Projecto de receptores integrados de banda larga

O presente trabalho tem como objectivo o estudo e projecto de receptores optimizados para sistemas de comunicações por fibra óptica de muito alto débito (10Gb/s e 40Gb/s), com a capacidade integrada de compensação adaptativa pós-detecção da distorção originada pela característica de dispersão cromática e de polarização do canal óptico. O capítulo 1 detalha o âmbito de aplicabilidade destes receptores em sistemas de comunicações ópticas com multiplexagem no comprimento de onda (WDM) actuais. O capítulo apresenta ainda os objectivos e principais contribuições desta tese. O capítulo 2 detalha o projecto de um amplificador pós-detecção adequado para sistemas de comunicação ópticos com taxa de transmissão de 10Gb/s. São discutidas as topologias mais adequadas para amplificadores pós detecção e apresentados os critérios que ditaram a escolha da topologia de transimpedância bem como as condições que permitem optimizar o seu desempenho em termos de largura de banda, ganho e ruído. Para além disso são abordados aspectos relacionados com a implementação física em tecnologia monolítica de microondas (MMIC), focando em particular o impacto destes no desempenho do circuito, como é o caso do efeito dos componentes extrínsecos ao circuito monolítico, em particular as ligações por fio condutor do monólito ao circuito externo. Este amplificador foi projectado e produzido em tecnologia pHEMT de Arsenieto de Gálio e implementado em tecnologia MMIC. O protótipo produzido foi caracterizado na fábrica, ainda na bolacha em que foi produzido (on-wafer) tendo sido obtidos dados de caracterização de 80 circuitos protótipo. Estes foram comparados com resultados de simulação e com desempenho do protótipo montado num veículo de teste. O capítulo 3 apresenta o projecto de dois compensadores eléctricos ajustáveis com a capacidade de mitigar os efeitos da dispersão cromática e da dispersão de polarização em sistemas ópticos com débito binário de 10Gb/s e 40Gb/s, com modulação em banda lateral dupla e banda lateral única. Duas topologias possíveis para este tipo de compensadores (a topologia Feed-Forward Equalizer e a topologia Decision Feedback Equaliser) são apresentadas e comparadas. A topologia Feed-Forward Equaliser que serviu de base para a implementação dos compensadores apresentados é analisada com mais detalhe sendo propostas alterações que permitem a sua implementação prática. O capítulo apresenta em detalhe a forma como estes compensadores foram implementados como circuitos distribuídos em tecnologia MMIC sendo propostas duas formas de implementar as células de ganho variável: com recurso à configuração cascode ou com recurso à configuração célula de Gilbert. São ainda apresentados resultados de simulação e experimentais (dos protótipos produzidos) que permitem tirar algumas conclusões sobre o desempenho das células de ganho com as duas configurações distintas. Por fim, o capítulo inclui ainda resultados de desempenho dos compensadores testados como compensadores de um sinal eléctrico afectado de distorção. No capítulo 4 é feita uma análise do impacto da modulação em banda lateral dupla (BLD) em comparação com a modulação em banda lateral única (BLU) num sistema óptico afectado de dispersão cromática e de polarização. Mostra-se que com modulação em BLU, como não há batimento entre portadoras das duas bandas laterais em consequência do processo quadrático de detecção e há preservação da informação da distorção cromática do canal (na fase do sinal), o uso deste tipo de modulação em sistemas de comunicação óptica permite maior tolerância à dispersão cromática e os compensadores eléctricos são muito mais eficientes. O capítulo apresenta ainda resultados de teste dos compensadores desenvolvidos em cenários experimentais de laboratório representativos de sistemas ópticos a 10Gb/s e 40Gb/s. Os resultados permitem comparar o desempenho destes cenários sem e com compensação eléctrica optimizada, para os casos de modulação em BLU e em BLD, e considerando ainda os efeitos da dispersão na velocidade de grupo e do atraso de grupo diferencial. Mostra-se que a modulação BLU em conjunto com compensação adaptativa eléctrica permite um desempenho muito superior á modulação em BLD largamente utilizada nos sistemas de comunicações actuais. Por fim o capítulo 5 sintetiza e apresenta as principais conclusões deste trabalho.

Chalcogenide thin films for solar cells: growth and properties

Thin film solar cells have in recent years gained market quota against traditional silicon photovoltaic panels. These developments were in a large part due to CdTe solar panels on whose development started earlier than their competitors. Panels based on Cu(In,Ga)Se2 (CIGS), despite being more efficient in a laboratory and industrial scale than the CdTe ones, still need a growth technology cheaper and easier to apply in industry. Although usually presented as a good candidate to make cheap panels, CIGS uses rare and expensive materials as In and Ga. The price evolution of these materials might jeopardize CIGS future. This thesis presents three different studies. The first is the study of different processes for the incorporation of Ga in a hybrid CIGS growth system. This system is based on sputtering and thermal evaporation. This technology is, in principle, easier to be applied in the industry and solar cells with efficiencies around to 7% were fully made in Aveiro. In the second part of this thesis, a new material to replace CIGS in thin film solar cells is studied. The growth conditions and fundamental properties of Cu2ZnSnSe4 (CZTSe) were studied in depth. Suitable conditions of temperature and pressure for the growth of this material are reported. Its band gap energy was estimated at 1.05 eV and the Raman scattering peaks were identified. Solar cells made with this material showed efficiencies lower than 0.1%. Finally, preliminary work regarding the incorporation of selenium in Cu2ZnSnS4 (CZTS) thin films was carried out. The structural and morphological properties of thin films of Cu2ZnSn(S,Se)4 have been studied and the results show that the incorporation of selenium is higher in films with precursors rather with already formed Cu2SnS3 or Cu2ZnSnS4 thin films. A solar cell with 0.9 % of efficiency was prepared.

Estudo e aplicação de amplificadores ópticos para suporte a redes banda larga

Esta tese tem como foco principal a análise dos principais tipos de amplificação óptica e algumas de suas aplicações em sistemas de comunicação óptica. Para cada uma das tecnologias abordadas, procurou-se definir o estado da arte bem como identificar as oportunidades de desenvolvimento científico relacionadas. Os amplificadores para os quais foi dirigido alguma atenção neste documento foram os amplificadores em fibra dopada com Érbio (EDFA), os amplificadores a semicondutor (SOA) e os amplificadores de Raman (RA). Este trabalho iniciou-se com o estudo e análise dos EDFA’s. Dado o interesse científico e económico que estes amplificadores têm merecido, apenas poucos nichos de investigação estão ainda em aberto. Dentro destes, focá-mo-nos na análise de diferentes perfis de fibra óptica dopada de forma a conseguir uma optimização do desempenho dessas fibras como sistemas de amplificação. Considerando a fase anterior do trabalho como uma base de modelização para sistemas de amplificação com base em fibra e dopantes, evoluiu-se para amplificadores dopados mas em guias de onda (EDWA). Este tipo de amplificador tenta reduzir o volume físico destes dispositivos, mantendo as suas características principais. Para se ter uma forma de comparação de desempenho deste tipo de amplificador com os amplificadores em fibra, foram desenvolvidos modelos de caixa preta (BBM) e os seus parâmetros afinados por forma a termos uma boa modelização e posterior uso deste tipo de amplificiadores em setups de simulação mais complexos. Depois de modelizados e compreendidos os processo em amplificadores dopados, e com vista a adquirir uma visão global comparativa, foi imperativo passar pelo estudo dos processos de amplificação paramétrica de Raman. Esse tipo de amplificação, sendo inerente, ocorre em todas as bandas de propagação em fibra e é bastante flexível. Estes amplificadores foram inicialmente modelizados, e algumas de suas aplicações em redes passivas de acesso foram estudadas. Em especial uma série de requisitos, como por exemplo, a gama de comprimentos de onda sobre os quais existem amplificação e os altos débitos de perdas de inserção, nos levaram à investigação de um processo de amplificação que se ajustasse a eles, especialmente para buscar maiores capacidades de amplificação (nomeadamente longos alcances – superiores a 100 km – e altas razões de divisão – 1:512). Outro processo investigado foi a possibilidade de flexibilização dos parâmetros de comprimento de onda de ganho sem ter que mudar as caractísticas da bomba e se possível, mantendo toda a referenciação no transmissor. Este processo baseou-se na técnica de clamping de ganho já bastante estudada, mas com algumas modificações importantes, nomeadamente a nível do esquema (reflexão apenas num dos extremos) e da modelização do processo. O processo resultante foi inovador pelo recurso a espalhamentos de Rayleigh e Raman e o uso de um reflector de apenas um dos lados para obtenção de laser. Este processo foi modelizado através das equações de propagação e optimizado, tendo sido demonstrado experimentalmente e validado para diferentes tipos de fibras. Nesta linha, e dada a versatilidade do modelo desenvolvido, foi apresentada uma aplicação mais avançada para este tipo de amplificadores. Fazendo uso da sua resposta ultra rápida, foi proposto e analisado um regenerador 2R e analisada por simulação a sua gama de aplicação tendo em vista a sua aplicação sistémica. A parte final deste trabalho concentrou-se nos amplificadores a semiconductor (SOA). Para este tipo de amplificador, os esforços foram postos mais a nível de aplicação do que a nível de sua modelização. As aplicações principais para estes amplificadores foram baseadas em clamping óptico do ganho, visando a combinação de funções lógicas essenciais para a concepção de um latch óptico com base em componentes discretos. Assim, com base num chip de ganho, foi obtido uma porta lógica NOT, a qual foi caracterizada e demonstrada experimentalmente. Esta foi ainda introduzida num esquema de latching de forma a produzir um bi-estável totalmente óptico, o qual também foi demonstrado e caracterizado. Este trabalho é finalizado com uma conclusão geral relatando os subsistemas de amplificação e suas aplicacações.

À sombra de um passado por contar: banda de música de Santiago de Riba-Ul

O presente trabalho inscreve-se no domínio da etnomusicologia e propõe-se abordar o fenómeno das bandas filarmónicas em Portugal numa perspetiva eminentemente histórica, usando como recursos metodológicos a pesquisa de arquivo e o trabalho de campo. Tomando como exemplo a Banda de Música de Santiago de Riba-Ul, que se autodesigna “a banda mais antiga de Portugal”, e tentando contextualizar a sua existência bicentenária no movimento filarmónico nacional, procurei explorar o desenvolvimento que o grupo teve ao longo do tempo, a partir da análise de documentos de arquivo em articulação com os discursos produzidos no domínio da oralidade e do periodismo local. Através da análise histórica reflito sobre movimentos semelhantes aos da implantação de agrupamentos filarmónicos na Europa, a estrutura democratizada e multifuncional que foram adquirindo ao longo do tempo, os espaços de ocorrência, o repertório e os instrumentos. Tendo em conta que o universo das bandas filarmónicas em Portugal representa ainda um domínio pouco estudado, procurei neste trabalho definir um quadro conceptual sobre as diferentes designações associadas a agrupamentos de sopro em Portugal, que recorre à história como ferramenta essencial para o entendimento da relação entre designações e conceitos a partir da análise de funções, contextos e repertórios. Através do estudo de caso da Banda de Música de Santiago de Riba-Ul, procurei estabelecer um modelo de análise que é revelador sobre o modo como, em muitos casos, as bandas usam o passado (real, imaginado, vivido ou mitificado), transformado em memória semântica, como forma de legitimar o presente, convertendo a própria banda num documento/monumento. Esse estatuto conduz à reprodução do modelo de polivalência que o passado lhe legou e oferece-lhe, através de um jogo coerente de cumplicidades no seio do movimento filarmónico em Portugal, um lugar cativo no macrocosmos que aqui defini como um campo social institucional, a partir da proposta conceptual de Bourdieu.

Reconfigurable transmitters for software-defined radios

Flexible radio transmitters based on the Software-Defined Radio (SDR) concept are gaining an increased research importance due to the unparalleled proliferation of new wireless standards operating at different frequencies, using dissimilar coding and modulation schemes, and targeted for different ends. In this new wireless communications paradigm, the physical layer of the radio transmitter must be able to support the simultaneous transmission of multi-band, multi-rate, multi-standard signals, which in practice is very hard or very inefficient to implement using conventional approaches. Nevertheless, the last developments in this field include novel all-digital transmitter architectures where the radio datapath is digital from the baseband up to the RF stage. Such concept has inherent high flexibility and poses an important step towards the development of SDR-based transmitters. However, the truth is that implementing such radio for a real world communications scenario is a challenging task, where a few key limitations are still preventing a wider adoption of this concept. This thesis aims exactly to address some of these limitations by proposing and implementing innovative all-digital transmitter architectures with inherent higher flexibility and integration, and where improving important figures of merit, such as coding efficiency, signal-to-noise ratio, usable bandwidth and in-band and out-of-band noise will also be addressed. In the first part of this thesis, the concept of transmitting RF data using an entirely digital approach based on pulsed modulation is introduced. A comparison between several implementation technologies is also presented, allowing to state that FPGAs provide an interesting compromise between performance, power efficiency and flexibility, thus making them an interesting choice as an enabling technology for pulse-based all-digital transmitters. Following this discussion, the fundamental concepts inherent to pulsed modulators, its key advantages, main limitations and typical enhancements suitable for all-digital transmitters are also presented. The recent advances regarding the two most common classes of pulse modulated transmitters, namely the RF and the baseband-level are introduced, along with several examples of state-of-the-art architectures found on the literature. The core of this dissertation containing the main developments achieved during this PhD work is then presented and discussed. The first key contribution to the state-of-the-art presented here consists in the development of a novel ΣΔ-based all-digital transmitter architecture capable of multiband and multi-standard data transmission in a very flexible and integrated way, where the pulsed RF output operating in the microwave frequency range is generated inside a single FPGA device. A fundamental contribution regarding the simultaneous transmission of multiple RF signals is then introduced by presenting and describing novel all-digital transmitter architectures that take advantage of multi-gigabit data serializers available on current high-end FPGAs in order to transmit in a time-interleaved approach multiple independent RF carriers. Further improvements in this design approach allowed to provide a two-stage up-conversion transmitter architecture enabling the fine frequency tuning of concurrent multichannel multi-standard signals. Finally, further improvements regarding two key limitations inherent to current all-digital transmitter approaches are then addressed, namely the poor coding efficiency and the combined high quality factor and tunability requirements of the RF output filter. The followed design approach based on poliphase multipath circuits allowed to create a new FPGA-embedded agile transmitter architecture that significantly improves important figures of merit, such as coding efficiency and SNR, while maintains the high flexibility that is required for supporting multichannel multimode data transmission.

Novos materiais com polioxotungstatos substituídos com metais d e f

The present work reports studies on the new compounds obtained by the combination of polyoxoanions derived from the Keggin and Lindquist structures with several cations. The studies were first focused on the monolacunary Keggin polyoxoanions [PW11O39M(H2O)]n- (M = FeIII, MnIII and n = 4; M = CoII and n = 5) and its combination with the organic cation 1-butyl-3-methylimidazolium (Bmim+). The association of Bmim+ cation with the polyoxoanion [PW11O39Fe(H2O)]4- allowed to isolate for the first time both the monomeric and the dimeric [PW11O39Fe)2O]10- anions, with the same cation and using simple bench techniques by pH manipulation. Studies regarding the stability of these inorganic species in solution indicated that both species are present in solution in equilibrium. However, the inability to up until now isolate the dimeric unit through simple bench methods, lead to the hypothesis that the cation had a role to play in the selective precipitation of either the monomer or the dimer. Repetition of the same procedures with the polyoxoanions [SiW11O39Fe(H2O)]5- and [PW11O39M(H2O)]n- (M = FeIII, MnIII and n = 4; M = Co and n = 5), afforded only the corresponding monomeric compounds, (Bmim)5[SiW11O39FeIII(H2O)]· 4H2O (3), (Bmim)5[PW11O39CoII(H2O)]· 0.5 H2O, (4) and (Bmim)5[PW11O39MnIII(H2O)]· 0.5 H2O (5). Moreover, the combination of Bmim+ and the polyoxotungstate [PW11O39Co(H2O)]5- afforded two different crystal structures, depending on the synthetic conditions. Thus, a ratio Bmim+:POM of 5:1 and the presence of K+ cations (due to addition of KOH) led to a formula Na2K(Bmim)2[PW11.2O39Co0.8(H2O)]·7H2O (4a), whilst a ratio Bmim:POM of 7:1 led to the formation of a crystal with the chemical formula Na2(Bmim)8[PW11O39Co(H2O)]2·3H2O (4b). Electrochemical studies were performed with carbon paste electrodes modified with BmimCl to investigate the influence of the Bmim+ cation in the performance of the electrodes. The voltametric measurements obtained from solutions containing the anions [PW11O39]7- and [SiW11O39]8- are presented. Results pointed to an improvement of the acquired voltametric signal with a slight addition of BmimCl (up to 2.5% w/w), specially in the studies regarding pH variation. Additional synthesis were carried out with both the cations Omim+ and THTP+.

Dependable wireless real-time communications for open environments

Wireless communication technologies have become widely adopted, appearing in heterogeneous applications ranging from tracking victims, responders and equipments in disaster scenarios to machine health monitoring in networked manufacturing systems. Very often, applications demand a strictly bounded timing response, which, in distributed systems, is generally highly dependent on the performance of the underlying communication technology. These systems are said to have real-time timeliness requirements since data communication must be conducted within predefined temporal bounds, whose unfulfillment may compromise the correct behavior of the system and cause economic losses or endanger human lives. The potential adoption of wireless technologies for an increasingly broad range of application scenarios has made the operational requirements more complex and heterogeneous than before for wired technologies. On par with this trend, there is an increasing demand for the provision of cost-effective distributed systems with improved deployment, maintenance and adaptation features. These systems tend to require operational flexibility, which can only be ensured if the underlying communication technology provides both time and event triggered data transmission services while supporting on-line, on-the-fly parameter modification. Generally, wireless enabled applications have deployment requirements that can only be addressed through the use of batteries and/or energy harvesting mechanisms for power supply. These applications usually have stringent autonomy requirements and demand a small form factor, which hinders the use of large batteries. As the communication support may represent a significant part of the energy requirements of a station, the use of power-hungry technologies is not adequate. Hence, in such applications, low-range technologies have been widely adopted. In fact, although low range technologies provide smaller data rates, they spend just a fraction of the energy of their higher-power counterparts. The timeliness requirements of data communications, in general, can be met by ensuring the availability of the medium for any station initiating a transmission. In controlled (close) environments this can be guaranteed, as there is a strict regulation of which stations are installed in the area and for which purpose. Nevertheless, in open environments, this is hard to control because no a priori abstract knowledge is available of which stations and technologies may contend for the medium at any given instant. Hence, the support of wireless real-time communications in unmanaged scenarios is a highly challenging task. Wireless low-power technologies have been the focus of a large research effort, for example, in the Wireless Sensor Network domain. Although bringing extended autonomy to battery powered stations, such technologies are known to be negatively influenced by similar technologies contending for the medium and, especially, by technologies using higher power transmissions over the same frequency bands. A frequency band that is becoming increasingly crowded with competing technologies is the 2.4 GHz Industrial, Scientific and Medical band, encompassing, for example, Bluetooth and ZigBee, two lowpower communication standards which are the base of several real-time protocols. Although these technologies employ mechanisms to improve their coexistence, they are still vulnerable to transmissions from uncoordinated stations with similar technologies or to higher power technologies such as Wi- Fi, which hinders the support of wireless dependable real-time communications in open environments. The Wireless Flexible Time-Triggered Protocol (WFTT) is a master/multi-slave protocol that builds on the flexibility and timeliness provided by the FTT paradigm and on the deterministic medium capture and maintenance provided by the bandjacking technique. This dissertation presents the WFTT protocol and argues that it allows supporting wireless real-time communication services with high dependability requirements in open environments where multiple contention-based technologies may dispute the medium access. Besides, it claims that it is feasible to provide flexible and timely wireless communications at the same time in open environments. The WFTT protocol was inspired on the FTT paradigm, from which higher layer services such as, for example, admission control has been ported. After realizing that bandjacking was an effective technique to ensure the medium access and maintenance in open environments crowded with contention-based communication technologies, it was recognized that the mechanism could be used to devise a wireless medium access protocol that could bring the features offered by the FTT paradigm to the wireless domain. The performance of the WFTT protocol is reported in this dissertation with a description of the implemented devices, the test-bed and a discussion of the obtained results.

Laser assisted directional solidification of zirconia-based eutectics

Directionally solidified zirconia-based eutectic (DSE) fibres were obtained using the laser floating zone (LFZ) method. Two systems were investigated: zirconia-barium zirconate and zirconia-mullite. The purpose was to take advantage of zirconia properties, particularly as an ionic conductor and a mechanical rein-forcement phase. The influence of processing conditions in the structural and microstructural characteristics and their consequences on the electrical and mechanical behaviour were the focus of this thesis. The novel zirconia-barium zirconate eutectic materials were developed in order to combine oxygen ionic conduction through zirconia with protonic conduction from barium zirconate, promoting mixed ionic conduction behaviour. The mi-crostructure of the fibres comprises two alternated regions: bands having coarser zirconia-rich microstructure; and inter-band regions changing from a homogeneous coupled eutectic, at the lowest pulling rate, to columnar colony microstructure, for the faster grown fibres. The bands inter-distance increases with the growth rate and, at 300 mm/h, zirconia dendrites develop enclosed in a fine-interpenetrated network of 50 vol.% ZrO2-50 vol.% BaZrO3. Both phases display contiguity without interphase boundaries, according to impedance spec-troscopy data. Yttria-rich compositions were considered in order to promote the yttrium incorporation in both phases, as revealed by Raman spectroscopy and corroborated by the elemental chemical analysis in energy dispersive spectros-copy. This is a mandatory condition to attain simultaneous contribution to the mixed ionic conduction. Such results are supported by impedance spectrosco-py measurements, which clearly disclose an increase of total ionic conduction for lower temperatures in wet/reduction atmospheres (activation energies of 35 kJ/mol in N2+H2 and 48 kJ/mol in air, in the range of 320-500 ºC) compared to the dry/oxidizing conditions (attaining values close to 90 kJ/mol, above 500 ºC). At high temperatures, the proton incorporation into the barium zirconate is un-favourable, so oxygen ion conduction through zirconia prevails, in dry and oxi-dizing environments, reaching a maximum of 1.3x10-2 S/cm in dry air, at ~1000 ºC. The ionic conduction of zirconia was alternatively combined with another high temperature oxygen ion conductor, as mullite, in order to obtain a broad elec-trolytic domain. The growth rate has a huge influence in the amount of phases and microstructure of the directionally solidified zirconia-mullite fibres. Their microstructure changes from planar coupled eutectic to dendritic eutectic mor-phology, when the growth rate rises from 1 to 500 mm/h, along with an incre-ment of tetragonal zirconia content. Furthermore, high growth rates lead to the development of Al-Si-Y glassy phase, and thus less mullite amount, which is found to considerably reduce the total ionic conduction of as-grown fibres. The reduction of the glassy phase content after annealing (10h; 1400 ºC) promotes an increase of the total ionic conduction (≥0.01 S/cm at 1370 °C), raising the mullite and tetragonal zirconia contents and leading to microstructural differ-ences, namely the distribution and size of the zirconia constituent. This has important consequences in conductivity by improving the percolation pathways. A notable increase in hardness is observed from 11.3 GPa for the 10 mm/h pulled fibre to 21.2 GPa for the fibre grown at 500 mm/h. The ultra-fine eutectic morphology of the 500 mm/h fibres results in a maximum value of 534 MPa for room temperature bending strength, which decreases to about one-fourth of this value at high temperature testing (1400 ºC) due to the soft nature of the glassy-matrix.

Estudo de defeitos em filmes finos de ZnO depositados por Rf-sputtering

Neste trabalho foram estudados diferentes filmes finos de ZnO depositados por Rf-Sputtering. Filmes finos de ZnO com diferentes propriedades óticas foram obtidos intencionalmente variando os parâmetros de deposição. De modo a correlacionar as propriedades óticas e estruturais com os parâmetros de deposição, foram utilizadas diferentes técnicas de caracterização avançadas, tais como, fotoluminescência, microscopia de força atómica, difração de raios- X e retrodispersão de Rutherford. Este trabalho centra-se na discussão e análise das bandas de emissão vermelha, verde e azul, comumente observadas em amostras de ZnO e cuja natureza tem sido objeto de grande controvérsia na literatura. A utilização de técnicas de caracterização estrutural revelou-se de extrema importância para correlacionar as propriedades físicas de composição e estrutura com os centros óticos observados nos filmes. Nesta base, foram propostos e discutidos diferentes modelos de recombinação ótica associados à qualidade estrutural dos filmes, considerando modelos de camadas que descrevem a heterogeneidade lateral e em profundidade. Desta análise verificou-se a presença de heterogeneidade estrutural e composicional, que aumenta a complexidade na compreensão da correlação dos parâmetros de deposição com as propriedades óticas dos filmes. Foi discutida a limitação e validade de diferentes modelos tendo em conta a presença da heterogeneidade existente nos filmes estudados. Este trabalho contribui assim para uma melhor compreensão da complexidade de interação dos diferentes defeitos e o seu efeito nas propriedades óticas, nomeadamente o papel dos defeitos de interface, na superfície, nas fronteiras de grão e junto ao substrato.

Nanocomposites as probes for biodetection by SERS

The present work aimed to explore the potential of new nanocomposites based on carbon nanostructures and metal nanoparticles for the detection of biomolecules through surface enhanced Raman scattering (SERS). In a first step, polyvinyl alcohol composites were prepared incorporating silver nanoparticles by two different reduction procedures. At first without introduction of carbon nanostructures. These composites showed good results for the SERS identification of nucleic acids. Next, the synthesis and characterization of graphene oxide was studied to be used in the preparation of silver and gold nanocomposites. The reduction of this nanomaterial with different chemical agents was explored, since its reduction degree may be a determinant factor for the application envisaged (biomolecules interaction). The preparation of the nanocomposites with silver and gold was performed with different reducing agents. The SERS activity of these new nanocomposites was then explored in the presence of different analytes, varying the experimental conditions for Raman spectra acquisition. It was interesting to verify that the silver containing nanocomposites presented the particularity to intensify the graphene D and G bands. It is also important to highlight that a new eco-friendly reducing agent was tested for the synthesis of the graphene oxide composites, an Eucalyptus Globulus extract. Other variable introduced was the preparation of gold nanostars synthesized with hydroxylamine in the presence of graphene oxide, which allowed the preparation of a new nanocomposite with SERS potential. Fibrous membranes were also prepared by electrospinning with the aim to prepare SERS supports with adequate topography and porosity for the formation of nanoparticles agglomerates for the creation of the so-called hot-spots and also to allow the penetration of the analyte molecules. The polymers polyvinyl alcohol and polyacrylonitrile were selected for electrospinning. Using this technique, electrospun mantles with silver and gold nanoparticles and nanocomposites were prepared. Several variables were studied, such as the introduction of the nano-fillers during the electrospinning process, later deposition of the nano-fillers on the simple electrospun polymeric fibres and surface functionalization of the simple polymeric membranes to link the nano-fillers. At last, the potentialities of using carbon nanotubes forests, produced by chemical vapor deposition and coated with gold film by sputtering, as new SERS substrates were explored. It was found that the SERS detection of DNA bases and ADN itself is possible using these substrates.

Development of infrared spectroscopy for assessing bacterial quality in foods

Rapid and specific detection of foodborne bacteria that can cause food spoilage or illness associated to its consumption is an increasingly important task in food industry. Bacterial detection, identification, and classification are generally performed using traditional methods based on biochemical or serological tests and the molecular methods based on DNA or RNA fingerprints. However, these methodologies are expensive, time consuming and laborious. Infrared spectroscopy is a reliable, rapid, and economic technique which could be explored as a tool for bacterial analysis in the food industry. In this thesis it was evaluated the potential of IR spectroscopy to study the bacterial quality of foods. In Chapter 2, it was developed a calibration model that successfully allowed to predict the bacterial concentration of naturally contaminated cooked ham samples kept at refrigeration temperature during 8 days. In this part, it was developed the methodology that allowed the best reproducibility of spectra from bacteria colonies with minimal sample preparation, which was used in the subsequent work. Several attempts trying different resolutions and number of scans in the IR were made. A spectral resolution of 4 cm-1, with 32 scans were the settings that allowed the best results. Subsequently, in Chapter 3, it was made an attempt to identify 22 different foodborne bacterial genera/species using IR spectroscopy coupled with multivariate analysis. The principal component analysis, used as an exploratory technique, allowed to form distinct groups, each one corresponding to a different genus, in most of the cases. Then, a hierarchical cluster analysis was performed to further analyse the group formation and the possibility of distinction between species of the same bacterial genus. It was observed that IR spectroscopy not only is suitable to the distinction of the different genera, but also to differentiate species of the same genus, with the simultaneous use of principal component analysis and cluster analysis techniques. The utilization of IR spectroscopy and multivariate statistical analysis were also investigated in Chapter 4, in order to confirm the presence of Listeria monocytogenes and Salmonella spp. isolated from contaminated foods, after growth in selective medium. This would allow to substitute the traditional biochemical and serological methods that are used to confirm these pathogens and that delay the obtainment of the results up to 2 days. The obtained results allowed the distinction of 3 different Listeria species and the distinction of Salmonella spp. from other bacteria that can be mistaken with them. Finally, in chapter 5, high pressure processing, an emerging methodology that permits to produce microbiologically safe foods and extend their shelf-life, was applied to 12 foodborne bacteria to determine their resistance and the effects of pressure in cells. A treatment of 300 MPa, during 15 minutes at room temperature was applied. Gram-negative bacteria were inactivated to undetectable levels and Gram-positive showed different resistances. Bacillus cereus and Staphylococcus aureus decreased only 2 logs and Listeria innocua decreased about 5 logs. IR spectroscopy was performed in bacterial colonies before and after HPP in order to investigate the alterations of the cellular compounds. It was found that high pressure alters bands assigned to some cellular components as proteins, lipids, oligopolysaccharides, phosphate groups from the cell wall and nucleic acids, suggesting disruption of the cell envelopes. In this work, bacterial quantification and classification, as well as assessment of cellular compounds modification with high pressure processing were successfully performed. Taking this into account, it was showed that IR spectroscopy is a very promising technique to analyse bacteria in a simple and inexpensive manner.

Functional alkali tantalate 2D structures for microelectronics and related applications

Alkali tantalates and niobates, including K(Ta / Nb)O3, Li(Ta / Nb)O3 and Na(Ta / Nb)O3, are a very promising ferroic family of lead-free compounds with perovskite-like structures. Their versatile properties make them potentially interesting for current and future application in microelectronics, photocatalysis, energy and biomedics. Among them potassium tantalate, KTaO3 (KTO), has been raising interest as an alternative for the well-known strontium titanate, SrTiO3 (STO). KTO is a perovskite oxide with a quantum paraelectric behaviour when electrically stimulated and a highly polarizable lattice, giving opportunity to tailor its properties via external or internal stimuli. However problems related with the fabrication of either bulk or 2D nanostructures makes KTO not yet a viable alternative to STO. Within this context and to contribute scientifically to the leverage tantalate based compounds applications, the main goals of this thesis are: i) to produce and characterise thin films of alkali tantalates by chemical solution deposition on rigid Si based substrates, at reduced temperatures to be compatible with Si technology, ii) to fulfil scientific knowledge gaps in these relevant functional materials related to their energetics and ii) to exploit alternative applications for alkali tantalates, as photocatalysis. In what concerns the synthesis attention was given to the understanding of the phase formation in potassium tantalate synthesized via distinct routes, to control the crystallization of desired perovskite structure and to avoid low temperature pyrochlore or K-deficient phases. The phase formation process in alkali tantalates is far from being deeply analysed, as in the case of Pb-containing perovskites, therefore the work was initially focused on the process-phase relationship to identify the driving forces responsible to regulate the synthesis. Comparison of phase formation paths in conventional solid-state reaction and sol-gel method was conducted. The structural analyses revealed that intermediate pyrochlore K2Ta2O6 structure is not formed at any stage of the reaction using conventional solid-state reaction. On the other hand in the solution based processes, as alkoxide-based route, the crystallization of the perovskite occurs through the intermediate pyrochlore phase; at low temperatures pyrochlore is dominant and it is transformed to perovskite at >800 °C. The kinetic analysis carried out by using Johnson-MehlAvrami-Kolmogorow model and quantitative X-ray diffraction (XRD) demonstrated that in sol-gel derived powders the crystallization occurs in two stages: i) at early stage of the reaction dominated by primary nucleation, the mechanism is phase-boundary controlled, and ii) at the second stage the low value of Avrami exponent, n ~ 0.3, does not follow any reported category, thus not permitting an easy identification of the mechanism. Then, in collaboration with Prof. Alexandra Navrotsky group from the University of California at Davis (USA), thermodynamic studies were conducted, using high temperature oxide melt solution calorimetry. The enthalpies of formation of three structures: pyrochlore, perovskite and tetragonal tungsten bronze K6Ta10.8O30 (TTB) were calculated. The enthalpies of formation from corresponding oxides, ∆Hfox, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -203.63 ± 2.84 kJ/mol, - 358.02 ± 3.74 kJ/mol, and -1252.34 ± 10.10 kJ/mol, respectively, whereas from elements, ∆Hfel, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -1408.96 ± 3.73 kJ/mol, -2790.82 ± 6.06 kJ/mol, and -13393.04 ± 31.15 kJ/mol, respectively. The possible decomposition reactions of K-deficient KTa2.2O6 pyrochlore to KTaO3 perovskite and Ta2O5 (reaction 1) or to TTB K6Ta10.8O30 and Ta2O5 (reaction 2) were proposed, and the enthalpies were calculated to be 308.79 ± 4.41 kJ/mol and 895.79 ± 8.64 kJ/mol for reaction 1 and reaction 2, respectively. The reactions are strongly endothermic, indicating that these decompositions are energetically unfavourable, since it is unlikely that any entropy term could override such a large positive enthalpy. The energetic studies prove that pyrochlore is energetically more stable phase than perovskite at low temperature. Thus, the local order of the amorphous precipitates drives the crystallization into the most favourable structure that is the pyrochlore one with similar local organization; the distance between nearest neighbours in the amorphous or short-range ordered phase is very close to that in pyrochlore. Taking into account the stoichiometric deviation in KTO system, the selection of the most appropriate fabrication / deposition technique in thin films technology is a key issue, especially concerning complex ferroelectric oxides. Chemical solution deposition has been widely reported as a processing method to growth KTO thin films, but classical alkoxide route allows to crystallize perovskite phase at temperatures >800 °C, while the temperature endurance of platinized Si wafers is ~700 °C. Therefore, alternative diol-based routes, with distinct potassium carboxylate precursors, was developed aiming to stabilize the precursor solution, to avoid using toxic solvents and to decrease the crystallization temperature of the perovskite phase. Studies on powders revealed that in the case of KTOac (solution based on potassium acetate), a mixture of perovskite and pyrochlore phases is detected at temperature as low as 450 °C, and gradual transformation into monophasic perovskite structure occurs as temperature increases up to 750 °C, however the desired monophasic KTaO3 perovskite phase is not achieved. In the case of KTOacac (solution with potassium acetylacetonate), a broad peak is detected at temperatures <650 °C, characteristic of amorphous structures, while at higher temperatures diffraction lines from pyrochlore and perovskite phases are visible and a monophasic perovskite KTaO3 is formed at >700 °C. Infrared analysis indicated that the differences are due to a strong deformation of the carbonate-based structures upon heating. A series of thin films of alkali tantalates were spin-coated onto Si-based substrates using diol-based routes. Interestingly, monophasic perovskite KTaO3 films deposited using KTOacac solution were obtained at temperature as low as 650 °C; films were annealed in rapid thermal furnace in oxygen atmosphere for 5 min with heating rate 30 °C/sec. Other compositions of the tantalum based system as LiTaO3 (LTO) and NaTaO3 (NTO), were successfully derived as well, onto Si substrates at 650 °C as well. The ferroelectric character of LTO at room temperature was proved. Some of dielectric properties of KTO could not be measured in parallel capacitor configuration due to either substrate-film or filmelectrode interfaces. Thus, further studies have to be conducted to overcome this issue. Application-oriented studies have also been conducted; two case studies: i) photocatalytic activity of alkali tantalates and niobates for decomposition of pollutant, and ii) bioactivity of alkali tantalate ferroelectric films as functional coatings for bone regeneration. Much attention has been recently paid to develop new type of photocatalytic materials, and tantalum and niobium oxide based compositions have demonstrated to be active photocatalysts for water splitting due to high potential of the conduction bands. Thus, various powders of alkali tantalates and niobates families were tested as catalysts for methylene blue degradation. Results showed promising activities for some of the tested compounds, and KNbO3 is the most active among them, reaching over 50 % degradation of the dye after 7 h under UVA exposure. However further modifications of powders can improve the performance. In the context of bone regeneration, it is important to have platforms that with appropriate stimuli can support the attachment and direct the growth, proliferation and differentiation of the cells. In lieu of this here we exploited an alternative strategy for bone implants or repairs, based on charged mediating signals for bone regeneration. This strategy includes coating metallic 316L-type stainless steel (316L-SST) substrates with charged, functionalized via electrical charging or UV-light irradiation, ferroelectric LiTaO3 layers. It was demonstrated that the formation of surface calcium phosphates and protein adsorption is considerably enhanced for 316L-SST functionalized ferroelectric coatings. Our approach can be viewed as a set of guidelines for the development of platforms electrically functionalized that can stimulate tissue regeneration promoting direct integration of the implant in the host tissue by bone ingrowth and, hence contributing ultimately to reduce implant failure.

Coupling of magnetic, strain and electric polarization fields in the structure of multiferroic material

The comprehensive study on the coupling of magnetism, electrical polarization and the crystalline lattice with the off-stoichiometric effects in self-doped multiferroic hexagonal h-LuMnxO3±δ (0.92≤x≤1.12) ceramic oxides was carried out for the PhD work. There is a complex coupling of the three ferroic degrees. The cancelation of the magnetic moments of ions in the antiferromagnetic order, electric polarization with specific vortex/antivortex topology and lattice properties have pushed researchers to find out ways to disclose the underlying physics and chemistry of magneto-electric and magneto-elastic couplings of h-RMnO3 multiferroic materials. In this research work, self-doping of Lu-sites or Mn-sites of h-LuMnxO3±δ ceramics prepared via solid state route was done to pave a way for deeper understanding of the antiferromagnetic transition, the weak ferromagnetism often reported in the same crystalline lattices and the ferroelectric properties coupled to the imposed lattice changes. Accordingly to the aim of the PhD thesis, the objectives set for the sintering study in the first chapter on experimental results were two. First, study of sintering off-stoichiometric samples within conditions reported in the bibliography and also extracted from the phase diagrams of the LuMnxO3±δ, with a multiple firings ending with a last high temperature step at 1300ºC for 24 hours. Second, explore longer annealing times of up to 240 hours at the fixed temperature of 1300 ºC in a search for improving the properties of the solid solution under study. All series of LuMnxO3±δ ceramics for each annealing time were characterized to tentatively build a framework enabling comparison of measured properties with results of others available in literature. XRD and Rietveld refinement of data give the evolution the lattice parameters as a function to x. Shrinkage of the lattice parameters with increasing x values was observed, the stability limit of the solid solution being determined by analysis of lattice parameters. The evolution of grain size and presence of secondary phases have been investigated by means of TEM, SEM, EDS and EBSD techniques. The dependencies of grain growth and regression of secondary phases on composition x and time were further characterized. Magnetic susceptibility of samples and magnetic irreversibility were extensively examined in the present work. The dependency of magnetic susceptibility, Neel ordering transition and important magnetic parameters are determined and compared to observation in other multiferroics in the following chapter of the thesis. As a tool of high sensitivity to detect minor traces of the secondary phase hausmannite, magnetic measurements are suggested for cross-checking of phase diagrams. Difficulty of previous studies on interpreting the magnetic anomaly below 43 K in h-RMnO3 oxides was discussed and assigned to the Mn3O4 phase, with supported of the electron microscopy. Magneto-electric coupling where AFM ordering is coupled to dielectric polarization is investigated as a function of x and of sintering condition via frequency and temperature dependent complex dielectric constant measurements in the final chapter of the thesis. Within the limits of solid solubility, the crystalline lattice of off-stoichiometric ceramics was shown to preserve the magneto-electric coupling at TN. It represents the first research work on magneto-electric coupling modified by vacancy doping to author’s knowledge. Studied lattices would reveal distortions at the atomic scale imposed by local changes of x dependent on sintering conditions which were widely inspected by using TEM/STEM methods, complemented with EDS and EELS spectroscopy all together to provide comprehensive information on cross coupling of distortions, inhomogeneity and electronic structure assembled and discussed in a specific chapter. Internal interfaces inside crystalline grains were examined. Qualitative explanations of the measured magnetic and ferroelectric properties were established in relation to observed nanoscale features of h-LuMnxO3±δ ceramics. Ferroelectric domains and topological defects are displayed both in TEM and AFM/PFM images, the later technique being used to look at size, distribution and switching of ferroelectric domains influenced by vacancy doping at the micron scale bridging to complementary TEM studies on the atomic structure of ferroelectric domains. In support to experimental study, DFT simulations using Wien2K code have been carried out in order to interpret the results of EELS spectra of O K-edge and to obtain information on the cation hybridization to oxygen ions. The L3,2 edges of Mn is used to access the oxidation state of the Mn ions inside crystalline grains. In addition, rehybridization driven ferroelectricity is also evaluated by comparing the partial density of states of the orbitals of all ions of the samples, also the polarization was calculated and correlated to the off-stoichiometric effect.

Critical magnetic elds in superconducting systems with semi-metallic bands

In this work, we study the Zeeman splitting effects in the parallel magnetic field versus temperature phase diagram of two-dimensional superconductors with one graphene-like band and the orbital effects of perpendicular magnetic fields in isotropic two-dimensional semi-metallic superconductors. We show that when parallel magnetic fields are applied to graphene and as the intraband interaction decreases to a critical value, the width of the metastability region present in the phase diagram decreases, vanishing completely at that critical value. In the case of two-band superconductors with one graphene-like band, a new critical interaction, associated primarily with the graphene-like band, is required in order for a second metastability region to be present in the phase diagram. For intermediate values of this interaction, a low-temperature first-order transition line bifurcates at an intermediate temperature into a first-order transition between superconducting phases and a second-order transition line between the normal and the superconducting states. In our study on the upper critical fields in generic semi-metallic superconductors, we find that the pair propagator decays faster than that of a superconductor with a metallic band. As result, the zero field band gap equation does not have solution for weak intraband interactions, meaning that there is a critical intraband interaction value in order for a superconducting phase to be present in semi-metallic superconductors. Finally, we show that the out-of-plane critical magnetic field versus temperature phase diagram displays a positive curvature, contrasting with the parabolic-like behaviour typical of metallic superconductors.