Orientador: Marcelo Orgaghi Orlandi / Banca: Maria Aparecida Z. Bertochi / Banca: Sonia Maria Zanetti / O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi / Resumo: Nos últimos anos, o interesse pelos materiais nanoestruturados vem permitindo que esta seja uma das áreas de maior evolução científica. O estudo das propriedades destes materiais caminha em passos largos e os resultados indicam que existem muitas vantagens em se utilizar materiais em escala reduzida. Neste trabalho, nanoestruturas de óxido de estanho foram sintetizadas pelo método de redução carbotérmica a fim de verificar o comportamento desses materiais como sensores de gás. Os materiais coletados após as sínteses foram caracterizados por difração de raios X (DRX), microscopia eletrônica de varredura de alta resolução (MEV-FEG), microscopia eletrônica de transmissão (MET), análise de área de superfície específica (BET), espectroscopia de absorção na região do ultravioleta e do visível (UV-Vis) e medidas elétricas em corrente contínua. Os resultados mostraram que o controle da atmosfera de síntese permite obter nanoestruturas de óxido de estanho com diferentes estados de oxidação (SnO, SnO2 e Sn3O4). Pelas análises por MEV-FEG foi possível observar que o material crescido na fase SnO é constituído por nanofitas e discos enquanto os materiais crescidos nas fases SnO2 e Sn3O4 são constituídos unicamente por nanofitas. As análises por MET mostraram que os materiais sintetizados são monocristalinos e não apresentam defeitos superficiais aparentes. A partir dos resultados das análises por BET, verificou-se que os materiais têm baixa área superficial devido à ausência de poros na superfície das nanoestruturas. Por meio dos espectros de UV-Vis foi observado que os materiais crescidos em diferentes fases apresentam valores distintos de bandgap. A caracterização elétrica dos materiais permitiu analisar o comportamento das nonoestruturas como sensor de NO2, H2 e CO e os resultados... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In recent years the interest in nanostructured materials has permited it to be an area of great scientific developments. The study of properties of these materials moving in leaps and bounds and results show that there are many advantages in using small-scale materials. In this study were synthesized tin oxide nanostructures by carbothermal reduction method to verify the behavior of these materials as a gas sensor. The materials collected after the synthesis were characterized by X-ray diffraction (XRD), high resolution scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), analysis of specific surface area (BET), ultraviolet and visible absorption spectroscopy (UV-Vis) and dc electrical measurements. The results showed that by control synthesis atmosphere it is possible to obtain tin oxide nonomaterials with different oxidation states (SnO, SnO2 and Sn3O4). By FEG-SEM analysis it was observed that the material grown on SnO phase consists of nanoribbons and disks while the grown materials in SnO2 and Sn3O4 phases consist solely of nanoribbons. The TEM analysis showed that the materials synthesized are monocrystalline and show no apparent surface defects. From the results of analyzes by BET, it was found that the materials exhibit low values of surface area due to absence of porous on the surface of nanostructures. Through UV-Vis spectra was observed that the materials grown in different phases have different values of bandgap. The electrical characterization of materials enabled to analyze the behavior of the nanostructures as NO2, H2 and CO sensors and the results showed that all materials exhibit n-type semiconductor behavior and a sensitivity and response time dependent on the concentration of gas and temperature. The best results were achieved when the nanostructures (especially SnO disks) were exposed to NO2 at temperatures... (Complete abstract click electronic access below) / Mestre

Made available in DSpace on 2014-06-11T19:30:19Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-03-27Bitstream added on 2014-06-13T19:00:08Z : No. of bitstreams: 1 suman_ph_me_bauru.pdf: 8662064 bytes, checksum: f9b4c3f0d58b2511c3daba558e51dec2 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Nos últimos anos, o interesse pelos materiais nanoestruturados vem permitindo que esta seja uma das áreas de maior evolução científica. O estudo das propriedades destes materiais caminha em passos largos e os resultados indicam que existem muitas vantagens em se utilizar materiais em escala reduzida. Neste trabalho, nanoestruturas de óxido de estanho foram sintetizadas pelo método de redução carbotérmica a fim de verificar o comportamento desses materiais como sensores de gás. Os materiais coletados após as sínteses foram caracterizados por difração de raios X (DRX), microscopia eletrônica de varredura de alta resolução (MEV-FEG), microscopia eletrônica de transmissão (MET), análise de área de superfície específica (BET), espectroscopia de absorção na região do ultravioleta e do visível (UV-Vis) e medidas elétricas em corrente contínua. Os resultados mostraram que o controle da atmosfera de síntese permite obter nanoestruturas de óxido de estanho com diferentes estados de oxidação (SnO, SnO2 e Sn3O4). Pelas análises por MEV-FEG foi possível observar que o material crescido na fase SnO é constituído por nanofitas e discos enquanto os materiais crescidos nas fases SnO2 e Sn3O4 são constituídos unicamente por nanofitas. As análises por MET mostraram que os materiais sintetizados são monocristalinos e não apresentam defeitos superficiais aparentes. A partir dos resultados das análises por BET, verificou-se que os materiais têm baixa área superficial devido à ausência de poros na superfície das nanoestruturas. Por meio dos espectros de UV-Vis foi observado que os materiais crescidos em diferentes fases apresentam valores distintos de bandgap. A caracterização elétrica dos materiais permitiu analisar o comportamento das nonoestruturas como sensor de NO2, H2 e CO e os resultados... / In recent years the interest in nanostructured materials has permited it to be an area of great scientific developments. The study of properties of these materials moving in leaps and bounds and results show that there are many advantages in using small-scale materials. In this study were synthesized tin oxide nanostructures by carbothermal reduction method to verify the behavior of these materials as a gas sensor. The materials collected after the synthesis were characterized by X-ray diffraction (XRD), high resolution scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), analysis of specific surface area (BET), ultraviolet and visible absorption spectroscopy (UV-Vis) and dc electrical measurements. The results showed that by control synthesis atmosphere it is possible to obtain tin oxide nonomaterials with different oxidation states (SnO, SnO2 and Sn3O4). By FEG-SEM analysis it was observed that the material grown on SnO phase consists of nanoribbons and disks while the grown materials in SnO2 and Sn3O4 phases consist solely of nanoribbons. The TEM analysis showed that the materials synthesized are monocrystalline and show no apparent surface defects. From the results of analyzes by BET, it was found that the materials exhibit low values of surface area due to absence of porous on the surface of nanostructures. Through UV-Vis spectra was observed that the materials grown in different phases have different values of bandgap. The electrical characterization of materials enabled to analyze the behavior of the nanostructures as NO2, H2 and CO sensors and the results showed that all materials exhibit n-type semiconductor behavior and a sensitivity and response time dependent on the concentration of gas and temperature. The best results were achieved when the nanostructures (especially SnO disks) were exposed to NO2 at temperatures... (Complete abstract click electronic access below)

The objective of this dissertation is to investigate several dynamical approaches aiming to improve the sensitivity and performance of microelectromechanical systems (MEMS) resonant sensors. Resonant sensors rely on tracking shifts in the dynamic features of microstructures during sensing, such as their resonance frequency. We aim here to demonstrate analytically and experimentally several new concepts aiming to sharpen their response, enhance the signal to noise ratio, and demonstrate smart functionalities combined into a single resonator. The dissertation starts with enhancing the excitations of the higher order modes of vibrations of clamped-clamped microbeam resonators. The concept is based on using partial electrodes with shapes that induce strong excitation of the mode of interest. Using a half electrode, the second mode is excited with a high amplitude of vibration. Also, using a two-third electrode configuration is shown to amplify the third mode resonance amplitude compared with the full electrode under the same electrical loading conditions. Then, we demonstrate the effectiveness of higher order mode excitation and metal organic frameworks (MOFs) functionalization for improving the sensitivity and selectivity of resonant gas sensors. Also, using a single mode only, we show the possibility of realizing a smart switch triggered upon exceeding a threshold mass when operating the resonator near the dynamic pull-in instability. The second part of the dissertation deals with the dynamics of the microbeam under a two-source harmonic excitation. We experimentally demonstrate resonances of an additive and subtractive type. It is shown that by properly tuning the frequency and amplitude of the excitation force, the frequency bandwidth of the resonator is controlled. Finally, we employ the multimode excitation of a single resonator to demonstrate smart functionalities. By monitoring the frequency shifts of two modes, we experimentally demonstrate the effectiveness of this technique to measure the environmental temperature and gas concentration. Also, we present a hybrid sensor and switch device, which is capable of accurately measuring gas concentration and perform switching when the concentration exceeds a specific (safe) threshold. In contrast to the single mode operation, we show that monitoring the third mode enhances sensitivity, improves accuracy, and lowers the sensor sensitivity to noise.

Nanoscale materials are not new in either nature or physics. However, the recent technological improvements have given scientists new tools to understand and quantify phenomena that occur naturally due to quantum confinement effects. In general, these phenomena induce remarkable optical, magnetic, and electronic properties in nanoscale materials in contrast to their bulk counterpart. In addition, scientists have recently developed the necessary tools to control and exploit these properties in electronic devices, in particular field effect transistors, magnetic memories, and gas sensors. In the present thesis we implement theoretical and computational tools for analyzing the ground state and electronic transport properties of nanoscale materials and their performance in electronic devices. The ground state properties are studied within density functional theory using the SIESTA code, whereas the transport properties are investigated using the non-equilibrium Green's functions formalism implemented in the SMEAGOL code. First we study Si-based systems, as Si nanowires are believed to be important building blocks of the next generation of electronic devices. We derive the electron transport properties of Si nanowires connected to Au electrodes and their dependence on the nanowire growth direction, diameter, and length. At equilibrium Au-nanowire distance we find strong electronic coupling between electrodes and nanowire, resulting in low contact resistance. For the tunneling regime, the decay of the conductance with the nanowire length is rationalized using the complex band structure. The nanowires grown along the (110) direction show the smallest decay and the largest conductance and current. Due to the high spin coherence in Si, Si nanowires represent an interesting platform for spin devices. Therefore, we built a magnetic tunneling junction by connecting a (110) Si nanowire to ferromagnetic Fe electrodes. We have find a substantial low bias magnetoresistance of ~ 200%, which halves for an applied voltage of about 0.35 V and persist up to 1 V. In order to account for shallow impurities coming from bulk Si, the nanowire is doped with either P or B atoms (n or p type). Doping in general decreases the magnetoresistance as soon as the conductance is no longer dominated by tunneling. On the other hand, we study the electron transport properties of Si nanotubes connected to Au electrodes. The general properties turn out to be largely independent of the nanotube chirality, diameter, and length. However, the tunneling conductance of Si nanotubes is found to be significantly larger than in Si nanowires, while having a comparable band gap. For this reason we simulate a Si nanotube field effect transistor by applying an uniform potential gate. Our results demonstrate very high values of the transconductance, outperforming the best commercial Si field effect transistors, combined with low values of the subthreshold swing. Phosphorene (monolayer black P) is the only elemental two-dimensional material besides graphene that can be mechanically exfoliated and also can support electronics. Specific dislocations of the atoms in the phosphorene lattice generate another stable two-dimensional allotrope with buckled honeycomb lattice, blue P. We demonstrate structural stability of monolayer zigzag and armchair blue P nanotubes by means of molecular dynamics simulations. The vibrational spectrum and electronic band structure are determined and analyzed as functions of the tube diameter and axial strain. The nanotubes are found to be semiconductors with a sensitive indirect band gap that allows flexible tuning. We study the adsorption of CO, CO2, NH3, NO, and NO2 molecules on blue P nanotubes. They are found to surpass the gas sensing performance of other nanoscale materials. Investigations of the gas adsorption and induced charge transfer indicate that blue P nanotubes are highly sensitive to N-based molecules, in particular NO2, due to covalent bonding. The current-voltage characteristics of nanotubes connected to Au electrodes is used to evaluate the change in resistivity upon adsorption. The observed selectivity and sensitivity properties make blue P nanotubes superior gas sensors for a wide range of applications. Using black P and blue P nanoribbons, we configure field effect transistors with atomically perfect junctions by using armchair nanoribbons as semiconducting channel and zigzag nanoribbons as metallic leads. We characterize the devices and observe a performance superior to Si-based devices, with on/off ratio of ~ 103, low subthreshold swing of ~ 60 mV/decade, and high transconductance of ~ 104 S/m.

Ce sujet de thèse porte sur l’élaboration et l’étude de l’effet de la substitution du fer par le cobalt sur les propriétés physiques (structurales, morphologiques et magnétiques) et particulièrement la détection des deux gaz réducteurs NH3 et CO des composés La0,8Ca0,1Pb0,1Fe1-xCoxO3 (x = 0,00 ; 0,05 ; 0,10 ; 0,15 et 0,20). La diminution du volume a été, par la suite, confirmée par l’approximation SGGA+U en utilisant la théorie fonctionnelle de la densité (DFT). De même l’étude morphologique a révélé des micrographies poreuses présentant des particules agrégées et agglomérées de taille nanométrique et de forme irrégulière. Les analyses structurales et morphologiques nous ont permis de prédire que le composé avec x = 0,05 peut être considéré comme un bon candidat pour l’application dans le domaine de la détection des gaz. Les résultats des mesures électriques ont montré que la résistance diminue pour des taux de Co inférieurs à 0,10 puis augmente avec des taux supérieurs. De même les réponses électriques sous gaz ont montré que nos composés sont capables de détecter des gaz, avec une variation de la résistance électrique aisément mesurable suite à l’exposition sous différentes concentrations des deux gaz (NH3 et CO) et de déduire que le composé La0,8Ca0,1Pb0,1Fe0,95Co0,05O3 (x = 0,05) présente la meilleure réponse envers les deux gaz testés / This thesis deals with the elaboration and study of the effect of iron substitution by cobalt on the physical properties (structural, morphological and magnetic) and particularly the detection of the two reducing gases NH3 and CO of the compounds La0.8Ca0.1Pb0.1Fe1-xCoxO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20). The decrease of valume was subsequently confirmed by the SGGA + U approximation using the Density Functional Theory (DFT). Similarly, the morphological study reveals porous micrographs presenting aggregated and agglomerated particles of nanometric size and irregular shape. Structural and morphological analyzes predicted that the compound with x = 0.05 could be considered as a good candidate for application in the field of gas detection. The results of the electrical measurements have shown that the resistance decreases for Co rate below 0.10 and then increases with higher rate. Similarly, electrical responses under gas have shown that our compounds are able to detect gases, with a variation of the electrical resistance easily measurable following exposure under different concentrations of both gases (NH3 and CO) and to deduce that the compound La0.8Ca0.1Pb0.1Fe0.95Co0.05O3 (x = 0.05) presents the best response towards the two tested gases

Les progrès récents dans les nanomatériaux présentent un fort potentiel pour la réalisation de capteurs de gaz avec de nombreux avantages tels que : la grande sensibilité de détection de molécule unique, le faible coût et la faible consommation d'énergie. Le graphène, isolé en 2004, est l'un des meilleurs candidats prometteurs pour le développement de futurs nanocapteurs en raison de sa structure à deux dimensions, sa conductivité élevée et sa grande surface spécifique. Chaque atome de la monocouche de graphène peut être considéré comme un atome de surface, capable d'interagir même avec une seule molécule de l'espèce gazeuse ou de vapeur cible, ce qui conduit finalement à un capteur ultrasensible.Dans cette thèse, des composants à base de graphène ont été fabriqués et caractérisés. Les films de graphène ont été synthétisés par dépôt chimique à phase vapeur (CVD) sur des substrats de verre. La spectroscopie Raman a été utilisée pour analyser la qualité et le nombre de couches de graphène. La microscope à force atomique (AFM) et la microscopie électronique à balayage (MEB) ont été également réalisées pour analyser la qualité du graphène. Après la caractérisation de couches de graphène, des dispositifs résistifs à base de graphène ont été fabriquées : quatre électrodes identiques ont été évaporées thermiquement et directement sur le film de graphène comme des électrodes métalliques. La caractérisation électrique a été réalisée à l'aide de Keithley-4200.La réponse de dispositif Intrinsèque a été étudiée sous différents conditions (pression, humidité, exposition à la lumière). Le dispositif a été fonctionnalisé de manière non covalente avec le complexe organométallique (Ru (II) trisbipyridine) et son effet sous exposition à la lumière a été étudié. La réponse de dispositif était reproductible même après de nombreux cycles en présence et en absence de la lumière. Les approches théoriques et expérimentales ainsi que les résultats obtenus au cours de cette thèse ouvrent un moyen de comprendre et de fabriquer des futurs dispositifs de détection de gaz à base du graphène fonctionnalisé de manière non covalente / Recent advances in nanomaterials provided a strong potential to create a gas sensor with many advantages such as high sensitivity of single molecule detection, low cost, and low power consumption. Graphene, isolated in 2004, is one of the best promising candidate for the future development of nanosensors applications because of its atom-thick, two-dimensional structures, high conductivity, and large specific surface areas. Every atom of a monolayer graphene can be considered as a surface atom, capable of interacting even with a single molecule of the target gas or vapor species, which eventually results in the ultrasensitive sensor response.In this thesis work, graphene films were synthesized by Chemical Vapor Deposition (CVD) on the glass substrate. Raman spectroscopy was used to analyze the quality and number of layers of graphene. Atomic Force Microscope (AFM) and Scanning Electron Microscopy (SEM) were also performed to analyze the quality of graphene. After the characterization of graphene films, graphene based resistive devices (four identical electrodes are thermally evaporated directly onto the graphene film as metal electrodes) were fabricated. The electrical characterization has been carried out using Keithley-4200.Intrinsic device response was studied with different external condition changes (pressure, humidity, light illumination). The device was non-covalently functionalized with organometallic complex (Ru(II) trisbipyridine) and the its light exposure response was studied. The observed device response was reproducible and similar after many cycles of on and off operations. The theoretical and experimental approaches and the results obtained during the thesis are opening up a way to understand and fabricate future gas sensing devices based on the non-covalentely functionalized graphene.

Indiana University-Purdue University Indianapolis (IUPUI) / The nanotechnology materials have been used for high sensitivity sensing devices due to their ability to alter their properties in response to the environmental parameters such as temperature, pressure, gas, electromagnetic, and chemicals. The features of employing nanoparticles on top of graphene thin film have driven the hypothesis of achieving high sensing nanotechnology devices. This study demonstrates a novel approach for designing a low noise nanoparticle based gas sensing device with internet of things (IoT) capability. The system is capable of minimizing cross-talk between multiple channels of amplifiers arranged on one chip using guard rings. Graphene mono-layer is utilized as sensing material with the sensitivity catalyzed by addition of gold nano-particles on its surface. The signal from the sensing unit is received by an offset cancellation amplifying system using a system on chip (SoC) approach. IoT capability of the sensing device is developed using FRDM K64f micro-controller board which sends messages on IoT platform when a gas is sensed. The message is received by an application created and sent as an email or message to the user. This study details the mathematical models of the graphene based gas sensing devices, and the interface circuitry that drives the differential potentials, resulting from the sensing unit. The study presents the simulation and practical model of the device, detailing the design approach of the processing unit within the SoC system and wireless implementation of it. The sensing device was capable of sensing gas concentration from 5% to 100% using both the resistive and capacitive based models. The I-V characteristics of the FET sensing device was in agreeable with the other models. The SoC processing unit was designed using cadence tools, and simulation results showed very high CMRR that enable the amplifier to sense a very low signal received from the gas sensors. The cross talk noise was reduced by surrounding guard rings around the amplifier circuits. The layout was accomplished with 45nm technology and simulation showed an offset voltage of 17μV.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20385号 / 工博第4322号 / 新制||工||1670(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 三浦 清貴, 教授 田中 勝久, 教授 木村 俊作 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The selectivity mechanism of the UV-light activated metal oxide semiconductor (MOS) gas sensors was studied. A reaction model based on two competing redox processes was presented to solve the selectivity problem. A concept named dynamic equilibrium of adsorbed oxygen concentration was brought about in this model and two reaction responses were discussed: (1) when most of the MOS surface is adsorbed with oxygen, the resistance of the MOS gas sensor will decrease upon the injection of reducing agents (RAs); (2) when most of the MOS surface is not adsorbed with oxygen, the resistance of the MOS gas sensor will increase upon the injection of RAs. Finally, experiments were conducted on ZnO MOS gas sensors to prove the proposed hypothesis of the reaction mechanism. / Thesis / Master of Applied Science (MASc)

This dissertation focuses on an uncooled Mid-Wave Infra-Red (MWIR) detector was developed by doping an n-type 4H-SiC with Ga using the laser doping technique. 4H-SiC is one of the polytypes of crystalline silicon carbide, a wide bandgap semiconductor. The dopant creates an energy level of 0.30 eV, which was confirmed by optical spectroscopy of the doped sample. This energy level corresponds to the MWIR wavelength of 4.21 µm. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the semiconductor. This process modifies the electron density, which changes the refraction index and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam such as a He-Ne laser. This capability of measuring the detector response remotely makes it a wireless optical detector. The variation of refraction index was calculated as a function of absorbed irradiance based on the reflectance data for the as-received and doped samples. A distinct change was observed for the refraction index of the doped sample, indicating that the detector is suitable for applications at 4.21 µm wavelength. The Ga dopant energy level in the substrate was confirmed by optical absorption spectroscopy. Secondary ion mass spectroscopy (SIMS) of the doped samples revealed an enhancement in the solid solubility of Ga in the substrate when doping is carried out by increasing the number of laser scans. Higher dopant concentration increases the number of holes in the dopant energy level, enabling photoexcitation of more electrons from the valence band by the incident MWIR photons. The detector performance improves as the dopant concentration increases from 1.15×1019 to 6.25×1020 cm-3. The detectivity of the optical photodetector is found to be 1.07×1010 cm·Hz1/2/W for the case of doping with 4 laser passes. The noise mechanisms in the probe laser, silicon carbide MWIR detector and laser power meter affect the performance of the detector such as the responsivity, noise equivalent temperature difference (NETD) and detectivity. For the MWIR wavelength 4.21 and 4.63 µm, the experimental detectivity of the optical photodetector of this study is found to be 1.07×1010 cm·Hz1/2/W, while the theoretical value is 2.39×1010 cm·Hz1/2/W. The values of NETD are found to be 404.03 and 15.48 mK based on experimental data for an MWIR radiation source of temperature 25°C and theoretical calculation respectively. The doped SiC also has a capability of gas detection since gas emission spectra are in infrared range. Similarly, the sensor is based on the semiconductor optics principle, i.e., an energy gap is created in a semiconductor by doping it with an appropriate dopant to ensure that the energy gap matches with an emission spectral line of the gas of interest. Specifically four sensors have been fabricated by laser doping four quadrants of a 6H-SiC substrate with Ga, Al, Sc and P atoms to detect CO2, NO, CO and NO2 gases respectively. The photons, which are emitted by the gas, excite the electrons in the doped sample and consequently change the electron density in various energy states. This phenomenon affects the refraction index of the semiconductor and, therefore, the reflectivity of the semiconductor is altered by the gas. The optical response of this semiconductor sensor is the reflected power of a probe beam, which is a He-Ne laser beam in this study. The CO2, NO, CO and NO2 gases change the refraction indices of Ga-, Al-, Sc- and Al-doped 6H-SiC, respectively, more prominently than the other gases tested in this study. Hence these doped 6H-SiC samples can be used as CO2, NO, CO and NO2 gas sensors respectively.