Thèses sur le sujet « Surface electronic propertie »

Adsorbates on metal surfaces have attracted recently the scientific interest both from a fundamental point of view (quantum confinement) and in perspective of application in technology. In particular the electronic properties of such reduced symmetry systems are strictly related to their low dimensionality. An accurate theoretical description of their spectral properties has necessarily to deal with the absence of periodicity that characterizes these systems in one (thin films) or more (adatoms) directions. The embedding method allows to overcome this drawback considering a really infinite system both on vacuum and bulk side. In this thesis the ab initio spectral properties of single adatoms (alkali-metal atom and Ba) on Cu(111) and thin overlayers (K/Cu(111), Bi/Cu(100), O/Fe(100)) are analyzed, also in comparison with experimental results. The capabilities of the theoretical method adopted allow to evidence the role of the substrate band structure on the adsorbates induced electronic states. In particular the aspects related to the resonant charge transfer from the adsorbate's states to the bulk continuum are analyzed. This process represents an elastic decay channel for the surface electronic states and contributes to the elastic lifetime, that we can estimate according to the description of a continuous substrate band structure. The results presented for single adatoms evidence the blockade effect that a surface projected energy gap of the substrate produces on the resonant charge transfer. On the other hand the results relative to the overlayer allows to analyze the complex hybridization mechanism between surface features and substrate states in different points of the surface Brillouin zone. In addition the theoretical description of the electronic properties of overlayers on metal surfaces is devoted in this thesis also to the simulation of experimental findings, namely STM and STS images and photoemission spectra, showing the predictive character of the theoretical approach used.

Diamond is of interest as an advanced functional material, since the extreme physical properties of diamond, suggests it is ideally suited to a range of new demanding applications. In this context, the thesis explores basic surface chemical properties of diamond thin films, along with electrochemical, electronic and electron emission processes involving this material. New experiments are reported concerning the nature of surface conductivity on diamond. Measurements clearly show that the conductivity only arises if a hydrogenated diamond surface is exposed to water vapour, in the presence of chemical species capable of acting as electron acceptors. The conduction properties of surface conductive diamond in aqueous solution are also studied, and the first detailed electrochemical investigations of this material are described. Comparative electrochemical studies of nanocrystalline and boron-doped diamond have been performed. Investigations of electrode stability, and the accessible "potential window" are described, as well as the behaviour of a range of 'redox' systems, including transition metal complexes, metal deposition/stripping, and bio-related organic species. Significant differences between the behaviour of nanodiamond and microcrystalline boron-doped material are observed. A range of surface chemical and threshold photoemission studies of diamond thin films are reported. The results indicate that quantum photoyields (QPYs) are insensitive to the diamond "quality", although the wavelength selectivity is dependent on it. The adsorption of oxygen strongly reduces the QPY, although this only occurs slowly in the presence of O₂ because of a low reactive sticking probability. Much more rapid uptake of oxygen and consequent reduction of photoyield is observed in the presence of atomic O or electronically excited dioxygen O₂*. The presence of alkali metals on the diamond surface increases the QPY, and reduces the sensitivity of the QPY to surface oxygen. Significant differences between the surface chemical properties of Li, and other adsorbed akali metals (K and Cs) are observed.

The trend over the last 50 years of down-scaling the silicon transistor to achieve faster computations has led to doubling of the number of transistors and computation speed over about every two years. However, this trend cannot be maintained due to the fundamental limitations of silicon as the main material for the semiconducting industry. Therefore, there is an active search for exploration of alternate materials. Among the possible candidates that can may [sic] be able to replace silicon is graphene which has recently gained the most attention. Unique properties of graphene include exceedingly high carrier mobility, tunable band gap, huge optical density of a monolayer, anomalous quantum Hall effect, and many others. To be suitable for microelectronic applications the material should be semiconductive, i.e. have a non-zero band gap. Pristine graphene is a semimetal, but by the virtue of doping the graphene surface with different molecules and radicals a band gap can be opened. Because the electronic properties of all materials are intimately related to their atomic structure, characterization of molecular and electronic structure of functionalizing groups is of high interest. The ab-inito (from the first principles) calculations provide a unique opportunity to study the influence of the dopants and thus allow exploration of the physical phenomena in functionalized graphene structures. This ability paves the road to probe the properties based on the intuitive structural information only. A great advantage of this approach lies in the opportunity for quick screening of various atomic structures. We conducted a series of ab-inito investigations of graphene functionalized with covalently and hapticly bound groups, and demonstrated possible practical usage of functionalized graphene for microelectronic and optical applications. This investigation showed that it is possible [to] produce band gaps in graphene (i.e., produce semiconducting graphene) of about 1 eV, without degrading the carrier mobility. This was archived by considering the influence of those adducts on electronic band structure and conductivity properties.

In dieser Arbeit wurden einerseits die elektronischen Eigenschaften von Graphenen und andererseits die Verwendung von Graphenen und Kohlenstoff-basierten Hybridmaterialien als transparente Elektroden untersucht. Entsprechend ist der erste, umfangreichere Teil der Arbeit Grundlagen-orientiert und fokussiert auf die elektrostatische Wechselwirkung zwischen Graphen und dem Substrat Glimmer. Der zweite, kleinere Teil befasst sich mit der Entwicklung leitfähiger Tinten auf der Basis von Graphenen und anderen Kohlenstoff-basierten Hybridmaterialien für Anwendungen in der druckbaren Elektronik, insbesondere für die Herstellung transparenter Elektroden. Graphen auf Glimmer ist ein sehr wohldefiniertes System, in dem das Graphen über mehrere Quadratmikrometer atomar flach ist. Schichtdickenabhängige Variationen des Oberflächenpotentials von einzel- und mehrlagigen Graphenen auf Glimmer wurden mittels Kelvin Probe Rasterkraftmikroskopie untersucht. Damit konnte die elektrostatische Abschirmlänge von Graphen auf Glimmer bestimmt werden. Lokale Variationen des Oberflächenpotentials innerhalb einer Graphenlage, verursacht durch eingeschlossene Wasserschichten zwischen Graphen und Glimmer, wurden mit Rasterkraftmikroskopie, elektrostatischer Rasterkraftmikroskopie und der Raman-Spektroskopie untersucht. Dies ermöglichte es, die Dotierung von Graphen durch eingeschlossene Wasserschichten zu quantifizieren. Außerdem wurde gezeigt, dass Graphen auf molekular modifiziertem Glimmer lokal auf der Nano-Skala dehnbar ist. Dabei wurde der Glimmer durch das Aufbringen von dendronisierten Polymeren verschiedener Generationen auf Nanometer-Skala modifiziert. Dies eröffnet neue Möglichkeiten, die lokalen elektronischen Eigenschaften von Graphen durch Dehnung zu kontrollieren.Schließlich wurden Kohlenstoff-basierte leitfähige Tinten hergestellt, daraus transparente Elektroden hergestellt, und die Formulierungen der Tinten für das Drucken auf Plastiksubstrate optimiert.
This work focusses on the electronic properties of graphene on the one hand, and on the application of graphenes and other carbon-based hybrid materials for transparent electrodes on the other hand. Accordingly, the first part of the work, which is the larger one, is of fundamental nature and focusses on the electronic interaction between graphene and mica as a substrate. The second, smaller part deals with the design of novel conductive inks based on graphene and other carbon-based hybrid materials for applications in printed electronics, in particular for the production of transparent electrodes. Graphene on mica is a very well defined system, which provides atomically flat graphene extending over several square micrometers. Layer-dependent surface potential variations of single and few layered graphenes on mica were probed with Kelvin Probe Force Microscopy. This allowed to estimate the screening length of graphene on mica. Local variations of the surface electrostatic potential above single layer graphene, originating from confined fluid interfacial monolayers of water between the mica and the graphene, were monitored with Scanning Force Microscopy, Electrostatic Scanning Force Microscopy and Raman spectroscopy. This allowed to quantify the doping of graphene by the confined water layers. Exfoliation of graphene onto adsorbed nanostructures on mica allowed to control the strain of graphene at the nano-scale. Nanostructuring was achieved by first coating mica with submonolayers of dendronized polymers of different generations and subsequently depositing graphene. This approach provides new opportunities for the control of the electronic properties of graphene by strain.Finally, novel conducting carbon-based inks were designed and transparent electrodes were fabricated therefrom. The formulations of the inks were optimized for printing on plastic substrates.

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003.
Includes bibliographical references (leaves 106-110). Also available in electronic version. Access restricted to campus users.

Metal oxides are of immense technological importance. Their wide variety of structural and electronic characteristics leads to a flexibility unrivalled by other groups of materials. However, there is still much debate about the fundamental properties of some of the most widely used oxides, including TiO₂ and In₂O₃. This work presents high quality, in-depth characterisation of these two oxides in pure and doped form, including soft and hard X-ray photoelectron spectroscopy and X-ray diffraction. Bulk samples as well as thin film samples were prepared analysed. For the preparation of thin films a high quality sol-gel dip-coating method was developed, which resulted in epitaxial films. In more detail the organisation of the thesis is as follows: Chapter 1 provides an introduction to key ideas related to metal oxides and presents the metal oxides investigated in this thesis, In₂O₃, Ga₂O₃, Tl₂O₃, TiO₂, and SnO₂. Chapter 2 presents background information and Chapter 3 gives the practical details of the experimental techniques employed. Chapters 4 presents reciprocal space maps of MBE-grown In₂O₃ thin films and nanorods on YSZ substrates. Chapters 5 and 6 investigate the doping of In₂O₃ bulk samples with gallium and thallium and introduce a range of solid state characterisation techniques. Chapter 7 describes the development of a dip-coating sol-gel method for the growth of thin films of TiO₂ and shows 3D reciprocal space maps of the resulting films. Chapter 8 concerns hard x-ray photoelectron spectroscopy of undoped and Sn-doped TiO₂. Chapter 9 interconnects previous chapters by presenting 2D reciprocal space maps of nano structured epitaxial samples of In₂O₃ grown by the newly developed sol-gel based method. Chapter 10 concludes this thesis with a summary of the results.

Über die besonders hohe Effizienz von Halid-Perowskit (HaP)-basierten optoelektronischen Bauteilen wurde bereits in der Literatur berichtet. Um die Entwicklung dieser Bauteile voranzutreiben, ist ein umfassendes und verlässliches Verständnis derer elektronischen Struktur, sowie der Energielevelanordnung (ELA) an HaP Grenzflächen von größter Bedeutung. Demzufolge beschäftigt sich die vorliegende Arbeit mit der Untersuchung i) der Bandstruktur von Perowskit-Einkristallen, um ein solides Fundament für die Darlegung der elektronischen Eigenschaften von polykristallinen Dünnschichten zu erarbeiten, und mit ii) den Einflüssen von Oberflächenzuständen auf die elektronische Struktur der Oberfläche, sowie deren Rolle bei der Kontrolle von ELA an HaP Grenzflächen. Die Charakterisierung erfolgt überwiegend mithilfe von Photoelektronenspektroskopie (PES) und ergänzenden Messmethoden wie Beugung niederenergetischer Elektronen an Oberflächen, UV-VIS-Spektroskopie, Rasterkraftmikroskopie und Kelvin-Sonde. Erstens weist die Banddispersion von zwei prototypischen Perowskit-Einkristallen eine starke Dispersion des jeweiligen oberen Valenzbandes (VB) auf, dessen globales Maximum in beiden Fällen am R-Punkt in der Brillouin-Zone liegt. Dabei wird eine effektive Lochmasse von 0.25 m0 für CH3NH3PbBr3, bzw. von ~0.50 m0 für CH3NH3PbI3 bestimmt. Basierend auf diesen Ergebnissen werden die elektronischen Spektren von polykristallinen Dünnschichten konstruiert und es wird dadurch aufgezeigt, dass eine Bestimmung der Valenzbandkantenposition ausgehend von einer logarithmischen Intensitätsskala aufgrund von geringer Zustandsdichte am VB Maximum vorzuziehen ist. Zweitens stellt sich bei der Untersuchung der elektronischen Struktur von frisch präparierten Perowskit-Oberflächen heraus, dass die n-Typ Eigenschaft eine Folge der Bandverbiegung ist, welche durch donatorartige Oberflächenzustände hervorgerufen wird. Des Weiteren weisen die PES-Messungen an Perowskiten mit unterschiedlichen Zusammensetzungen aufgrund von Oberflächenphotospannung eine Anregungslichtintensitätsabhängigkeit der Energieniveaus von bis zu 0.7 eV auf. Darüber hinaus wird die Kontrolle von ELA durch gezielte Variation der Oberflächenzustandsdichte gezeigt, wodurch sich unterschiedliche ELA-Lagen (mit Abweichungen von über 0.5 eV) an den Grenzflächen mit organischen Akzeptormolekülen erklären lassen. Die vorliegenden Ergebnisse verhelfen dazu, die starke Abweichung der in der Literatur berichteten Energieniveaus zu erklären und somit ein verfeinertes Verständnis des Funktionsprinzips von perowskit-basierten Bauteilen zu erlangen.
Optoelectronic devices based on halide perovskites (HaPs) and possessing remarkably high performance have been reported. To push the development of such devices even further, a comprehensive and reliable understanding of their electronic structure, including the energy level alignment (ELA) at HaPs interfaces, is essential but presently not available. In an attempt to get a deep insight into the electronic properties of HaPs and the related interfaces, the work presented in this thesis investigates i) the fundamental band structure of perovskite single crystals, in order to establish solid foundations for a better understanding the electronic properties of polycrystalline thin films and ii) the effects of surface states on the surface electronic structure and their role in controlling the ELA at HaPs interfaces. The characterization is mostly performed using photoelectron spectroscopy, together with complementary techniques including low-energy electron diffraction, UV-vis absorption spectroscopy, atomic force microscopy and Kelvin probe measurements. Firstly, the band structure of two prototypical perovskite single crystals is unraveled, featuring widely dispersing top valence bands (VB) with the global valence band maximum at R point of the Brillouin zone. The hole effective masses there are determined to be ~0.25 m0 for CH3NH3PbBr3 and ~0.50 m0 for CH3NH3PbI3. Based on these results, the energy distribution curves of polycrystalline thin films are constructed, revealing the fact that using a logarithmic intensity scale to determine the VB onset is preferable due to the low density of states at the VB maximum. Secondly, investigations on the surface electronic structure of pristine perovskite surfaces conclude that the n-type behavior is a result of surface band bending due to the presence of donor-type surface states. Furthermore, due to surface photovoltage effect, photoemission measurements on different perovskite compositions exhibit excitation-intensity dependent energy levels with a shift of up to 0.7 eV. Eventually, control over the ELA by manipulating the density of surface states is demonstrated, from which very different ELA situations (variation over 0.5 eV) at interfaces with organic electron acceptor molecules are rationalized. Our findings further help to explain the rather dissimilar reported energy levels at perovskite surfaces and interfaces, refining our understanding of the operational principles in perovskite related devices.

"There is plenty of room at the bottom." This bold and prophetic statement from Nobel laureate Richard Feynman back in 1950s at Cal Tech launched the Nano Age and predicted, quite accurately, the explosion in nanoscience and nanotechnology. Now this is a fast developing area in both science and technology. Many think this would bring the greatest technological revolution in the history of mankind. To understand electronic and optical properties of nanostructures, the following problems have been studied. In particular, intensity of mid-infrared light transmitted through a metallic diffraction grating has been theoretically studied. It has been shown that for s-polarized light the enhancement of the transmitted light is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating enhancement can be increased by a few orders of magnitude. The spatial distribution of the transmitted light is highly nonuniform with very sharp peaks, which have the spatial widths about 10 nm. Furthermore, under the ultra fast response in nanostructures, the following two related goals have been proved: (a) the two-photon coherent control allows one to dynamically control electron emission from randomly rough surfaces, which is localized within a few nanometers. (b) the photoelectron emission from metal nanostructures in the strong-field (quasistationary) regime allows coherent control with extremely high contrast, suitable for nanoelectronics applications. To investigate the electron transport properties of two dimensional carbon called graphene, a localization of an electron in a graphene quantum dot with a sharp boundary has been considered. It has been found that if the parameters of the confinement potential satisfy a special condition then the electron can be strongly localized in such quantum dot. Also the energy spectra of an electron in a graphene quantum ring has been analyzed. Furthermore, it has been shown that in a double dot system some energy states becomes strongly localized with an infinite trapping time. Such states are achieved only at one value of the inter-dot separation. Also a periodic array of quantum dots in graphene have been considered. In this case the states with infinitely large trapping time are realized at all values of inter-dot separation smaller than some critical value.

Tungsten trioxide WO3 shows promising perspectives in many fields of research. Surface states appear to play a fundamental role on the overall functionality of the material. Experimental evidences showed the capability to consistently obtain meta-stable polar surfaces, characterized by large amount of excess charge on the surface. In this study, we investigate the WO2 terminated polar (001) surface, by density functional theory calculations. Our data suggest a tri-dimensional nature of the excess electrons, delocalized across the entire slab, compared to the usually observed two-dimensional electron gas behavior, in the case of common transitional metal oxide. The material is expected to be able to host excess charge also in localized states (namely polarons). Furthermore, this work also suggests a deep coupling between the antiferroelectric distorsions, the oxygen vacancies at the topmost layer, and the dispersion of the excess charge. Our results, constitute overall solid bases to further investigate these connections and a valid starting point towards the opportunity to tune the electronic properties of the WO3 surface.

The main focus of the thesis is to have better understanding of the atomic and electronic structures, vibrational dynamics and thermodynamics of metallic surfaces and bi-metallic nanoparticles (NPs) via a multi-scale simulational approach. The research presented here involves the study of the physical and chemical properties of metallic surfaces and NPs that are useful to determine their functionality in building novel materials. The study follows the  bottom-up approach for which the knowledge gathered at the scale of atoms and NPs serves as a base to build, at the macroscopic scale, materials with desired physical and chemical properties. We use a variety of theoretical and computational tools with different degrees of accuracy to study problems in different time and length scales. Interactions between the atoms are derived using both Density Functional Theory (DFT) and Embedded Atom Method (EAM), depending on the scale of the problem at hand. For some cases, both methods are used for the purpose of comparison. For revealing the local contributions to the vibrational dynamics and thermodynamics for the systems possessing site-specific environments, a local approach in real-space is used, namely Real Space Green s Function method (RSGF). For simulating diffusion of atoms/clusters and growth on metal surfaces, Molecular Statics (MS) and Molecular Dynamics (MD) methods are employed.
Ph.D.
Department of Physics
Sciences
Physics PhD

La création de matériaux multifonctionnels en ayant recours à l’auto-assemblage de molécules spécifiques est d’un grand intérêt dans le domaine des nanotechnologies. Dans ce contexte un réel effort a été produit afin d’obtenir le contrôle total de l’auto-organisation de molécules π-conjuguées ; ceci dans le but de créer des architectures supramoléculaires anisotropes et de créer des nano-objets électriquement actifs. Il est en effet connu que l’ordre, à un niveau supramoléculaire, affecte de façon importante les propriétés électroniques d’assemblages moléculaires. Dans ce travail de thèse nous nous sommes intéressé à l’auto-assemblage de différents systèmes moléculaires menant à la formation de nanostructures supramoléculaires, ces dernières résultant d’un équilibre entre les interactions intramoléculaires, intermoléculaires ainsi que celles existant à l’interface. Les propriétés structurelles, dynamiques, électriques et électroniques de surfaces solides ont été étudiées à l’échelle du nanomètre en utilisant les techniques de microscopie en champs proche (ou SPM pour Scanning Probe Microscopy). Les techniques utilisées durant cette thèse vont du microscope à effet tunnel (STM) et spectroscopie à effet tunnel (STS) aux techniques dérivées des microscopies à force atomique(SFM) telles que Kelvin Probe Microscopy (KPFM) et le Conducting Probe Force Microscopy (C-AFM). Les nanostructures qui ont été développées dans le cadre de ce travail ne sont pas intéressantes uniquement en tant que nano-constructions sur des surfaces solides mais aussi en tant que candidats en vue d’applications dans le domaine de l’électronique (supra)moléculaire. Afin d’explorer l’utilisation d’interactions de faible intensité menant à la constitution d’assemblages supramoléculaires fonctionnels, nous avons étudié les systèmes suivants:i)Un composant thio-tryophénylène alkylé connu pour former une phase cristal liquide discotique grâce à la présence d’interactions π-π entre les molécules; ce composant ayant de plus des propriétés électroniques potentiellement intéressantes. Ii)Un hexaazatriphenylène fonctionnalisé par des amides, intéressant comme potentiel porteur d’électrons aux vues de la nature chimique de son noyau conjugué. Ce composant montre de plus une phase cristal liquide discotique résultant de la formation de liaisons hydrogènes entre ses fonctions amides. Iii)Une diade HBC-PérylèneMonoImide formant un système Donneur-Accepteur monomoléculaire. Iv)Des composant HBC fonctionnalisés à l’aide de nucléotides, afin d’exploiter les propriétés directionnelles et de reconnaissance des liaisons hydrogènes entre les nucléotides. V)Des composants « cavitands » en vue de former des assemblages hôte-invité dont la courbure sera contrôlable. En ce qui concerne les dérivés de triphenylène, nous avons porté notre attention, en collaboration avec le groupe du Prof. Yves Geerts de l’université Libre de Bruxelles (Belgique), sur des composants hexaazatriphenylenes et des thio-triphenylène alkylés. Ces derniers sont des molécules de cristaux liquides discotiques possédant d’intéressantes propriétés électroniques. Nous avons suivi leur auto-assemblage à l’interface liquide –solide et nous avons observé la formation de monocouches ayant deux motifs coexistants sur la surface basale du graphite. De plus, l’évolution temporelle des joints de domaine au sein d’une monocouche polycrystalline a révélé un « Ostwal ripening » en 2D. Les propriétés électriques ont été investiguées, à la fois à l’échelle de la molécule à l’aide de STS et à l’échelle des ensembles supramoléculaires à l’aide de C-AFM. D’autre part les expériences utilisant un microscope à force atomique ont montré la formation de nanofils allongés sur la surface de substrats isolants, comme par exemple des substrats de mica muscovite et de SiOx. Des approches ont été développées afin de créer des auto-assemblages utilisant les nanofils cités précédemment en les incluant entre deux nano-électrodes, ceci en vue de l’étude de leur comportement électrique dans une telle configuration. D’autre part, les molécules de hexathialkylhexaazatriphénylènes ne montrent pas de phase cristal liquide colonnaire comme escompté, ceci étant probablement du à leur grande charge négative portée par les atomes d’azote; ce qui pourrait entraîner une répulsion entre les noyaux des molécules plus proches voisines. En conséquence, la formation de liaisons hydrogènes entre les différentes fonctions amide a été utilisée afin de contrebalancer et surpasser les répulsions coulombiennes. Nous avons ensuite étudié l’auto-organisation et les propriétés électroniques de molécules d’azatriphénylène spécialement synthétisées pour l’occasion. . Ces études ont révélé la formation d’architectures colonnaires dans différents environnements. Les molécules discales sont alors maintenues ensemble grâce aux liaisons hydrogène entre les fonctions amide. En particulier, un réseau de fibres a été formé, ce dernier exhibant une hauteur caractéristique comparable au diamètre d’une molécule. Les propriétés électroniques aux échelles de la molécule et de grands ensembles ont été étudiées sur des substrats conducteurs (HOPG). Les investigations STM à l’interface liquide-solide mettent en avant la formation de monocouches ordonnées. De plus, l’analyse des niveaux d’énergie contribuant au contraste dans les images STM, en complément de calculs de chimie quantique (effectués en collaboration avec le groupe du Dr. Jérôme Cornil de l’université de Mons, Belgique) , montre de façon remarquable la contribution des niveaux électroniques localisés au niveau des groupes amides. Finalement,des mesures utilisant le KPFM ont permis de déterminer le potentiel de surface des couches auto-organisées. . En collaboration avec le groupe du Professeur Klaus Müellen du Max-Planck Institut à Mainz, nous avons étudié de nombreux dérivés de HBC. En exploitant les propriétés directionnelles ainsi que les propriétés de reconnaissance des liaisons hydrogènes, nous avons obtenu des architectures branchées à partir de composants hybrides HBC-nucléotide, et ceci sur des surfaces isolantes (mica et SiOx) mais aussi conductrices (HOPG). De plus, grâce à la combinaison d’études STM et SFM nous avons investigué une diade HBC-PerylèneMonoImide sur graphite à l’interface liquide-solide et sur films secs, ce système étant un système Donneur Accepteur monomoléculaire. Dans une autre partie, en collaboration avec le groupe du Prof Enrico Dalcanale à l’université de Parme (Italie), nous avons étendu notre étude à l’auto-arrangement sur les surfaces de « cavitands », ces molécules pouvant former au travers d’une reconnaissance hôte-invité des polymères supramoléculaires ayant une courbure contrôlée. Les « cavitands » se sont avérés être des systèmes versatiles pour la formation des assemblages hôte-invité. Les études en solution ont montré la formation de polymères formés par ces « cavitands », et leur capacité d’agrégation a été quantifiée. Nous avons ensuite transféré ces nano-structures de la solution sur des surfaces atomiquement planes où nous avons pu visualiser des architectures en bâtonnet. Un recourbement en deux dimensions de bâtonnets uniques a pu être observé et a pu être corrélé au degré de courbure des polymères obtenu par simulation. En résumé, nous avons étudié l’auto-organisation d’une large gamme de systèmes moléculaires et nous en avons déterminé les propriétés structurelles électriques et électroniques de l’échelle mésoscopique jusqu’à l’échelle de la molécule. Les composants dérivés des triphénylènes ont montré des propriétés les rendant intéressant pour de potentielles applications tirant partie de leur conduction quasi unidimensionnelle. De plus, nous avons vu que la reconnaissance entre les nucléotides pouvait être exploitée afin de contrôler l’auto-assemblage de HBC. Finalement, les « cavitands » se sont montrés être de bons candidats pour la formation de polymères ayant une courbure contrôlée
The generation of multifunctional materials by self-assembly of specifically designed molecules, through supramolecular chemistry approaches, currently gathers a great interest in nanotechnology. In this context a great deal of effort has been devoted to achieve a full control of the self-organization of -conjugated molecules into highly ordered, anisotropic supramolecular architectures as spatially confined electrically active nano-objects. It is indeed well known that the order at the supramolecular level strongly affects the electronic properties of molecular based assemblies. In this thesis we report on the self-assembly on surfaces of different molecular systems into supramolecular nanostructures, as obtained by balancing the interplay of intramolecular, intermolecular, and interfacial interactions. On solid substrates structure, dynamics, electrical and electronic properties have been investigated on the nanometer scale making use primarily of Scanning Probe Microscopies, in particular Scanning Tunneling Microscopy and Scanning Force Microscopy (SFM) based approaches, such as Scanning Tunneling Spectroscopy (STS), Kelvin Probe Force Microscopy (KPFM) and Conducting Probe Force Microscopy (C-AFM). The nanostructures that have been developed are not only of interest as nano-constructions on solid surfaces, but exhibit properties that make them candidates for applications in the field of (supra)molecular electronics. In order to explore the use of weak interactions to for functional supramolecular assemblies at surfaces, we have studied the following systems:i)an alkylated thio-triphenylene compound known to from a discotic liquid crystal phase through  stacking, with potentially interesting electronic properties;ii)an amide functionalized hexaazatriphenylene, of interest as potential electron carrier in view of the nature of its conjugated core, and forming a discotic liquid crystal phase through the formation of Hydrogen bonds between amide units;iii)a HBC-PeryleneMonoImide dyad, as a single molecule Donor-Acceptor system;iv) HBC-nucleotide functionalized compounds, to exploit directionality and recognition properties of hydrogen bonds between nucleotides;v)Cavitand compounds as molecular building blocks for host-guest assemblies with controlled curvature. Among triphenylene derivatives, we have centered our attention, in collaboration with the group of Prof. Yves Geerts of the Université Libre de Bruxelles (Belgium), on hexaazatriphenylenes, and on an alkylated thio-triphenylene compound. This latter discotic is a liquid crystal forming molecule holding interesting electronic properties. We followed its self-organization at the solid-liquid interface observing a packing into monolayers bearing two coexisting structural motifs on the basal plane of graphite. The temporal evolution of domain boundaries in a polycrystalline monolayer, explored by recording series of subsequent STM images, revealed an Ostwald ripening phenomenon, i. E. Coarsening in two-dimensional molecular polycrystals. The electrical properties have been investigated at both the single molecule level, using STS, and at the ensemble (supramolecular) level by means of C-AFM. On the other hand SFM microscopy experiments highlighted the formation of edge-on nanowires on the surface of electrically insulating substrates, such as muscovite mica and SiOx. Approaches have been explored to drive the self-assembly of the aforementioned nano-wires embedded in the gap between nano- electrodes, and to study their electrical behavior in such a configuration. In a different way, hexathialkylhexaazatriphenylenes do not form columnar liquid crystalline phases like the corresponding triphenylene derivatives, probably due to the large negative charges on the nitrogen atoms that lead to a repulsion of adjacent cores. Hydrogen bonds between peripheral amide units have been therefore used to counterbalance and overcome this Coulombic repulsion. We have studied the self-organization and electronic properties of a specially designed and synthesized azatriphenylene. Such studies revealed in different environments the formation of columnar architectures, where the single molecular discs are held together by H-bonds between amide moieties. In particular a network of fibers was formed, exhibiting a height comparable to the molecular diameter. The electronic properties both at the single molecule as well as at the ensemble level have been investigated. On electrically conductive substrates such as highly oriented pyrolitic graphite (HOPG), STM investigations at the solid-liquid interface highlighted the formation of ordered monolayers and the analysis of the energy levels contributing to the contrast in the current STM images, supported by quantum-chemical calculations (pursued in collaboration with the group of Dr. Jérôme Cornil of the Université de Mons (Belgium)), revealed a significant contribution of the electronic levels localized on the amide groups. Furthermore KPFM measurements allowed us to determine the surface potential of the self-organized layers. In collaboration with the group of Prof. Klaus Müllen of the MPI of Maintz (Germany), we have studied various HBC-derivatives. Exploiting the directionality and recognition properties of hydrogen bonds between nucleotides, branched architectures have been obtained from hybrid solutions of HBC-nucleotide functionalized compounds on both insulating (mica and SiOx) and conductive (HOPG) surfaces, employing different room temperature deposition methods. Furthermore through combined STM and SFM studies we have investigated the behaviour of a HBC-PeryleneMonoImide dyad on HOPG at the solid liquid interface, and on dry films, as a single molecule Donor-Acceptor system. In addition, in collaboration with the group of Prof. Enrico Dalcanale at the University of Parma (Italy), we have extended our studies of self assembly on surfaces to cavitand compounds as system that can from, through host-guest recognition, supramolecular polymers with controlled curvature. Cavitands are indeed versatile molecular building blocks for host-guest assemblies. The design, crystal structure determination and self-assembly in solution and on surfaces of supramolecular polymers from cavitand based compounds have been subject of investigation. Studies in solution provided evidence for the formation of polymers from specifically designed cavitands, and the aggregation behavior was quantified. The successful translation of the given self-assembly procedure from solution to surfaces required a comprehensive understanding and control over various boundary conditions. Rod-like architectures have been in this way visualized on surface. A bending in two dimensions of the single rods has also been observed and it has been correlated to the degree of curvature of the polymer as obtained form simulation studies. In summary, we have studied the self organization of a variety of molecular systems into supramolecular architectures on surfaces, exploring structural, electrical and electronic properties down to the nanoscale. Triphenylene based compounds have proved to exhibit properties that make them interesting for potential applications as quasi-1D charge carrier systems for electrical conduction. Moreover, we have shown how nucleoside recognition can be exploited to drive the self-assembly of HBCs from solution to surfaces. In addition cavitand compounds have shown to be versatile molecular building blocks to form polymers with potentially controlled curvature

Quantum dots (QDs) have drawn a special attention since recent past due to their properties such as broad range absorption ability, size tunable narrow emission, high extinction coefficients, and charge carriers ability. Nonetheless, imbalanced surface passivation/defects leads to the appearance of surface trap states inside the band gap, affecting both radiative and non-radiative dynamics. Experimentally, it is difficult to explore the effect of surface states as they are optically inactive. However, computations provide valuable insights to these characteristics. We performed calculations using density functional theory (DFT) and time-dependent DFT (TDDFT) to provide our insights to such effects. Firstly, we performed DFT studies to understand the effect of QD- ligand interactions on their photophysical properties. Our studies on thiols passivated CdSe QDs showed that passivation of their surface by equilibrium concentration of neutral thiols and negatively charged thiolates is essential to achieve photoluminescence (PL) enhancement. Additionally, we investigated the effect of surface defects on photophysical properties of silicon QDs. Our results showed that defects introduce mid gap states inside band-gap. Absorption spectra showed the appearance of dark/semi-dark states at the first energy band, proving that the surface states quench PL efficiency in QDs. Secondly, we studied the effect of QD-QD interactions on their optoelectronic properties. In collaboration with the experimental group from Prof. Hobbies’ lab, we studied interactions between defective and non-defective QDs. Calculated Forster Resonance Energy Transfer rates suggest that all the trap states in a defective QD would be filled by the excited electrons from the non-defective QD and thus emission happens from the highest bright energy state. We proposed this as the reason for the experimental observation of the increased on-time blinking and overall enhancement of PL in these QDs. Furthermore, in collaboration with experimentalists from Los-Alamos National Lab, we have provided our insights into the chemical engineering of self-assembling of PbSe QDs into (100) directed 2D nanoplates. Our surface energy calculations on the oriented attachments revealed that 2D nanoplates grown in (100) are more feasible than 3D quantum dots. Overall, our calculations not only supported the experimental findings, but also provided solutions to questions raised by experimentalists.
Department of Energy (DOE) CAREER: DE-SC008446
Center for Computationally Assisted Science and Technology (CCAST)

This thesis summarizes the results concerning the manufacture of medical implants for bone replacement using electron beam melting (EBM) which is an additive manufacturing (AM) technology, and aims to satisfy the engineering needs for the medical functionality of manufacturing technology. This thesis has focused on some microscopic properties for surfaces and bone integration. The process parameters of EBM manufacturing were studied to ascertain whether they have impacts on surface appearance, as surface properties have impacts on bone integration and implant performance. EBM manufacturing uses an electron beam to melt metal powder onto each layer in a manner akin to welding. The electron beam is controlled by process parameters that may be altered to a certain extent by the operator. There are individual process parameters for every material, and new parameters are set when developing new materials. In this thesis, process parameters in default settings were altered to ascertain whether it was possible to specify process parameters for implant manufacturing. The blood chamber model was used for thromboinflammation validation, using human whole blood. The model is used to identify early reactions of coagulation and immunoreactions. The material used in this study was Ti6Al4V-ELI, which is corrosion resistant and has the same surface oxide layers as titanium, and CoCr-F75, which has high stiffness, is wear-resistant and is commonly used in articulating joints. The study shows that among the process parameters researched, a combination of speed and current have the most impact on surface roughness and an interaction of parameters were found using design of experiment (DOE). As-built EBM surfaces show thrombogenicity, which in previous studies has been associated with bone ingrowth. Surface structure of as-build EBM manufactured surfaces are similar to implants surfaces described by Pilliar (2005), but with superior material properties than those of implants with sintered metals beads. By altering the process parameters controlling the electron beam, surface roughness of as-build parts may be affected, and the rougher EBM manufactured surfaces tend to be more thrombogen than the finer EBM manufactured surfaces. As-build EBM manufactured surfaces in general show more thrombogenicity than conventional machined implants surfaces.
Denna avhandling behandlar tillverkning av medicinska implantat för integration i ben. I fokus är den additiva tillverkningstekniken ”elektronstrålesmältning” ( Electron Beam Melting –EBM), en av flera tekniker som populärt beskrivs med termen 3D-skrivare. Avhandlingen fokuserar på mikroskopiska ytegenskaper och dess inverkan på benintegration. Processparametrarna för EBM-tillverkning studerades för att fastställa hur de påverkar ytans utseende, efter som ytegenskaper har effekt på implantatens funktion. EBM-tillverkning använder en elektronstråle som likt svetsning smälter ihop metallpulver. Elektronstrålen styrs av processparametrar som till viss mån kan justeras av maskinoperatören. Det finns individuella processparametrar för varje material och nya parametrar utvecklas till varje ny legering. I denna avhandling har ”grundinställningarnas processparametrar” studerats för att ta reda på om det är möjligt att ställa in specifika parametrar till implantattillverkning. Med hjälp av blodkammarmetoden, som använder humant blod, har thromboinflammatoriska egenskaper undersökts. Metoden identifierar tidiga koagulations- och immunologiska reaktioner. Legeringarna som undersökts i denna studie var Ti6Al4V-ELI, som är korrosionsbeständigt med samma uppsättning oxider på ytan som titan har, och CoCr-F75, en legering som har hög styvhet, är slitstarkt och är vanligt förekommande i implantat för leder. Bland de undersökta processparametrarna visar en kombination av hastighet och ström ha mest inverkan på ytjämnhet och en interaktion mellan parametrar identifierades med hjälp av försöksplanering. EBM-tillverkade ytor visade på thrombogena egenskaper som i tidigare studier kan relateras till god integration i benvävnad. Ytstrukturen hos EBM-tillverkade ytor liknar de implantatytor som Pilliar (2005) beskriver, men materialegenskaperna är bättre än de materialegenskaper som implantat, med sintrad yta, har. Genom att ändra processparametrarna som styr elektronstrålen kan ytstrukturen påverkas. Grövre EBM-tillverkade ytor tenderar att vara mer thrombogena än de finare EBM-tillverkade ytorna är. Obehandlade EBM-tillverkade ytor i allmänhet är mer thrombogena än vad konventionellt framställda implantatytor är.

This thesis concerns new hydrogen- and polarity-related effects in the photoluminescence of ZnO single crystal wafers and the relationship between surface electron accumulation and surface hydroxyl coverage on different ZnO surfaces. A comparative study of the low temperature photoluminescence of various types of hydrothermal and melt-grown ZnO wafers revealed several new hydrogen-related exciton recombination lines and a number of consistent polarity-related differences in the PL emission from different crystallographic surfaces. Temperature-dependent PL measurements were extensively used to distinguish the ground and excited state transitions involved in these effects. ZnO samples of different surface polarity were annealed in oxygen and nitrogen gases and in hydrogen-containing forming gas mixtures in an attempt to identify the origin of these new PL features. The well known aluminium-related I_₆ recombination line was resolved into two separate features in hydrothermal ZnO, and the new component I6-H (3.36081 eV) was found to repeatedly quench and then re-emerge after annealing in oxygen and forming gas, respectively. A model involving an aluminium - lithium - hydrogen defect complex was proposed for I6-H and further tested via hydrogen and deuterium implantation experiments on hydrothermal ZnO wafers with different lithium concentrations. These experiments also provided evidence for the involvement of a different lithium-hydrogen defect complex in other hydrogen-related emission lines I₄b,c (3.36219 eV and 3.36237 eV) unique to hydrothermal ZnO. In addition, a broad Gaussian-shaped feature observed in the near-band-edge PL emission from the O-polar (000‾1), a-plane (11‾20) and r-plane (1‾102) faces of ZnO was shown to be surface sensitive and also related to hydrogen. The involvement of hydrogen in the chemical and electronic properties of different ZnO surfaces was also investigated. The thermal stability of the hydroxyl termination and the associated downward surface band bending on the polar and non-polar surfaces of ZnO was studied by synchrotron and real-time photoelectron spectroscopy, both during and after annealing and subsequent H₂O/H₂ dosing in ultra-high vacuum conditions. On the O-polar face, the band bending could be reversibly switched over a range of approximately 0.8 eV by adjusting the surface H-coverage using simple UHV heat treatments and atmospheric exposure. A transition from electron accumulation to electron depletion on the O-polar face was observed at a H-coverage of approximately 0.9 monolayers. In contrast, the downward band bending on the Zn-polar face was significantly more resilient and electron-depleted surfaces could not be prepared by heat treatment alone. This was also the case for in situ cleaving in UHV conditions which failed to produce hydroxyl-free surfaces due to migration of hydrogen from the bulk to the cleaved surface. Interestingly, the thermal stability of the hydroxyl termination on the a-plane (11‾20) and m-plane (10‾10) surfaces was signiifcantly lower than on the polar faces due to the availability of a lower energy desorption pathway and the electrostatic stability of these non-polar surfaces in their clean, bulk terminated form. The surface band bending on the non-polar ZnO surfaces was also found to be directly related to their OH coverage with a transition from downward to upward band bending, similar to that observed on the O-polar face, as the OH coverage was reduced. Thermal admittance spectroscopy and deep level transient spectroscopy was used to investigate the effect of lithium removal on the defect nature of hydrothermal ZnO. A number of new defects were introduced by the high temperature (1100-1400°C) annealing/re-polishing process used to reduce the lithium concentration, particularly E₁₉₀ (also known as T2) which is thought to be related to Zn vacancies. Significantly, both the E₅₀ defect level and the I6-H PL emission line were absent after lithium (and hydrogen) removal suggesting an association of both these features with the same aluminium - lithium - hydrogen defect complex.

Avec la miniaturisation des composants optoélectroniques, contrôler la surface de leur constituants actifs devient prépondérant. C’est en particulier vrai pour les nanofils semi-conducteurs dont la géométrie favorise un rapport surface sur volume élevé. L’objectif de cette thèse consiste donc à mener une étude précise de la structure cristallographique et électronique de leur surface et à déterminer à quel point cette surface affecte leurs propriétés physiques globales. Ce travail commence par une description détaillée de la croissance des nanofils III-V en insistant sur l’intérêt de fabriquer des ensembles de nanofils uniformes, condition nécessaire pour assurer une grande reproductibilité des résultats. Il se poursuit par un éclairage sur une technique de choix pour analyser la surface des nanofils, la microscopie à effet tunnel, et une technique d’encapsulation des nanofils pour préserver leur surface de toute contamination. L’intérêt de ces deux techniques est démontré au travers de l’étude de la surface de nanofils GaAs et InAs pour expliquer comment la désorption d’une couche protectrice d’arsenic conduit à des morphologies de surface différentes. L’expertise ainsi acquise est alors mise à profit pour caractériser des nanofils GaAs cœur-coquille, dont la coquille est fabriquée à basse température. Au travers de l’identification des défauts rencontrés dans la coquille, cette dernière se révèle posséder des propriétés similaires à celles de films GaAs fabriqués à basse-température. La durée de vie limitée des porteurs de charge photoexcités est alors exploitée pour étudier les effets induits par les défauts sur les propriétés d’émission THz de nanofils à base de GaAs
With the size reduction of optoelectronic devices, controlling the surface of semiconductor materials is becoming crucial to optimize their performances. This is particularly true for one-dimensional systems such as semiconductor nanowires that are subject to high surface-to-volume ratio. The aim of this thesis is therefore to perform a comprehensive study of the surface properties of III-V semiconductor nanowires and to determine to what extent they affect their overall properties. Starting with a description of the basic principles that govern their growth in order to obtain nanowire ensembles with a good uniformity, we then highlight a surface science tool, scanning tunneling microscopy, and a surface preparation technique, based on the use of a protective arsenic layer, that are key to further understand the structural and electronic properties of the surface of self-catalysed GaAs and InAs semiconductor nanowires. In the fourth part of this work, we apply these techniques to analyse the structural and electronic properties of GaAs core-shell nanowires consisting of a thin shell grown at low temperature. We show the similarity of the shell properties with low-temperature grown GaAs thin film through the identification of their point defects and finally compare the THz properties of these nanowires with GaAs nanowires. The importance of the shell in the dynamics of the free charge carriers is demonstrated from the analysis of the THz waveforms

Organic molecular electronic materials are used in many applications such as chemical sensors, p-n junction devices, photovoltaics and xerography. Chlorogallium phthalocyanine (GaPc-Cl), shown by previous research in this group to have exceptional photoelectrochemical properties and sensitivity to chemical dopants such as oxygen and hydrogen, is suspected to be influenced by growth conditions and subsequent exposure to ambient conditions. GaPc-Cl films were grown on interdigitated array microcircuits in an ultra high vacuum chamber and several solid state parameters (in vacuum) were measured. This yielded information concerning: structural trap concentration; chemical impurity concentration and energetic levels; trap depths; ohmic or space charge limited current behavior; quantum efficiencies; and, photocurrent sensitivity to changes in illumination intensity. Films with higher chemical impurity concentrations had larger rise and decay times; high dark currents and low dark activation energies; larger photoactivation energies; a poor contrast of photo-to-dark current; extended ohmic regions in dark current-voltage curves; enhanced absorbed light quantum efficiencies; and low sensitivity of photocurrent to changes in light intensity. Chemical dopants (O₂, NH₃, NO₂ and TCNQ) were then chemisorbed on the films and uptake curves were obtained by monitoring dark and/or photocurrent responses. Solid state measurements were repeated for comparison and contrast to the native state. O₂ and NH₃ cause irreversible dark and photocurrent decreases followed by reversible dark and photocurrent increases. TCNQ and NO₂ caused immediate reversible photo and dark current increases. Solid state parameters varied depending on whether the dopant was surface-bound (TCNQ), or could intercalate into the bulk (O₂, NH₃ and NO₂). XPS and UPS experiments were conducted on native GaPc-Cl or TCNQ films and bilayers of these two compounds. UPS established HOMO levels for GaPc-Cl and TCNQ and substantiated feasibility of GaPc-Cl oxidation; however, no useful XPS/UPS information was obtained for reactions between GaPc-Cl and TCNQ primarily because of overlapping spectral regions. XPS quantitation of TCNQ peak areas established identities of carbon atoms responsible for them. TCNQ deposition on various work function metals demonstrated the identity of TCNQ carbons susceptible to reduction.

A one dimensional analysis of the boundary conditions of the electron energy eigenfunc-tion at a sharp interface between two crystals was made. An attempt to evaluate these conditions in terms of known band structure was made. It was concluded that this cannot be done in general. It was shown, however, that if the interface has the proper symmetry properties, the boundary conditions can be expressed in terms of only one unknown, energy-dependent parameter. It was concluded that setting this parameter equal to one gives boundary conditions which, though more general, are equivalent to the commonly used effective mass boundary conditions when they are applicable. It was concluded from numerical results for the transmission coefficient of the symmetric interface, that in general, these boundary conditions, which depend only on known band structure, do not give a good approximation to the exact answer. Since the energy dependence of the parameter mentioned above is described quite well qualitatively using the nearly free electron approximation or the tight-binding approximation, the applicability of any boundary conditions depending only on band structure can be predicted using these simple theories. The exact numerical results were calculated using the transfer matrix method. It was also concluded that the presence of symmetry in the interface either maximizes or minimizes the transmission coefficient. A tight-binding calculation showed that the transmission coefficient depends on an interface parameter which is independent of band structure. The transmission coefficient is maximized when this parameter is ignored. It was concluded that the effective mass equation is of little use when applied to this problem. Some transfer matrix results pertaining to the barrier and the superlattice were obtained.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

Cette thèse porte sur les propriétés de transport anormal des oxydes de métaux de transition, en particulier de la surface de SrTiO₃ ou de l’interface entre SrTiO₃ et LaAlO₃. Dans ces systèmes on observe l’apparition de gaz d’électrons bidimensionnels. Des mesures d’Effet Hall non linéaire indiquent que ces gaz sont constitués de plusieurs sortes de porteurs de charge, et que leurs populations varient de manière non monotone sous l’effet du dopage électrostatique. L’effet des propriétés électrostatiques et des corrélations électroniques sur ces variations sont discutées. Celles-ci sont à l’origine de réponses remarquables en ce qui concerne la conversion du spin en charge dans ces systèmes à l’aide d’un modèle de liaisons fortes et de la théorie de la réponse linéaire. Les effets conjoints du spin-orbite atomique et de la brisure de symétrie d’inversion à l’interface verrouille les nombres quantiques de spin, de caractère orbital et d’impulsion des électrons, et induit des textures de spin complexe dans l’espace réciproque. Ces textures sont responsables de l’apparition des effets Edelstein et Hall de spin dans ces hétérostructures et sont caractéristiques de la nature multi-orbitale de ces systèmes électroniques. Enfin nous conduirons une étude ab initio des hétérostructures STO/LAO/STO pour expliquer les observations expérimentales de nouvelles manières de former un gaz d’électrons à ces interfaces d’oxydes. Nous discuterons des rôles respectifs de la chimie, de l’électrostatique et des défauts dans l’apparition de ce gaz
The anomalous transport properties of transition metal oxides, in particular the surface of SrTiO₃ or at the interface between SrTiO₃ and LaAlO₃ is investigated in this thesis. These systems host two-dimensional electron gases. Nonlinear Hall Effect measurements suggest that several species of carriers are present in these systems, and that their population is varying on a nontrivial manner upon electrostatic doping. The role of the electrostatics properties of the electron gas and of the electronic correlations are discussed in this light. Next we discuss the spin to charge conversion of these systems thanks to tight-binding modeling and linear response theory. The complex interplay between atomic spin-orbit coupling and the inversion symmetry breaking at the interface leads to a complex spin-orbital-momentum locking of the electrons, inducing spin textures. These spin textures are responsible for the appearance of the Edelstein and Spin Hall Effect in these heterostructures and are characteristic of the multi-orbital character of these electronic systems. Finally an ab initio study of STO/LAO/STO heterostructures is performed to explain experimental evidence of new ways to produce an electron gas at this interface. The respective roles of the chemistry, electrostatics and defects are discussed

The thesis demonstrates some of the possibilities of using light to measure and understand microscopic properties of noble metal and nickel surfaces. The work consist mainly of three parts:

1. The design and construction of a complete reflection anisotropy spectroscopy (RAS) system, connected to a ultra-high vacuum (UHV) chamber where samples can be cleaned and measured at pressure levels less than 1∙10^-10mbar.

2. Theoretical progress in classical local field effect calculations of resonat dipoles at (110) surfaces of noble metals. Methods to solve the plane-wise dipole-interaction coefficients are developed. It is investigated how variations in local-field effect model parameters inclluding effective surface hieght and plasma-frequency, change the shape of the calculated spectra. In addition, it is discussed how imperfections in the surface, such as steps and vacancies, can be inlcuded in the model.

3. RAS measurements of clean and reconstructed surfaces Ag(110), Cu(110), Au(110), Au (100), Pt(100) and Ni(110) surfaces. The spectra for each of these surfaces are compared to theoretical model spectra. Using expressions for the screened dipole-dipole interaction we have found that the surface local-field effect contribute to the reflection anistropy of all the above surfaces except for Pt(100). Transitions between surface states at the Y point of the surface Brillouin zone are responsible for features in the spectra of Ag(110), Cu(110). Many of the above surfaces have a reflection anisotropy that can be described by a phenomenological shift or broadening of the permittivity near a critical-point energy.

Das Verhalten der elektronischen Eigenschaften von gespaltenen, aus der Schmelze gezüchteten In2O3-(111) Kristallen wurde bei Deposition von Edelmetallen, In und Sn mittels winkelaufgelöster Photoelektronen-Spektroskopie untersucht. Die Stöchiometrie, strukturelle Qualität und Kristall-Orientierung, die Oberflächenmorphologie und die Elektronenkonzentration wurden jeweils mittels energiedispersiver Röntgenspektroskopie, Laue-Beugung, Raster Tunnel-Mikroskopie (STM) und Hall-Effekt untersucht. Die Ähnlichkeit der fundamentalen und Oberflächen-Bandlücken kann auf das fast flache Verhalten der Bänder auf der gespaltenen Oberfläche der Kristalle zurückgeführt werden. Die Grenzflächen von Ag und Au/In2O3 zeigen Schottky-Verhalten, während ein ohmscher in Cu, In und Sn /In2O3-Kontakten beobachtet wurde. Aufgrund der Übereinstimmung zwischen optischen und Oberflächen-Bandlücken, der Bildung eines Gleichrichterkontaktes und des Auftretens der Oberflächenphotospannung auf der frischen Kristalloberfläche kann gefolgert werden, dass SEAL nicht eine intrinsische Eigenschaft der gespaltenen Oberfläche der untersuchten Kristalle ist. Des Weiteren wurden bei dicker Au- und Cu-Beschichtung von In2O3 bei Raumtemperatur Shockley-artige Oberflächenzustände beobachtet. Zusätzlich wurde die erste Phase des Wachstums von Cu und In auf In2O3 von der Ausbildung eines 2-dimensionalen Elektrongases (2DEG) begleitet, welches bei dickeren Schichten verschwand, die von dem auf reinen Oberflächen von dünnen In2O3- Filmen gemessenen 2DEG verschieden sind. Nach Messung der Austrittarbeit von In2O3 und den jeweils untersuchten Metallen in situ und unter Verwendung der Schottky-Mott-Regel trat außer bei Ag/In2O3 eine deutliche Abweichung auf. Die experimentellen Ergebnisse stimmen auch mit fortgeschrittenen Theorien, die auf dem Elektronegativitätskonzept und MIGS–Modellen basieren, nicht überein.
The behavior of the electronic properties of as-cleaved melt-grown In2O3 (111) single crystals was studied upon noble metals, In and Sn deposition using angle-resolved photoemission spectroscopy. The stoichiometry, structural quality and crystal orientation, surface morphology, and the electron concentration were examined by energy dispersive X-ray spectroscopy, Laue diffraction, scanning tunneling microscopy (STM), and Hall-effect measurement, respectively. The similarity of the measured-fundamental and surface-band gaps reveals the nearly flat behavior of the bands at the as-cleaved surface of the crystals. Ag and Au/In2O3 interfaces show Schottky behavior, while an ohmic one was observed in Cu, In, and Sn/In2O3 contacts. From agreement of the bulk and surface band gaps, rectifying contact formation as well as the occurrence of photovoltage effect at the pristine surface of the crystals, it can be deduced that SEAL is not an intrinsic property of the as-cleaved surface of the studied crystals. Moreover, for thick Au and Cu overlayer regime at room temperature, Shockley-like surface states were observed. Additionally, the initial stage of Cu and In growth on In2O3 was accompanied by the formation of a two dimensional electron gas (2DEG) fading away for higher coverages which are not associated with the earlier-detected 2DEG at the surface of In2O3 thin films. The application of the Schottky-Mott rule, using in situ-measured work functions of In2O3 and the metals, showed a strong disagreement for all the interfaces except for Ag/In2O3. The experimental data also disagree with more advanced theories based on the electronegativity concept and metal-induced gap states models.

This thesis describes the experimental investigations into the transverse mode structure of nearly hemispherical microcavities. The nearly hemispherical niicrocavity structures are fabricated electrocemically through a template of self assembled latex spheres. Controlling the electrochemical parameters, such as the electrochemical solution and electrode potential. allows a wide range of uearly henuspherical rnicrocavities to be realised. The spatial intensity profiles arid resonant frequeucies of the transverse modes of nearly hemispherical nucrocavities are measured experimentally for a wide range of cavity lengths amid mirror curvatures. The experimental mode profiles are radially symmetric Gauss-Laguerre modes, but do not display the frequency degeneracies typical of large scale optical cavities. The nearly hemispherical mnicrocavity samples are compared to investigate how the cavity parameters. such as cavity length and mirror curvature, affect the experimental spatial intensity profiles and resonant frequencies of the transverse modes. Higher order modes are observed despite the fact that they are forbidden due to the symmetrical coupling geometry. rrhe symmetry breaking is shown to be produced by the surface roughness of the curved nnrror. The frequency degeneracy lifting which occurs in the nearly hemispherical niicrocavity structures can he explained and modelled by considering non-parabolic elements in the cavity. A nmnher of mathematical models for the cavity propagation are developed based on paraxial theory. rrliese models are analysed and the predictions made from the models are compared with the experimental profiles and frequencies. The basic agreement between theory and experinient shows that the paraxial theory is able to model the cavity modes. However, the spectrum and the mnode profiles are cpnte sensitive functions of the geometry of the cavity amid the surface roughness of the cavity mirrors. The nearly hemispherical mnicrocavities are structures which offer a new fabrication technique allowing inexpensive and a ummconmplicated method of fabrication. An important feature of the nearly hemispherical microcavities is the tunablity, and the ease in which this can be achieved. The structures are also empty, and this will allow them, in the future, to be easily filled with functional optical nmaterials such as liquid crystals.

During recent years, new types of materials have been discovered with unique properties. One family of such materials are two-dimensional materials, which include graphene and MXene. These materials are stronger, more flexible, and have higher conductivity than other materials. As such they are highly interesting for new applications, e.g. specialized in vivo drug delivery systems, hydrogen storage, or as replacements of common materials in e.g. batteries, bulletproof clothing, and sensors. The list of potential applications is long for these new materials. As these materials are almost entirely made up of surfaces, their properties are strongly influenced by interaction between their surfaces, as well as with molecules or adatoms attached to the surfaces (surface groups). This interaction can change the materials and their properties, and it is therefore imperative to understand the underlying mechanisms. Surface groups on two-dimensional materials can be studied by Transmission Electron Microscopy (TEM), where high energy electrons are transmitted through a sample and the resulting image is recorded. However, the high energy needed to get enough resolution to observe single atoms damages the sample and limits the type of materials which can be analyzed. Lowering the electron energy decreases the damage, but the image resolution at such conditions is severely limited by inherent imperfections (aberrations) in the TEM. During the last years, new TEM models have been developed which employ a low acceleration voltage together with aberration correction, enabling imaging at the atomic scale without damaging the samples. These aberration-corrected TEMs are important tools in understanding the structure and chemistry of two-dimensional materials. In this thesis the two-dimensional materials graphene and Ti3C2Tx MXene have been investigated by low-voltage, aberration-corrected (scanning) TEM. High temperature annealing of graphene covered by residues from the synthesis is studied, as well as the structure and surface groups on single and double Ti3C2Tx MXene. These results are important contributions to the understanding of this class of materials and how their properties can be controlled.

En este trabajo se han realizado cálculos de primeros principios para estudiar las propiedades físicas de superficies y materiales laminares. Los cálculos se basan en la obtención de las propiedades electrónicas por medio de la Teoría del Funcional de la Densidad, con la que se obtienen la energía y fuerzas atómicas para cada sistema estudiado. De esta forma, se realizaron cálculos de optimización estructural y de dinámica molecular, que proporcionan información sobre las estructuras de equilibrio y la dinámica atómica en función de las condiciones externas (tales como presión y temperatura). Los cálculos realizados se han centrado en diferentes sistemas con interés experimental, siempre en estrecha colaboración con distintos grupos experimentales. En el campo de superficies, se han estudiado problemas relacionados con el crecimiento de láminas delgadas de Cobre para metalización de circuitos microelectrónicos, la adsorción de fullerenos sobre superficies de Germanio, y la medición de ondas de densidad de carga mediante microscopía de efecto túnel en bronces azules. En materiales laminares, el trabajo se ha centrado en comprender el efecto de la presión sobre las propiedades estructurales y electrónicas de semiconductores laminares III-VI, así como en explicar la aparición de nuevas fases no-laminares para varios de estos sistemas a altas presiones.
In this work we have made first-principles calculations to study the physical properties of surfaces and layered materials. The calculations are based on obtaining the electronic properties through the Density Functional Theory, with which the energy and atomic forces for each system studied are derived. Thus, structural optimization calculations and molecular dynamics were carried out. They provide information about the equilibrium structure and the atomic dynamic as a function of external conditions (such as temperature and pressure). Calculations have focused on different systems with experimental interest, always in strong collaboration with different experimental groups. In the field of surfaces we have been studied problems associated with the growth of thin films of copper for metallization of microelectronic circuits, the adsorption of fullerenes on Germanium surfaces, and the measurement of charge density waves by scanning tunneling microscopy in bronze blue. In layered materials, the work has focused on understanding the effect of pressure on the structural and electronic properties of layered semiconductor III-VI, as well as explaining the emergence of new no-layered phases for several of these systems at high pressures.

Wide band gap metal oxides have recently become one of the most investigated materials in surface science. Among these metal oxides especially TiO2 attracts great interest, because of its wide range applications, low cost, biocompatibility and ease of analysis by all experimental techniques. The usage of TiO2 as a component in solar cell technology is one of the most investigated applications of TiO2 . The wide band gap of TiO2 renders it inecient for isolated use in solar cells. TiO2 surface are therefore coated with a dye in order to increase eciency. This type of solar cells are called dye sensitized solar cells . The eciency of dye sensitized solar cells is directly related with the absorbed light portion of the entire solar spectrum by the dye molecule. Inspite of the early dyes, recent dye molcules, which are called wider wavelength response dye molecules, can absorb a larger portion of entire solar spectrum. Thus, the eciency of dye sensitized solar cells is increased by a considerably amount. In this thesis the electronic structure of organic rings, which are the fundamental components of the dye molecules, adsorbed on anatase (001) surface is analyzed using density functionaltheory. The main goal is to obtain a trend in the electronic structure of the system as a function of increasing ring number. Electronic structure analysis is conducted through band structure and density of states calculations. Results are presented and discussed in the framework of dye sensitized solar cells theory.

In this thesis, the effects of various chemical treatments on the surface modification of bulk InAs are investigated. The study focuses on the chemical processes that occur upon the exposure of the surface to sulphur-, chlorine- and bromine-containing solutions and oxygen, and the resulting changes to the electronic structure of the surface, as deduced from photoluminescence (PL) measurements, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Raman scattering and scanning electron microscopy (SEM). Three processing treatments were evaluated: i) treatment with sulphur-based solutions (Na2S:9H2O, (NH4)2S + S, [(NH4)2S / (NH4)2SO4] + S); ii) etching in halogen-based solutions (bromine-methanol and HCl: H2O); and iii) thermal oxidation. A significant overall enhancement in PL response was observed after chemical treatment or thermal oxidation, which is associated with a reduction in surface band bending. These changes correlate with the removal of the native oxide, in addition to the formation of well-ordered layers of In-S (or In-As)O as a passivating layer, indicating that electronic passivation occurs at the surface. The passivating effect on sulphide treated surfaces is unstable, however, with an increase in band bending, due to reoxidation, observed over periods of a few days. The lowest re-oxidation rate was observed for ([(NH4)2S / (NH4)2SO4] + S). Etching in HCl:H2O and Br-methanol solutions of appropriate concentrations and for moderate times (1 min) resulted in smooth and defect-free InAs surfaces. Etching completely removed the native oxides from the surface and enhanced the PL response. The adsorption of bromine and chlorine onto the InAs surface led to the formation of As-Brx , In-Brx, As-Clx and In-Clxcompounds (x = 1, 2, 3), as inferred from changes in the In 3d3/2; 5/2 and As 3d core level binding energies. The etch rate was found to decrease because of strong anisotropic effects. The improvements in surface properties were reversed, however, if the concentrations of the etchants increased or the etch time was too long. In the worst cases, pit formation and inverted pyramids with {111} side facets were observed. Surface treatments or thermal oxidisation significantly enhanced the PL intensity relative to that of the as-received samples. This was due to a reduction in the surface state density upon de-oxidation, or in some cases, to the formation of a well ordered oxide layer on the surface. The overall increase in PL intensity after treatment is ascribed to a reduction in band bending near the surface. This allows several welldefined peaks not observed or reported previously for bulk InAs (with a carrier concentration n~2x1016 cm-3), to be studied. A combination of PL and XPS measurements before and after the various treatments was used to identify the chemical nature of the impurities giving rise to bound exciton recombination in InAs (111).

Solid-state dye-sensitized solar cells (ssDSCs) offer the possibility of high power conversion efficiencies (PCEs) of over 20%. However, after more than a decade of research, devices still barely reach over 7% PCEs. In this thesis, limitations to device performance are studied in detail, and solutions for future advancement are put forward. In the first part of the thesis, factors limiting charge generation are explored by studying the crystallization environment of mesoporous TiO2 self-assembled through block copolymers. It was found that the density and distribution of sub band gap states are a function of the synthesis conditions and critically affect the performance characteristics of the self-assembled titania used in ssDSCs. As a result, the self-assembled mesoporous oxide system presented in this thesis outperforms for the first time the conventional nanoparticle based electrodes fabricated and tested under the same conditions, with demonstrated PCEs of over 5%. In chapters 6, 7, and 8, the factors limiting the diffusion length and hence, the thickness of the fabricated devices, are carefully examined. Previous literature points towards insufficient pore-filling of the hole transporting material (HTM) as the main limiting factor. In chapter 6, a pore-filling study is shown where a new technique to evaluate the pore-filling fraction of the HTM in the conventional mesoporous metal oxide electrode is also presented and conclude that sufficient pore-filling of thick films can easily be achieved. Another usual strategy to extend the electron lifetime in the devices and thus, the charge diffusion length, involving thin film coatings of insulating metal oxides is examined in chapter 7, with satisfactory results for SnO2-based ssDSCs. The diffusion length can also be extended if the factors limiting the diffusion of charges through the device are identified and removed, as presented in chapter 8. Finally, a study on the stability of the ssDSC is presented in chapter 9. The developments achieved enable long term stability to be effectively targeted, and represent a key milestone towards commercial realization of ssDSCs.

A scanning tunneling microscope has been used to study the surface properties of CdTe crystals. Coupled with scanning tunneling spectroscopy (STS) and atomic force microscopy, the surface morphology, structure and electronic properties of CdTe and CdZnTe surfaces have been studied. We have systematically investigated the three low index surface planes of the cubic crystal, that is the {100}, {110} and {111} surface planes. In addition, wet chemically treated surfaces were also examined. Clean surfaces were prepared in ultra-high vacuum conditions using argon ion sputtering and annealing. For each surface we imaged and recorded the surface reconstructions and morphologies. For the (100) surface, a mixed c(2x2)+(2x1) surface phase was found, where steps on the surface were found to preferentially align along <100> directions. For the (110) surface, tunneling spectroscopy was used to investigate the surface electronic structure of the (1x1) reconstruction. Using theoretical calculations of the tunneling current, we were able to match theory to experiment and discern the various vacuum tunneling processes for both n-type and semi-insulating material. For the (111) surface, a (2x2) reconstruction consisting of a cadmium vacancy structure was found. For the (-1-1-1) surface, a very disordered c(8x4) reconstruction was observed, consisting of a complicated tellurium terminated chain structure. For both faces, a large amount of faceting was observed to occur with the facets formed by {311} planes. Finally, wet chemically treated surfaces, important for the construction of many semiconductor devices, were investigated. Here the change in surface morphology for a variety of different common surface preparation methods was observed and, using STS, various surface electronic states were identified.