenen charakteristischen Gr{\"o}ßen. Die experimentellen Spektren zeigen oft viele Resonanzen, deren Interpretation aufgrund fehlender Referenzmaterialien schwierig ist. In solchen F{\"a}llen bietet es sich an, die Spektren mittels quantenchemischer Methoden zu simulieren. Der daf{\"u}r erforderliche mathematisch-physikalische Methodenkatalog wird im zweiten Kapitel der Arbeit er{\"o}rtert. Das erste von mir untersuchte System ist Graphen. In experimentellen Arbeiten wurde die Oberfl{\"a}che mittels Bromplasma modifiziert. Die im Anschluss gemessenen NEXAFS-Spektren unterscheiden sich maßgeblich von den Spektren der unbehandelten Oberfl{\"a}che. Mithilfe periodischer DFT-Rechnungen wurden verschiedene Gitterdefekte sowie bromierte Systeme untersucht und die NEXAFS-Spektren simuliert. Mittels der Simulationen k{\"o}nnen die Beitr{\"a}ge verschiedener Anregungszentren analysiert werden. Die Berechnungen erlauben den Schluss, dass Gitterdefekte maßgeblich f{\"u}r die entstandenen Ver{\"a}nderungen verantwortlich sind. Polyvinylalkohol (PVA) wurde als zweites System behandelt. Hierbei sollte untersucht werden, wie groß der Einfluss der Molekularbewegung auf die Verbreiterung der Peaks im XP-Spektrum ist. Des Weiteren wurde untersucht, wie groß der Einfluss von intermolekularen Wechselwirkungen auf die Peakpositionen und Peakverbreiterung ist. F{\"u}r die Berechnung dieses Systems wurde eine Kombination aus molekulardynamischen und quantenchemischen Methoden verwendet. Als Strukturen dienten Oligomermodelle, die unter dem Einfluss eines (ab initio) Potentials propagiert wurden. Entlang der erstellten Trajektorie wurden Schnappsch{\"u}sse der Geometrien extrahiert und f{\"u}r die Berechnung der XP-Spektren verwendet. Die Spektren werden bereits mithilfe klassischer Molekulardynamik sehr gut reproduziert. Die erhaltenen Peakbreiten sind verglichen mit dem Experiment allerdings zu klein. Die Hauptursache der Peakverbreiterung ist die Molekularbewegung. Intermolekulare Wechselwirkungen verschieben die Peakpositionen um 0.6 eV zu kleineren Anregungsenergien. Im dritten Teil der Arbeit stehen die NEXAFS-Spektren von ionischen Fl{\"u}ssigkeiten (ILs) im Fokus. Die experimentell gefundenen Spektren zeigen eine komplexe Struktur mit vielen Resonanzen. In der Arbeit wurden zwei ILs untersucht. Als Geometrien verwenden wir Clustermodelle, die aus experimentellen Kristallstrukturen extrahiert wurden. Die berechneten Spektren erlauben es, die Resonanzen den Anregungszentren zuzuordnen. Außerdem kann eine erstmals gemessene Doppelresonanz simuliert und erkl{\"a}rt werden. Insgesamt kann die Interpretation der Spektren mithilfe der Simulation signifikant erweitert werden. In allen Systemen wurde zur Berechnung des NEXAFS-Spektrums eine auf Dichtefunktionaltheorie basierende Methode verwendet (die sogenannte Transition-Potential Methode). G{\"a}ngige wellenfunktionsbasierte Methoden, wie die Konfigurationswechselwirkung mit Einfachanregungen (CIS), zeigen eine starke Blauverschiebung, wenn als Referenz eine Hartree-Fock Slaterdeterminante verwendet wird. Wir zeigen, dass die Verwendung von kernnah-angeregten Determinanten sowohl das resultierende Spektrum als auch die Anregungsenergien deutlich verbessert. Des Weiteren werden auch Referenzen aus Dichtefunktionalrechnungen getestet. Zus{\"a}tzlich werden auch Referenzen mit gebrochenen Besetzungszahlen f{\"u}r kernnahe Elektronen verwendet. In der Arbeit werden die Resultate der verschiedenen Referenzen miteinander verglichen. Es zeigt sich, dass Referenzen mit gebrochenen Besetzungszahlen das Spektrum nicht weiter verbessern. Der Einfluss der verwendeten Elektronenstrukturmethode ist eher gering.}, language = {de} }