Made available in DSpace on 2014-06-12T23:14:32Z (GMT). No. of bitstreams: 2 arquivo7642_1.pdf: 1977405 bytes, checksum: 9ee0b0f45dde924aa155dc2b1018f9f8 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2011 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Os organofosforados caracterizam-se como uma classe importante de pesticidas, atuando como inibidores da atividade enzimática da acetil colinesterase (AChE). Dentre os organofosforados de uso permitido no Brasil, destacamos o clorpirifós (CPF). Neste trabalho, desenvolveu-se novas metodologias para a obtenção de Qds de CdTe, em meio aquoso, utilizando-se ultrassom de alta potência para acelerar o processo de obtenção dos NCs. Em seguida, os Qds obtidos foram incorporados em hidrogéis de agarose, para avaliação do comportamento de suas propriedades espectroscópicas em diferentes concentração do polissacarídeo desta matriz. Por fim, avaliou-se a capacidade de detecção dos Qds de CdTe incorporados em hidrogéis de agarose a 0,50 %(m/v), na presença de diferentes concentrações de CPF (0.1, 0.5 e 1.0 ppm), complexados nas cavidades de moléculas de β-CD. As amostras de Qds sintetizadas foram caracterizadas estruturalmente e espectroscopicamente pelas técnicas de MET e DRX (estrutural) e Absorbância UV-vis e espectroscopia de fluorescência e excitação. As amostras incorporadas nos hidrogéis de agarose e os experimentos de detecção dos pesticidas foram caracterizados por espectroscopia de fluorescência. Os resultados obtidos mostraram a viabilidade das metodologias desenvolvidas, tanto para a obtenção dos Qds com tamanho médio abaixo de 5 nm e com boa fluorescência e fotoestabilidade, quanto para a detecção dos pesticidas em meio aquoso, comprovando que é possível o desenvolvimento de sensores baseados em Qds e ciclodextrinas, sem a necessidade de processos complicados de conjugação covalente entre estes compostos, para a aplicação na detecção de CPF em concentrações abaixo de 1 ppm, em meio aquoso

Title: Recombination centers in semiinsulating CdTe Author: Jakub Zázvorka Department / Institute: Institute of Physics of Charles University Supervisor of the master thesis: prof. Ing. Jan Franc, DrSc., Institute of Physics of Charles University Abstract: The properties of CdTe for application as a radiation detector are influenced through the presence of deep levels in the bang gap. These energy levels complicate the charge collection and the detector efficiency. Contactless resistivity mapping (COREMA) represents a good option for material characterization without the necessity of metal contacts application. The time-dependent charge measurement was investigated on an adjusted apparatus in FMF Freiburg. Theoretical model of charge transport based on band bending on the sample surface was proposed and a non-exponential behavior was calculated. Using this, the resulted parameter tendencies and their connection with deep level trap or recombination center were explained. A correlation was observed between resistivity, photoconductivity and a near midgap level photoluminescence. Parameter profiles were explained using the theory of Fermi level shift relative to the near midgap level. Three deep levels were observed on samples grown at the Charles University in Prague. Their photoluminescence supports the...

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La nanospintronique basée sur les semi-conducteurs implique la combinaison des fonctions nanoélectroniques et magnétiques au sein d’une nanostructure unique. Une méthode intéressante consiste en la préparation d’un semi-conducteur magnétique dilué (DMS), dans lequel le ferromagnétisme induit par les trous est obtenu par le dopage de type P. Les DMS II-VI permettent de contrôler séparément les porteurs et la concentration du Mn, puisque les ions de Mn ne sont pas des dopants électriques. Les nanofils (NW) de semi-conducteurs II-VI représentent un système modèle permettant de concevoir la fonction d’onde et la déformation au sein d’heterostructures parfaitement contrôlées et contenant les impuretés magnétiques. Ceci nécessite une croissance optimale d’heterostructures dans la géométrie des nanofils et un contrôle précis du niveau du dopant, ce qui constitue une des difficultés majeures dans l’état actuel des recherches.Dans ce contexte, mon travail de Thèse s’est focalisé sur trois principaux axes de recherche : la croissance par épitaxie par jets moléculaires de nanofils de ZnTe catalysés par nanoparticules d’Au, la maîtrise du rapport de forme des boites quantiques de CdTe insérées dans les fils de ZnTe, et le dopage azote des fils de ZnTe.Concernant la croissance des fils de ZnTe, le problème de la variabilité des temps d’incubation d’un fil à l’autre a été étudié à l’aide de la technique des marqueurs. Une nouvelle méthode basée sur la préparation du catalyseur d’Au sous flux de Zn a démontré son efficacité dans la suppression de variabilité des temps d’incubation, réduisant la dispersion des longueurs sur un même échantillon d’un facteur 10 à un facteur 2, et augmentant le taux de succès des fils verticaux de 20% à 80%. Des mesures complémentaires de diffraction des rayons X ont fourni des informations supplémentaires sur l’importance de l’orientation relative entre le catalyseur d’Au et le substrat ZnTe(111)B.Le rapport de forme des boites quantiques de CdTe est un moyen pour maîtriser leur état fondamental (entre trou léger et trou lourd). Ceci peut être obtenu en contrôlant le temps de croissance des boite quantiques, mais demande (1) la suppression de la croissance latérale (responsable de la formation de boites parasites) et (2) la reproductibilité d’un échantillon à l’autre, basée sur une maîtrise de la température de croissance avec une précision meilleure que 10 degrés. Ceci a été validé dans nos conditions de croissance par une étude croisée de croissance d’heterostructures de ZnTe avec multiples boites quantiques de CdTe et de caractérisation par microscopie électronique.Je présenterai les résultats des croissances par épitaxie pa jets moléculaires et des caractérisations des nanofils de ZnTe /ZnTe:N cœur/coquille avec dopage azoté. Des transistors à effet de champ basé sur des nanofils isolés ont été fabriqués pour la caractérisation électrique. Nous avons obtenu des fils de ZnTe/ZnTe:N cœur/coquille présentant des densités de charges de 6×18 trous/cm3 à température ambiante, du même ordre que la densité critique de Mott pour le ZnTe. / Semiconductor nano-spintronics requires combining magnetism and nanoelectronics functions into a single semiconductor nanostructure. An attractive method consists in preparing diluted magnetic semiconductors (DMS), where hole-mediated ferromagnetism appears with p-type doping. II-VI DMS allow to control separately the carriers and the Mn concentrations, since the Mn ions are not electrical dopants. II-VI semiconductor nanowires (NWs) are a model system offering the possibility to engineer the wavefunction and the strain in well-chosen heterostructures containing the magnetic impurities. This requires an optimal growth of NW-based heterostructures, and the possibility to control the doping level, which is a major challenge in present NW research. In this context, my PhD project has been devoted to three main investigation axis: the growth by molecular beam epitaxy (MBE) of Au-catalyzed ZnTe NWs, the control of the aspect ratio of CdTe quantum dots (QDs) embedded in ZnTe NWs, and the nitrogen doping of ZnTe NW.Concerning the growth of ZnTe NWs, the problem of an incubation time different from NW to NW has been studied using a marker technique. A new method involving the preparation of Au catalyst under Zn flux has been demonstrate to efficiently suppress differences in the incubation times, reducing the length dispersion in the same sample to factor of 2 instead of 10 and improving the yield of vertical NWs of 80% instead of 20%. Complementary XRD experiments gave further information about the importance of the relative orientation between the Au catalysts and the ZnTe(111)B growth substrate.The aspect ratio of CdTe QDs is an important way to control the QD ground state (between light hole and heavy hole). This can be achieved by changing the growth time of the QDs, but requires (1) the suppression of the lateral growth (giving parasitic QDs) and (2) reproducibility from a sample to another which relies on a precise control of the growth temperature within a very narrow window of 10° C. This was demonstrated in our growth conditions with a coupled study of growth of multi-QD-NW CdTe-ZnTe heterostructures and transmission electron microscopy characterization.Then the results of the growth by molecular beam epitaxy and characterization of nitrogen doped ZnTe/ZnTe:N core/shell NWs will be presented. Single NW based field effect transistor were realized by electron beam lithography for electrical characterization. We were able to obtain ZnTe/ZnTe:N core/shell NWs showing a p-type carrier density of 6 ×18 holes/cm3 at room temperature, of the same order as the Mott critical density in ZnTe.

La compréhension des propriétés électriques de nano-objets est essentielle pour le développement s des nanotechnologies. La microscopie à force Kelvin (KFM) est une des techniques les plus utiles pour cartographier simultanément la topographie et la différence de potentiel de contact (CPD) à l'échelle nanométrique. Après 20 ans de développement, la KFM est principalement utilisé dans des conditions normales de pression et de température, permettant d'effectuer, de manière simple, de multiples analyses comparatives. Toutefois, sous ultra-vide (UHV), comme la surface est contrôlée et que la sensibilité est meilleure, des mesures plus précises et plus fiables sont réalisables. Dans la première partie, la mesure KFM sous atmosphère ambiante est améliorée en développant la technique simple-passage à la fois en modulation de fréquence (FM) et en modulation d'amplitude (AM). Une électronique externe Nanonis a été adaptée sur les AFMs commerciaux (Dimension 3100 et MultiMode, Bruker). Une étude comparative avec le mode Lift a été réalisée sur des couches de graphène épitaxié sur échantillon de SiC. L'effet de la distance pointe-échantillon sur le contraste et la résolution est décrit ainsi que l'influence des paramètres expérimentaux. Une amélioration significative du contraste et de la résolution est clairement observée sur les résultats obtenus par la technique simple passage en modulation de fréquence, indépendamment de la distance pointe-échantillon. Dans une deuxième partie, la technique KFM est développée sous vide secondaire. Le travail instrumental est réalisé sur un AFM EnviroScope de chez Bruker, qui a été équipé d'une électronique externe Nanonis, permettant de mesurer simultanément la topographie en mode non-contact et la CPD en modulation d'amplitude ou de fréquence. Les résultats montrent que la CPD mesurée est comparable à celle obtenue avec une mesure sous ultravide. Enfin, après avoir posé les bases à la fois expérimentale et théorique de la KFM, cette technique est utilisée pour caractériser les hétérostructures CdTe/CdS en films minces utilisés pour les applications de cellules solaires. Un protocole de préparation d'échantillon sur la tranche a été spécialement développé. L'hétérojonction CdTe/CdS est étudiée sous polarisation à la fois dans l'obscurité et sous éclairement. L'influence de l'épaisseur de la couche de CdS est également étudiée pour comprendre son effet dramatique sur le rendement des cellules solaires. / The development of nanoscience makes the understanding of the electrical properties of nano-objects essential. The Kelvin Force Microscopy (KFM) is one of the most useful techniques to map at the nanoscale and simultaneously both the topography and contact potential difference (CPD). After 20 years of development, KFM is mainly operated under air at normal pressure, allowing to perform, in an easy way, multiple comparative analyses. However, under UHV, as the surface is controlled and the sensitivity improved, more accurate and reliable measurements can be achieved. In the first part, KFM under ambient atmosphere is improved by developing the single-scan method using either a frequency modulation (FM) or an amplitude modulation (AM) mode. An external Nanonis electronic has been implemented on commercial AFMs (Dimension 3100 and MultiMode, Bruker). A comparative study with the common Lift-mode is done by imaging epitaxial graphene layers on SiC sample. The tip-sample separation effect on the CPD contrast and resolution is described as well as experimental settings. It is shown that higher contrasts are obtained using single-scan frequency modulation KFM regardless the tip-sample operating distance. In a second part, the KFM technique under secondary vacuum is developed. The instrumental work is carried out with an EnviroScope AFM from Bruker. We outfitted our Veeco's AFMs with an external Nanonis electronic to perform simultaneously the acquisition of topography and CPD using either the amplitude or the frequency modulation mode. The upgrade of the electronic has raised compatibility issues. Our results show that the comparable results are obtained with KFM under UHV. Finally, having laid down both the experimental and theoretical groundwork of the KFM, this technique is used to characterize CdTe/CdS heterostructures used in thin films solar cell application. A protocol for the cross section sample preparation has been specifically developed. The CdTe/CdS heterojunction is studied under polarization both in dark and under illumination. The influence of the CdS layer thickness is also studied to understand its dramatic effect on the solar cell efficiency.

Made available in DSpace on 2015-03-26T13:35:20Z (GMT). No. of bitstreams: 1 texto completo.pdf: 1927502 bytes, checksum: 3b216b20861fe585d8cac540c9638d0b (MD5) Previous issue date: 2006-12-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In the present work, we study films CdTe electrodeposited galvanostatically from aqueous acidic solution (pH<1) onto n-type monocrystalline silicon wafer (111). The optimum current density to obtain good crystalline quality CdTe films was found to be 0,3 mA/cm2. The films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and photoacoustic spectroscopy (PAS). The CdTe films were deposited at room temperature or at 85 ºC and covered evenly the substrate surface. The as-prepared films were non-reflecting and presented a grey color which showed dependence with current density and temperature The SEM pictures revealed a granular structure, without indications of faces that evidenced some crystalline arrangement. The XRD patterns of the films deposited at 85 ºC show peaks associated to the CdTe cubic phase. Additionally, peaks corresponding to a hexagonal phase of the elemental cadmium and tellurium were also detected. However, in the XRD patterns of the films which were grown at low temperature, no peaks were associated to CdTe. This film was passed by chemical treatment in saturated solution of CdCl2 followed by thermal annealing at 420 and 450 °C in nitrogen atmosphere. The XRD patterns of the films after annealing showed only signals associated to the cubic phase of CdTe. These results suggest improvement in the crystallinity of the films grown at room temperature and reduction of the amount of present elemental Te e Cd. Change in microstructure due to the annealing with CdCl2 was observed in the SEM pictures, which show completely recrystallized grains. The measurements of optical absorption in the films grown after annealing by PAS, showed an absorption band around the expected band gap energy value of CdTe. Our experimental results also showed that the CdTe films adhered better to unpolished surfaces. / No presente trabalho, estudamos filmes finos de CdTe eletrodepositados galvanostaticamente em meio aquoso ácido (pH<1) sobre silício monocristalino, tipo-n, (111). A densidade de corrente de 0,3 mA/cm2 permitiu a obtenção de filmes de melhor qualidade. Os filmes foram caracterizados por microscopia eletrônica de varredura (SEM), difração de raios-X (XRD) e espectroscopia fotoacústica (PAS). Obtivemos depósitos que cobriram efetivamente o substrato tanto a temperatura ambiente quanto a 85 ºC. Os filmes depositados são cinza escuros e a tonalidade varia com a temperatura de deposição e a densidade de corrente. A SEM revelou nos filmes uma estrutura granular, sem indicações de faces que pudessem evidenciar algum arranjo cristalino. A XRD dos filmes depositados a 85 ºC mostrou picos associados à fase cúbica do CdTe e também picos correspondendo à fase hexagonal do telúrio e do cádmio livres. Porém a XRD dos filmes depositados a temperatura ambiente não mostrou picos que pudessem ser associados ao CdTe. Os filmes foram submetidos a tratamento térmico a 420 e 450 ºC, em atmosfera de nitrogênio, depois de umedecidos com solução saturada de CdCl2. A XRD dos filmes depois do tratamento térmico mostrou picos intensos associados à fase cúbica do CdTe, com orientação preferencial dos grãos na direção (111), tanto nos filmes depositados a temperatura ambiente quanto a 85 ºC. Os resultados mostraram também uma redução na concentração do Te e Cd livres. Micrografias SEM mostraram recristalização da superfície do filme devido ao tratamento térmico. As medidas de absorção óptica por PAS, efetuadas em filmes tratados termicamente, revelaram uma banda de absorção em torno do valor esperado do gap óptico do CdTe. Nossos experimentos mostraram também que os filmes de CdTe aderem melhor em substratos de silício não polidos.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2017-02-13T14:42:08Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) dissertação para imprimir.pdf: 1944013 bytes, checksum: 446b8ae7597e9bfcfe53826edaa4a028 (MD5) / Made available in DSpace on 2017-02-13T14:42:08Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) dissertação para imprimir.pdf: 1944013 bytes, checksum: 446b8ae7597e9bfcfe53826edaa4a028 (MD5) Previous issue date: 2014-08-20 / FACEPE / Neste trabalho foram sintetizados quantum dots de CdTe e CdTe:Gd3+ tendo o ácido mercapto succínico como estabilizante, visando uma potencial aplicação como agente de contraste bimodal para imagem ótica e magnética. A difração de raios-X evidenciou a formação dos QDs. Após quantificação dos íons dopantes por Espectrometria de Emissão Ótica com Plasma Indutivo Acoplado (ICP-OES), verificou-se 75,7% dos íons Gd3+ adicionados ficaram na nanopartícula. A grande quantidade de íons aderidos à nanopartícula, além da observação do aumento da relaxividade com a diminuição do pH e o deslocamento do estiramento das carbonilas por espectroscopia de infravermelho, são fatores que indicam que os íons Gd3+ aderiram aos QDs pela coordenação com o estabilizante e não por dopagem. A presença do íon gadolínio não alterou o deslocamento do máximo de absorção dos QDs e possivelmente interferiu na cinética de estabilização das superfícies, pois houve a diminuição da intensidade da luminescência dos mesmos. / In this work were synthesized quantum dots of CdTe and CdTe:Gd3+ having mercapto succinic acid (MSA) as a stabilizer, looking forward their use as a bimodal contrast agent for optical and magnetic images. The X-ray diffraction showed the formation of QDs. The quantification of dopants by Optical Emission Spectrometry with Inductive Coupled (ICPOES) Plasma, showed that 75.7% of Gd3+ ions were added in the nanoparticle. The large amount of ions bonded to the nanoparticle and the observation of increased relaxivity with decreasing pH and the observed shift of the stretching of carbonyls by infrared spectroscopy, are factors that indicated that Gd3+ ions were adhered to the QDs by coordinating with the stabilizer and not by doping. The presence of gadolinium ions did not change the absorption maximum band of the QDs, but probably changed the stabilization kinetics at the QD surface, due to the observed luminescence intensity decreasing.

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Signal formation in a photon-counting x-ray/gamma-ray imaging detector is a complex process resulting in detector signals governed by multiple random effects. Recovering maximum possible information about event attributes of interest requires a systematic collection of calibration data and analysis provided by estimation theory. In this context, a likelihood model provides a description of the connection between the observed signals and the event attributes. A quantitative measure of how well the measured signals can be used to produce an estimate of the parameters is given by Fisher Information analysis. In this work, we demonstrate several applications of the Fisher Information Matrix (FIM) as a powerful and practical tool for investigating and optimizing potential next-generation x-ray/gamma-ray detector designs, with an emphasis on medical-imaging applications. Using FIM as a design tool means to explore the physical detector design choices that have a relationship with the FIM through the likelihood function, how are they interrelated, and determining whether it is possible to modify any of these choices to yield or retain higher values for Fisher Information. We begin by testing these ideas by investigating a new type of a semiconductor detector, a Cadmium Telluride (CdTe) detector with double-sided-strip geometry developed by our collaborators at the Japan Aerospace Exploration Agency (JAXA). The statistical properties of the detector signals as a function of interaction positions in 3D (x, y, z) are presented with mathematical expressions as well as experimental data from measurements using synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory. We show the computation of FIM for evaluating positioning performance and discuss how various detector parameters, that are identified to affect FIM, can be used in detector optimization. Next, we show the application of FIM analysis in a detector system based on multi-anode photomultiplier tubes coupled to a monolithic scintillator in the design of smart electronic read-out strategies. We conclude by arguing that a detector system is expected to perform the best when the hardware is optimized jointly with the estimation algorithm (simply referred to as the "software" in this context) that will be used with it. The results of this work lead to the idea of a detector development approach where the detector hardware platform is developed concurrently with the software and firmware in order to achieve optimal performance.