Diluted magnetic semiconductor oxides (DMSOs) have received great attention recently due to their outstanding applications in optoelectronic and spintronic devices. Ever since the initial observation of ferromagnetism at room temperature in cobalt-doped titania, extensive effort is concentrated on preparation of transition metal doped wide band gap semiconductors, especially Mn- doped ZnO. Compared to Mn-doped ZnO, magnetic interactions in SnO! and In!O! semiconductors have been underexplored. SnO! and In!O! semiconductors have many applications, owing to their high charge carrier density and mobility as well as high optical transparency. Investigation on electronic structure changes induced by dopants during the synthesis procedure can effectively influence magnetic interactions between charge carriers. In this work, a combination of structural and spectroscopic methods was used to probe as-synthesized SnO! and In!O! nanocrystals doped with Mn!! and Mn!! as precursors. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy are powerful techniques to explore formal oxidation state of manganese dopant, electronic environment, number of nearest neighbors around the absorbent, and bond lengths to the neighboring atoms. Analysis reveals the presence of multiple oxidation states in the doped nanocrystals, and establishes a relation between !"!! ratio and expansion or contraction of lattice parameters. !"!! Although doping semiconductors are crucial for manipulating the functional properties, the influence of dopants on nanocrystals structure is not well understood. Nanocrystalline films prepared from colloidal Mn-doped SnO! and In!O! nanocrystals through spin coating process exhibit ferromagnetic behavior in temperatures ranging from 5 K to 300 K. Magnetic transformation from paramagnetic in free-standing Mn-doped nanocrystals to strong ferromagnetic ordering in nanocrystalline films is attributed to the formation of extended structural defects, e.g., oxygen vacancies at the nanocrystals interface. Magnetic circular dichroism (MCD) studies clearly show that Mn!! occupies different symmetry sites in indium oxide, when bixbyite and rhombohedral In!O! nanocrystals (NCs) are compared.

Diluted magnetic semiconductor oxides (DMSOs) have received great attention recently due to their outstanding applications in optoelectronic and spintronic devices. Ever since the initial observation of ferromagnetism at room temperature in cobalt-doped titania, extensive effort is concentrated on preparation of transition metal doped wide band gap semiconductors, especially Mn- doped ZnO. Compared to Mn-doped ZnO, magnetic interactions in SnO! and In!O! semiconductors have been underexplored. SnO! and In!O! semiconductors have many applications, owing to their high charge carrier density and mobility as well as high optical transparency. Investigation on electronic structure changes induced by dopants during the synthesis procedure can effectively influence magnetic interactions between charge carriers. In this work, a combination of structural and spectroscopic methods was used to probe as-synthesized SnO! and In!O! nanocrystals doped with Mn!! and Mn!! as precursors. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy are powerful techniques to explore formal oxidation state of manganese dopant, electronic environment, number of nearest neighbors around the absorbent, and bond lengths to the neighboring atoms. Analysis reveals the presence of multiple oxidation states in the doped nanocrystals, and establishes a relation between !"!! ratio and expansion or contraction of lattice parameters. !"!! Although doping semiconductors are crucial for manipulating the functional properties, the influence of dopants on nanocrystals structure is not well understood. Nanocrystalline films prepared from colloidal Mn-doped SnO! and In!O! nanocrystals through spin coating process exhibit ferromagnetic behavior in temperatures ranging from 5 K to 300 K. Magnetic transformation from paramagnetic in free-standing Mn-doped nanocrystals to strong ferromagnetic ordering in nanocrystalline films is attributed to the formation of extended structural defects, e.g., oxygen vacancies at the nanocrystals interface. Magnetic circular dichroism (MCD) studies clearly show that Mn!! occupies different symmetry sites in indium oxide, when bixbyite and rhombohedral In!O! nanocrystals (NCs) are compared.

Made available in DSpace on 2014-12-17T15:41:47Z (GMT). No. of bitstreams: 1 WashingtonCMGpdf.pdf: 1072479 bytes, checksum: d37434aed1d2ae898ee7a9fd3701e0d7 (MD5) Previous issue date: 2009-04-29 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / This work deals with the application of X-Ray Absorption Spectroscopy on the study of the behavior of Cu2+ ions in inverse micelles. The formation of copper nanoparticles in water-in-oil microemulsions in pseudo-ternary systems of cetyl trimethylammonium Bromide (CTAB) surfactant, butanol co-surfactant, heptane as oil phase and aqueous solutions of CuSO4.5H2O, and NaBH4. The microemulsions were prepared with a fixed percentage (60 %) of oil phase and a variable water to tensoative proportion. It was observed an increase on Cu2+ reduction by the sodium borohydride in microemulsions with 13 % of aqueous phase, independent of the reaction time. For the microemulsions in which the aqueous phase is composed only by the CuSO4 solution, it was observed that the color of the solution depends on the water to surfactant ratio. These changes in color were attributed to a competition for the hidratation water between the polar head of the tensoative and Cu2+ ions with the eventual substitution of oxygen by bromine atoms in the first coordination shell of Cu2+ ions / Neste trabalho estudou-se aplica??o da espectroscopia de absor??o de raios x no estudo do comportamento dos ?ons Cu2+ em micelas inversas. Estudou-se a forma??o de nanopart?culas de cobre em microemuls?es ?gua-em-?leo de sistemas pseudo-tern?rios tensoativo (brometo de hexadeciltrimetilam?nio, CTAB)-cotensoativo (butanol) / fase oleosa (heptano) / fase aquosa (solu??es de CuSO4.5H2O, e NaBH4). As microemuls?es foram preparadas fixando em 60% a fase ?leo (Heptano) e variando 40% de fase aquosa com tensoativo/cotensoativo na mesma propor??o, sendo que a fase aquosa do agente redutor NaBH4 foi usada em excesso de 0,4 mol.L-1 e 0,8 mol.L-1 em rela??o 0,2 mol.L-1 de fase aquosa de CuSO4.5H2O. Observou-se uma maior redu??o dos ?ons Cu2+ pelo boridreto de s?dio em microemuls?es com 13% de ?gua, independente do tempo de rea??o. Para as microemuls?es em que a fase aquosa corresponde a solu??o de CuSO4, observou-se uma depend?ncia na colora??o em fun??o da rela??o entre as concentra??es de ?gua/tensoativo. Essa mudan?a de colora??o foi atribu?da a uma competi??o pelas mol?culas de ?guas entre a cabe?a polar do tensoativo e os ?ons Cu2+ com uma eventual substitui??o do oxig?nio da primeira esfera de coordena??o por bromo

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No. of bitstreams: 1 curcio_al_me_rcla.pdf: 1672370 bytes, checksum: fd59b862449a04ac385f3661da6430f3 (MD5) Previous issue date: 2016-02-29 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Nanocristais tem sido extensivamente investigados nos últimos anos devido à sua ampla gama de aplicações em vários dispositivos tais como sensores, células solares, lasers, fotocatalisadores, fotodetectores, detectores de infravermelhos, diodos emissores de luz, materiais eletroluminescentes e outros materiais emissores de luz. Semicondutores nanocristalinos apresentam propriedades eletrônicas intermediárias entre aqueles de estrutura molecular e sólidos macrocristalinos, proporcionando uma ampla gama de aplicações. Entre estes materiais, o sulfeto de zinco (ZnS) puro ou dopado tem recebido notável atenção por causa de suas propriedades estruturais ópticas, versatilidade e potencial para várias aplicações tecnológicas. O ZnS é um típico semicondutor II-VI, com um gap direto de 3,6 eV à temperatura ambiente e aproximadamente 40 meV de energia de gap, sendo um bom material luminescente utilizado em telas, sensores e lasers. Como material de gap largo, o ZnS pode facilmente hospedar diferentes metais de transição como centros luminescentes. Entre estes íons de metais de transição para estruturas dopadas, os íons Cu2+e Mn2+ são atraentes pelas emissões de luz características e por apresentarem propriedades eficientes para aplicações como luminóforos. A inserção desses íons na estrutura do ZnS proporcionam defeitos que resultam em emissão no verde para os íons Cu2+e emissão no laranja para os íons Mn2+. Neste estudo, as amostras de ZnS pura e dopadas com Cu2+ e Mn2+ foram preparados pelo método solvotermal, que demonstra ser um processo eficaz para preparar nanopartículas. Uma vez preparadas, as estruturas das amostras nanoestruturadas foram caracterizadas e correlacionada s com propriedades fotoluminescentes. Os resultados de difração de raios X mostram que as amostras de ZnS foram cristalizadas completamente sem a presença de fases secundárias e os difratogramas correspondem à estrutura blenda cúbica de zinco com grupo espacial F-43m. Os espectros de XANES (X-ray Absorption Near Edge Structure) teóricos e experimentais na borda K do Zn indicam que a incorporação de átomos de Mn na matriz ZnS causam a formação de vacâncias de Zn e S, a qual é confirmada por ajustes de espectros EXAFS (Extended X-ray Absorption Fine Structure). Estas vacâncias estão relacionadas com um desvio para o vermelho observado no pico do espectro de fotoluminescência devido a adição de Mn na estrutura do ZnS. Para o ZnS puro, o pico é centrado em ~ 504 nm, relativo as vacâncias de S na amostra nanoestruturada. À medida que se aumenta a porcentagem de Mn na matriz ZnS, uma emissão no amarelo-laranja centrada em ~ 590 nm pode ser observada, associada com a transição 4T1-6A1 no interior de níveis 3d de Mn2+. A adição de íons Cu2+ ao ZnS resulta em um alargamento no pico do espectro de fotoluminescência decorrente de emissão no azul-verde, que está relacionada a recombinação de elétrons de níveis de defeitos mais profundos dos estados t2 do Cu próximos da banda de valência. / Nanocrystals has been extensively investigated in recent years due to its wide range of applications in various devices light emitting materials such as sensors, solar cells, lasers, photocatalysts, photodetectors, IR detectors, light emitting diodes and others. Nanocrystalline Semiconductors have electronic properties between those intermediate molecular macrocristalinos and solid structure, providing a wide range of applications. Among these materials, zinc sulfide (ZnS) pure or doped has received considerable attention because of its optical structural properties, versatility and potential for several technological applications. The ZnS is a typical II-VI semiconductor with a direct band gap of 3.6 eV at room temperature and about 40 meV in energy gap, and a good luminescent material for constrution of displays, lasers and sensors. As wide band gap material, ZnS can easily host different transition metals as luminescent centers. Among these ions of transition metal doped structures, Cu2+ and Mn2+ ions are attractive for light emission characteristics and for having effective properties for applications such as phosphors. The addition of these ions in ZnS structure provide defects that result in emission in the green for the Cu2+ ions and emission in orange for the Mn2+ ions. In this study, samples of pure ZnS and doped with Cu2+ and Mn2+ ions were prepared by solvotermal method, which demonstrate to be an effective process for preparing nanoparticles. Once prepared, the structures of the nanostructured samples were characterized and correlated with photoluminescent properties. The results of X-ray diffraction showed that the ZnS samples were completely crystallized without the presence of secondary phases and XRD patterns correspond to the structure of zinc blende to cubic space group F-43m. spectra XANES (X-ray Absorption Near Edge Structure) theoretical and experimental in the Zn K edge indicates that the inclusion of Mn atoms in the ZnS matrix cause the formation of Zn and S vacancies, which is confirmed by spectral adjustments EXAFS (Extended X-ray Absorption Fine Structure). These vacancies are associated with a red shift observed in the photoluminescence spectrum peak due to the addition of Mn in ZnS structure. For pure ZnS, the peak is centered at ~ 504 nm concerning the vacancies in the S nanostructured sample. As it increases the percentage of Mn in the ZnS matrix, in yellow-orange emission centered at ~ 590 nm can be observed, associated with the transition 4 T1- 6A1 inside 3d levels of Mn2+. Adding Cu2+ to the ZnS results in a broadening of the peak of the photoluminescence spectrum due to emission in blue-green, which is related to recombination deeper defect levels of electrons of t2 Cu states near the valence band.

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

L’identification des mécanismes de relâchement des Produits de Fission (PF) hors de combustible nucléaire irradié lors d’un accident grave est primordiale pour le développement de codes capables d’estimer précisément le terme source (nature et quantité des radionucléides émis dans l’environnement). Parmi les différents PF, les Ba, Cs, Mo et le Ru sont particulièrement intéressants, car ils peuvent interagir entre eux ou avec d’autres éléments et donc affecter leur relâchement. Dans le cadre de cette thèse, deux axes de travail ont été mis en place avec l’objectif d’identifier les phases chimiques présentes avant l’accident et leur évolution au cours de l’accident lui-même. L’approche expérimentale a consisté à reproduire les conditions d’un accident nucléaire à l’échelle du laboratoire, en utilisant des échantillons de combustibles irradiés et des matériaux modèles (UO₂ vierge dopés en 12 PF). Le principal avantage de ces derniers est l’utilisation de méthodes de spéciation chimique comme la Spectroscopie d’Absorption des rayons X, qui n’est pas aujourd’hui encore disponible pour les combustibles irradiés. Trois échantillons de combustible irradié ont été étudies, représentatifs de l’état initial (i.e. avant l’accident), d’une étape intermédiaire en température (1773K) et d’un état avancé d’accident nucléaire (2873K). Pour les matériaux modèles, plusieurs séquences accidentelles (de 573K à 1973K) ont été réalisés. Les résultats expérimentaux ont permis d’établir un nouveau mécanisme de relâchement des PF en en fonction des conditions oxydantes et réductrices du scénario accidentel. Ces résultats ont démontré aussi l’importance des matériaux modèles pour l’étude des accidents nucléaires graves, en complémentarité aux combustibles irradiés. / The identification of Fission Products (FP) release mechanism from irradiated nuclear fuels during a severe accident is of main importance for the development of codes for the estimation of the source-term (nature and quantity of radionuclides released into the environment). Among the many FP Ba, Cs, Mo and Ru present a particular interest, since they may interact with each other or other elements and thus affect their release. In the framework of this thesis, two work axes have been set up in order to identify, firstly, the chemical phases initially present before the accident and, secondly, their evolution during the accident itself. The experimental approach consisted in reproducing nuclear severe accidents conditions at laboratory scale using both irradiated fuels and model materials (natural UO₂ doped with 12 FP). The advantage of these latter is the possibility of using characterization methods such as X-ray Absorption Spectroscopy which are not available for irradiated fuels. Three irradiated fuel samples have been studied, representative to an initial state (before the accident), to an intermediate stage (1773K) and to an advanced stage (2873K) of a nuclear severe accident. Regarding to model materials, many accident sequences have been carried out, from 573 to 1973K. Experimental results have allowed to establish a new release mechanism, considering both reducing and oxidizing conditions during an accident. These results have also demonstrated the importance of model materials as a complement to irradiated nuclear fuels in the study of nuclear severe accidents.

<p>In this thesis we have performed core-electron spectroscopy studies of gas phase molecular systems starting with smaller diatomic, continuing with triatomic and extending our research to more complex polyatomic ones. We can subdivide the results presented here into two categories: the first one focusing on electronic fine structure and effect of the chemical bonds on molecular core-levels and the other one dealing with nuclear dynamics induced by creation of a core hole. In our research we have mostly used synchrotron radiation based techniques such as X-ray Photoelectron (XPS), X-ray Absorption (XAS), normal and Resonant Auger (AES and RAS, respectively) and Energy-Selected Auger Electron PhotoIon COincidence (ES-AEPICO) spectroscopies.</p><p>We have demonstrated that resonant Auger spectroscopy can be used to aid interpretation of the features observed in XAS for Rydberg structures in the case of Cl₂ and C1s⁻¹π*¹ states of allene molecules. The combined use of high-resolution spectroscopy with <i>ab initio</i> calculations can help the interpretation of strongly overlapped spectral features and disentangle their complex profiles. This approach enabled us to determine the differences in the lifetimes for core-hole 2p sublevels of Cl₂ which are caused by the presence of the chemical bond. We have shown that contribution in terms of the Mulliken population of valence molecular orbitals is a determining factor for resonant enhancement of different final states and fragmentation patterns reached after resonant Auger decays in N₂O.</p><p>We have also performed a systematic study of the dependence of the C1s resonant Auger kinetic energies on the presence of different substituents in CH₃X compounds. For the first time we have studied possible isomerization reaction induced by core excitation of acetylacetone. We could observe a new spectral feature in the resonant Auger decay spectra which we interpreted as a signature of core-excitation-induced keto-enol tautomerism.</p>

In this thesis we have performed core-electron spectroscopy studies of gas phase molecular systems starting with smaller diatomic, continuing with triatomic and extending our research to more complex polyatomic ones. We can subdivide the results presented here into two categories: the first one focusing on electronic fine structure and effect of the chemical bonds on molecular core-levels and the other one dealing with nuclear dynamics induced by creation of a core hole. In our research we have mostly used synchrotron radiation based techniques such as X-ray Photoelectron (XPS), X-ray Absorption (XAS), normal and Resonant Auger (AES and RAS, respectively) and Energy-Selected Auger Electron PhotoIon COincidence (ES-AEPICO) spectroscopies. We have demonstrated that resonant Auger spectroscopy can be used to aid interpretation of the features observed in XAS for Rydberg structures in the case of Cl2 and C1s−1π*1 states of allene molecules. The combined use of high-resolution spectroscopy with ab initio calculations can help the interpretation of strongly overlapped spectral features and disentangle their complex profiles. This approach enabled us to determine the differences in the lifetimes for core-hole 2p sublevels of Cl2 which are caused by the presence of the chemical bond. We have shown that contribution in terms of the Mulliken population of valence molecular orbitals is a determining factor for resonant enhancement of different final states and fragmentation patterns reached after resonant Auger decays in N2O. We have also performed a systematic study of the dependence of the C1s resonant Auger kinetic energies on the presence of different substituents in CH3X compounds. For the first time we have studied possible isomerization reaction induced by core excitation of acetylacetone. We could observe a new spectral feature in the resonant Auger decay spectra which we interpreted as a signature of core-excitation-induced keto-enol tautomerism.

Ever since the discovery of exotic phenomena like high temperature (Tc) superconductivity in the cuprates and colossal magnetoresistance in the manganites, strongly correlated electron systems have become the center of attention in the field of condensed matter physics research. This renewed interest has been further kindled by the rapid development of sophisticated experimental techniques and tremendous computational power. Computation plays a pivotal role in the theoretical investigation of these systems, because one cannot explain their complicated phase diagrams by simple, exactly solvable models. Among the plethora of experimental techniques, various kinds of high energy electron spectroscopies are fast gaining importance due to the multitude of physical properties and phenomena which they can access. However the physical processes involved and the interpretation of the spectra obtained from these spectroscopies are extremely complex and require extensive theoretical modelling. This thesis is concerned with the theoretical modelling of a certain class of high energy electron spectroscopies, viz. the core-level electron spectroscopies, for strongly correlated systems of various kinds. The spectroscopies covered are Auger electron spectroscopy (AES), core-level photoemission spectroscopy (core-level PES) and X-ray absorption spec- troscopy (XAS), which provide non-magnetic information, and also X-ray magnetic circular and linear dichroism (XMCD and XMLD), which provide magnetic information. .

The Phthalocyanines (Pc’s) are a group of macro-cyclic molecules, widely investigated due to the possibility to use them in a variety of applications. Electronic and geometrical structure investigations of molecular model systems of Pc’s adsorbed on surfaces are important for a deeper understanding of the functionality of different Pc-based devices. Here, Pc’s monolayers and films, deposited on different surfaces, were investigated by X-ray Photoelectron Spectroscopy (XPS), X-ray Absorption Spectroscopy (XAS) and Scanning Tunneling Microscopy (STM). In addition Density Functional Theory (DFT) simulations were performed. For molecular films of Metal-free (H2Pc) and Iron (FePc) Pc’s, on surfaces, it is found that the intermolecular interaction is weak and the molecules arrange with their molecular plane mainly perpendicular to the surface. Several monolayer systems were characterized, namely H2Pc and FePc adsorbed on Graphite, ZnPc on InSb(001)-c(8x2), H2Pc on Al(110) and on Au(111). For all the studied monolayers it was found that the molecules are oriented with their molecular plane parallel to the surface. The electronic structure of the molecules is differently influenced by interaction with the surfaces. For H2Pc adsorbed on Graphite the nearly negligible effect of the surface on the molecular electronic structure allowed STM characterization of different molecular orbitals. A strong interaction is instead found in the case of H2Pc on Al(110) resulting in molecules strongly adsorbed, and partly dissociated. Modifications of the electronic and geometrical structure induced by alkali doping of H2Pc films and monolayers were characterized. It is found both for the H2Pc film on Al(110) and monolayer adsorbed on Au(111), that the molecular arrangement is changed upon doping by Potassium and Rubidium, respectively. Potassium doping of the H2Pc films results in a filling of previously empty molecular orbitals by a charge transfer from the alkali to the molecule, with significant modification of the molecular electronic structure.