Dissertations / Theses on the topic 'Electronic spectroscopies'

Extraordinary chemical and physical properties have been discovered from the studies of two-dimensional (2D) materials, ever since the successful exfoliation of graphene, the first 2D material. Theoretical investigations of electronic structure and spectroscopies of these materials play a fundamental role in deep understanding the various properties. In particular, the band structure and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy can provide critical information near the Fermi level. In this thesis, we performed first-principles density functional theory calculations to study the electronic structure and NEXAFS spectra of four materials, including three 2D materials and one bulk material. The three 2D materials are atomically thin bismuth telluride, nitrogen and boron nitride doped graphenes. The bulk material is lithium intercalated graphite, an analogue of lithium doped graphene. Our studies provide important electronic property information of the studied materials, and are helpful for understanding their properties and developing potential applications.

QC 20160425

Clusters are finite ensembles of atoms or molecules with sizes in the nanometer regime (i.e. nanoparticles). This thesis present results on the geometric and electronic structure of homogeneous and heterogeneous combinations of atoms and molecules. The systems have been studied with synchrotron radiation and valence, core and Auger electron spectroscopic techniques.

The first theme of the thesis is that of mixed clusters. It is shown that by varying the cluster production technique both structures that are close to that predicted by equilibrium considerations can be attained as well as far from equilibrium structures.

Electronic processes following ionization constitute the second theme. The post-collision interaction phenomenon, energy exchange between the photo- and the Auger electrons, is shown to be different in clusters of argon, krypton and xenon. A model is proposed that takes polarization screening in the final state into account. This result is of general character and should be applicable to the analysis of core level photoelectron and Auger electron spectra of insulating and semi-conducting bulk materials as well.

Interatomic Coloumbic Decay is a process that can occur in the condensed phases of weakly bonded systems. Results on the time-scale of the process in Ne clusters and mixed Ar/Ne clusters are herein discussed, as well observations of resonant contributions to the process. In analogy to Auger vis-à-vis Resonant Auger it is found that to the ICD process there is a corresponding Resonant ICD process possible. This has later been observed in other systems and by theoretical calculations as well in subsequent works by other groups.

Delocalization of dicationic valence final states in the hydrogen bonded ammonia clusters and aqueous ammonia has also been investigated by Auger electron spectroscopy. With those results it was possible to assign a previously observed feature in the Auger electron spectrum of solid ammonia.

L'efficacité du processus photovoltaïque dépend du matériau actif à travers la structure de bande et la dynamique des porteurs de charge. Dans cette thèse, nous avons relié les propriétés électroniques et la dynamique de relaxation à la structure atomique des matériaux utilisés pour deux technologies différentes de cellules solaires, celle à base d’hétérostructures de silicium, et celle à base de pérovskites hybrides organiques-inorganiques. Dans les cellules solaires de silicium, nous avons analysé l'influence des défauts sur les propriétés électroniques des hétérostructures de silicium amorphe (a-Si:H/a-SIC:H/c-Si) par spectroscopies des niveaux de coeur et de la bande de valence. En particulier, nous avons quantifié le nombre de liaisons pendantes induites dans la couche a-Si:H par irradiation, et nous avons identifié les états électroniques qui leur sont associés. Enfin nous avons expliqué les transitions précédemment observées par photoluminescence. Dans les cellules solaires à pérovskite hybride, nous avons corrélé la structure atomique, la structure électronique et la dynamique électronique pour des pérovskites bi- et tridimensionnelles. Dans ce but nous avons utilisé tout un panel de techniques complémentaires: diffraction des rayons X, spectroscopie de photoémission résolue en angle, spectroscopie de photoémission inverse et photoémission à deux photons résolue en temps. Pour la pérovskite bidimensionnelle (C₆H₅C₂H₄NH₃)₂PbI₄, nous avons déterminé expérimentalement les bandes de valence et de conduction et nous les avons comparées aux simulations de la fonction spectrale. Pour la pérovskite tridimensionnelle CH₃NH₃PbI₃, nous avons aussi déterminé les structures de bande expérimentale et simulée. Des signatures spectrales très larges ont été observées expérimentalement, ce qui relaxe les conditions de transition optique avec un impact éventuel sur l'efficacité des cellules solaires. Tant dans les expériences que dans les calculs, nous observons que le poids spectral suit une périodicité cubique alors que le système est structurellement dans une phase tétragonale. Cette contradiction apparente s'explique par la largeur spectrale des bandes, qui cache le repliement dû à la distorsion tétragonale. En ce qui concerne la dynamique de relaxation, nous avons observé que les porteurs photoexcités se thermalisent dans une échelle de temps subpicoseconde par couplage aux vibrations des cations organiques. À des échelles de temps plus longues (10~100 picosecondes), la diffusion électronique contrôle la dynamique. Cette dynamique est affectée par les défauts induits par recuit, qui localisent les électrons photoexcités pendant plus de 300 picosecondes
The efficiency of the photovoltaic process depends on the electronic band structure of the active material and the charge carrier dynamics. In this thesis, we have studied how these issues are related to the atomic structure in materials for two different technologies of solar cells, namely silicon heterostructure solar cells, and hybrid organic-inorganic perovskite solar cells. In silicon heterostructure solar cells, we have analyzed the impact of defects on the electronic properties of amorphous silicon heterostructures (a-Si:H/a-SIC:H/c-Si) by core level and valence band spectroscopies. In particular, we have quantified the number of dangling bonds inside a-Si:H layer upon irradiation, we have identified the electronic states associated to them, and we have understood the transitions previously observed by photoluminescence. In perovskite solar cells, we have correlated the atomic structure, the electronic structure and the electronic dynamics for two- and three-dimensional hybrid organic-inorganic perovskites. We have used with this goal a whole panel of complementary techniques: X-ray diffraction, angle-resolved photoemission spectroscopy, inverse photoemission spectroscopy, and time-resolved two-photon photoemission. In the two-dimensional perovskite (C₆H₅C₂H₄NH₃)₂PbI₄, the valence and conduction bands have been determined experimentally and compared to spectral function simulations. In the three-dimensional perovskite CH₃NH₃PbI₃, we have again determined the band structure and simulated it. Very broad spectral features have been experimentally observed, which relax the optical transition conditions impacting in the solarcell efficiencies. In both experiments and calculations, we observe that the spectral weight follows a cubic periodicity while the system is structurally in the tetragonal phase. This apparent contradiction is explained by the band broadness, which hides the band folding of the tetragonal distortion. As for the relaxation dynamics, we have observed that the photoexcited carriers thermalize in a subpicosecond time scale through the coupling to organic cation vibrations. At longer timescales (10~100 picoseconds), the electron diffusion controls the dynamics. This dynamics is affected by the annealing-induced defects, which localize the photoexcited electrons for more than 300 picoseconds

2007/2008
In the course of this thesis I present an experimental investigation into various strongly correlated transition metal and rare earth compounds using core-level spectroscopic techniques, which have augmented the understanding of the electronic properties of these different systems. I will discuss the orbital occupation and the symmetry of the states near the Fermi level, responsible for the variety of ground states shown by a variety of strongly correlated systems such as misfit cobaltates, strontium ruthenates, metallic manganese and rare earth compounds.
XXI Ciclo
1978

Cette thèse présente une étude des propriétés électroniques de systèmes de basse dimension à base de métaux et de semiconducteurs. La première partie de l'étude traite le confinement de l'état de Shockley dans des nanostructures tridimensionnelles d'Ag(111), par des mesures STM/STS à très basse température (5 K). Nous avons d'abord analysé en détail la structure en énergie et la distribution spatiale des modes confinés. Nous avons ensuite mis à profit la nature discrète du spectre en énergie pour étudier le temps de vie des quasiparticules. Un comportement typique de liquide de Fermi est mis en évidence, et nous montrons que le mécanisme de diffusion dominant est associé au couplage électron-phonon. La contribution extrinsèque provenant du confinement partiel de l'onde électronique a également été obtenue. Une loi d'échelle est observée avec la taille des nanostructures, ce qui permet d'extraire un coefficient de réflexion plus important que dans de simples ilôts monoatomiques. La seconde partie de l'étude est consacrée aux couches ultra-minces semiconductrices obtenues par dépôts d'alcalins (K, Rb, Cs) sur la surface Si(111):B-[racine]3. Ce travail résout la controverse concernant la nature de l'état fondamental de ce système, et notamment l'origine de la reconstruction 2[racine]3 obtenue à la saturation du taux de couverture. La compréhension en amont de la structure cristallographique permet d'élucider les propriétés électroniques. Nous montrons qu'une approche à un électron, conduisant à un isolant de bandes, décrit le système de manière convaincante, malgré l'indication de forts effets polaroniques. Ce résultat est le fruit d'une étude approfondie combinant des techniques diverses et complémentaires (LEED, ARPES, XPS, STM/STS et calcul DFT)
This thesis is devoted to the electronic properties of low-dimensional systems based on metal and semiconducting materials. The first part deals with the Shockley state confinement in Ag(111) nanostructures, by means of very-low temperature (5 K) STM/STS measurements. We study the electronic structure and spatial distribution of the confined modes. Then the discrete nature of the electronic spectrum allows one to yield the quasiparticule lifetime. A Fermi-liquid behaviour is evidenced and we show that the dominant decay mechanism is attributed to the electron-phonon coupling. The extrinsic contribution arising from the partial confinement of the electronic wave is obtained as well. A scaling law with the nanostructure width is demonstrated, from which we deduce a higher reflection amplitude than in monoatomic islands. In the second part of the thesis, we study semiconducting ultra-thin films produced by alkali (K, Rb, Cs) deposition on the Si(111):B-[root of]3 surface. This work solves the controversy concerning the ground state of this system, and especially the nature of the 2[root of]3 surface recontruction obtained at saturation coverage. Prior understanding of the crystallographic structure allows to elucidate the electronic properties. We show that a one-electron picture, leading to a band insulator scenario, gives a good description of the system, in spite of strong polaronic effects. This conclusion results from an in-depth, combined study of complementary techniques (LEED, ARPES, XPS, STM/STS and DFT calculations)

2011/2012
Nel vasto scenario dei materiali fortemente correlati gli ossidi dei metalli di transizione hanno attratto enorme interesse a causa delle loro interessanti proprietà fisiche, come ad esempio, la superconduttività nei cuprati e la magnetoresistenza gigante nelle manganiti. In particolare, il mio interesse è stato rivolto ad una specifica classe di materiali, per i quali la dimensionalità è il parametro più importante. Le attività sperimentali sono state focalizzate verso due sistemi: la manganite Pr0.5Ca1.5MnO4 dopata a metà e a strato singolo (hd-PCMO) e la famiglia dei rutenati Srn+1RunO3n+1 (n=1,2,3). Entrambi questi sistemi esibiscono fenomeni affascinanti strettamente legati ad una complicata interazione tra i gradi di libertà del reticolo cristallino, di spin, di carica, ed orbitale, dove la dimensionalità cristallina gioca un ruolo cruciale. Con il mio progetto di dottorato ho studiato alcune proprietà dei materiali sopracitati per mezzo di spettroscopie con raggi X, come l’emissione risonante di raggi X (RXES) e l’assorbimento di raggi X (XAS) statico e risolto in tempo. Tutte le misure sono state condotte utilizzando la linea di luce BACH (linea di luce per dicroismo avanzato) dell’anello di accumulazione Elettra della Elettra-Sincrotrone Trieste. Il sistema hd-PCMO presenta una transizione di ordinamento di carica ed orbitale (CO-O) ad una temperatura TCO relativamente elevate, i.e. 340 K, accompagnata da una distorsione strutturale ortorombica, dove i portatori di carica fortemente correlati eg del Mn si ordinano in sotto-reticoli cristallografici separati (stato di carica ordinato) con un carattere orbitale specifico (stato di ordinamento orbitale). Inoltre, hd-PCMO presenta anche una risposta reticolare anomala ad una temperatura 20 K sopra la temperatura di Neél TN, che è associata ad un inatteso accoppiamento spin-reticolo. Poiché mancava uno studio degli stati elettronici non occupati del PCMO, misure dipendenti dalla temperatura per mezzo del dicroismo lineare (XLD) sono state realizzate alle soglie K dell’ossigeno e L3 del Mn al fine di spiegare il ruolo della topologia orbitale dei Mn 3d – O 2p. I dati sperimentali, supportati da calcoli ab-initio LDA+U, ci danno informazioni sulla ridistribuzione di carica e sui cambiamenti delle p-DOS alla transizione CO-O e a quella antiferromagnetica (AFM). I risultati ottenuti mostrano che l’interazione competitiva tra la distorsione locale atomica, necessaria per permettere l’ordinamento CO, e le dinamiche di carica del meccanismo di hopping regolano lo stato orbitale dei portatori di carica. Inoltre, sulla base di studi teorici che predicono la formazione di fasi orbitali e strutturali transienti “nascoste” per mezzo della stimolazione ottica, abbiamo studiato le DOS non occupate dello stato metastabile indotto otticamente nel PCMO per mezzo della XAS risolta in tempo, che offre uno strumento unico per misurare le DOS proiettate in sito ed in simmetria degli stati metastabili della materia. Le misure XAS risolte in tempo alla soglia K dell’ossigeno sono state realizzate per mezzo di un nuovo apparato sperimentale disponibile a BACH, che si basa su un laser Ti:zaffiro (impulsi di pompa) con tasso di ripetizione variabile sincronizzato con gli impulsi a 500 MHz dei raggi X (impulsi di sonda). L’evoluzione temporale degli spettri XAS attraverso la transizione CO-O fotoindotta otticamente risulta differente rispetto alle misure XAS adiabatiche, dimostrando l’esistenza di una “fase nascosta” fotoindotta nel PCMO, la cui natura è ancora sconosciuta. I rutenati Srn+1RunO3n+1 (n=1,2,3) sono emersi come una famiglia importante di peroschiti a causa dell’evoluzione inattesa e senza precedenti dal comportamento anisotropico ferro- o metamagnetico del Sr4Ru3O10 (n=3) dipendente dalla direzione del campo magnetico, all’ aumentato paramagnetismo di Pauli vicino all’ordinamento magnetico del Sr3Ru2O7 (n=2) e, infine, alla superconduttività a bassa temperature in Sr2RuO4 (n=1). Nonostante vengano riportati numerosi studi sulle proprietà strutturali e magnetiche di questi composti, l’evoluzione delle strutture elettroniche occupate e non occupate non è stata investigata in dettaglio. Quindi, la dipendenza delle strutture elettroniche e l’ibridizzazione degli stati 2p dell’ossigeno sono state investigate combinando la spettroscopia XAS alla soglia K dell’ossigeno (transizione 2p-1s) dipendente dalla polarizzazione e la spettroscopia RXES. Una sezione del capitolo 3 è dedicata ad illustrare un setup sperimentale sviluppato recentemente per esperimenti XAS risolti in tempo sfruttando la struttura temporale “multibunch” dell’anello di accumulazione del sincrotrone. Sfruttando le potenzialità di questo setup, la transizione di superficie semiconduttore-metallo nel germanio cristallino è stata fotoindotta ed il set completo di dati viene discusso. Lo schema della mia tesi di dottorato è il seguente. Il primo capitolo presenta una panoramica dell’intero lavoro. Il secondo capitolo è diviso in due sezioni. La prima sezione introduce il lettore alla fisica orbitale ed alle transizioni di fase elettroniche nei metalli di transizione a ridotta dimensionalità, con un excursus sullo stato dell’arte dei composti 3d del manganese e la famiglia 4d dei rutenati. L’intento della seconda sezione è quello di spiegare l’importanza delle tecniche spettroscopiche nei raggi X molli come strumenti per investigare le proprietà elettroniche dei solidi. La descrizione delle spettroscopie XAS e RXES vengono riviste più in dettaglio nel capitolo 3, che include anche la descrizione dell’apparato sperimentale della beamline BACH e del laboratorio T-ReX al Sincrotrone Elettra. Il capitolo 4 è dedicato alla teoria funzionale di densità (DFT) ed alla approssimazione locale di densità più U (LDA+U) ed ai dettagli del modello del sistema hd-PCMO. Il capitolo 5, che presenta i casi studiati, è diviso in due sezioni: il caso del PCMO, che include le misure XAS statiche e risolte in tempo, ed il caso della serie Ruddlesden-Popper dei rutenati di Sr investigate per mezzo della RXES. Nel capitolo finale vengono presentati i commenti finali su questo lavoro.
In the vast scenario of strongly correlated-electron materials transition-metal oxides have attracted enormous interest because of their interesting physical properties, including for example, superconductivity in cuprates and colossal magnetoresistance in manganites. In particular, my interest was directed to a particular class of materials, whose dimensionality is the most defining material parameter. With my Ph.D. project I deepened into some physical properties of these materials by means of core-levels spectroscopies such as resonant x ray emission (RXES) and static and time-resolved x ray absorption (XAS). All the measurements have been carried out at the beamline BACH (Beamline for Advanced diCHroism) at the Elettra light source facility in Trieste. The experimental activities focused on two case-study systems: the single layered half-doped Pr0.5Ca1.5MnO4 (hd-PCMO) and the layered Srn+1RunO3n+1 (n=1,2,3) family. Both these systems exhibit fascinating phenomena intimately related to a complicated interplay between the crystal lattice, spin, charge, and orbital degrees of freedom, where crystal dimensionality plays a crucial role. hd-PCMO exhibits a charge-orbital ordering (CO-O) transition at a remarkably high TCO, slightly above room temperature, accompanied by an orthorhombic structural distortion, where the strongly correlated Mn eg charge carriers order onto separate crystallographic sub-lattices (charge-ordered state) with a specific orbital character (orbital ordered state). Furthermore, hd-PCMO also displays an anomalous lattice response at temperatures 20K above the Neél temperature TN, which is associated to an unexpected spin-lattice coupling. Since a study of the PCMO unoccupied electronic states was lacking, temperature dependence measurements by XAS linear dichroism (XLD) have been performed at the O-K and Mn-L3 thresholds in order to elucidate the role of Mn 3d - O 2p orbital topology. The experimental data, supported by ab-initio LDA+U, shed light on the charge redistribution and p-DOS changes at the CO-O and antiferromagnetic (AFM) transitions. The results obtained show that the competitive interplay between the local atomic distortion, necessary for accomodating the CO-ordering, and the charge dynamics of the hopping mechanism regulates the orbital state of the charge carriers. Furthermore, on the basis of theoretical studies that predict the formation of transient “hidden” orbital and structural phases by optical stimulation, we have studied the unoccupied DOS of the optically induced metastable state in PCMO by means of time resolved XAS, which offers a unique tool to measure site and symmetry projected DOS of metastable states in matter. Tr-XAS measurements at the O-K edge have been carried out by means of a novel experimental apparatus available at BACH, which is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ∼ 500 MHz X-ray photon pulses (probe pulses). The time evolution of the XAS lineshapes across the optically photoinduced CO-O transition results different respect to the adiabatic XAS measurements, demonstrating the existence of a photoinduced “hidden phase” in PCMO, whose nature is still unknown. The layered Srn+1RunO3n+1 (n=1,2,3) have emerged as an important family of perovskites because of the unexpected and unprecedented evolution from anisotropic ferro- or metamagnetic behavior of Sr4Ru3O10 (n=3) dependent on the direction of the magnetic field, enhanced Pauli paramagnetism close to magnetic order of Sr3Ru2O7 (n=2) and, finally, to low-temperature superconductivity in Sr2RuO4 (n=1). Although numerous studies have been reported on the structural and magnetic properties of these compounds, the evolution of the occupied and unoccupied electronic structures were not investigated in detail. Thus, the dependence of electronic structures and the hybridization of O 2p states have been investigated by combining polarization dependent O K (2p-1s transition) XAS and RXES spectroscopies. A section of the chapter 3 is dedicated to illustrate a newly developed experimental setup for time-resolved XAS experiments by exploiting the multibunch time structure of a synchrotron storage ring. By exploiting the capabilities of this setup, the surface semiconductor-metal transition in crystalline germanium has been photoinduced and the complete set of data discussed. The outline of my Ph.D. thesis is the following. The first chapter presents an overview of the entire work. The second chapter is divided into two sections. The first section introduces the reader into the orbital physics and the electronic phase transitions in low dimensional transition metal oxides, with an excursus on the state of the art of 3d manganese compounds and the family of 4d Ruthenates. The second section is aimed to explain the importance of soft x-ray spectroscopic techniques as tools to investigate the electronic properties of solids. The description of XAS and RXES are reviewed in more details in chapter 3, which includes also the description of the experimental apparatus of BACH beamline and T-ReX lab at the Elettra synchrotron light source. Chapter 4 is dedicated to the Density Functional Theory (DFT) and Local Density Approximation plus U (LDA+U) theories and to the details of the modelling of the hd-PCMO system. Chapter 5, which presents the cases studied, is divided into two sections: the case of PCMO, including static and time resolved XAS measurements, and the case of Ruddlesden-Popper series of Sr Ruthenates investigated by means of RXES. In the final chapter the concluding remarks on this work are presented.
XXV Ciclo
1983

Le travail de thèse "Modification of the electronic structure of catalytic active metal centres upon adsorption of molecules : a XAS and XES study" porte sur l'étude de la structure électronique des métaux de transition utilisés dans les catalyseurs par les techniques spectroscopiques d'absorption et d'émission de rayons durs (rayonnement synchrotron). Il s'agit, en particulier, d'utiliser les avancées dans les techniques spectroscopiques des rayons X pour l'étude des matériaux nanostructurés en conditions in situ. Le premier chapitre de la thèse, "X-Ray Spectroscopy: an Overview", présente brièvement les techniques spectroscopiques disponibles sur la ligne de lumière ID26 de l'Installation Européenne de Rayonnement Synchrotron (en anglais European Synchrotron Radiation Facility (ESRF), Grenoble), où toutes les mesures présentées dans ce travail ont été effectuées. Dans le deuxième chapitre, "Study of the Electronic structure of the Ti-sites in TS-1 using X-ray spectroscopy", est décrite la caractérisation électronique des centres de Ti en TS-1, qui est un important catalyseur, employé dans de nombreuses usines à travers le monde. Nous montrons que la combinaison des techniques spectroscopiques d'absorption et d'émission des rayons avec des calculs de mécanique quantique permet d'obtenir des informations importantes sur la structure électronique des centres de Ti dans des conditions in situ. Le troisième chapitre, “Identification of the Ti-Ligands in a Silica Supported Ziegler-Natta Catalyst by X-Ray Emission Spectroscopy", présente l'étude d'un catalyseur (une variante du catalyseur Ziegler-Natta), qui affiche un degré élevé/important de désordre chimique. L'utilisation de la théorie fonctionnelle de la densité des états (DFT) pour l'interprétation des données expérimentales a permis de développer des modèles possibles pour l'environnement d'un ligand Ti. Le quatrième chapitre, "Observing the dd-Excitations in CPO-27-Ni using Resonant Inelastic X-ray Scattering", conclut la partie principale de la thèse. Il présente un exemple d'utilisation de la diffusion inélastique des rayons X (RIXS) en chimie pour la détermination des excitations du champ cristallin des ions Ni dans l'oxyde de nickel et d'une cage organique autour d'un Ni métallique (CPO-27-Ni). Le chapitre explique brièvement les différentes approches théoriques qui peuvent être utilisées pour l'interprétation des caractéristiques spectrales. De plus, l'adsorption de molécules sonde, comme le CO sur les centres de Ni en CPO-27-Ni, est discutée sur la base des données expérimentales RIXS. Le dernier chapitre synthétise les résultats obtenus et indique des perspectives futures. Enfin, dans les annexes sont reportés les dispositifs expérimentaux développés dans le cadre du travail de thèse pour des mesures résolues en temps, ainsi que le curriculum vitae et les publications du candidat
The purpose of this research project was to apply advanced X-ray based spectroscopic techniques for investigating the electronic structure of transition metals within catalysts and molecular sieves under in-situ conditions. Thus, the first chapter of the thesis, "X-Ray Spectroscopy: an Overview " briefly presents the spectroscopic techniques available at ID26, beamline of the European Synchrotron Radiation Facility (Grenoble) where all the measurements reported in this work have been obtained. In the second chapter, "Study of the Electronic structure of the Ti-sites in TS-1 using hard X-ray spectroscopy", it is reported the electronic characterization of the Ti centres in titanium silicalite-1 (TS-1), that is a relevant catalyst employed in industrial plants worldwide. The chapter shows that the combination of X-ray absorption and X-ray emission spectroscopy with quantum mechanical calculations is effective to obtain important insights on the electronic structure of the Ti-centres under in-situ conditions. The third chapter entitled “Identification of the Ti-Ligands in Silica Supported Ziegler-Natta Catalyst by X- Ray Emission Spectroscopy" presents the study of a variant of the Ziegler-Natta catalyst. The chapter discusses the interpretation of the valence emission lines within the theoretical framework provided by the density functional theory (DFT) and proposes possible models for the Ti-ligand-environment. The fourth chapter, entitled "Observing the dd-Excitations in CPO-27-Ni using Resonant Inelastic X-ray Scattering", concludes the main part of the thesis. It presents the application of resonant inelastic X-ray scattering (RIXS) for obtaining the crystal field excitations of the Ni ions within nickel oxide and within a Ni- metal-organic-framework (CPO-27-Ni). The chapter briefly describes the different theoretical approaches that can be used for the interpretation of the spectral features and discusses the adsorption of probe molecules like H2O, CO and H2S on the Ni centres of CPO-27-Ni. The last chapter (Chapter five) drawn a series of conclusions concerning the performed investigations and indicates possible future research directions. In Appendix A entitled "Pump and Probe Time Resolved Experiments at ID26" it is reported the description of the experimental setup co-developed and co-realized by the candidate for time resolved experiments. The appendix also accounts for the scientific outcome of the performed pump and probe measurements. The curriculum vitae and the publications list of the candidate are respectively reported in Appendix B and C

Ce travail porte sur le développement de nouvelles méthodes de caractérisation in-situ basées sur les spectroscopies électroniques XPS et MM-EPES associées à des calculs théoriques obtenus grâce à des simulations Monte-Carlo afin de réaliser des études quantitatives fines et précises. La première partie de ce travail, a été consacrée à l’analyse quantitative de signaux XPS et MM-EPES. Pour cela, dans un premier temps, la fonction de correction de l’analyseur hémisphérique (HSA) qui est une combinaison de l’aire d’analyse (A) et de la transmission (T) a été déterminée en utilisant une nouvelle méthode basée sur des images élastiques. Pour la première fois, la dépendance de A en énergie cinétique des électrons a été mise en évidence. Avec l’utilisation de cette nouvelle fonction, une méthode de caractérisation in situ basée sur la modélisation théorique des signaux XPS et MM-EPES a été développée. Cette méthode a permis d’étudier le dépôt d’un film d’or sur un substrat de silicium oxydé et a montré une grande précision dans le cas de très faibles quantités de matière déposée (< 2 nm) alors que les techniques microscopiques classiques se sont révélées inefficaces. La deuxième partie a porté sur le développement d’une nouvelle technique d’imagerie in-situ appelé MM-EPEM qui consiste à scanner la surface par un faisceau d’électrons et de collecter les électrons rétrodiffusés élastiquement afin de construire une image en intensité de la surface. Les étapes d’obtention des images MM-EPEM et les procédures d’exploitation de ces dernières ont été décrites et optimisées. Ensuite, cette technique a été utilisée pour l’étude de l’état de surface de dépôts d’or sur différents substrats. Cette technique s’avère être non destructive et très sensible aux éléments présents à la surface. Et elle permet de déterminer la cartographie chimique et la nano-organisation de la surface
This thesis focuses on the development of new in-situ methods of characterization based on the electron spectroscopies (XPS and EPES) coupled with theoretical calculations obtained through Monte-Carlo simulations in order to perform very precise quantitative studies. The first part of this thesis was devoted to quantitative studies of XPS and MM-EPES measurements. Firstly, the correction function of a hemispherical analyzer (HSA) which is a combination of the analysis area (A) and the transmission (T) was determined using a new method based on the elastic images. For the first time, the dependence of A on the kinetic energy of electrons was highlighted. Using this function, an in-situ method based on the combination of XPS and MM-EPES modeling was setting up. This method was used to determine the organization of gold film deposed on oxidized silicon substrate. Measurements show that this method is able to determine surface parameters when the microscopy techniques do not give any information in the case of a small quantity of gold deposit (less than 2 nm). The second part of this work was directed towards developing a new generation of microscopy called MM-EPEM which is based on the detection of elastic electrons. The stages required to obtain these images are well described and optimized here. The MM-EPEM images processing was used to study gold growth on different substrates. This technique is a non-destructive method and allows the operator to construct chemical tomography and to determine the nano-organization of the surface

The details of the electronic structure and bonding in ferrocenes and early transition metal bent metallocenes are probed by photoelectron spectroscopy. The fundamental electronic interaction of the methyl group substituted for a hydrogen on a metal-coordinated cyclopentadienyl ring is shown by a combined core and valence pe spectroscopic study of a series of methyl-substituted ferrocenes. Shifts of core and valence ionization energies upon methyl substitution are equivalent and additive for the iron atom. Knowledge of the core ionization energy shifts for both carbon and iron allow the relative changes in atomic charges upon methyl substitution to be found. In these molecules, the methyl group is found not to be an inductive electron donor as is commonly assumed. The primary electronic effect of methyl substitution is on the valence orbitals of the cyclopentadienyl ring. Methylation of the cyclopentadienyl rings of ferrocene causes a dramatic redistribution of valence electron density and greatly increases the covalent nature of metal-ring bonding. An understanding of the electronic effect of methylation of metal-coordinated cyclopentadienyl rings is used to establish the relative locations of ring π and Cl ionizations in the pe spectra of group IV and V early transition metal bent metallocene dichlorides with both unsubstituted cyclopentadienyl and pentamethylcyclopentadienyl ligands. The differences in chloride and methyl ligand bonding to an early transition metal center are reflected in the photoelectron data of the dichlorides and related dimethyls. The relative differences in metal-chlorine and metal-carbon bond strengths are also shown in the pe data. The relationship between bond strengths and ionization energies is detailed for early transition metal bent metallocenes of niobium and tantalum with a variety of ligands. The relative bond strength/ionization energy information for metal-hydrogen and metal-carbon bonds is shown to help in understanding the stability of niobocene and tantalocene ethylene-hydride complexes and their resistance to intramolecular olefin insertion. Evidence from the pe data concerning the electron distribution as well as the relative bond strengths in these ethylene-hydride complexes is found supporting the consideration of these complexes more properly as metallacyclopropane-hydrides.

Principles of transition metal electronic structure are presented to enable an understanding of the activation of C-H and C=C bonds by metals. A multitechnique approach utilizing core and valence photoelectron spectroscopies (p.e.s.) and molecular orbital calculations has been used to gain these insights. In the first half of the dissertation three principles are developed: ligand additivity, core-valence ionization correlation, and ring methylation. In the latter half of the dissertation these principles are seen to be crucial for understanding ionization data for the C-H and C=C activated species. Additive (with respect to ligand substitution) electronic effects, including additive core and valence ionization potentials, are shown in the p.e.s. of phosphine substituted molybdenum carbonyls. These additive effects demonstrate that the electronic effects of ligand substitution are predictable from empirical models. The core-valence ionization correlation enables direct comparison of XPS (core) and UPS (valence) ionization data and allows separation of bonding and overlap induced valence shift effects from Coulombic and relaxation shift effects. In the study of trimethylphosphine substituted cyclopentadienylmanganese tricarbonyl complexes, both the ligand additivity and core-valence ionization correlation principles are less valid than for the molybdenum carbonyl complexes because of loss of the very influential carbonyl backbonding. Methylation of the cyclopentadienyl ring in this system adds another independent variable of electronic structure perturbation and enables separation of the one-center and two-center Coulombic contributions to the core shifts. The above principles are used in the later chapters to show that the initial activation of the C-H bond in alkenylmanganese tricarbonyl complexes is dominated by the interaction of the C-H sigma bonding level with empty metal acceptor levels. The activation stops at the agostic stage rather than proceeding to full β-hydribe abstraction because there is, in these molecules, no gain in the number of pi electrons between the allyl and diene hydride endpoints of the abstraction cycle. Activation of the C=C bond in the cyclopentadienylmetal olefins is similar for Co and Rh complexes despite little similarity in the valence ionization spectra. The spectral differences are largely caused by the relaxation energy differences between Co and Rh. These complexes also provide interesting examples of electron delocalization through the metal. Permethylation of the cyclopentadienyl ring shifts the olefin pi ligand ionizations more than the expected Coulombic shift.

Les composés de cérium et d'ytterbium sont des systèmes qui présentent des propriétés magnétiques, électroniques et thermodynamiques exceptionnelles à basse température, tels que l'effet Kondo, les fermions lourds, la valence intermédiaire et des ondes de densité de charge. Ces propriétés sont dues aux interactions électron-électron très fortes des états f et à l'hybridation des états f avec les électrons de conduction. Dans la première partie de ce travail de thèse, nous étudions la structure électronique de l'YbPd, un composé cubique à haute température, qui présente des ondes de densité de charge (CDW) incommensurable et commensurable dans en dessous de T_1 = 130 K et T_2 = 105 K, respectivement. En dessous de 105 K, les sites magnétique Yb^{3+} et non magnétique Yb^{2.6+} sont disposés alternativement le long de l'axe c, en une structure tétragonale présentant un ordre de valence. La spectroscopie de photoémission de rayons X durs (HAXPES) des états 3d de l'Yb montre des caractéristiques Yb^{2+} et Yb^{3+} indépendantes de la température au-dessus de T_1, et un changement de valence des deux sites cristallographiques d'Yb dans la phase CDW. Des calculs des spectres HAXPES au seuil 3d de l'Yb avec un modèle à une impureté d'Anderson simplifié montrent un bon accord avec la dépendance en température de la valence et fournissent les températures de Kondo des deux sites d'Yb. Les résultats indiquent une évolution de la valence intermédiaire dans la phase cubique à un ordre CDW statique à longue portée dans la phase tétragonale. Nous proposons que le mécanisme Kondo est à l'origine de l'onde de densité de charge. La diffusion inélastique résonante des rayons X est une technique très efficace pour étudier les excitations élémentaire dans une large variété de systèmes tels que les excitations de transfert de charge, les excitations de champ cristallin et de spin-orbite. Dans la deuxième partie de ce travail, le RIXS combiné avec des simulations à base du modèle à une impureté d'Anderson et la théorie des multiplets a été utilisé pour l'étude des composés de cérium avec différentes températures de Kondo T_K. Nous avons démontré que toutes les excitations de charge observées en les spectres de photoémission et photoémission inverse combinés peuvent être mises en évidence par la spectroscopie RIXS simplement en faisant varier l'énergie du photon incident autour du seuil M_5 du cérium. Nous discutons également de la dépendance drastique de la structure f^0 dans les spectres RIXS en fonction de la polarisation et de la forte dépendance en température de la structure de type fluorescence, qui rappelle la dépendance à la température de la résonance de Kondo
Cerium and ytterbium compounds are systems that exhibit outstanding magnetic, electronic, and thermodynamic properties at low temperatures, such as the Kondo effect, heavy fermions, intermediate valence, and charge density waves. These properties are due to the very strong electron-electron interactions of the f states and the hybridization of the f states with the conduction electrons. In the first part of this thesis work, we investigate the temperature-dependent electronic structure of YbPd, a Kondo mixed-valent cubic compound at high temperature, which exhibits incommensurate and commensurate charge-density-wave (CDW) ordering below T_1 = 130 K and T_2 = 105 K, respectively. Below 105 K, magnetic Yb^{3+} and nonmagnetic Yb^{2.6+} are arranged alternatively along the c axis, in a tetragonal structure exhibiting valence order. Hard x-ray photoemission spectroscopy of Yb 3d states show temperature-independent Yb^{2+} and Yb^{3+} features indicating single-Yb site dynamic valence fluctuations above T_1, and a clear temperature dependent valence change of the two crystallographic Yb sites in the CDW phase. Simplified single-impurity Anderson model calculations of the Yb 3d states show good agreement with the temperature-dependent valency change and provide site-dependent Kondo temperatures. The results indicate an evolution from dynamic mixed-valence in the cubic phase to long-range static CDW order in the tetragonal phase, which is driven by the difference in Kondo energies of the two phases. Resonant inelastic x-ray scattering (RIXS) is a very efficient technique to study elementary excitations in a large variety of systems, such as charge transfer excitations, the crystal field, and spin-orbit excitations. In the second part of this work, we investigated cerium compounds with different Kondo temperatures using RIXS at Ce M_5-edge combined with simulation using the full multiplet theory and the single impurity Anderson model. We demonstrated that all the charge excitations observed in combined photoemission and inverse photoemission can be evidenced by RIXS spectroscopy just by varying the incident photon energy around the Ce M_5-edge. We also discuss the drastic polarization dependence of the f^0 structure in RIXS spectra and the strong temperature dependence of the fluorescence-like structure, which is reminiscent of the temperature dependence of the Kondo resonance

Thesis (Ph. D.)--Ohio State University, 2004.
Title from first page of PDF file. Document formatted into pages; contains xvii, 208 p.; also includes graphics (some col.). Includes bibliographical references (p. 189-208).

Polymeric nanocomposites have received an exceptional amount of attention over the recent years as they have the ability to possess enhanced properties. The use of nanosized phases in composite materials, as opposed to their microsized counterpart, delivers characteristics which allow nanodielectric systems to operate at an increased performance and improved efficiency. The requirements of the polymeric system can easily be tailored to suit speci�c applications with as little as 2 wt.% filler loading, whilst maintaining the typical weight of the virgin material. With the transition from micrometric to nanomeric phases, the volume of the interfacial region increases dramatically and this is where the mechanisms behind nanocomposite behaviour are believed to occur. As the potential for nanodielectrics is endless, the importance of in-depth studies into the �ller-matrix interface is fundamental. Many studies have already partaken in research which uses organosilanes as a coupling agent, however few the quantity of organosilane as a variable parameter, or compared the use of hydrous and anhydrous functionalisation methods. This study investigates the consequences of introducing differently functionalised nanosilicas into epoxy systems; a number of spectroscopic techniques (Raman spectroscopy, Fourier transform infrared spectroscopy and combustion analysis) were employed to quantify the level of surface modification on the surface of silica nanoparticles, before mixing methods were developed in an attempt to reach nanoparticle homogeneity in an epoxy matrix. Scanning electron microscopy was employed to investigate the dispersion state of the filler with respect to the degree of functionalisation, whilst data from AC breakdown studies, differential scanning calorimetry and dielectric spectroscopy were analysed to determine the effects of differently functionalised nanosilica in a dielectric system. Theinvestigation shows how condensation reactions within the interphase has an infuence dielectric behaviour, and highlights how changes in the stoichiometry of the epoxy system alters the polymerarchitecture to have an effect on the electrical properties of the nanocomposites. Further studies explore the use of confocal Raman spectroscopy as a tool in probing the nanofiller-matrix interface. A simulation based on the scattering of incident photons was compared with empirical data from a range of dielectric �lms; modi�cations to the scattering photon approach relates physically obtained values for bulk attenuation directly to those observed in confocal Raman depth profiles. Although it was found that the revised model was able to produce confocal Raman depth profiles that closely match experimental data from the nanocomposite films, the nature of nanoparticle agglomeration during functionalisation and the typical resolution of confocal Raman systems do not allow for the detection of chemical changes on the filler.

Les plasmons de surface (SP) sont des ondes électromagnétiques se propageant à l'interface entre deux milieux, typiquement un métal et un diélectrique. Les plasmons de surface ont la capacité de confiner le champ électromagnétique dans de très petite région de l’espace, typiquement quelques nanomètres, c’est à dire bien en dessous de la limite de diffraction de la lumière. Une conséquence de ce confinement sub-longueur d’onde de la lumière est que leur observation nécessite une résolution spatiale nanométrique - ce qui exclut l’utilisation de techniques optiques standard. Néanmoins, le microscope électronique en transmission à balayage (STEM) est un outil particulièrement adapté à l'étude des plasmons de surface car il emploie des électrons rapides ayant une longueur d’onde typique comprise entre 1 et 10 picomètres. Ainsi, durant la dernière décennie, les spectroscopies électroniques appliquées à la nano-optique se sont fortement développées, parmi elle comptent : la spectroscopie de perte d'énergie électronique (EELS), la spectroscopie cathodoluminescence (CL) ou l'interférométrie de Hanbury Brown et Twiss (HBT) appliquée à la CL. Dans cette thèse, j’ai exploré différents problèmes ouverts de la plasmonique et de la nano-optique dans le cadre particulier de la microscopie électronique. Dans le chapitre 3, je présente un formalisme prenant en compte à la fois la nature quantique et relativiste des expériences d’EELS en faisant appel notamment à des éléments de théorie quantique des champs. Dans le chapitre 4, nous démontrons que la réalisation d’une expérience d’EELS avec de tels faisceaux permet de mesurer des propriétés jusqu’alors inatteignable à l’échelle du nanomètre telle que la phase des plasmons, leurs chiralité optique voire même leur longueur de cohérence. Dans le chapitre 5, je présente plusieurs résultats théoriques et expérimentaux concernant des expériences de couplage. En particulier, j’étudie le phénomène contre-intuitif d’auto-hybridation qui est une conséquence de la nature non-hermitienne du problème aux valeurs propres associé aux résonances de plasmon et établit une analogie avec les systèmes quantiques ouverts. Enfin, au chapitre 6, je discute des récentes mesures de phonon réalisées dans un STEM grâce au développement de monochromateur électroniques
Surface plasmons (SP) are electromagnetic waves propagating at the interface between two media typically a metal and a dielectric. SPs can confine electromagnetic fields in very short volumes (typically one to few nanometers), well below the light diffraction limit. This property has a tremendous number of applications ranging from fundamental physics (e.g. quantum optics) to applications (e.g. cancer therapy). However, the price to pay is that SPs suffer from huge ohmic losses in the metal which leads to very short lifetimes (typically few femtoseconds). Theoretically, this presence of dissipation dramatically hardens the theoretical description of SPs. Another consequence of the sub-wavelength confinement of light associated with SPs is that their observation requires a nanometric resolution - which excludes the use of standard optical techniques. Yet, the scanning transmission electron microscope (STEM) is a particularly suitable tool to study SPs as it employs fast electrons with typical wavelength from 1 to 10 picometers. Thus, the last decade has seen the tremendous development of electron-based spectroscopies applied to nano-optics such as electron energy loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL) or STEM- Hanbury Brown and Twiss interferometry (HBT). In this thesis, I explored different open problems of plasmonics and nano-optics under the scope of electron microscopy and spectroscopies. In chapter 3, I develop a formalism taking into account both the quantum and relativistic nature of EELS experiments using elements of quantum field theory. In chapter 4, I apply the latter formalism to the case of EELS measurements of SPs using electrons with shaped phase. In chapter 5, I give several theoretical and experimental results on coupling experiments involving SPs. Particularly, I demonstrate a counterintuitive type of coupling, the so-called self- hybridization which is a consequence of the non-Hermitian nature of the LSP eigenproblem and draw analogy with open quantum system. Finally, in chapter 6, I discuss the recent result on vibrational EELS in monochromated STEM

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.

Surface structure, chemical composition, bonding configuration, film polarity, and electronic properties of InN layers grown by high pressure chemical vapor deposition (HPCVD) have been investigated. Sputtering at an angle of 50-70 degrees followed by atomic hydrogen cleaning (AHC) was successful in removing the carbon contaminants. AHC is found to be the most effective cleaning process to remove oxygen contaminants from InN layers in an ultrahigh vacuum (UHV) system and produced a well ordered surface. Auger electron spectroscopy (AES) confirmed the cleanliness of the surface, and low energy electron diffraction (LEED) yielded a 1×1 hexagonal pattern demonstrating a well-ordered surface. High resolution electron energy loss spectra (HREELS) taken from the InN layers exhibited loss features at 550 cm-1, 870 cm-1 and 3260 cm-1 which were assigned to Fuchs-Kliewer phonon, N-H bending, and N-H stretching vibrations, respectively. Assignments were confirmed by observation of isotopic shifts following atomic deuterium dosing. No In-H species were observed indicating N-termination of the surface and N-polarity of the film. Broad conduction band plasmon excitations were observed centered at 3100 cm-1 to 4200 cm-1 in HREEL spectra acquired with 25 eV electrons, for a variety of samples grown with different conditions. Infrared reflectance data shows a consistent result with HREELS for the bulk plasma frequency. The plasmon excitations are shifted about 300 cm-1 higher in HREEL spectra acquired using 7 eV electrons due to the higher plasma frequency and carrier concentration at the surface than in the bulk, demonstrating a surface electron accumulation. Hydrogen completely desorbed from the InN surface upon annealing for 900 s at 425 ºC or upon annealing for 30 s at 500 ºC. Fitting the coverage versus temperature for anneals of either 30 or 900 s indicated that the desorption was best described by second order desorption kinetics with an activation energy and pre-exponential factor of 1.3±0.2 eV and 10-7.3±1.0 cm2/s, respectively. Vibrational spectra acquired from HREEL can be utilized to explain the surface composition, chemical bonding and surface termination, and film polarity of InN layers. The explanation of evidence of surface electron accumulation and extraction of hydrogen desorption kinetic parameters can be performed by utilizing HREEL spectra.

The huge growth in nanotechnology brings with it an increasing need for techniques capable of structural and chemical analysis on the nanoscale. Two such techniques are electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and x-ray absorption spectroscopy in the scanning transmission x-ray microscope (STXM-XAS). Despite the different probe-specimen interactions of EELS and XAS, these techniques both excite transitions from core states into unoccupied excited states, providing remarkably similar chemical information, but with different spatial and spectral resolutions, as well as different electron/photon induced damage properties. Due to advances in the fabrication of diffractive x-ray optics, soft x-ray probes in the STXM can now achieve ~15 nm spatial resolution, while the STEM probe can routinely reach sub-nanometre dimensions. At the same time, the spectral performance of EELS is becoming more competitive with the ~0.1 eV resolutions of x-ray instruments, due to the development of monochromators in the electron microscope. While the spatial and spectral performances of these two techniques are continually converging, the gaps which still exist may be bridged by employing a correlative approach. The motivation of this thesis is to apply both STEM-EELS and STXM-XAS to a range of nanoscale systems, and to investigate the benefits and limitations of this correlative approach. In order to understand the biodegradation of carbon nanotubes (CNTs), spectral signatures corresponding to graphitic carbon have been used to map CNT aggregates within the cellular environment. The nature and distribution of functional groups on oxidised CNTs have also been characterised using EELS, and compared to XAS data from the literature. Lastly, wear debris generated from CoCr hip prostheses has been investigated within explanted tissue. For the first time, detailed chemical analysis was performed of debris particles, which were found to be composed of mainly oxidised Cr as well as trace amounts of oxidised Co. Additionally, some nanoparticles were observed to have metallic Co and Cr cores.

In this thesis, two sets of quantum cascade lasers were studied. The first set is three Ga0.3In0.7As/A1As(Sb)/lnP strain compensated QCLs emitting at 3.3 μrn at room temperature. The three QCL samples have the same active region design but differ in their waveguide doping. The losses within the samples were investigated by the Fabry- Perot fringes contrast technique which provides probing the QCLs with both transverse electric (TE) and transverse magnetic (TM) polarizations for measuring the waveguide losses and the resonance intersubband absorption respectively. We found that reducing the waveguide doping slightly, results in a reduction in the waveguide losses by more than 50%. Additionally, when the injector doping is reduced, the resonance absorption within the QCL structures is reduced too. The second set of the studied quantum cascade lasers is three Gao41n06As/ Alo58Ino42As strain-compensated on InP substrates emitting in the vicinity of 5 urn. The three lasers have different active region designs. Those active regions are double phonon design, single phonon design and zero phonon design. The main difference among those designs is their extraction rate. The double phonon design has the most efficient extraction while the zero phonon design has least and the single phonon is in the middle. The three lasers show performance with the same order as their active region extraction efficiencies which means that the laser with the active region design having the most efficient extraction shows the highest performance. A comparative investigation of those samples was performed by using the broad-band mid infrared transmission technique. This technique enables the mid-infrared spectroscopic study of the electronic distribution in quantum cascade lasers whilst under operating conditions. Using the TE polarization, the waveguide of the samples was investigated in a wide range of wavelength (1.2-15 μrn). On the other hand, the TM polarization was used for investigating the electron distribution in a range of temperature and bias condition by probing the intersubband transitions in the QCL devices. The gain build- up, the resonance intersubband absorption evolution and the transparency current were also investigated in a range of temperature and bias for the three samples. The dispersive gain was observed for all the samples at high temperatures however for the zero phonon design it commenced at a temperature as low as 80 K. A new experimental method for measuring the transparency current from the transmission spectra is introduced. The transparency current measured contributes about 70% to the threshold current for our samples. The analysis of the obtained transmission spectra also revealed that gain onset is achieved at a certain voltage value for all the temperatures which we call "voltage threshold". The electron temperature for the zero phonon design was also predicted from the analysis of the obtained transmission spectra. It was found that the electron temperature measures about a 100 K higher than the lattice temperature.

In the present thesis the electronic properties of two organic molecules were studied by means of density functional theory (DFT) in connection to their possible applications in organic photovoltaics and molecular spintronics respectively. The first analysed system is the C60 derivative PCBM extensively used in polymer solar cells for the charge separation process. Since fullerenes have been shown to undergo modifications as a result of light exposure, investigating their electronic structure is the first step in elucidating the photodegradation process. The electronic excitations from core levels to unoccupied molecular orbitals reveal not only the empty level structure of the molecule, but provide additional information related to the chemical bonds involving a specific atom type. In this way, they represent a means of determining the chemical changes that the molecule might withstand. The electronic transitions from carbon 1s core levels to unoccupied states are explained for the unmodified PCBM by a joint theoretical (DFT) and experimental study using the near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The second investigated system is the transition metal phthalocyanine with a manganese atom as the metal center. Manganese phthalocyanine (MnPc) is a single molecular magnet in which the spin switch process can be triggered by various methods. It has been shown, for instance, that the adsorption of hydrogen to the Mn center changes the spin state of the molecule from 3/2 to 1. More interestingly, the process is reversible and can be controlled, opening up the possibility of using MnPc as a quantum bit in magnetic memory devices. Up to this date, the d orbital occupation in MnPc has been under a long debate, both theoretical and experimental studies revealing different configurations. In this thesis the electronic structure of the phthalocyanine is thoroughly analysed by means of DFT and the calculated results are compared to photoelectron spectroscopy measurements. The combination of theoretical and experimental tools reveals that in gas phase at high temepratures the molecule exhibits a mixed electronic configuration. In this light, the possible control of the specific electronic state of the central metal represents an interesting prospect for molecular spintronics.

La prédiction de propriétés spectroscopiques moléculaires et l’interprétation de spectres expérimentaux nécessitent de faire appel à la théorie. Une première étape consiste à se limiter à la spectroscopie électronique dans l’approximation de Born-Oppenheimer ce qui consiste à considérer les noyaux de la molécule comme étant fixes et les états électroniques indépendants les uns des autres. L’objectif de cette thèse est d’étudier la structure électronique de petites molécules organiques et organométalliques dans l’approximation de Born-Oppenheimer dans un premier temps avant d’aller au delà en prenant en compte des effets tels que le couplage vibronique ou le couplage spin-orbite entre les états électroniques. Le premier chapitre est consacré aux méthodes ab initio utilisées pour obtenir les résultats présentés dans les chapitres ultérieurs. Une première partie est consacrée aux méthodes de structure électronique dans l’approximation de Born-Oppenheimer, elle est suivie d’une partie qui traite des effets de couplage vibronique et spin-orbite. Le second chapitre présente une étude de la structure électronique et des courbes d’énergie potentielle des carbènes de métaux de transition MCH+2 (M=Fe, Co, Ni). Le troisième chapitre rapporte les spectres vibroniques des fluoroéthylènes obtenus par simulation et comparés aux spectres expérimentaux afin d’identifier l’origine de différentes contributions spectroscopiques. Enfin le dernier chapitre traite des effets spin-orbite dans l’eau et ses homologues lourds (H2X avec X=O, Te, Po)
Theory is sometime necessary to predict molecular spectroscopic properties and interpret experimental spectra. A first step study can be limited to the electronic spectroscopy in Born-Oppenheimer approximation which consists in considering nuclei fixed and electronic states independent from each other. The scope of this thesis is to first study the electronic structure of small organic and orgnometallic molecules in the Born-Oppenheimer approximation and ultimatly go beyond by taking into account effects such as vibronic or spin-orbit couplings between electronic states. The first chapter is dedicated to the ab initio methods used to obtain the results presented in the following chapters. Electronic structure methods in the Born-Oppenheimer approximation are first presented followed by the methods that treat vibronic and spin-orbit couplings. The second chapter is a study of the electronic structure and potential energy curves of MCH+2 (M=Fe, Co, Ni) transition metal carbenes. Chapter three reports simulated vibronic spectra of fluoroethylenes, they are compared to experimental spectra to indentify the origin of the different spectroscopic contributions. A last chapter deals with the spin-orbit effects in water and its heavy homologous (H2X with X=O, Te, Po)

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In the recent years, electronic devices have become smaller and faster, as well as more powerful and efficient. However, the size of the building blocks of the current electronics - i.e. the transistors - is quickly approaching the limits of manufacture. The need of a new generation of devices based on novel mechanisms is today essential. In this scenario, well-defined interfaces between oxide materials have produced novel electronic systems displaying a spectacular variety of properties with promising potentialities for future devices, such as colossal magnetoresistance, high-temperature superconductivity, magnetism at the interface between non magnetic oxides, as well as two-dimensional electron gas between two oxide insulators. Indeed, interfaces between perovskite oxides - e.g. the LaAlO3/SrTiO3 – have shown properties at the nanometer scale that are qualitatively different from their single building blocks, allowing one to engineer novel functionalities through the growth of epitaxial heterostructures. Nevertheless, both atomic and electronic reconstructions could be present in oxide interfaces when a polar discontinuity occurs at the junction and the possibility to probe non-destructively the cation depth profiles can provide further insight into the oxide heterointerface physics. This is also true for less-ordered systems - i.e. amorphous thin films - which are playing a key role in the development of new architectures in photovoltaic applications, such as the CdxSnyOz/TiO2 heterojunction. In this Thesis, the combination of chemical and structural information on a local scale (i.e. at the interface and the nearby few atomic layers) has been obtained by combining an effective modeling of angle-resolved x-ray photoemission data, with synchrotron based electron spectroscopy techniques. It is shown how in oxides the interfacial electronic properties can be driven by complex substitutional effects across the interface, such as stoichiometry gradients, cation vacancies and interdiffusion, as well as by the presence of interfacial/surface oxygen vacancies.

It is generally believed that the Si technology is going to hit a road block soon. Amongst all the potential candidates, graphene shows the most promise as replacement material for the aging Si technology. This has caused a tremendous stir in the scientific community. This excitement stems from the fact that graphene exhibits unique electronic properties. Physically, it is a two-dimensional network of sp₂bonded carbon atoms. The unique symmetry of two equivalent sublattices gives rise to a linear energy dispersion for the charge carriers. As a consequence, the charge carriers behave like massless Dirac particles with a constant speed of c/300, where c is the speed of light. The sublattice symmetry gives rise to unique half-integer quantum hall effect, Klein's paradox, and weak antilocalization. In this research work, I was able to successfully study the growth and electronic structure of EG on SiC(0001), in ultra-high vacuum and low-vacuum furnace environment. I used STM to study the growth at an atomic scale and macroscopic scale. With STM imaging, I studied the distinct properties of commonly observed interface region (layer 0), first graphene layer, and the second graphene layer. I was able to clearly resolve graphene lattice in both layer 1 and 2. High resolution imaging of the defects showed a unique scattering pattern. Raman spectroscopy measurements were done to resolve the layer dependent signatures of EG. The characteristic Raman 2D peak was found to be suppressed in layer 1, and a single Lorentzian was seen in layer 2. Ni metal islands were grown on EG by e-beam deposition. STM/ STS measurements were done to study the changes in doping and the electronic structure of EG with distance from the metal islands.

La découverte en 2008 des supraconducteurs à base de fer a ouvert un nouveau champ d'investigation de la supraconductivité à haute température critique. En particulier, la phase nématique de ces matériaux pourrait jouer un rôle prépondérant dans le mécanisme de la supraconductivité. Nous avons étudié le composé FeSe par spectroscopie Raman, à pression ambiante et sous pression hydrostatique. Celui-ci ne possède pas d'ordre magnétique statique à pression ambiante, ce qui en fait un composé de choix pour l'étude de l'ordre nématique. Nous avons observé les fluctuations nématiques de charge. Leur évolution dans la phase tétragonale prouve l'existence d'une instabilité nématique d'origine électronique, qui gouverne la transition structurale. Dans la phase orthorhombique, le comportement des phonons souligne le rôle du couplage spin-phonon dans la transition nématique. Par ailleurs, la forme de la réponse Raman supraconductrice est compatible avec l'existence de deux gaps de symétrie s, dont un est anisotrope. Sous pression hydrostatique, les fluctuations nématiques s'atténuent rapidement. Le point critique quantique électronique associé se situe à très basse pression, peu avant l'apparition de l'ordre magnétique. Les fluctuations nématiques disparaissent complètement vers 2 GPa, quand la transition structurale passe de second ordre à premier ordre. C'est également proche de cette pression que se produit une anomalie dans le comportement des phonons, qui indique une modification de la structure électronique du système. Nos mesures révèlent en outre l'existence d'un pseudogap. Sa température d'apparition chute simultanément à la disparition de la phase magnétique, quand la température critique de supraconductivité atteint son maximum. Enfin, la réponse Raman de l'état supraconducteur à 7.8 GPa montre une signature claire d'un gap plein
The discovery in 2008 of the iron-based superconductors opened a new field of investigation of high-temperature superconductivity. In particular, the nematic phase of these materials may play a major role in the mecanism of superconductivity. We studied the FeSe compound using Raman spectroscopy, at ambient pressure and under hydrostatic pressure. This material does not display any static magnetic order at ambient pressure and is therefore an excellent choice to study the nematic order. We observed the charge nematic fluctuations. Their evolution in the tetragonal phase proves the existence of an electronic nematic instability, which drives the structural transition. In the orthorhombic phase, the behaviour of the phonons underlines the role of the spin-phonon coupling in the nematic transition. Besides, the shape of the superconducting Raman response is compatible with the existence of two s-wave gaps, one of which is anisotropic. Under hydrostatic pressure, the nematic fluctuations reduce rapidly. The associated electronic quantum critical point is situated at very low pressure, just before the appearance of magnetic order. The nematic fluctuations completely disappear around 2 GPa, when the structural transition changes from second order to first order. An anomaly of the phonons also occurs close to this pressure, which indicates a modification of the electronic structure of the system. Our measurements additionally reveal the existence of a pseudogap. Its temperature of appearance reduces significantly simultaneously to the disappearance of magnetic order, when the critical temperature of superconductivity reaches its maximum. Finally, the Raman response in the superconducting state at 7.8 GPa shows a clear signature of a full gap

Ce travail expérimental porte sur la synthèse du silicène hétéroépitaxié sur des substrats Si(111) passivés par des atomes de bore ou d’argent. L’adsorption à température contrôlée d’une quantité de Si proche d’une monocouche sur les substrats B:Si(111)-(√3×√3)R30° et Ag/Si(111)-(√3×√3)R30°, mène à la formation, sur chaque substrat, d’une couche bidimensionnelle de Si, compacte et bien ordonnée, qui adopte la symétrie du substrat, que nous attribuons au silicène hétéroépitaxié. Nous avons utilisé le LEED, AES, IPES/ARIPES. La mesure du courant absorbé (TCS) et l’évolution du travail d’extraction indiquent de fortes perturbations des propriétés électroniques de la couche de charge d’espace du substrat. Celle-ci s’accompagne de la disparition des états de surface de chaque substrat, remplacés par de nouveaux états électroniques UB et U0 caractéristiques, aux profils de dispersion sans rapport avec la symétrie (√3×√3)R30°. Sur Ag/Si(111)-(√3×√3)R30°, la disparition de l’état bidimensionnel S1 d’électrons quasi-libres, à la dispersion parabolique, provoque une transition métal/isolant. Les positions énergétiques des états UB et U0, éloignées de EF, indiquent une interaction significative avec le substrat, bien que non-covalente, impliquant un transfert de charges spatialement inhomogène du silicène avec le substrat. Mesurés par ARIPES, les profils de la dispersion des états inoccupés UB et U0 caractéristiques de la couche 2D ordonnée de Si apparaissent compatibles avec la symétrie d’une monocouche de silicène orientée, qui fait correspondre la direction Γ-M√3 de la première zone de Brillouin de la reconstruction (√3×√3)R30° avec la direction Γ-KSilicene du silicène
We realized the heteroepitaxy of silicene on Si(111) substrates passivated in two ways, either by B or Ag atoms. We deposited Si atoms on the UHV-prepared substrates B:Si(111)-(√3×√3)R30° and Ag/Si(111)-(√3×√3)R30° kept at controlled temperatures. we used LEED, AES, IPES/ARIPES. According to LEED, the adsorption of roughly one monolayer of Si on both substrates leads to the formation of a compact Si bidimensional layer which adopts the (√3×√3)R30° symmetry of the substrates. TCS and the evolution of the work function indicate strong perturbations of the substrate space-charge layer, while IPES reveals the disappearance of the surface states characteristic of each substrate. On Ag/Si(111)-(√3×√3)R30°, the disappearance of the well-known S1 « free-electron-like » surface state induces a metal/insulator transition. Instead of these surface states, new unoccupied electronic states UB and U0 appear which are associated to silicene on each substrate, with dispersions profiles which do not show the characteristics of a (√3×√3)R30° symmetry. Their limited overall bandwidths (resp. ~0.3 and ~0.45 eV) indicate rather large effective masses for electrons and suggest possible correlation effects which could justify the rather large measured bandgaps (resp. 2 eV and ~1 eV). The positions of UB and U0, far from the Fermi level, manifest a non-covalent but sizable interaction silcene/substrate, most probably a charge transfer which may be spatially inhomogeneous. The dispersion profiles of UB and U0 measured by ARIPES are compatible with the symmetry of a silicene layer with the Γ-KSilicene direction oriented along the Γ-M√3 direction of the reconstruction (√3×√3)R30°

A detailed investigation has been undertaken into the field induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated and dielectric coated metallic electrodes. These processes have been investigated using two dedicated experimental systems that were developed for this study. The first is a novel combined photo/field emission microscope, which employs a UV source to stimulate photo-electrons from the sample surface in order to generate a topographical image. This system utilises an electrostatic lens column to provide identical optical properties under the different operating conditions required for purely topographical and combined photo/field imaging. The system has been demonstrated to have a resolution approaching 1m. Emission images have been obtained from carbon emission sites using this system to reveal that emission may occur from the edge triple junction or from the bulk of the carbon particle. An existing UHV electron spectrometer has been extensively rebuilt to incorporate a computer control and data acquisition system, improved sample handling and manipulation and a specimen heating stage. Details are given of a comprehensive study into the effects of sample heating on the emission process under conditions of both bulk and transient heating. Similar studies were also performed under conditions of both zero and high applied field. These show that the properties of emission sites are strongly temperature and field dependent thus indicating that the emission process is `non-metallic' in nature. The results have been shown to be consistent with an existing hot electron emission model.

Depuis sa découverte, les propriétés électroniques exceptionnelles du graphène ont fait l'objet d'un nombre impressionnant d'études, faisant émerger un nouveau domaine de recherche autour des cristaux bidimensionnels. La spectroscopie Raman permet d'accéder de façon rapide, non destructive et sélective en symétrie, à la dynamique des électrons et à leur couplage avec les autres degrés de liberté d'un matériau. Jusqu'au présent, cependant, cette technique a été réservée presque exclusivement à la caractérisation des propriétés vibrationnelles du graphène, qui ne sondent qu'indirectement ses propriétés électroniques. Dans ce travail je mets en évidence le signal Raman électronique de mono- et multi-couches de graphène en le modulant avec une tension de grille. Pour cela j'ai combiné des techniques avancées de fabrication de dispositifs avec un microscope Raman spécialement conçu pour cet objectif. Grâce à l'effet du champ électrique, le continuum Raman électronique du graphène dû aux transitions inter-bande à travers le cône de Dirac, a été identifié et son intensité quantifiée pour la première fois. Les spectres, avec la présence d'un blocage de Pauli des excitations électroniques, sont en excellent accord avec les prévisions théoriques. Les mesures résolues en polarisation ont mis en évidence une propriété originale de la spectroscopie Raman: le fait d'être une sonde privilégiée des excitations électroniques chirales. Cette propriété, attribuée à un phénomène d'interférences quantiques entre les amplitudes de diffusion, ouvre des prospectives très intéressantes dans l'étude d'autres cristaux bidimensionnels et des phases topologiques
Since its discovery, the exceptional electronic properties of graphene have been studied in an impressive number of academic works, giving birth to a new research field dealing with two-dimensional crystals. Raman spectroscopy is a quick, non-destructive and symmetry-selective way to probe the dynamics of electrons and to their coupling with the other degrees of freedom of a material. Until now, nonetheless, this technique had been almost exclusively reserved to the characterization of graphene's vibrational properties, which probe its electronic properties only indirectly. In this work I unravel the electronic Raman signal of mono- and multi-layer graphene tuning it with a gate voltage. In order to do so, I combined advanced techniques of device fabrication with a Raman microscope specifically designed for this goal. By means of the electric field effect, I identified and quantified for the first time the intensity of the electronic Raman continuum of graphene due to the inter-band transitions through the Dirac cone. The spectra, with the presence of a Pauli blocking of electronic excitations, match perfectly with theoretical expectations. The polarization resolved measurements revealed an original property of Raman spectroscopy: it is a unique probe of chiral electronic excitations. This property, attributed to a quantum interferences phenomenon between scattering amplitudes, opens very interesting perspectives in the study of other two-dimensional crystals and of topological phases

Brazing may be described as more of an art than a science , there is little comprehension of the basic mechanisms governing brazing in air or vacuum. Consequently the behaviour of the Gold 18% Nickel brazing alloy in vacuum was studied using the novel techniques of hot-stage scanning electron microscopy with simultaneous bulk and surface analysis. The work demonstrates that careful control of experimental conditions allows both Auger electron and X-Ray spectroscopies (AES and EDXA) to be carried out at high spatial and energy resolutions , with good signal to noise in a realistic analyical time. Combining post-brazing examination of samples heated in a UHV oven , with real-time studies of braze powder/substrate combinations on a heating stage in an Auger microprobe , revealed several important mechanisms. Vacuum brazing is a two-stage process; oxide penetration followed by wetting and spreading via a suboxide route. AES analysis showed that oxide penetration is achieved by reduction as a result of low oxygen partial pressures and high carbon activities , in the system as a whole , or locally (for example under or around a molten braze droplet) , leaving an oxide-free surface , or a discontinuous , penetrable oxide. After penetration has occurred wetting and spreading continues along the metal/metal-oxide interface. Flow may be impeded by residual oxide resisting the advancing liquid which is trying to disbond it. Alternatively , interdiffusion of the braze metal and substrate allows pick-up of elements which raise the melting temperature of the braze alloy and causes solidification. EDX analysis and digital element mapping during interdiffusion and solidification illustrates that the periphery of the braze pools are most affected , and that further elevating the temperature initially caused remelting , but eventually resulted in second phase formation in accord with the Au/Ni/Fe equilibrium diagram.

An (e,2e) electron scattering spectrometer has been constructed and used for the first time to investigate the spectral momentum density of the valence bands of a solid target. This technique provides fundamental information about the electronic structure of both crystalline and amorphous solids. The three fundamental quantities, the band structure, electron density of states, and electron momentum distribution can be simultaneously derived from the measured (e,2e) cross section. A review of single electron and (e,2e) scattering theory is given with an emphasis on scattering from solids. The effects of multiple scattering are discussed and a method of deconvoluting those effects from the measured (e,2e) cross section is developed. There is a detailed description of the spectrometer design and operation with particular attention given to the electron optics and voltage distribution. The algorithms and software for computer aided data acquisition and analysis are also outlined, as is error analysis. The techniques employed in the preparation and characterization of extremely thin film samples of a-C and single crystal graphite are described. An analysis of the data taken for a-C samples is given. The data are compared with the results of complementary experiments and theory for graphite, diamond, and a-C which are given in a review of the literature. The existence of a definite dispersion relation ε(q) in amorphous carbon is demonstrated. The a-C band structure appears to be more similar to that of graphite than to that of diamond, however it differs significantly from both in some respects. The measured spectral momentum density seems compatible with a model of a-C based on small, randomly-oriented islands of quasi-2D graphite-like continuous random network structures. However, no definitive interpretations can be made until higher resolution experiments are performed on both a-C and single crystal graphite.
Ph. D.