Sub-femtomolar Isothermal Desoprtion and Reaction Kinetics on Microhotplate Sensor Platforms

The population of adsorbates on a semiconductor surface directly influences the physical and chemical properties of the semiconductor. In the case of a metal-oxide semiconductor, the adsorbing species can change its electrical conductivity, a phenomenon which forms the operating principle of gas sensors. The interaction of adsorbed oxygen species on a metaloxide surface with reducing or oxidizing gases leads to an increase or decrease in electrical conductivity respectively. Miniature gas sensors called microhotplates (developed at the National Institute of Standards and Technology) are excellent surface science tools to explore surface reactions on semiconducting metal-oxide films. This thesis outlines how the desorption kinetics may be modeled in situations where the effects of finite heating rate, and system pumping rate are intertwined with the desorption rate, and how it is possible to estimate these time constants from isothermal desorption of sub-femtomolar coverages. Benzoic acid on reduced SnO2 was used as a model system to demonstrate the technique. It was observed to adsorb at coverages below 0.005 monolayers with an activation energy for desorption of 97 kJ/mol. The uptake, reaction pathways, and desorption kinetics of 2-propanol on TiO2 and SnO2 films were studied to demonstrate new microhotplate-based techniques to probe the fundamental surface processes that lead to electrical conductivity changes in chemiresistive gas sensors. Uptake and pulsed desorption measurements showed that reproducible coverages of 2-propanol could be prepared during low temperature adsorption, while interlaced, mass-resolved desorption pulses quantified indications of conversion to propene on oxidized TiO2 and SnO2 that correlate with conductivity changes. Fractional isothermal desorption data for 2-propanol on the oxidized TiO2 film suggest that the surface is energetically heterogeneous. A Monte Carlo model gives an average binding energy of 102 kJ/mol with a standard deviation of 15.7 kJ/mol, assuming diffusion is negligible on the timescale of the microhotplate’s heating pulse. The technique can thus show how a microsensor platform can provide a better understanding of the principles of sensor operation by determining, from sub-femtomolar quantities of adsorbates on a single microsensor, coverage, pumping speed, desorption rate, and reactivity of surface interactions and their effect on the sensing film conductivity

New Fabrication Methodologies for the Development of Low Power Gas Sensors Based on Semiconducting Nanowires

[eng] The air and environment quality is, nowadays, one of the main political concerns of the governmental institutions. Suitable gas detection is nowadays an important requirement, which is provided by gas sensors. Metal oxide semiconductors are the most common materials used as semiconducting gas sensors. They can be used with different type of operational mechanisms, like resistive, capacitive or optical based sensors, among others. The main objective of this dissertation is to contribute towards the improvement of gas sensors based on semiconducting nanowires. The easy integration of nanowires in low consumption devices is the fundamental idea that guides this dissertation, and the subsequent characterization of the sensors. Furthermore, the low power consumption of the device is a constant condition of the presented procedures, implemented by means of MEMS substrate that has been used as platform for all the fabricated sensors. The easy integration of nanowire-based devices by using localized growth on top of sensing platforms has been developed, by a site-specific growth of SnO2 and Ge nanowires on top of micromembranes and microhotplates, on the sensing area of the gas sensors. The fabrication procedure allows in single process the growth of NW networks on top of the electronic platforms that will be used for the gas sensing, an important step forward towards the integration of nanowires on electronic devices. On the other hand, the fabricated devices can be used as gas sensors readily after the growth. The gas sensing behaviour of SnO2 networks have been characterized towards different gases; specifically, the kinetics of ammonia response in dry and humid air has been analysed in detail. In addition, the influence of water vapour is analysed, and thus, the chemical paths of the interaction with ammonia have been related to the operating temperature. Furthermore, the synthesized meshes of Ge NWs have been for the first time, at the best of our knowledge, studied as a gas sensor. The chemical interactions towards oxidizing and reducing gases are analysed, paying important attention on the structural characterization, which results primordial for the analysis of sensing behaviour. A procedure based on electron beam lithography is explored in order to fabricate individual nanowire-based devices on top of microhotplates and micromembranes. The experimental procedure for that is detailed in the different steps. The individual nanowires have also studied as a gas sensor, whose results are discussed and compared to their mesh counterpart.[spa] La tesis titulada New Fabrication Methodologies for the Development of Low Power Gas Sensors Based on Semiconducting Nanowires, se enmarca dentro de los sensores de gas para la monitorización ambiental de la calidad del aire, con el objetivo de detectar la presencia de gases nocivos para la salud humana. El trabajo desarrollado se basa en el uso de sensores de gas resistivos, es decir, que la adsorción de un gas en la superficie del sensor da lugar a un cambio en la conductividad del sensor, el cual repercute en un cambio de su resistencia medible experimentalmente. Los materiales utilizados son los óxidos metálicos, materiales semiconductores de banda prohibida ancha (entre 2 y 4 eV). Generalmente, los óxidos metálicos necesitan ser calentados a una temperatura por encima de 150 ºC para promover la interacción con los gases adsorbidos, que se lleva a cabo mediante la denominada quimisorción, una interacción localizada que implica una transferencia de carga entre el semiconductor y la especie de gas. Dado que el gas se adsorbe en la superficie del material, el mecanismo es considerado como superficial, cuya sensibilidad depende en gran medida, entre otros efectos, del ratio superficie/volumen. Así pues, las nanoestructuras aparecen como candidatas óptimas para ser utilizadas como sensor de gas. En la tesis mencionada, se utilizan estructuras unidimensionales, concretamente nanohilos semiconductores como parte activa. El principal desafío para la comercialización de sensores basados en nanohilos es, actualmente, la integración de estos en un dispositivo de forma eficiente y escalable. Un nuevo método de fabricación que mejor la integración de los nanohilos en sus plataformas electrónicas ha sido desarrollado, basado en el crecimiento localizado de nanohilos monocristalinos sobre microplacas calefactoras, es decir, membranas de dimensiones micrométricas que contienen un microcalefactor enterrado. El calefactor es utilizado para proporcionar la temperatura necesaria para la termólisis del precursor durante el crecimiento, y para calentar y promover la reacción durante el sensado de gas. Mediante este proceso han sido crecidos nanohilos de SnO2 y de Ge, en forma de redes con múltiples nanohilos conectados entre sí. Los dispositivos fabricados han sido caracterizados como sensores de gas. Los nanohilos de dióxido de estaño presentan la máxima respuesta ante amoníaco. Los mecanismos químicos que se dan lugar durante la interacción entre el amoníaco y el semiconductor han sido analizados a partir de la respuesta cinética y los distintos fenómenos observados, en aire seco y húmedo. Los nanohilos de Ge han sido estudiados por primera vez como sensores de gas, manteniendo una temperatura de 100 ºC. El comportamiento de tipo p de los nanohilos ha sido determinado a partir de medidas de efecto campo. Los nanohilos presentan una capa de óxido de Ge en la superficie, de alrededor de 1 nm, que posee un papel clave en la interacción, ya que contiene sitos de adsorción para el sensado de gas. El mecanismo de sensado de Ge se concluye como consistente con un óxido metálico de tipo p. Por otro lado, han sido fabricados sensores de gas basados en nanohilos individuales de SnO2 mediante un proceso basado en litografía por haz de electrones, sobre micromembranas calefactores suspendidas. El proceso ha sido integrado y adaptado para el tipo de micromembranas, que presentan rugosidades en la superficie. Los dispositivos fabricados han sido caracterizados como sensor de gas y comparados con las redes de nanohilos del mismo material

Fabrication and gas sensing properties of pure and au-functionalised W03 nanoneedle-like structures, synthesised via aerosol assisted chemical vapour deposition method

En esta tesis doctoral, se ha investigado y desarrollado un nuevo método de CVD asistido por aerosol (AACVD), que permite el crecimiento de nanoestructuras de WO3 intrínsecas y funcionalizadas con Au. Así mismo se han depositado capas policristalinas de SnO2 para aplicaciones de detección de gases. La síntesis de materiales nanoestructurados, la fabricación de dispositivos y sus propiedades de detección de gases, han sido estudiadas. El método AACVD fue utilizado para la síntesis y la deposición directa de capas activas encima de sustratos de alúmina y también sobre substratos micromecanizados (microhotplates), lo que demuestra la compatibilidad entre la tecnología de silicio y la deposición de la capas activas nanoestructuradas. En la tesis se ha demostrado que las capas nanoestructuradas de WO3 funcionalizadas con oro tienen una sensibilidad mejor que las intrínsecas frente a algunos gases relevantes y al mismo tiempo se ha producido un cambio de selectividad.In this doctoral thesis, it has been investigated and developed the Aerosol Assisted Chemical Vapour Deposition (AACVD) method for direct in-situ growth of intrinsic and Au-functionalised nanostructured WO3, as well as SnO2-based devices for gas sensing applications. The nanostructured material synthesis, device fabrication and their gas sensing properties have been studied. AACVD method was used for synthesis and direct deposition of sensing films onto classical alumina and microhotplate gas sensor substrates, demonstrating the compatibility between the microhotplate fabrication process and the sensing nanostructured layer deposition. The effect of Au nanoparticles on the gas sensor’s response was measured and presented in this thesis. The test results revealed that the addition of Au nanoparticles to the WO3 nanoneedles has increased the sensor’s response towards the tested gases (i.e. EtOH). It was therefore demonstrated that the Au-functionalisation has an enhancing effect on the gas sensing properties of WO3 nanoneedle

Synthesis and integration of one-dimensional nanostructures for chemical gas sensing applications

The need for improved measurement technology for the detection and monitoring of gases has increased tremendously for maintenance of domestic and industrial health and safety, environmental surveys, national security, food-processing, medical diagnostics and various other industrial applications. Among the several varieties of gas sensors available in the market, solid-state sensors are the most popular owing to their excellent sensitivity, ruggedness, versatility and low cost. Semiconducting metal oxides such as tin oxide (SnO2), zinc oxide (ZnO), and tungsten oxide (WO3) are routinely employed as active materials in these sensors. Since their performance is directly linked to the exposed surface area of the sensing material, one-dimensional nanostructures possessing very high surface to volume ratios are attractive candidates for designing the next generation of sensors. Such nano-sensors also enable miniaturization thereby reducing power consumption. The key to achieve success in one-dimensional nanotechnologies lies in assembly. While synthesis techniques and capabilities continue to expand rapidly, progress in controlled assembly has been sluggish due to numerous technical challenges. In this doctoral thesis work, synthesis and characterization of various one-dimensional nanostructures including nanotubes of SnO2, and nanowires of WO3 and ZnO, as well as their direct integration into miniature sensor platforms called microhotplates have been demonstrated. The key highlights of this research include devising elegant strategies for growing metal oxide nanotubes using carbon nanotubes as templates, substantially reducing process temperatures to enable growth of WO3 nanowires on microhotplates, and successfully fabricating a ZnO nanowire array based sensor using a hybrid nanowire-nanoparticle assembly approach. In every process, the gas-sensing properties of one-dimensional nanostructures were observed to be far superior in comparison with thin films of the same material. Essentially, we have formulated simple processes for improving current thin film sensors as well as a means of incorporating nanostructures directly into miniature sensing devices. Apart from gas sensing applications, the approaches described in this work are suitable for designing future nanoelectronic devices such as gas-ionization, capacitive and calorimetric sensors, miniature sensor arrays for electronic nose applications, field emitters, as well as photonic devices such as nanoscale LEDs and lasers

Microfabricated Formaldehyde Gas Sensors

Formaldehyde is a volatile organic compound that is widely used in textiles, paper, wood composites, and household materials. Formaldehyde will continuously outgas from manufactured wood products such as furniture, with adverse health effects resulting from prolonged low-level exposure. New, microfabricated sensors for formaldehyde have been developed to meet the need for portable, low-power gas detection. This paper reviews recent work including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, and nanowire-based sensors. This paper also investigates the promise of polymer-based sensors for low-temperature, low-power operation

Non-silicon Microfabricated Nanostructured Chemical Sensors For Electric Nose Application

A systematic investigation has been performed for Electric Nose , a system that can identify gas samples and detect their concentrations by combining sensor array and data processing technologies. Non-silicon based microfabricatition has been developed for micro-electro-mechanical-system (MEMS) based gas sensors. Novel sensors have been designed, fabricated and tested. Nanocrystalline semiconductor metal oxide (SMO) materials include SnO2, WO3 and In2O3 have been studied for gas sensing applications. Different doping material such as copper, silver, platinum and indium are studied in order to achieve better selectivity for different targeting toxic gases including hydrogen, carbon monoxide, hydrogen sulfide etc. Fundamental issues like sensitivity, selectivity, stability, temperature influence, humidity influence, thermal characterization, drifting problem etc. of SMO gas sensors have been intensively investigated. A novel approach to improve temperature stability of SMO (including tin oxide) gas sensors by applying a temperature feedback control circuit has been developed. The feedback temperature controller that is compatible with MEMS sensor fabrication has been invented and applied to gas sensor array system. Significant improvement of stability has been achieved compared to SMO gas sensors without temperature compensation under the same ambient conditions. Single walled carbon nanotube (SWNT) has been studied to improve SnO2 gas sensing property in terms of sensitivity, response time and recovery time. Three times of better sensitivity has been achieved experimentally. The feasibility of using TSK Fuzzy neural network algorithm for Electric Nose has been exploited during the research. A training process of using TSK Fuzzy neural network with input/output pairs from individual gas sensor cell has been developed. This will make electric nose smart enough to measure gas concentrations in a gas mixture. The model has been proven valid by gas experimental results conducted

Development and Application of Integrated Silicon-in-Plastic Microfabrication in Polymer Microfluidic Systems

Polymer-based microfluidic devices can offer a number of advantages over conventional devices, and have found many applications in chemical and biological analysis. In order to fully develop a lab-on-chip (LOC) device, the functional components, such as sensors and actuators, tend to be assembled to complete a functional device. But the integration of silicon chips into polymer-based microfluidic systems remains a virtually unexplored area. In this work, a novel silicon-in-plastic microfabrication technology is developed, which involves seamlessly integrating individual microfabricated silicon chips into a larger polymer substrate, where the silicon components provide functionality, and the plastic substrate provides system-level fluid handling. This technology employs low-cost polymer substrates and simple polymer processing techniques which are amenable to mass production. The fabrication and testing of two polymer microfluidic systems using the silicon-in-plastic technology are presented in this dissertation. The first integrated microsystem is a water-based chemical monitoring system based on microhotplate gas sensor and polymer microfluidics. The chemical monitoring system is designed to sample a water source, extract solvent present within the aqueous sample into the vapor phase, and direct the solvent vapor past the integrated gas sensor for analysis. Design, fabrication, and characterization of a prototype system are described, and results from illustrative measurements performed using methanol, toluene, and 1,2-dichloroethane in water are presented. The second one is an integrated UV absorbance detection system that uses silicon-in-plastic technology to seamlessly integrate bare photodiode chips into a polymer microfluidic system. Detection platforms fabricated using this approach exhibit excellent detection limits down to 1.5 x 10 8 M for bovine serum albumin (BSA) as a model protein. In addition to providing high sensitivity, sub-nanoliter detection volumes are enabled by the use of direct photodetector integration. The fabrication methodology is detailed, and system performance metrics including minimum detection limit, detection volume, dynamic range, and linearity are reported

Integrated Micro-Machined Hydrogen Gas Sensor. Final Report

This report details our recent progress in developing novel MEMS (Micro-Electro-Mechanical Systems) based hydrogen gas sensors. These sensors couple novel thin films as the active layer on a device structure known as a Micro-HotPlate. This coupling has resulted in a gas sensor that has several unique advantages in terms of speed, sensitivity, stability and amenability to large scale manufacture. This Phase-I research effort was focused on achieving the following three objectives: (1) Investigation of sensor fabrication parameters and their effects on sensor performance. (2) Hydrogen response testing of these sensors in wet/dry and oxygen-containing/oxygen-deficient atmospheres. (3) Investigation of the long-term stability of these thin film materials and identification of limiting factors. We have made substantial progress toward achieving each of these objectives, and highlights of our phase I results include the demonstration of signal responses with and without oxygen present, as well as in air with a high level of humidity. We have measured response times of <0.5 s to 1% H{sub 2} in air, and shown the ability to detect concentrations of <200 ppm. These results are extremely encouraging and suggest that this technology has substantial potential for meeting the needs of a hydrogen based economy. These achievements demonstrate the feasibility of using micro-hotplates structures in conjunction with palladium+coated metal-hydride films for sensing hydrogen in many of the environments required by a hydrogen based energy economy. Based on these findings, they propose to continue and expand the development of this technology in Phase II

Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented