This thesis reports on the elemental growth of oxide thin films including TiO_x, BaO_x and Ba_xTi_yO_z by Ti/Ba deposition and oxidation. The films were grown on two different substrates, Au(111) and SrTiO₃(001), and studied using a variety of surface characterisation techniques. On the reconstructed Au(111) surface, three different TiO_x structures were obtained with increasing Ti amounts deposited: a (2 × 2) Ti₂O₃ honeycomb structure, a pinwheel structure that is the result of a Moiré pattern, and a triangular island TiO_1.30 structure. The structures arise from raised Ti coverages and have increased Ti densities. Although Ba deposited on the reconstructed Au(111) has a weak interaction with the substrate, the BaO_x thin films can grow epitaxially and lift the Au(111) reconstruction. Two well-ordered phases, a (6 × 6) and a (2√3 × 2√3) BaO_x structure, were obtained which may have octopolar-based surface structures. For Ba & Ti deposition on Au(111), a locally ordered (5 × 5) BaxTiyOz structure was observed in the sub-monolayer regime. What is more interesting is the possible formation of a BaO-TiO surface alloy with short-range ordering achieved by Ba deposition on the (2 × 2) Ti₂O₃-templated Au(111) surface. This is the first time that surface-alloying has been observed for oxides. When Ti is deposited onto the SrTiO₃(001) surface, it is incorporated into the substrate by forming a variety of Ti-rich SrTiO₃ surface reconstructions, such as c(4 × 2), (6 × 2), (9 × 2) and (√5 ×√5)-R26.6°. Ti deposition provides a completely different route to obtaining these reconstructions at much lower anneal temperatures than the previously reported preparation procedures involving sputtering and annealing the SrTiO₃ sample. Anatase islands with (1 × 3) and (1 × 5) periodicities were also formed by increasing the Ti deposition amount and post-annealing. Reconstructed SrTiO₃ substrate surface has a lattice that differs from the bulk crystal and affects the epitaxial growth of BaO, however, a locally ordered BaO_x structure was observed on the sputtered substrate with a growth temperature of 300 °C. Depositing Ba & Ti on SrTiO₃(001) results in the formation of BaO_x clusters and the Ti incorporation into the substrate, forming the familiar Ti-rich SrTiO₃ surface reconstructions.

Unter Verwendung von Dichtefunktionaltheorie werden die katalytischen Eigenschaften von Cerdioxidoberflächen mit verschiedenen Terminierungen untersucht. Cerdioxid wird auch als Trägermaterial in der heterogenen Katalyse eingesetzt, um Aktivität, Selektivität und Stabilität der aktiven Komponente zu erhöhen. In dieser Arbeit werden geträgerte Vanadiumoxidcluster diskutiert. Dabei wird die oxidative Dehydrierung von Methanol als Modellreaktion zur Aktivierung von C-H-Bindungen genutzt. Ceroxidpartikel werden oft in wässriger Lösung synthetisiert. Damit hängt die Form der Nanokristallite direkt von der relativen Stabilität der unterschiedlichen Terminierungen in der Gegenwart von Wasser ab. Außerdem ist Wasser an zahlreichen Reaktionen entweder als Produkt, Edukt oder Lösungsmittel beteiligt. Aus diesem Grund werden auch die Wasser-Oberflächenwechselwirkungen untersucht. Des Weiteren wird die Genauigkeit von drei verschiedenen Funktionalen (B3LYP, HSE und PBE+U) durch den Vergleich mit experimentellen Daten evaluiert. Diese beinhalten Barrieren, die mittels Temperatur-programmierter Desorptionsspektroskopie erhalten wurden, und Schwingungsspektren. / Density functional theory is applied to study the catalytic properties of ceria surfaces with different terminations. Ceria is also used as a support material in heterogeneous catalysis to improve activity, selectivity, and stability of the active component. In this work, supported vanadia clusters are discussed. The oxidative dehydrogenation of methanol is used as a model reaction for C–H bond activation. Ceria catalysts are often prepared in aqueous solution. As a result, the shape of ceria nanocrystallites depends on the relative stability of the different surface terminations in the presence of water. Furthermore, many reactions involve water either as a product, as a reagent, or as a solvent. Hence, water–surface interactions are studied as well. Furthermore, the accuracy of three different functionals (B3LYP, HSE, and PBE+U) is assessed by comparison to experimental data such as barriers obtained via temperature-programmed desorption and infrared spectra.

Diese Arbeit ist ein Beitrag zur theoretischen Chemie sowie zur Oberflächenchemie. Durch Kombination von computergestützten und experimentellen Untersuchungen wird die atomare Struktur von dünnen SiO2-Filmen auf Ru(0001)-Unterlagen, von eisendotierten SiO2-Filmen auf diesen Unterlagen und von H2O-Filmen auf MgO(001)-Oberflächen bestimmt. Die atomaren Strukturmodelle wurden entweder mit dem neu entworfenen und im Paket DoDo implementierten genetischen Algorithmus oder mittels auf Sachkenntnis gestützter Vermutungen erhalten. Die simulierten Eigenschaften der so erhaltenen Strukturen stimmen sehr gut mit den experimentellen Daten (Raster-Tunnel-Mikroskopie, Infrarot-Spektroskopie) überein. Die erfolgreiche Strukturbestimmung mithilfe des DoDo-Programms zeigt, dass genetische Algorithmen zur systematischen und extensiven Erkundung der Energielandschaften 2D-periodischer Systeme geeignet sind. / This work is a contribution in the field of theoretical chemistry and surface science. The joint computational and experimental studies investigated the atomic structure of ultrathin silica and iron-doped silica films formed on the Ru(0001) surface and water films formed on the MgO(001) surface. The atomic structure models were obtained using either the educated guess approach or the genetic algorithm that was designed and implemented within the DoDo package. The properties simulated for the resulting models are in a very good agreement with the experimental data (scanning tunnelling microscopy, infrared spectroscopy). The successful structure determination using the DoDo program shows that the genetic algorithm technique is capable of systematic and extensive exploration of the energy landscapes for 2D-periodic systems.

No description available.

Core- and Valence Photoelectron Spectroscopy (PES), X-ray- and Ultraviolet-Visible Absorption Spectroscopy (XAS and UV-Vis), Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) calculations are used to study the electronic and geometrical structure of a class of macro-cyclic molecules, Phthalocyanines (Pc), on surfaces. These molecules are widely studied due to their application in many different fields. Multilayer and monolayer coverages of Iron Phthalocyanine (FePc) and metal-free Phthalocyanine (H2Pc) deposited on different surfaces are investigated in order to get insight in the electronic and geometrical structure of the obtained overlayers, of crucial importance for the understanding of the film functionality. Sublimation of molecular thick films on Si(100) and on conducting glass results in films with molecules mainly oriented with their molecular plane orthogonal to the surface. Ex-situ deposited H2Pc films on conductive glass show different molecular orientation and morphology with respect to the vacuum sublimated films. We study the monolayer adsorption structure of FePc and H2Pc and compare our results with other Pc’s adsorbed on graphite. We find that the molecular unit cell and the superstructure is characteristic for each Pc adsorbed on graphite, even if the geometrical size of the compared molecules is the same. The PE- and XA- spectra of FePc on graphite are essentially identical for the mono- and multilayer preparations, evidencing weak intermolecular and molecular-substrate interactions of van der Waals nature. Furthermore, we characterize Pc’s on InSb (001)-c(8x2). The substrate In rows are observed to be the adsorption site for Pc’s. We find that the growth of the two-dimensional islands of FePc is prolonged in the [-110] direction, in contrast to ZnPc adsorbed on the same substrate at room temperature. We interpret this result as an indication that the adsorption is controlled by the substrate corrugation observed at 70 K.

No description available.

NaCl et KCl sont des matériaux à large bande interdite d’énergie, qui sont de plus en plus utilisés sous forme de couches minces en sciences des surfaces et dans des études de spectroscopie de molécules individuelles, pour découpler électroniquement des molécules organiques d’un substrat métallique. De plus, la réactivité de NaCl et KCl, sous forme de cristaux massifs, sous irradiation électronique ou photonique, a été beaucoup étudiée depuis les années 1970s. Dans ce mémoire de thèse, nous présentons les résultats d’une étude détaillée de couches minces de NaCl et de KCl sur Ag(001) par microscopie et spectroscopie à effet tunnel (STM/STS), diffraction des électrons lents (LEED) et spectroscopie d’électrons Auger (AES) sous ultravide. Afin d’obtenir des couches minces de haute qualité sur une surface métallique et de contrôler les propriétés de ces couches (épaisseur, taille et orientation des domaines, etc.), nous étudions en détails la formation de la couche en fonction des paramètres de croissance. De plus, les mesures de topographie STM montrent que l’épaisseur apparente et le contraste STM des couches isolantes dépendent de la différence de potentiel pointe-surface. Nous décrivons les modifications induites par irradiation électronique d’halogénures d’alcalins dans le régime de couches ultraminces. Les cinétiques et produits de réaction sont examinés dans le cas de couches de NaCl sur Ag(001). Les modifications structurales et chimiques sous faisceau d’électrons d’énergie 52-60 eV et 3 keV sont étudiées en utilisant respectivement le LEED et l’AES. Les effets de l’irradiation sur la géométrie et l’épaisseur des couches (allant de 2 à 5 couches atomiques) sont mesurés en STM. Nous observons que la déplétion en chlore induite par l’irradiation suit une cinétique différente de celles précédemment décrites pour les couches épaisses et cristaux massifs de NaCl. Les atomes de sodium produits par la dissociation de NaCl diffusent vers les zones nues de la surface Ag(001), où ceux-ci forment des superstructures Na-Ag connues pour le système Na/Ag(001). La modification des couches est le résultat de deux processus, pouvant être interprétés comme une désorganisation rapide de la couche avec l’arrachement de molécules de NaCl des bords d’îlots, et une perte lente de l’ordre structural à l’intérieur des îlots de NaCl due à la formation de trous par déplétion du chlore. La cinétique de la croissance des superstructures Na-Ag est expliquée par la diffusion limitée sur la surface irradiée, en raison de l’agrégation de molécules de NaCl désordonnées aux bords de marches du substrat. Nous avons également entrepris l’étude par STM de molécules de PTCDA déposées sur le substrat métallique Ag(001) et sur le système KCl/Ag(001). Nous obtenons une monocouche de PTCDA sur Ag(001) qui présente un arrangement avec une maille carrée. Sur les films de KCl, des molécules de PTCDA isolées ainsi que des structures empilées compactes ont été trouvées. / NaCl and KCl are wide band gap materials that are increasingly used as thin films in surface science and single-molecule spectroscopy studies to electronically decouple organic molecules from a metal substrate. In addition, the reactivity of bulk NaCl and KCl crystals under electron irradiation has been widely studied since the year 1970 s. In this dissertation, we report a detailed investigation on the structures of two different thin films of NaCl and KCl grown on the Ag(001) substrate by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) in ultrathin vacuum condition. In order to obtain high-quality thin films and to control the properties of these films on the metal surface (film thickness, domain size, domain orientation, etc), we study in detail the film growth, especially as a function of the growth parameters. In addition, the apparent height measurements by STM show that the apparent thickness and the STM contrast of these two insulating films are bias dependent. We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52–60 eV and 3 keV is studied using LEED and AES, respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using STM. We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on thick NaCl films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges. We have also investigated the deposition of PTCDA molecules on the metallic substrate Ag(001) and on the KCl/Ag(001) system using STM. We obtain a monolayer of PTCDA molecules on Ag(001) that is arranged in a square unit cell. On KCl films, both individual PTCDA molecules and a densely packed structure are found.

This thesis concerns my theoretical calculations and simulations in comparison with experimental measurements acquired by means of surface science techniques on bare surfaces and molecules adsorbed on surfaces. In the beginning of the thesis I briefly describe the density functional theory (DFT) method, which is used for calculations of geometric and electronic structure of surfaces and absorbed molecules. It is followed by a quick overview of the scanning probe microscopy (SPM) and X-ray spectroscopy techniques that provided experimental context for my calculations. In the later part of my thesis I introduce publications on which I participated. In these works, direct results of my DFT calculations or simulations based on DFT outputs helped to reveal geometric and electronic structure of acetophenone adsorbed on Si(111) surface, on-surface oligomerized organic molecules and boron atoms used for substitational doping of graphene. The simulations also enable us to probe a laser initiated CO hydrogenation in real time. A big part of my work was the development of some simulation methods for SPM imaging of molecules on surfaces with flexible tip apexes. These simulation methods helped to create a comprehensive overview of SPM techniques performed with flexible tip apexes. 1

Les mono-couches auto-assemblées (SAMs) sont des systèmes de choix pour le développement de plateformes moléculaires aux propriétés physico-chimiques contrôlées. Il s’agit de monocouches organisées de molécules bi-fonctionnelles. Ces molécules se composent d’une fonction terminale modulable, séparée d’un groupement d’ancrage par un espaceur adapté. Ainsi, les propriétés des SAMs peuvent être ajustées pour le développement de systèmes électroniques moléculaires ou de capteurs (bio)-chimiques. De plus, des structurations chimiques supplémentaires peuvent être induites par irradiation.Les méthodes d’irradiation les plus courantes impliquent des particules de haute énergie. Les dommages induits résultent de plusieurs mécanismes en compétition (ionisations, excitations, dissociations). Dans cette thèse, les électrons lents (0-20 eV) sont utilisés comme particules primaires, et les processus d’interaction électron-SAM sont étudiés afin d’identifier les résonances d’attachement électronique. Aux énergies concernées, des processus dissociatifs sélectifs et efficaces peuvent être mis à profit pour proposer des stratégies d’irradiation menant à des modifications chimiques contrôlées et optimisées.Des SAMs modèles de thiols sur or sont étudiées par une technique de spectroscopie vibrationnelle de forte sensibilité de surface, la spectroscopie de perte d’énergie d’électrons lents (HREELS). Elle permet à la fois de caractériser les SAMs et de sonder les processus d'interaction électron-molécule. Les résultats obtenus concernent les : 1) SAMs aromatiques modèles de terphénylthiol (HS-(C₆H₄)₂-C₆H₅), stabilisables par réticulation sous irradiation. Une caractérisation vibrationnelle poussée de la SAM avant et après irradiation, en portant une attention particulière au comportement des modes d’élongation ν(CH), a permis d’opposer les processus de réactivité induite à 1, 6 et 50 eV.2) SAMs d’acide mercaptoundécanoïque (HS-(CH₂)₁ ₀ -COOH), les fonctions acides terminales permettant par exemple le greffage de peptides. L’interface SAM / environnement (COO-/COOH, eau résiduelle) a été caractérisée grâce à la forte sensibilité des modes d’élongation ν(OH) aux liaisons hydrogène. La démarche mise en place peut être facilement transposée à d’autres systèmes. / Self-Assembled Monolayers (SAMs) are good candidates to develop molecular platforms with controlled physico-chemical properties. A SAM is an ordered monolayer of bi-functionnal molecules. These molecules consist of an adjustable terminal function, separated from a headgroup by a chosen spacer chain. Thus, SAMs properties can be adjusted for the development of molecular electronic systems or (bio)-chemical sensors. Furthermore, additional chemical structuration can be induced by irradiation.Most current methods of irradiation involve high energy particles. The induced damages result from several competitive mechanisms (ionisations, excitations, dissociations). In this thesis, low energy electrons (0-20 eV) are used as primary particles, and the interaction processes between electrons and SAMs are studied in order to identify electron attachment resonances. At the associated energies, selective and effective dissociative processes can be induced to propose irradiation strategies leading to controlled and optimized chemical modifications.Model SAMs of thiols on gold are studied by a vibrational spectroscopy technique of strong surface sensibility, high resolution electron energy loss spectroscopy (HREELS). It allows at the same time to characterize SAMs and to probe electron-molecule interaction processes. The result obtained deal with : 1) Aromatic model SAMs of Terphenyl-thiol (HS-(C₆H₄)₂-C₆H₅), which can be stabilized by cross-linking under irradiation. Induced reactive processes at 1, 6 and 50 eV were compared and opposed, thanks to an advanced vibrational characterization of the SAM before and after irradiation, and by paying a particular attention to the stretching mode ν(CH) behaviour.2) Mercaptoundecanoic acid SAMs (HS-(CH₂)₁ ₀ -COOH), whose terminal functions allow for example the peptide anchoring. The interface SAM / environment (COO-/COOH, residual water) was characterized thanks to the strong sensitivity of the stretching modes ν(OH) to hydrogen bonding. The approach that was developped can be easily transposed to other systems.

Το ερευνητικό ενδιαφέρον για τους οργανικούς ημιαγωγούς είναι συνεχώς αυξανόμενο τα τελευταία χρόνια, καθώς η αγορά των οργανικών ηλεκτρονικών είναι από τις πιο αναπτυσσόμενες. Για την καλή απόδοση των διατάξεων αυτών σημαντικός είναι ο ρόλος των διεπιφανειών. Οι οικογένειες των n-type οργανικών ημιαγωγών naphthalene bisimides και perylene bisimides έχουν δείξει καλές αποδόσεις σε οργανικά τρανζίστορ. Στην παρούσα εργασία μελετήθηκαν οι διεπιφάνειες νέων οργανικών ημιαγωγών από τις παραπάνω οι οικογένειες οργανικών πάνω σε ανόργανα υποστρώματα με φασματοσκοπίες φωτοηλεκτρονίων. Μελετήθηκε ο σχηματισμός λεπτών υμενίων, πάχους έως τα 10 nm, τριών naphthalene οργανικών ημιαγωγών με διαφορετικό ενεργειακό χάσμα πάνω στον χρυσό και ενός perylene πάνω σε χρυσό και SiO2. Σκοπός ήταν να προσδιοριστεί η επίδραση των διαφορετικών υποκαταστατών του κεντρικού πυρήνα των naphthalene bisimides, στα ενεργειακά χαρακτηριστικά του ημιαγωγού και τα φράγματα έγχυσης των φορέων στην διεπιφάνεια με τον χρυσό. Ο τρόπος ανάπτυξης των όλων των οργανικών ημιαγωγών προσδιορίστηκε ως πολλαπλά στρώματα. Σε μια περίπτωση εντοπίστηκε ότι αλλάζει από οριζόντιο σε κάθετο ο προσανατολισμός των μορίων. Προσδιορίστηκαν όλα τα μεγέθη που χαρακτηρίσουν ενεργειακά την διεπιφάνεια. Συγκεκριμένα, σε όλες τις διεπιφάνειες εμφανίζεται ένα διεπιφανειακό δίπολο λόγω της αναδιάταξης του ηλεκτρονιακού νέφους της επιφάνειας του χρυσού από τα μόρια του οργανικού. Επίσης, οι τιμές των φραγμάτων έγχυσης των ηλεκτρονίων που υπολογίστηκαν είναι αρκετά μικρές που δείχνουν το n-type χαρακτήρα των οργανικών. Οι τιμές του δυναμικού ιονισμού που υπολογίστηκαν ήταν όλες μεγαλύτερες του 5, που είναι προϋπόθεση για τα τρανζίστορ να είναι σταθερά στον αέρα, ενώ σε μια περίπτωση η τιμή ήταν αρκετά μικρή, που δείχνει ότι ο συγκεκριμένος οργανικός ημιαγωγός μπορεί να έχει ambipolar χαρακτηριστικά. Τα αποτελέσματα έδειξαν ότι ο χρυσός μπορεί να χρησιμοποιηθεί αποτελεσματικά ως ηλεκτρόδιο σε τρανζίστορ με n-type οργανικούς ημιαγωγούς. Τέλος, από τα αποτελέσματα τονίστηκε ότι με την υποκατάσταση χημικών ομάδων στον κεντρικό πυρήνα του naphthalene, μια εύκολη διαδικασία, είναι δυνατόν να οδηγηθεί ενεργειακά η διεπιφάνεια προς την επιθυμητή κατεύθυνση. / In the recent years the interest on organic semiconductors is increased as the market of organic electronics is one of most promising. The interfaces between the organic semiconductors with metals or other materials are crucial for the performance of the devices. The study of interfaces by surface sensitive techniques could give useful information for the physics of metal-organic contacts and therefore it is possible the tuning and the improvement of the device performance. The n-type organic semiconductors derivatives of naphthalene bisimides and perylene bisimides, have shown good performance in OFETs. In this work, the interfaces of new synthesized naphthalene bisimides and perylene bisimides molecules with inorganic substrates have been studied by photoelectron spectroscopies. Thin films up to 10 nm thickness of three naphthalene organic semiconductors of different energy gap on Au substrates have been studied. The aim was to investigate the effect of the different substituents of the naphthalene core on the energy characteristics of the organic semiconductors and on the charge injection barriers at the interface. Moreover, the interface of one perylene n-type semiconductor deposited on Au and SiO2 was studied in order to examine the influence of the substrate on the growth mode and the electronic properties. The growth mode of all the organic semiconductors was characterized as simultaneous multilayers. In one case, the orientation of the organic molecules was changed from horizontal to vertical to the surface. In all the interfaces an interface dipole is formed during the early stage of deposition which is attributed to the reorganization of the electron cloud of the Au surface by the organic molecules when they are deposited on Au. The hole and electron injection barriers were also determined. The electron injection barriers were found to be small which indicates the n-type character of these organic molecules. In addition, the results displayed that the Au can be used efficiently as electrode in devices with these organic semiconductors. The ionization potentials of the organic semiconductors were measured and found to be above 5 eV for all and therefore, they are suitable for air-stable transistors. In the case of one organic semiconductor the ionization potential was measured close to the value of five. Thus, this organic semiconductor is suitable for ambipolar transistors. The valance band characteristics near the HOMO, as detected by the UPS spectra, showed that they are affected by the different substituents on the side groups of the imide. These results have shown that changing the substituents of the organic core, which is an easy process; it is possible to tune the energy levels and the electronic characteristics of the interface.