Spectroscopic, morphological and electrochromic characterization of layer-by-layer hybrid films of polyaniline and hexaniobate nanoscrolls

The combination of semiconducting oxides and polyaniline in the nanoscale range may result in hybrid materials having enhanced properties, such as electrochromism and charge capacity. This paper reports the spectroscopic, morphological and electrochromic characterization of hybrid films made up of hexaniobate one-dimensional (1D) nanoscrolls and polyaniline prepared by the layer-by-layer assembly technique (LbL). Secondary electron imaging and backscattered electron imaging techniques performed using a scanning electron microscope showed that polyaniline is adsorbed on the hexaniobate nanoscrolls, which confirms the combination of the components in the nanoscale domain. UV-VIS-NIR electronic spectra of the LbL hybrid films showed the absorption tail in the NIR region, assigned to delocalized polarons of the polyaniline. Resonance Raman spectra in the 1000-1700 cm(-1) range indicated that hybrid films present a higher relative intensity of polaron bands at 1337 and 1508 cm(-1) than pristine polyaniline in the emeraldine salt form. These results suggest that hexaniobate nanoscrolls induce a secondary doping of polyaniline. The cyclic voltammetry (CV) data for the hybrid film showed a specific capacity of 870 C cm(-3). According to CV results, the synergistic effect on charge storage properties of the hybrid material is attributed to the enhanced electroactivity of the hexaniobate component in the LbL film. Spectroelectrochemical experiments showed that the electrochromic efficiencies at 420 nm are ca. -41 and 24 cm(2) C-1 as the potential changes from 0.8 to -0.9 V and from -0.9 to -1.8 V, respectively, whereas at 800 nm the efficiencies are ca. -55 and 8 cm(2) C-1 for the same potential ranges. The electrochromic efficiencies and multi-colour character of the LbL film of hexaniobate nanoscrolls and polyaniline indicate that this novel hybrid material is an interesting modified electrode for electrochromic devices.

Transport Properties and Novel Sensing Applications of Organic Semiconducting Crystals

The present thesis is focused on the study of Organic Semiconducting Single Crystals (OSSCs) and crystalline thin films. In particular solution-grown OSSC, e.g. 4-hdroxycyanobenzene (4HCB) have been characterized in view of their applications as novel sensors of X-rays, gamma-rays, alpha particles radiations and chemical sensors. In the field of ionizing radiation detection, organic semiconductors have been proposed so far mainly as indirect detectors, i.e. as scintillators or as photodiodes. I first study the performance of 4HCB single crystals as direct X-ray detector i.e. the direct photon conversion into an electrical signal, assessing that they can operate at room temperature and in atmosphere, showing a stable and linear response with increasing dose rate. A dedicated study of the collecting electrodes geometry, crystal thickness and interaction volume allowed us to maximize the charge collection efficiency and sensitivity, thus assessing how OSSCs perform at low operating voltages and offer a great potential in the development of novel ionizing radiation sensors. To better understand the processes generating the observed X-ray signal, a comparative study is presented on OSSCs based on several small-molecules: 1,5-dinitronaphthalene (DNN), 1,8-naphthaleneimide (NTI), Rubrene and TIPS-pentacene. In addition, the proof of principle of gamma-rays and alpha particles has been assessed for 4HCB single crystals. I have also carried out an investigation of the electrical response of OSSCs exposed to vapour of volatile molecules, polar and non-polar. The last chapter deals with rubrene, the highest performing molecular crystals for electronic applications. We present an investigation on high quality, millimeter-sized, crystalline thin films (10 – 100 nm thick) realized by exploiting organic molecular beam epitaxy on water-soluble substrates. Space-Charge-Limited Current (SCLC) and photocurrent spectroscopy measurements have been carried out. A thin film transistor was fabricated onto a Cytop® dielectric layer. The FET mobility exceeding 2 cm2/Vs, definitely assess the quality of RUB films.

Crecimiento, fabricación y caracterización de heteroestructuras y nanocolumnas ordenadas basadas en nitruros del grupo III para aplicaciones sensoras

Esta memoria está basada en el crecimiento y caracterización de heteroestructuras Al(Ga)N/GaN y nanocolumnas ordenadas de GaN, y su aplicación en sensores químicos. El método de crecimiento ha sido la epitaxia de haces moleculares asistida por plasma (PAMBE). En el caso de las heteroestructuras Al(Ga)N/GaN, se han crecido barreras de distinto espesor y composición, desde AlN de 5 nm, hasta AlGaN de 35 nm. Además de una caracterización morfológica, estructural y eléctrica básica de las capas, también se han fabricado a partir de ellas dispositivos tipo HEMTs. La caracterización eléctrica de dichos dispositivos (carga y movilidad de en el canal bidimensional) indica que las mejores heteroestructuras son aquellas con un espesor de barrera intermedio (alrededor de 20 nm). Sin embargo, un objetivo importante de esta Tesis ha sido verificar las ventajas que podían tener los sensores basados en heteroestructuras AlN/GaN (frente a los típicos basados en AlGaN/GaN), con espesores de barrera muy finos (alrededor de 5 nm), ya que el canal de conducción que se modula por efecto de cambios químicos está más cerca de la superficie en donde ocurren dichos cambios químicos. De esta manera, se han utilizado los dispositivos tipo HEMTs como sensores químicos de pH (ISFETs), y se ha comprobado la mayor sensibilidad (variación de corriente frente a cambios de pH, Ids/pH) en los sensores basados en AlN/GaN frente a los basados en AlGaN/GaN. La mayor sensibilidad es incluso más patente en aplicaciones en las que no se utiliza un electrodo de referencia. Se han fabricado y caracterizado dispositivos ISFET similares utilizando capas compactas de InN. Estos sensores presentan peor estabilidad que los basados en Al(Ga)N/GaN, aunque la sensibilidad superficial al pH era la misma (Vgs/pH), y su sensibilidad en terminos de corriente de canal (Ids/pH) arroja valores intermedios entre los ISFET basados en AlN/GaN y los valores de los basados en AlGaN/GaN. Para continuar con la comparación entre dispositivos basados en Al(Ga)N/GaN, se fabricaron ISFETs con el área sensible más pequeña (35 x 35 m2), de tamaño similar a los dispositivos destinados a las medidas de actividad celular. Sometiendo los dispositivos a pulsos de voltaje en su área sensible, la respuesta de los dispositivos de AlN presentaron menor ruido que los basados en AlGaN. El ruido en la corriente para dispositivos de AlN, donde el encapsulado no ha sido optimizado, fue tan bajo como 8.9 nA (valor rms), y el ruido equivalente en el potencial superficial 38.7 V. Estos valores son más bajos que los encontrados en los dispositivos típicos para la detección de actividad celular (basados en Si), y del orden de los mejores resultados encontrados en la literatura sobre AlGaN/GaN. Desde el punto de vista de la caracterización electro-química de las superficies de GaN e InN, se ha determinado su punto isoeléctrico. Dicho valor no había sido reportado en la literatura hasta el momento. El valor, determinado por medidas de “streaming potential”, es de 4.4 y 4 respectivamente. Este valor es una importante característica a tener en cuenta en sensores, en inmovilización electrostática o en la litografía coloidal. Esta última técnica se discute en esta memoria, y se aplica en el último bloque de investigación de esta Tesis (i.e. crecimiento ordenado). El último apartado de resultados experimentales de esta Tesis analiza el crecimiento selectivo de nanocolumnas ordenadas de GaN por MBE, utilizando mascaras de Ti con nanoagujeros. Se ha estudiado como los distintos parámetros de crecimiento (i.e. flujos de los elementos Ga y N, temperatura de crecimiento y diseño de la máscara) afectan a la selectividad y a la morfología de las nanocolumnas. Se ha conseguido con éxito el crecimiento selectivo sobre pseudosustratos de GaN con distinta orientación cristalina o polaridad; templates de GaN(0001)/zafiro, GaN(0001)/AlN/Si, GaN(000-1)/Si y GaN(11-20)/zafiro. Se ha verificado experimentalmente la alta calidad cristalina de las nanocolumnas ordenadas, y su mayor estabilidad térmica comparada con las capas compactas del mismo material. Las nanocolumnas ordenadas de nitruros del grupo III tienen una clara aplicación en el campo de la optoelectrónica, principalmente para nanoemisores de luz blanca. Sin embargo, en esta Tesis se proponen como alternativa a la utilización de capas compactas o nanocolumnas auto-ensambladas en sensores. Las nanocolumnas auto-ensambladas de GaN, debido a su alta razón superficie/volumen, son muy prometedoras en el campo de los sensores, pero su amplia dispersión en dimensiones (altura y diámetro) supone un problema para el procesado y funcionamiento de dispositivos reales. En ese aspecto, las nanocolumnas ordenadas son más robustas y homogéneas, manteniendo una alta relación superficie/volumen. Como primer experimento en el ámbito de los sensores, se ha estudiado como se ve afectada la emisión de fotoluminiscencia de las NCs ordenadas al estar expuestas al aire o al vacio. Se observa una fuerte caída en la intensidad de la fotoluminiscencia cuando las nanocolumnas están expuestas al aire (probablemente por la foto-adsorción de oxigeno en la superficie), como ya había sido documentado anteriormente en nanocolumnas auto-ensambladas. Este experimento abre el camino para futuros sensores basados en nanocolumnas ordenadas. Abstract This manuscript deals with the growth and characterization of Al(Ga)N/GaN heterostructures and GaN ordered nanocolumns, and their application in chemical sensors. The growth technique has been the plasma-assisted molecular beam epitaxy (PAMBE). In the case of Al(Ga)N/GaN heterostructures, barriers of different thickness and composition, from AlN (5 nm) to AlGaN (35 nm) have been grown. Besides the basic morphological, structural and electrical characterization of the layers, HEMT devices have been fabricated based on these layers. The best electrical characteristics (larger carriers concentration and mobility in the two dimensional electron gas) are those in AlGaN/GaN heterostructures with a medium thickness (around 20 nm). However, one of the goals of this Thesis has been to verify the advantages that sensors based on AlN/GaN (thickness around 7 nm) have compared to standard AlGaN/GaN, because the conduction channel to be modulated by chemical changes is closer to the sensitive area. In this way, HEMT devices have been used as chemical pH sensors (ISFETs), and the higher sensitivity (conductance change related to pH changes, Ids/pH) of AlN/GaN based sensors has been proved. The higher sensibility is even more obvious in application without reference electrode. Similar ISFETs devices have been fabricated based on InN compact layers. These devices show a poor stability, but its surface sensitivity to pH (Vgs/pH) and its sensibility (Ids/pH) yield values between the corresponding ones of AlN/GaN and AlGaN/GaN heterostructures. In order to a further comparison between Al(Ga)N/GaN based devices, ISFETs with smaller sensitive area (35 x 35 m2), similar to the ones used in cellular activity record, were fabricated and characterized. When the devices are subjected to a voltage pulse through the sensitive area, the response of AlN based devices shows lower noise than the ones based on AlGaN. The noise in the current of such a AlN based device, where the encapsulation has not been optimized, is as low as 8.9 nA (rms value), and the equivalent noise to the surface potential is 38.7 V. These values are lower than the found in typical devices used for cellular activity recording (based on Si), and in the range of the best published results on AlGaN/GaN. From the point of view of the electrochemical characterization of GaN and InN surfaces, their isoelectric point has been experimentally determined. Such a value is the first time reported for GaN and InN surfaces. These values are determined by “streaming potential”, being pH 4.4 and 4, respectively. Isoelectric point value is an important characteristic in sensors, electrostatic immobilization or in colloidal lithography. In particular, colloidal lithography has been optimized in this Thesis for GaN surfaces, and applied in the last part of experimental results (i.e. ordered growth). The last block of this Thesis is focused on the selective area growth of GaN nanocolumns by MBE, using Ti masks decorated with nanoholes. The effect of the different growth parameters (Ga and N fluxes, growth temperature and mask design) is studied, in particular their impact in the selectivity and in the morphology of the nanocolumns. Selective area growth has been successful performed on GaN templates with different orientation or polarity; GaN(0001)/sapphire, GaN(0001)/AlN/Si, GaN(000- 1)/Si and GaN(11-20)/sapphire. Ordered nanocolumns exhibit a high crystal quality, and a higher thermal stability (lower thermal decomposition) than the compact layers of the same material. Ordered nanocolumns based on III nitrides have a clear application in optoelectronics, mainly for white light nanoemitters. However, this Thesis proposes them as an alternative to compact layers and self-assembled nanocolumns in sensor applications. Self-assembled GaN nanocolumns are very appealing for sensor applications, due to their large surface/volume ratio. However, their large dispersion in heights and diameters are a problem in terms of processing and operation of real devices. In this aspect, ordered nanocolumns are more robust and homogeneous, keeping the large surface/volume ratio. As first experimental evidence of their sensor capabilities, ordered nanocolumns have been studied regarding their photoluminiscence on air and vacuum ambient. A big drop in the intensity is observed when the nanocolumns are exposed to air (probably because of the oxygen photo-adsortion), as was already reported in the case of self-assembled nanocolumns. This opens the way to future sensors based on ordered III nitrides nanocolumns.

Desenvolvimento e caracterização do filtro óptico de interferência variável para detectores de alta resolução espectral e biossensores.

O presente trabalho está fundamentado no desenvolvimento de uma metodologia e/ou uma tecnologia de obtenção e caracterização de filtros ópticos de interferência de banda passante variável [C.M. da Silva, 2010] e de banda de corte variáveis, constituídos por refletores dielétricos multicamadas de filmes finos intercalados por cavidades de Fabry-Perot não planares com espessuras linearmente variáveis, que apresentam a propriedade do deslocamento linear da transmitância máxima espectral em função da posição, isto é, um Filtro de Interferência Variável (FIV). Este método apresenta novas e abrangentes possibilidades de confecção de filtros ópticos de interferência variável: lineares ou em outras formas desejadas, de comprimento de onda de corte variável (passa baixa ou alta) e filtros de densidade neutra variável, através da deposição de metais, além de aplicações em uma promissora e nova área de pesquisa na deposição de filmes finos não uniformes. A etapa inicial deste desenvolvimento foi o estudo da teoria dos filtros ópticos dielétricos de interferência para projetar e construir um filtro óptico banda passante convencional de um comprimento de onda central com camadas homogêneas. A etapa seguinte, com base na teoria óptica dos filmes finos já estabelecida, foi desenvolver a extensão destes conhecimentos para determinar que a variação da espessura em um perfil inclinado e linear da cavidade entre os refletores de Bragg é o principal parâmetro para produzir o deslocamento espacial da transmitância espectral, possibilitando o uso de técnicas especiais para se obter uma variação em faixas de bandas de grande amplitude, em um único filtro. Um trabalho de modelagem analítica e análise de tolerância de espessuras dos filmes depositados foram necessários para a seleção da estratégia do \"mascaramento\" seletivo do material evaporado formado na câmara e-Beam (elétron-Beam) com o objetivo da obtenção do filtro espectral linear variável de características desejadas. Para tanto, de acordo com os requisitos de projeto, foram necessárias adaptações em uma evaporadora por e-Beam para receber um obliterador mecânico especialmente projetado para compatibilizar os parâmetros das técnicas convencionais de deposição com o objetivo de se obter um perfil inclinado, perfil este previsto em processos de simulação para ajustar e calibrar a geometria do obliterador e se obter um filme depositado na espessura, conformação e disposição pretendidos. Ao final destas etapas de modelagem analítica, simulação e refinamento recorrente, foram determinados os parâmetros de projeto para obtenção de um determinado FIV (Filtro de Interferência Variável) especificado. Baseadas nos FIVs muitas aplicações são emergentes: dispositivos multi, hiper e ultra espectral para sensoriamento remoto e análise ambiental, sistemas Lab-on-Chip, biossensores, detectores chip-sized, espectrofotometria de fluorescência on-chip, detectores de deslocamento de comprimento de onda, sistemas de interrogação, sistemas de imageamento espectral, microespectrofotômetros e etc. No escopo deste trabalho se pretende abranger um estudo de uma referência básica do emprego do (FIV) filtro de interferência variável como detector de varredura de comprimento de ondas em sensores biológicos e químicos compatível com pós processamento CMOS. Um sistema básico que é constituído por um FIV montado sobre uma matriz de sensores ópticos conectada a um módulo eletrônico dedicado a medir a intensidade da radiação incidente e as bandas de absorção das moléculas presentes em uma câmara de detecção de um sistema próprio de canais de microfluidos, configurando-se em um sistema de aquisição e armazenamento de dados (DAS), é proposto para demonstrar as possibilidades do FIV e para servir de base para estudos exploratórios das suas diversas potencialidades que, entre tantas, algumas são mencionadas ao longo deste trabalho. O protótipo obtido é capaz de analisar fluidos químicos ou biológicos e pode ser confrontado com os resultados obtidos por equipamentos homologados de uso corrente.

Microchanneled chirped fibre Bragg gratings for simultaneous refractive index and temperature measurements

We report here the fabrication, charaterisation and refractive index sensing of two microchanneled chirped fiber Bragg gratings (MCFBGs) with different channel sizes (~550µm and ~1000µm). The chirped grating structures were UV-inscribed in optical fibre and the microchannels were created in the middle of the CFBGs by femtosecond (fs) laser assisted chemical etching method. The creation of microchannels in the CFBG structures gives an access to the external index liquid, thus inducing refractive index (RI) sensitivity to the structure. In comparison with previously reported FBG based RI sensors, for which the cladding layers usually were removed, the MCFBGs represent a more ideal solution for robust devices as the microchannel will not degrade the structure strength. The two MCFBGs were spectrally charaterised for their RI and temperature responses and both gratings exhibited unique thermal and RI sensitivities, which may be utilised for implementation of bio-chemical sensors with capability to eliminate temperature crosssensitivity.

Microchanneled chirped fibre Bragg gratings for simultaneous refractive index and temperature measurements

We report here the fabrication, charaterisation and refractive index sensing of two microchanneled chirped fiber Bragg gratings (MCFBGs) with different channel sizes (~550µm and ~1000µm). The chirped grating structures were UV-inscribed in optical fibre and the microchannels were created in the middle of the CFBGs by femtosecond (fs) laser assisted chemical etching method. The creation of microchannels in the CFBG structures gives an access to the external index liquid, thus inducing refractive index (RI) sensitivity to the structure. In comparison with previously reported FBG based RI sensors, for which the cladding layers usually were removed, the MCFBGs represent a more ideal solution for robust devices as the microchannel will not degrade the structure strength. The two MCFBGs were spectrally charaterised for their RI and temperature responses and both gratings exhibited unique thermal and RI sensitivities, which may be utilised for implementation of bio-chemical sensors with capability to eliminate temperature crosssensitivity.

Enhanced zinc oxide and graphene nanostructures for electronics and sensing applications

Zinc oxide and graphene nanostructures are important technological materials because of their unique properties and potential applications in future generation of electronic and sensing devices. This dissertation investigates a brief account of the strategies to grow zinc oxide nanostructures (thin film and nanowire) and graphene, and their applications as enhanced field effect transistors, chemical sensors and transparent flexible electrodes. Nanostructured zinc oxide (ZnO) and low-gallium doped zinc oxide (GZO) thin films were synthesized by a magnetron sputtering process. Zinc oxide nanowires (ZNWs) were grown by a chemical vapor deposition method. Field effect transistors (FETs) of ZnO and GZO thin films and ZNWs were fabricated by standard photo and electron beam lithography processes. Electrical characteristics of these devices were investigated by nondestructive surface cleaning, ultraviolet irradiation treatment at high temperature and under vacuum. GZO thin film transistors showed a mobility of ∼5.7 cm2/V·s at low operation voltage of <5 V and a low turn-on voltage of ∼0.5 V with a sub threshold swing of ∼85 mV/decade. Bottom gated FET fabricated from ZNWs exhibit a very high on-to-off ratio (∼106) and mobility (∼28 cm2/V·s). A bottom gated FET showed large hysteresis of ∼5.0 to 8.0 V which was significantly reduced to ∼1.0 V by the surface treatment process. The results demonstrate charge transport in ZnO nanostructures strongly depends on its surface environmental conditions and can be explained by formation of depletion layer at the surface by various surface states. A nitric oxide (NO) gas sensor using single ZNW, functionalized with Cr nanoparticles was developed. The sensor exhibited average sensitivity of ∼46% and a minimum detection limit of ∼1.5 ppm for NO gas. The sensor also is selective towards NO gas as demonstrated by a cross sensitivity test with N2, CO and CO2 gases. Graphene film on copper foil was synthesized by chemical vapor deposition method. A hot press lamination process was developed for transferring graphene film to flexible polymer substrate. The graphene/polymer film exhibited a high quality, flexible transparent conductive structure with unique electrical-mechanical properties; ∼88.80% light transmittance and ∼1.1742Ω/sq k sheet resistance. The application of a graphene/polymer film as a flexible and transparent electrode for field emission displays was demonstrated.

Synthesis and electron emission properties of aligned carbon nanotube arrays

Carbon nanotubes (CNTs) have become one of the most interesting allotropes of carbon due to their intriguing mechanical, electrical, thermal and optical properties. The synthesis and electron emission properties of CNT arrays have been investigated in this work. Vertically aligned CNTs of different densities were synthesized on copper substrate with catalyst dots patterned by nanosphere lithography. The CNTs synthesized with catalyst dots patterned by spheres of 500 nm diameter exhibited the best electron emission properties with the lowest turn-on/threshold electric fields and the highest field enhancement factor. Furthermore, CNTs were treated with NH3 plasma for various durations and the optimum enhancement was obtained for a plasma treatment of 1.0 min. CNT point emitters were also synthesized on a flat-tip or a sharp-tip to understand the effect of emitter geometry on the electron emission. The experimental results show that electron emission can be enhanced by decreasing the screening effect of the electric field by neighboring CNTs. In another part of the dissertation, vertically aligned CNTs were synthesized on stainless steel (SS) substrates with and without chemical etching or catalyst deposition. The density and length of CNTs were determined by synthesis time. For a prolonged growth time, the catalyst activity terminated and the plasma started etching CNTs destructively. CNTs with uniform diameter and length were synthesized on SS substrates subjected to chemical etching for a period of 40 minutes before the growth. The direct contact of CNTs with stainless steel allowed for the better field emission performance of CNTs synthesized on pristine SS as compared to the CNTs synthesized on Ni/Cr coated SS. Finally, fabrication of large arrays of free-standing vertically aligned CNT/SnO2 core-shell structures was explored by using a simple wet-chemical route. The structure of the SnO2 nanoparticles was studied by X-ray diffraction and electron microscopy. Transmission electron microscopy reveals that a uniform layer of SnO2 is conformally coated on every tapered CNT. The strong adhesion of CNTs with SS guaranteed the formation of the core-shell structures of CNTs with SnO2 or other metal oxides, which are expected to have applications in chemical sensors and lithium ion batteries.

Selective Preparation of Semiconducting Single-Walled Carbon Nanotubes: From Fundamentals to Applications

Aligned single-walled carbon nanotubes (SWNTs) synthesized by the chemical vapor deposition (CVD) method have exceptional potential for next-generation nanoelectronics. However, there are considerable challenges in the preparation of semiconducting (s-) SWNTs with controlled properties (e.g., density, selectivity, and diameter) for their application in solving real-world problems. This dissertation describes research that aims to overcome the limitations by novel synthesis strategies and post-growth treatment. The application of as-prepared SWNTs as functional devices is also demonstrated. The dissertation includes the following parts: 1) decoupling the conflict between density and selectivity of s-SWNTs in CVD growth; 2) investigating the importance of diameter control for the selective synthesis of s-SWNTs; 3) synthesizing highly conductive SWNT thin film by thiophene-assisted CVD method; 4) eliminating metallic pathways in SWNT crossbars by gate-free electrical breakdown method; 5) enhancing the density of SWNT arrays by strain-release method; 6) studying the sensing mechanism of SWNT crossbar chemical sensors.

Sintesi e Caratterizzazione di Elettrodi Modificati Chimicamente con nuovi complessi Carbenici del Ferro

The Chemically Modified Electrodes (CME) are widely used in electroanalytical chemistry as chemical sensors. The interest in the covalent anchoring of a redox mediator on the electrode surface is increasing, because it allows the sensibility and the selectivity of this kind of systems to improve. My work is situated in this field of research and involves the synthesis of new Iron(0) complexes that contain cyclopentadienone, N-heterocyclic carbene (NHC) and carbonyl ancillary ligands. These complexes have shown electrochemical properties similar to those of ferrocene (organometallic compound widely used as electrochemical sensor). These complexes have been properly functionalized with a EDOT group in the NHC ligand side chain that it was after used for the realization of Electrochemically Modified PEDOT thanks to copolymerization reaction between the functionalized complex and the EDOT in different amounts. All the synthetic steps were assisted by suitable characterizations (NMR, IR, ESI-MS, cyclic voltammetry and X-ray for the monomeric compound as imidazolium salt and NHC functionalized complexes; cyclic voltammetry, IR e SEM for the copolymers). The properties of the polymer as a selective sensor was preliminarily investigated for dopamine and 2-propanol.

Desenvolvimento de sensores químicos para aniões

O objectivo deste trabalho foi o desenvolvimento de sensores químicos para aniões em solução aquosa. Os sensores basearam-se em cristais piezoelétricos de quartzo revestidos com macrociclos pirrólicos. Sensores com porfirinas, hexafirinas, ftalocianinas e uma rede metalo-orgânica como filmes de reconhecimento foram avaliados no que respeita à reversibilidade, sensibilidade, seletividade e tempo de vida. A sensibilidade aos aniões presentes nas águas minerais destinadas ao consumo humano dos sensores com macrociclos pirrólicos dependia dos grupos substituintes na periferia do macrociclo. A presença de grupos substituintes NH potenciou a sensibilidade, sendo o valor mais elevado obtido com o grupo NH-tosilo. Técnicas espectroscópicas permitiram mostrar que a principal interação entre o anião e o composto se fazia por meio de pontes de hidrogénio entre o anião e o grupo NH. A sensibilidade destes compostos dependia ainda da quantidade do composto depositado sobre o cristal. Nenhum dos sensores era específico, mas apresentavam variações na seletividade com o composto pirrólico sobre o cristal. O sensor com uma membrana polimérica que incorporou uma rede metalo-orgânica construída com moléculas de uma ftalocianina de magnésio com grupos NH-tosilo ligados por iões cobre apresentou coeficientes de seletividade muito diferentes de todos os restantes sensores. Uma língua eletrónica construída com três dos novos sensores foi capaz de determinar simultaneamente as concentrações dos iões HCO3-, Cl- e SO42- em amostras de água mineral engarrafada, não tendo os valores obtidos sido estatisticamente diferentes (α=0,05) daqueles que foram obtidos por cromatografia iónica ou volumetria.

Obtenção de óxidos a base de níquel e cobalto para reação de oxidação parcial do metano

Nickel-based catalysts supported on alumina have been widely used in various reactions to obtain synthesis gas or hydrogen. Usually, higher conversion levels are obtained by these catalysts, however, the deactivation by coke formation and sintering of metal particles are still problems to be solved. Several approaches have been employed in order to minimize these problems, among which stands out in recent years the use of additives such as oxides of alkali metals and rare earths. Similarly, the use of methodologies for the synthesis faster, easier, applicable on an industrial scale and to allow control of the microstructural characteristics of these catalysts, can together provide the solution to this problem. In this work, oxides with spinel type structure AB2O4, where A represents divalent cation and B represents trivalent cations are an important class of ceramic materials investigated worldwide in different fields of applications. The nickel cobaltite (NiCo2O4) was oxides of spinel type which has attracted considerable interest due to its applicability in several areas, such as chemical sensors, flat panel displays, optical limiters, electrode materials, pigments, electrocatalysis, electronic ceramics, among others. The catalyst precursor NiCo2O4 was prepared by a new chemical synthesis route using gelatine as directing agent. The polymer resin obtained was calcined at 350°C. The samples were calcined at different temperatures (550, 750 and 950°C) and characterized by X ray diffraction, measurements of specific surface area, temperature programmed reduction and scanning electron microscopy. The materials heat treated at 550 and 750°C were tested in the partial oxidation of methane. The set of techniques revealed, for solid preparations, the presence of the phase of spinel-type structure with the NiCo2O4 NixCo1-xO solid solution. This solid solution was identified by Rietveld refinement at all temperatures of heat treatment. The catalyst precursors calcined at 550 and 750°C showed conversion levels around 25 and 75%, respectively. The reason H2/CO was around 2 to the precursor treated at 750°C, proposed reason for the reaction of partial oxidation of methane, one can conclude that this material can be shown to produce synthesis gas suitable for use in the synthesis Fischer-Tropsch process

Design. Fabrication and Characterization of Fiber Optic Sensors for Physical and Chemical Applications

Advent of lasers together with the advancement in fiber optics technology has revolutionized the sensor technology. Advancement in the telemetric applications of optical fiber based measurements is an added bonus. The present thesis describes variety of fiber based sensors using techniques like micro bending, long period grating and evanescent waves. Sensors to measure various physical and chemical parameters are described in this thesis.

Development of optical fiber sensors for selected chemical and physical sensing applications

The objective of the preset work is to develop optical fiber sensors for various physical and chemical parameters. As a part of this we initially investigated trace analysis of silica, ammonia, iron and phosphate in water. For this purpose the author has implemented a dual wavelength probing scheme which has many advantages over conventional evanescent wave sensors. Dual wavelength probing makes the design more reliable and repeatable and this design makes the sensor employable for concentration, chemical content, adulteration level, monitoring and control in industries or any such needy environments. Use of low cost components makes the system cost effective and simple. The Dual wavelength probing scheme is employed for the trace analysis of silica, iron, phosphate, and ammonia in water. Such sensors can be employed for the steam and water quality analysers in power plants. Few samples from a power plant are collected and checked the performance of developed system for practical applications.

Design and Development of Fiber Grating Based Chemical and Bio-Sensors

International School of Photonics