Modelling of the Chemical and Light Interactions in Individual Metal Oxide Nanowires for Sensing Applications

[spa]En la presente tesis se analizan desde el punto de vista de la modelización teórica los mecanismos de interacción de gases y luz con nanohilos individuales de óxidos metálicos. Desde este enfoque novedoso, se pretende alcanzar una mayor compresión de los mecanismos de transducción que dan lugar a la utilidad de dichos materiales en aplicaciones de sensado químico y de luz. Los resultados que aquí se presentan permiten comprender los detalles de las interacciones de gases, tanto oxidantes como reductores, sobre superficies de óxidos metálicos; siendo especialmente remarcable el descubrimiento del papel clave que juegan las vacantes de oxígeno en superficie en este proceso. Así mismo, se proponen técnicas experimentales tanto para la generación controlada de dichas vacantes como para su posterior detección. De este modo se das las herramientas necesarias para controlar a voluntad la respuesta a gases de estos materiales. Des de el punto de vista de la detección de luz, se demuestra que la gran relevancia de los efectos de superficie en los nanohilos permite obtener fotorespuestas muy por encima de las conseguidas con otras tecnologías pero, como contrapartida, la respuesta dinámica se ve severamente penalizada. Por ello se proponen y demuestran diversos métodos para modificar la influencia de la superficie en la respuesta total de los nanohilos. Finalmente, en base al conocimiento adquirido, se demuestra que es posible acoplar ambas interacciones (gas-óxido metálico y luz-óxido metálico) para ampliar las aplicaciones de estos materiales. En particular, no sólo se demuestra que es posible activar la respuesta a gases por medio de la iluminación, sino que también se presenta un modelo que describe cuantitativamente este fenómeno, abriendo las puertas a un gran número de aplicaciones de detección de gas a temperatura ambient

Modelling of the Chemical and Light Interactions in Individual Metal Oxide Nanowires for Sensing Applications

En la presente tesis se analizan desde el punto de vista de la modelización teórica los mecanismos de interacción de gases y luz con nanohilos individuales de óxidos metálicos. Desde este enfoque novedoso, se pretende alcanzar una mayor compresión de los mecanismos de transducción que dan lugar a la utilidad de dichos materiales en aplicaciones de sensado químico y de luz. Los resultados que aquí se presentan permiten comprender los detalles de las interacciones de gases, tanto oxidantes como reductores, sobre superficies de óxidos metálicos; siendo especialmente remarcable el descubrimiento del papel clave que juegan las vacantes de oxígeno en superficie en este proceso. Así mismo, se proponen técnicas experimentales tanto para la generación controlada de dichas vacantes como para su posterior detección. De este modo se das las herramientas necesarias para controlar a voluntad la respuesta a gases de estos materiales. Des de el punto de vista de la detección de luz, se demuestra que la gran relevancia de los efectos de superficie en los nanohilos permite obtener fotorespuestas muy por encima de las conseguidas con otras tecnologías pero, como contrapartida, la respuesta dinámica se ve severamente penalizada. Por ello se proponen y demuestran diversos métodos para modificar la influencia de la superficie en la respuesta total de los nanohilos. Finalmente, en base al conocimiento adquirido, se demuestra que es posible acoplar ambas interacciones (gas-óxido metálico y luz-óxido metálico) para ampliar las aplicaciones de estos materiales. En particular, no sólo se demuestra que es posible activar la respuesta a gases por medio de la iluminación, sino que también se presenta un modelo que describe cuantitativamente este fenómeno, abriendo las puertas a un gran número de aplicaciones de detección de gas a temperatura ambiente

First-principles study of NOx and SO2 adsorption onto SnO2(110) and SnO2 nanoparticles

Màster en Nanociència i Nanotecnologia, Facultat de Física, Universitat de Barcelona, Curs: 2006-2007 Tutor: Albert Cirera HernándezAn ab initio study of the adsorption of NOx and SO2 onto SnO2(110) surfaces is presented and related to gas-sensing applications. Using first-principles calculations (DFT-GGA approximation) the most relevant NO and NO2 adsorptions are analyzed by estimating their adsorption energies and charge transfers. The resulting values are compared with experimental desorption temperatures for NO and NO2. The adsorption of the poisoning agent SO2 is also analyzed. Optimum SnO2 working temperatures for minimum SO2 poisoning in NO2 sensing applications are discussed from the perspective of adsorption. In all cases, we observe that the surface reduction state has noticeable consequences on adsorption strength. We also present an ab initio thermodynamics study to analyze the stability of several surface reduction configurations with respect to the ambient oxygen partial pressure and the temperature of the material. Finally, the interaction of NO2 with spherical SnO2 nanoparticles smaller than 2nm is considered

Elastic tunneling charge transport mechanisms in silicon quantum dots / SiO2 thin films and superlattices

The role of different charge transport mechanisms in Si /SiO2 structures has been studied. A theoretical model based on the Transfer Hamiltonian Formalism has been developed to explain experimental current trends in terms of three different elastic tunneling processes: (1) trap assisted tunneling; (2) transport through an intermediate quantum dot; and (3) direct tunneling between leads. In general, at low fields carrier transport is dominated by the quantum dots whereas, for moderate and high fields, transport through deep traps inherent to the SiO 2 is the most relevant process. Besides, current trends in Si /SiO2 superlattice structure have been properly reproduced

The Power of Models: Modeling Power Consumption for IoT devices

Low-energy technologies in the Internet of Things (IoTs) era are still unable to provide the reliability needed by the industrial world, particularly in terms of the wireless operation that pervasive deployments demand. While the industrial wireless performance has achieved an acceptable degree in communications, it is no easy task to determine an efficient energy-dimensioning of the device in order to meet the application requirements. This is especially true in the face of the uncertainty inherent in energy harvesting. Thus, it is of utmost importance to model and dimension the energy consumption of the IoT applications at the pre-deployment or pre-production stages, especially when considering critical factors, such as reduced cost, life-time, and available energy. This paper presents a comprehensive model for the power consumption of wireless sensor nodes. The model takes a system-level perspective to account for all energy expenditures: communications, acquisition and processing. Furthermore, it is based only on parameters that can empirically be quantified once the platform (i.e., technology) and the application (i.e., operating conditions) are defined. This results in a new framework for studying and analyzing the energy life-cycles in applications, and it is suitable for determining in advance the specific weight of application parameters, as well as for understanding the tolerance margins and tradeoffs in the system

Machine-readable pattern for colorimetric sensor interrogation

We present a systematic methodology to generate machine-readable patterns embodying all the elements needed to carry out colorimetric measurements with conventional color cameras in an automated, robust and accurate manner. Our approach relies on the well-stablished machine-readable features of the QR Codes, to detect the pattern, identify the color reference elements and the colorimetric spots, to calibrate the color of the image and to conclude a quantitative measurement. We illustrate our approach with a NH3 colorimetric indicator operating at distinct color temperature ambient lights, demonstrating that with our design, consistent measurements can be achieved, with independence on the illumination conditions

Self-heating in pulsed mode for signal quality improvement: application to carbon nanostructures-based sensors

Sensor signal instability and drift are still unresolved challenges in conductometric gas sensors. Here, the use of self-heating effect to operate a gas sensor in a pulsed temperature modulation mode (pulsed self-heating operation) is presented as an effective method to enhance signal stability and reduce consumption figures down to a few W. The sensor operation temperature was pulsed periodically between two levels, obtaining two different sensing states from one single device driven with self-heating, i.e. free of heater. The signal differences between both operating points correlated well with gas concentrations and displayed no drift. This methodology is exemplified with a thorough study of the response of carbon nanofibers to humidity. Specifically, after analyzing the influence of the pulse characteristics (i.e. temperature variation, pulse period and pulse duty cycle) on the sensor performance, thumb rules to select suitable pulsing conditions are provided. The methodology is successfully extended to other target gases, such as NO2 and NH3. Finally, its implementation in a real-time sensing system with low computational requirements is demonstrated and discussed in detail

A transfer Hamiltonian model for devices based on quantum dot arrays

We present a model of electron transport through a random distribution of interacting quantum dots embedded in a dielectric matrix to simulate realistic devices. The method underlying the model depends only on fundamental parameters of the system and it is based on the Transfer Hamiltonian approach. A set of noncoherent rate equations can be written and the interaction between the quantum dots and between the quantum dots and the electrodes is introduced by transition rates and capacitive couplings. A realistic modelization of the capacitive couplings, the transmission coefficients, the electron/hole tunneling currents, and the density of states of each quantum dot have been taken into account. The effects of the local potential are computed within the self-consistent field regime. While the description of the theoretical framework is kept as general as possible, two specific prototypical devices, an arbitrary array of quantum dots embedded in a matrix insulator and a transistor device based on quantum dots, are used to illustrate the kind of unique insight that numerical simulations based on the theory are able to provide

Transport in quantum dot stacks using the transfer Hamiltonian method in self-consistent field regime

The non-coherent rate equation approach to the electrical transport in a serial quantum dot system is presented. The charge density in each quantum dot is obtained using the transfer Hamiltonian formalism for the current expressions. The interactions between the quantum dots and between the quantum dots and the electrodes are introduced by transition rates and capacitive couplings. Within this framework analytical expressions for the current and the charge in each quantum dot are presented. The effects of the local potential are computed within the self-consistent field regime. Despite the simplicity of the model, well-known effects are satisfactorily explained and reproduced. We also show how this approach can be extended into a more general case

Revisiting Colorimetric Gas Sensors: Compact, Versatile and Cost-Effective

We report on an inexpensive and very selective gas sensor implemented by simply combining colorimetric indicators casted on top of acetate-based transparent tape, with a commercial microchip adapted here to measure optical reflectance. This sensor can be easily reproduced (leading to quantitatively consistent results), refreshed and reconfigured to sense different target gases replacing only the colorimetric tape. The device may either work as sensor (CO2 and NH3) or dosimeter (Formaldehyde) depending on the targeted gas