O objetivo desta tese de doutorado foi estudar a influência da adição do íon estanho (Sn4+) à rede do composto SrTiO3 em substituição ao íon de titânio visando otimizar as propriedades elétricas desse composto e, como consequência, obter uma melhora de seu desempenho como um sensor de gás na forma de filmes finos. Para realizar a deposição destes filmes finos através dos métodos de Deposição por Feixe de Elétrons (EBD), alvos cerâmicos de composição SrTi1-xSnxO3 (STSO) com x = 0; 0,20; 0,40; 0,60; 0,80; 0,85; 0,90; 0,95; 1 foram obtidos através do método dos precursores poliméricos modificado. A sequência de formação de soluções sólidas foi determinada através do refinamento Rietveld das amostras STSO na forma de pó sinterizadas, obtidas através dos métodos dos precursores poliméricos e de reação de estado sólido, mostrando que a transição da fase cúbica Pm3̄m do composto SrTiO3 até a fase ortorrômbica Pnma do composto SrSnO3 não depende do método de síntese. As medidas de espectroscopia Raman e absorção de raios-X (XANES, na borda K do Ti) das amostras tanto na forma de pó, obtidas através do método dos precursores poliméricos e de reação de estado sólido, quanto na forma de filme fino obtidas por EBD revelaram a existência de uma desordem local na rede do composto SrTiO3 que diminui com o aumento da temperatura e com a diminuição da concentração de Sn. Os filmes finos STSO obtidos por EBD foram avaliados como sensores utilizando-se os gases O3 e NH3. Em medidas realizadas com o gás ozônio (O3), os resultados mostraram que os filmes finos de 100 nm de espessura apresentaram uma maior sensibilidade tendo a amostra com 60% de Sn com o melhor desempenho a 350°C para 0,15 ppm do gás. As análises de performance dos filmes STSO quanto a seletividade indicaram que não foram seletivos e que apresentaram uma maior resposta ao gás ozônio quando comparados ao gás NH3. / The objective of this work was to study the influence of the addition of tin ion (Sn4+) into the SrTiO3 compound lattice, to replace the titanium ion (Ti4+). The aim was to optimize the electrical properties of SrTiO3 compound and, as a consequence, to obtain an improvement of its performance as a gas sensor in the thin films samples. To perform the deposition of these thin films through Electron Beam Deposition (EBD), ceramic targets of composition SrTi1-xSnxO3 (STSO) with x = 0; 0.20; 0.40; 0.60; 0.80; 0.85; 0.90; 0.95; and 1 were obtained by the modified polymer precursor method. The solid solution formation sequence was determined by the Rietveld refinement of the STSO sintered powdered samples, obtained by both polymeric precursor and solid-state reaction methods, showing that the transition from the cubic Pm3̄m phase of the SrTiO3 compound to the orthorhombic Pnma phase of the SrSnO3 compound does not depend on the synthesis method. The measurements of Raman spectroscopy and absorption of X-rays (XANES, at Ti K-edge), of the powdered samples obtained by both synthesis methods and of the thin films obtained by EBD, revealed the existence of a local disorder in the SrTiO3 compound lattice which decreases with increasing of temperature and with decreasing of Sn concentration. The STSO thin films were evaluated as sensors using the O3 and NH3 gases. In measurements accomplished with the ozone gas (O3), the results showed that thin films of 100 nm thickness had a higher sensitivity. The sample having 60% of tin showed the best performance at 350°C for 0.15 ppm of ozone gas. The performance analysis related to the selectivity of the STSO films indicated they were not selective and that presented a higher response to the ozone gas when compared to the NH3 gas.

Resistive random access memory (RRAM) is a non-volatile memory technology based on resistive switching in a dielectric or semiconductor sandwiched between two different metals. Also known as memristors, these devices are potential candidates for a next-generation replacement for flash memory. In this thesis, bipolar resistive switching is reported for the first time in solution-deposited zinc-tin-oxide (ZTO). The impact of the compliance current on device operation, including the SET and RESET voltages, pre-SET, RESET and post-RESET currents, the resistance ratio between the low and high resistance states, retention, and the endurance, is investigated for an isolated Al dot/ZTO/Ir blanket device and for Al/ZTO/Pt crossbar RRAM devices. A gradual forming process is devised to improve device stability and performance. It is found that the device performance depends critically on the compliance current density that is used to limit the breakdown conduction during the SET operation. In addition, it was found that the conduction and switching mechanisms are consistent with the filament model of formation and rupture of conductive filaments. / Graduation date: 2012

<p>When the amorphous silicon (a-Si) dangling bonds are bonded to hydrogen the concentration of the dangling bond is decreased. The resulting film is called hydrogenated amorphous silicon (a-Si:H). The reduction in the dangling bonds concentration improves the optoelectrical properties of the film. The improved properties of a-Si:H makes it possible to manufacture electronic devices including a solar cell. A solar cell device based on the hydrogenated amorphous silicon (a-Si:H) was fabricated using the Hot-Wire Chemical Vapour Deposition (HWCVD). When an n-i-p solar cell configuration is grown, the norm is that the p-doped layer is deposited from a mixture of silane (SiH4) gas with diborane (B2H6). The boron atoms from diborane bonds to the silicon atoms and because of the number of the valance electrons, the grown film becomes a p-type film. Aluminium is a group 3B element and has the same valence electrons as boron, hence it will also produce a p-type film when it bonds with silicon. In this study the p-doped layer is grown from the co-deposition of a-Si:H from SiH4 with aluminium evaporation resulting in a crystallized, p-doped thin film. When this thin film is used in the n-i-p cell configuration, the device shows photo-voltaic activity. The intrinsic layer and the n-type layers for the solar cell were grown from SiH4 gas and Phosphine (PH3) gas diluted in SiH4 respectively. The individual layers of the solar cell device were characterized for both their optical and electrical properties. This was done using a variety of experimental techniques. The analyzed results from the characterization techniques showed the films to be of device quality standard. The analysed results of the ptype layer grown from aluminium showed the film to be successfully crystallized and doped. A fully functional solar cell was fabricated from these layers and the cell showed photovoltaic activity.<br /> &nbsp / </p>

A miniature, ultra-low power, sensitive, microbridge gas sensor has been developed.The heat loss from the bridge is a function of the thermal conductivity of thegas ambient. Miniature thermal conductivity sensors have been developed for gaschromatography systems [1] and microhotplates have been built with MEMS technologywhich operates within the mW range of power [2]. In this work a lower power microbridgewas built which allowed for the amplification of the effect of gas thermalconductivity on heat loss from the heated microbridge due to the increase inthe surface-to-volume ratio of the sensing element. For the bridge fabrication,CMOS compatible technology, nanolithography, and polysilicon surfacemicromachining were employed. Eight microbridges were fabricated on each die,of varying lengths and widths, and with a thickness of 1 μm. A voltagewas applied to the sensor and the resistance was calculated based upon thecurrent flow. The response has been tested with air, carbon dioxide, helium,and nitrogen. The resistance and temperature change for carbon dioxide was thegreatest, while the corresponding change for helium was the least. Thus the selectivity of the sensor todifferent gases was shown, as well as the robustness of the sensor. Another aspect of the sensor is that it hasvery low power consumption. The measuredpower consumption at 4 Volts is that of 11.5 mJ for Nitrogen, and 16.1 mJ forHelium. Thesensor responds to ambient gas very rapidly. The time constant not only showsthe fast response of the sensor, but it also allows for more accuratedetection, given that each different gas produces a different correspondingtime constant from the sensor. The sensor is able to detect differentconcentrations of the same gas as well. Fromthe slopes that were calculated, the resistance change at 5 Volts operation wasfound to be 2.05mΩ/ppm, 1.14 mΩ/ppm at 4.5 Volts, and 0.7 mΩ/ppm at 4 Volts. Thehigher voltages yielded higher resistance changes for all of the gases thatwere tested. Theversatility of the microbridge has been studied as well. Experiments were donein order to research the ability of a deposited film on the microbridge, inthis case tin oxide, to act as a sensing element for specific gases. In thissetup, the microbridge no longer is the sensing element, but instead acts as aheating element, whose sole purpose is to keep a constant temperature at whichit can then activate the SnO film, making it able to sense methane. In conclusion,the microbridge was designed, fabricated, and tested for use as an electrothermalgas sensor. The sensor responds to ambient gas very rapidly with differentlevels of resistance change for different gases, purely due to the differencein thermal conductivity of each of the gases. Not only does it have a fastresponse, but it also operates at low power levels. Further research has beendone in the microbridge's ability to act as a heating element, in which the useof a SnO film as the sensing element, activated by the microbridge, was studied. REFERENCES: 1. D. Cruz,J.P. Chang, S.K. Showalter, F. Gelbard, R.P. Manginell, M.G. Blain," Microfabricated thermal conductivity detector for themicro-ChemLabTM," Sensors andActuators B, Vol. 121 pp. 414-422, (2007). 2. A. G. Shirke, R. E. Cavicchi, S. Semancik, R. H. Jackson, B.G. Frederick, M. C. Wheeler. "Femtomolar isothermal desorption usingmicrohotplate sensors," J Vac Sci TechnolA, Vol. 25, pp. 514-526 (2007).

<p>When the amorphous silicon (a-Si) dangling bonds are bonded to hydrogen the concentration of the dangling bond is decreased. The resulting film is called hydrogenated amorphous silicon (a-Si:H). The reduction in the dangling bonds concentration improves the optoelectrical properties of the film. The improved properties of a-Si:H makes it possible to manufacture electronic devices including a solar cell. A solar cell device based on the hydrogenated amorphous silicon (a-Si:H) was fabricated using the Hot-Wire Chemical Vapour Deposition (HWCVD). When an n-i-p solar cell configuration is grown, the norm is that the p-doped layer is deposited from a mixture of silane (SiH4) gas with diborane (B2H6). The boron atoms from diborane bonds to the silicon atoms and because of the number of the valance electrons, the grown film becomes a p-type film. Aluminium is a group 3B element and has the same valence electrons as boron, hence it will also produce a p-type film when it bonds with silicon. In this study the p-doped layer is grown from the co-deposition of a-Si:H from SiH4 with aluminium evaporation resulting in a crystallized, p-doped thin film. When this thin film is used in the n-i-p cell configuration, the device shows photo-voltaic activity. The intrinsic layer and the n-type layers for the solar cell were grown from SiH4 gas and Phosphine (PH3) gas diluted in SiH4 respectively. The individual layers of the solar cell device were characterized for both their optical and electrical properties. This was done using a variety of experimental techniques. The analyzed results from the characterization techniques showed the films to be of device quality standard. The analysed results of the ptype layer grown from aluminium showed the film to be successfully crystallized and doped. A fully functional solar cell was fabricated from these layers and the cell showed photovoltaic activity.<br /> &nbsp / </p>

For polymer-matrix composites (PMCs) that contain insulating matrices and conducting fillers, the electrical properties of the PMCs are especially sensitive to the local concentration of the fillers in the matrix. For PMCs that have phase-segregated microstructures, better prediction of the properties is possible since enhanced control over the distribution of the filler in the matrix can be achieved. In this research, PMMA-ITO nanocomposites were chosen as the composite system in which to explore alternative microstructures, specifically highly phase-segregated microstructures. The microstructures were primarily controlled by varying the ITO particle size and concentration, and the fabrication parameters used to form the nanocomposites. The motivation for this research was to develop correlations between the microstructure and non-destructive measurements in order to improve the predictability of properties in percolating PMCs. As a result of this work, a novel phase-segregated microstructure was discovered, where ITO aggregate-wire structures self-assembled during the composite forming process. Structural analysis of the specimens was conducted primarily using transmission optical microscopy and scanning electron microscopy (SEM). Impedance spectroscopy and optical spectroscopy were the primary NDE characterization tools used for analyzing the variations among the specimens. Ultra-small angle x-ray scattering (USAXS) and stereological techniques were also used to describe the dimensions of the ITO aggregate-wire structures that self-assembled in the PMMA-ITO nanocomposites.

An aim of this work is a research of a deposition process of indium tin oxide by plasma assisted methods. The thesis deals with plasma diagnostics, e.g. Langmuir probe diagnostics and optical emission spectroscopy, as well as describes experiments of film deposition including their diagnostics, e.g. absorption spectroscopy, X-ray diffractometry and atomic force microscopy.

Made available in DSpace on 2016-06-02T20:15:21Z (GMT). No. of bitstreams: 1 3005.pdf: 10668736 bytes, checksum: 8ec8cb21968edc4feb09cb7616b0e9b2 (MD5) Previous issue date: 2010-05-07 / Universidade Federal de Minas Gerais / The structural and transport features of oxide nanostructures synthesized by a vapour phase aproach: the VLS and VS methods were investigated in this work. ITO and In2O3 nanowires were characterized by using XRD, HRTEM and FEG-SEM techniques. Both nanostructures were found to be body-centered cubic (bixbyite, point group Ia3) single crystals with a well defined growth direction. Raman spectroscopy was used in order to study the nanowires composition, crystalline character and the role of tin atoms in the In2O3 lattice (ITO) was studied as well. The influence of the structural disorder induced by doping was pointed as the main cause of the break of the selection rules in ITO and it was promptly recognized in the Raman spectrum. The metallic character observed in In2O3 micrometric wires was assigned to the electron-phonon scattering in agreement with the Bloch-Grüneisen theory. ITO samples with different sizes were analysed in the framework of the Bloch-Grüneisen theory and at high temperatures (T > 77 K) they were found to present a typical metallic character. It was observed at low temperatures (T < 77 K) and in small samples a negative temperature coefficient of resistance which is an evidence that quantum interference processes are present. A weak localized character was found in these samples as detected in magnetoresistance measurements. The electron s phase break was associated to the electronelectron scattering (T < 77 K) and the electron-phonon scattering (T > 77 K). The transport measurements in one-nanowire based FET provided data on the electron s mobility and density. Tin oxide nanobelts were also studied and their structural and electrical characterizations were obtained. In this case the association of several structural measurements provided that the samples are rutile-like single crystals (point group P42/mnm) grown by the VS mechanism. The transport measurements provided data on the nanobelts gap energy (3.8 eV) and on the transport mechanisms acting in different temperature ranges. An activated-like process and the variable range hopping were found to be present in different temperature range and additionally the localization length was determined. The influence of additional levels inside the gap caused by oxygen vacancies was studied by performing light and atmosphere-dependent experiments and as a result a photo-activated character was detected. Thermally stimulated current measurements provided evidence that only one level associated to the oxygen vacancies at 1.8 eV seems to contribute to the transport in SnO2 nanobelts. Triclinic single crystalline nanobelts were identified as the Sn3O4 phase and were analyzed by transport measurements. The samples were wide band gap semiconductors and the role of oxygen vacancies was identified by using PL and PC measurements. The semiconductor behavior was confirmed by the electron transport data, which pointed to the variable range hopping process as the main conduction mechanism (55 K < T < 398 K) and data on localization length and on the hopping distance were obtained. The presence of additional levels due to oxygen vacancies and tin interstitials was recognized in the samples by performing photo-activated and thermally stimulated current measurements. / Neste trabalho foram investigadas características estruturais e de transporte eletrônico em nanoestruturas óxidas sintetizadas por métodos baseados em fase de vapor: os métodos VLS e VS. Amostras de In2O3 e ITO foram caracterizadas quanto às suas características estruturais usando-se técnicas experimentais como XRD, HRTEM e FEG-SEM e comprovou-se que são monocristais cúbicos de corpo centrado (bixbyite) pertencentes ao grupo puntual Ia3 com direção preferencial de crescimento bem definida. A espectroscopia Raman foi utilizada como ferramenta fundamental para o estudo da composição destes materiais, confirmando a fase, o caráter monocristalino bem como a presença de dopantes na estrutura do In2O3 como no caso do ITO. Estudou-se ainda a influência da desordem estrutural causada pela dopagem nas estruturas já que esta se reflete diretamente em uma quebra na regra de seleção do material e portanto, no espectro Raman. O estudo dos mecanismos de transporte eletrônico em microfios de In2O3 mostrou uma característica essencialmente metálica nestes materiais, comprovada pela identificação do espalhamento elétron-fônon (teoria de Bloch-Grüneisen) como a principal fonte de espalhamento. Amostras de ITO com diferentes tamanhos também foram estudadas e observou-se, acima de 77 K, o aumento da resistência com o aumento da temperatura também caracterizado pela interação elétron-fônon. A observação de um coeficiente negativo de temperatura da resistência observado na amostra nanométrica e em baixas temperaturas aponta para a presença de processos quânticos de interferência originados principalmente da redução da dimensionalidade da amostra. De fato, a aplicação de um campo magnético mostrou a supressão desse comportamento em função da temperatura, comprovando assim que a chamada localização fraca encontra-se presente no nanofio de ITO. Nesse caso, a destruição da fase do elétron foi associada ao espalhamento elétron-elétron (T < 77 K) e ao espalhamento elétron-fônon (T > 77 K). O uso das referidas amostras como transistores de efeito de campo permitiu ainda a obtenção de parâmetros importantes como a mobilidade e a densidade de portadores nas amostras. Nanofitas de SnO2 também foram estudadas e suas propriedades estruturais e de transporte eletrônico foram obtidas. Nesse caso encontrou-se através de técnicas de medida variadas que as amostras são monocristais com estrutura do tipo rutila (grupo puntual P42/mnm) sintetizadas pelo método VS. Diferentes experimentos de transporte eletrônico permitiram a determinação do gap de energia deste material em 3.8 eV e ainda permitiram identificar a presença de diferentes mecanismos de transporte atuando em intervalos de temperatura bem determinados. De fato observou-se a transição de um comportamento de ativação térmica para um comportamento localizado e também ativado por fônons, o hopping donde se determinou o comprimento de localização eletrônico. A presença de níveis adicionais ao gap de energia foi estudada através de experimentos feitos em diferentes atmosferas e sob ação de luz ultravioleta visando explorar o caráter foto-ativado detectado nas amostras. Foi observado de medidas termicamente estimuladas a emissão termiônica de portadores através dos contatos elétricos o que indica que o único nível que parece contribuir com portadores livres nas nanofitas de SnO2 é aquele detectado em 1.8 eV. Amostras monocristalinas com estrutura triclínica, com morfologia de fita e cuja fase foi identificada como sendo Sn3O4 foram também investigadas. A presença de vacâncias de oxigênio e de um gap largo de energia foram observadas através de experimentos de PL e PC. O hopping foi identificado em um grande intervalo de temperaturas (55 K < T < 398 K) como o principal mecanismo de transporte eletrônico observado nas amostras o que comprova a presença de localização e também indica que as amostras se comportam como um semicondutor. Adicionalmente, parâmetros como o comprimento de localização e a distância de pulo dos elétrons foram calculadas. A presença de vacâncias de oxigênio nestas amostras foi ainda estudada através de medidas foto-ativadas pela luz ultravioleta e em diferentes atmosferas de medida, e também por experimentos de TSC donde obteve-se evidências adicionais sobre a presença de outras fontes de elétrons livres como vacâncias superficiais ou interstícios de estanho, contribuindo para o transporte nestas amostras.

abstract: Redox reactions are crucial to energy transduction in biology. Protein film electrochemistry (PFE) is a technique for studying redox proteins in which the protein is immobilized at an electrode surface so as to allow direct exchange of electrons. Establishing a direct electronic connection eliminates the need for redox­active mediators, thus allowing for interrogation of the redox protein of interest. PFE has proven a versatile tool that has been used to elucidate the properties of many technologically relevant redox proteins including hydrogenases, laccases, and glucose oxidase. This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from <italic>Pyrococcus furiosus</italic> (<italic>Pf</italic>SHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis. / Dissertation/Thesis / Doctoral Dissertation Biochemistry 2014

Philosophiae Doctor - PhD / When the amorphous silicon (a-Si) dangling bonds are bonded to hydrogen the concentration of the dangling bond is decreased. The resulting film is called hydrogenated amorphous silicon (a-Si:H). The reduction in the dangling bonds concentration improves the optoelectrical properties of the film. The improved properties of a-Si:H makes it possible to manufacture electronic devices including a solar cell. A solar cell device based on the hydrogenated amorphous silicon (a-Si:H) was fabricated using the Hot-Wire Chemical Vapour Deposition (HWCVD). When an n-i-p solar cell configuration is grown, the norm is that the p-doped layer is deposited from a mixture of silane (SiH4) gas with diborane (B2H6). The boron atoms from diborane bonds to the silicon atoms and because of the number of the valance electrons, the grown film becomes a p-type film. Aluminium is a group 3B element and has the same valence electrons as boron, hence it will also produce a p-type film when it bonds with silicon. In this study the p-doped layer is grown from the co-deposition of a-Si:H from SiH4 with aluminium evaporation resulting in a crystallized, p-doped thin film. When this thin film is used in the n-i-p cell configuration, the device shows photo-voltaic activity. The intrinsic layer and the n-type layers for the solar cell were grown from SiH4 gas and Phosphine (PH3) gas diluted in SiH4 respectively. The individual layers of the solar cell device were characterized for both their optical and electrical properties. This was done using a variety of experimental techniques. The analyzed results from the characterization techniques showed the films to be of device quality standard. The analysed results of the ptype layer grown from aluminium showed the film to be successfully crystallized and doped. A fully functional solar cell was fabricated from these layers and the cell showed photovoltaic activity. / South Africa