Design, Optimization and Characterization of Metal Oxide Nanowire Sensors

En aquesta tesi, he estudiat i desenvolupat un mètode de deposició química en fase vapor assistit per aerosol (AACVD), per al creixement directe de nanoagulles d'òxid de tungstè funcionalitzades o intrínseques. Els dipòsits s'han realitzat sobre diferents substrats trasndcutors per a la seva aplicació a la detecció de gasos. Aquesta tècnica ofereix la possibilitat de co-dipositar els metalls amb els òxids metàl•lics emprant una sola etapa de deposició. La síntesi del material nanoestructurat, la fabricació del dispositiu, la caracterització dels materials i la detecció de gasos han estat investigades. El mètode AACVD es va emprar per al creixement i la integració directa de la pel•lícula sensible sobre substrats ceràmics (alúmina), MEMS (micro hotplates) i polimèrics flexibles, el que demostra la seva compatibilitat i idoneïtat per al creixement de nanoestructures d'òxid metàl•lics sobre una àmplia gamma de substrats transductors. A més, el mètode AACVD s'ha implementat també en un reactor de paret freda per créixer les nanoestructures de WO3, emprant l'escalfament localitzat que permeten aconseguir les microresistencias calefactores integrades en alguns dels transdcutors emprats. Totes les pel•lícules sintetitzades en aquesta tesi doctoral es componien de nanoagulles de WO3 pur o de WO3 funcionalitzat amb nanopartícules d'or (Au), platí (Pt), òxid de coure (Cu2O) o pal•ladi (Pd). Es van utilitzar diverses tecnologies d'anàlisi per caracteritzar la morfologia, l'estructura i la composició de les pel•lícules produïdes. Els resultats van mostrar que el nostre mètode és eficaç per al creixement de nanoagulles cristal•lines de WO3 decorades amb nanopartícules de metalls / òxids metàl•lics, a temperatures moderades (és a dir, 380 ° C), amb eficàcia en els seus costos i amb temps de fabricació curts, directament sobre l'element transdcutor amb vista a obtenir sensors de gasos. Els estudis de detecció de gasos han mostrat que aquest nanomaterial híbrid té una excel•lent sensibilitat i selectivitat en comparació amb mostres de WO3 pur. A més, els nanocompostos Cu2O / WO3 i Pd / WO3 han demostrat posseir una excel•lent sensibilitat i selectivitat cap als gasos H2S i H2, respectivament.En esta tesis, he estudiado y desarrollado un método de deposición química en fase vapor asistido por aerosol (AACVD), para el crecimiento directo de nanoagujas de óxido de tungsteno funcionalizadas o intrínsecas. Los depósitos se han realizado sobre distintos sustratos transdcutores para su aplicación a la detección de gases. Esta técnica ofrece la posibilidad de co-depositar los metales con los óxidos metálicos empleando una sola etapa de deposición. La síntesis del material nanoestructurado, la fabricación del dispositivo, la caracterización de los materiales y la detección de gases han sido investigadas. El método AACVD se empleó para el crecimiento y la integración directa de la película de sensible sobre sustratos cerámicos (alúmina), MEMS (micro hotplates) y poliméricos flexibles, lo que demuestra su compatibilidad e idoneidad para el crecimiento de nanoestructuras de óxido metálicos sobre una amplia gama de sustratos transductores. Además, el método AACVD se ha implementado también en un reactor de pared fría para crecer las nanoestructuras de WO3, empleando el calentamiento localizado que permiten conseguir las microresistencias calefactoras integradas en algunos de los transductores empleados. Todas las películas sintetizadas en esta tesis doctoral se componían de nanoagujas de WO3 puro o de WO3 funcionalizado con nanopartículas de oro (Au), platino (Pt), óxido de cobre (Cu2O) o paladio (Pd). Se utilizaron diversas tecnologías de análisis para caracterizar la morfología, la estructura y la composición de las películas producidas. Los resultados mostraron que nuestro método es eficaz para el crecimiento de nanoagujas cristalinas de WO3 decoradas con nanopartículas de metales / óxidos metálicos, a temperaturas moderadas (es decir, 380 ° C), con eficacia en sus costes y con tiempos de fabricación cortos, directamente sobre el elemento trasndcutor con vistas a obtener sensores de gases. Los estudios de detección de gases han mostrado que este nanomaterial híbrido tiene una excelente sensibilidad y selectividad en comparación con muestras de WO3 puro. Además, los nanocompuestos Cu2O / WO3 y Pd / WO3 han demostrado poseer una excelente sensibilidad y selectividad hacia los gases H2S y H2, respectivamente.In this thesis, I have studied and developed aerosol assisted chemical vapour deposition (AACVD) methods for the direct growth of non-functionalized and functionalized tungsten oxide nanoneedles, onto different transducer substrates, for gas sensing applications. This technique gives the possibility to co-deposit metals with metal oxides nanostructures within a single step deposition. The nanostructured material synthesis, device fabrication, material characterization and gas sensing performance have been investigated. AACVD method was employed for the direct growth and integration of the sensing film onto ceramic (alumina), MEMS (silicon micro hotplates) and flexible polymeric substrates, demonstrating its compatibility and suitability for growing metal oxide nanostructures onto a wide spectrum of sensor substrates. Furthermore, AACVD based on the localized heating of substrates employing their embedded resistive microheaters has been also performed and developed for the growth of WO3 nanostructures, using a cold wall reactor. All the synthesized films used in this doctoral thesis were composed of pure WO3 nanoneedles or WO3 nanoneedles functionalized with either gold (Au), platinum (Pt), cuprous oxide (Cu2O) or palladium (Pd) nanoparticles. Various analytical techniques were used to characterize the morphology, the structure and the composition of the produced films. The results showed that our method is effective for growing single crystalline WO3 nanoneedles decorated with metals/metal oxides nanoparticles at moderate temperatures (i.e., 380 °C), with cost effectiveness and short fabrication times, directly onto transducers in view of obtaining gas sensors. The gas sensing studies performed showed that these hybrid nanomaterials have excellent sensitivity and selectivity compared to pristine WO3 samples. Cu2O/WO3 and Pd/WO3 nanocomposites have shown excellent sensitivity and selectivity toward H2S and H2 gases respectively

Deposition and Characterisation of SnS Thin Films for Application in Photovoltaic Solar Cell Devices

Thin films of SnS have been deposited onto heated glass substrates using the thermal evaporation method and the chemical and physical properties of the layers determined and correlated to the deposition conditions and to post-deposition heat treatments. In particular scanning electron microscopy, energy dispersive X-ray analysis, X-ray di.ractrometry and Raman studies were used to determine the material properties, transmittance and reflectance spectroscopy to determine the optical constants and 4-probe and van der Pauw measurements to determine the electrical properties. The results indicate that for a wide range of deposition conditions it is possible to produce high quality layers of SnS that are free from pin-holes and cracks, that are made of densely packed grains, and that adhere strongly to the substrate. For substrate temperatures between 280°C to 360°C it is possible to produce single phase SnS layers. The energy bandgap of these layers was in the range 1.3eV to 1.35eV, was direct, and had an optical absorption coefficient α > 105 cm-1 for photons with energies greater than the energy bandgap. The electrical properties indicate that all the layers are p-conductivity type with resistivities in the range 40Ωcm to 100Ωcm. Solar cell devices were fabricated in the superstrate and substrate configurations using n-type cadmium sulphide (CdS) and zinc indium diselenide (ZIS) buffer layers to partner the p-type SnS. The devices were investigated by measuring the I-V characteristics in the dark, to determine the predominant conduction mechanisms, the I-V characteristics under illumination to determine the open-circuit voltage V, the short circuit current density Jsc, the fill factor FF and solar conversion efficiency of the devices, C-V studies to determine the doping profile in the SnS and the built-in voltage at the junction and spectral response measurements to determine the minority carrier diffusion length in the p-SnS. Devices made with CdS as the n-type partner had a high density of interface states (1.36 x 1011 F C-1cm-2) with low photovoltaic parameters and a negative band offset of -0.36 eV obtained (as measured using x-ray photoelectron spectroscopy). The best devices made were substrate configuration solar cells in which the back contact on glass was molybdenum and the bu.er layer was ZIS. These devices have Voc = 472 mV, Jsc = 16.1 mA/cm2 , FF = 0.38 and a solar conversion efficiency of 2.9%. This is a world record efficiency for SnS-based solar cells at the time of submission of this PhD thesis

Process and Post-annealing Optimisation of SnS Thin Films with Alternative Buffer layers

Tin sulphide (SnS) is an environmentally friendly, Earth abundant and easy to fabricate thin film solar absorber for photovoltaic solar cell application. This work examines the properties of thermally evaporated SnS thin films, as a function of deposition parameters. Films were also subjected to a range of post-deposition treatments in vacuum, atmospheric pressure, chlorine and selenium ambient. SnS solar absorber layers were successfully deposited at low temperature (100 oC) to a thickness range from 100 to 3500 nm using thermal evaporation. Grain growth was partly dependent on the layer thickness where a progressive increase in grain size was noticed with increasing film thickness from 100 to 1500 nm; above 1500 nm thickness no further visible increase in the grains could be seen. Films grown to a thickness of 800 nm are found to be near stoichiometry with optimum energy bandgap compared to the thinner or thicker films. However, the SnS thin films showed strong dependence on substrate temperature. The temperature dependent study reveals that higher substrate temperatures lead to an increase in adatoms mobility, thereby promoting coalescences of smaller grains to form bigger grains. The increase in grain size with substrate temperature however stagnates after 350 oC such that further increasing the temperature does not induce further grain growth. Samples deposited at 350 oC substrate temperature were stoichiometric (Sn/S = 1.00) and with energy bandgap of 1.37 eV. Texture coefficient calculations showed that (111) orientation is more likely associated with the substrate temperatures 300 oC while, the (040) diffraction plane is related to higher temperatures (350 oC). Photoluminescence measurements demonstrated that controlling the film composition and optical bandgap is critical to produce a film that will luminesce, a requisite for any implementation in solar devices. On the other hand, the type of susbtrate material was found to significantly influence the properties of the SnS absorber films.The substrates studied include soda lime glass (SLG), quartz (Q), indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) coated glass, molybdenum (Mo) coated SLG and quartz. ii Films composition remains stoichiometric (Sn/S = 1.00 0.01) across the range of substrates. For the Na-free samples, reduction in micro-strain followed an increase in grain size. Unlike kesterite or chalcopyrite materials, the absence of Na in the substrate induces a significant grain growth with the average grain size increasing from 0.14 μm on SLG to 0.32 μm on quartz, ITO and FTO. SnS absorber layers deposited at 350 oC (thickness of 800 nm) were subjected to heat treatment in diverse environments such as vacuum (P = 10-6 mbar, 60 min), nitrogen (P=1000 mbar, 60 min) and selenium (20 min under 10 mbar argon pressure) for temperatures greater than the growth temperature (400-500 oC). Vacuum annealing was ineffective in both inducing grain growth and achieving recrystallisation. Nitrogen ambient revealed a recrystallised structure with slight increase in grain sizes and ~6% decrease in the bandgap compared to the reference 1.37 eV for the as-grown layer due to loss of sulphur (Sn/S ratio increased from 1.00 to 1.27 following anneal). The incorporation of Se led to substantial increase in grains with an average grain size of ~2.0 µm compared to 0.14 µm for as-grown films, with a nearly complete sulphur substitution by selenium. In addition, Se incorporation minimised voids while reducing the bandgap to 1.28 eV, improving photoluminescence yield and the open circuit voltage. Finally, this thesis explores a range of n-type buffer layers in order to fabricate devices. Numerical simulations show that ZnS buffer layer has potential to replace conventional CdS in fabricating SnS-based solar cells as it offers the most appropriate band alignment. Working devices could only be fabricated when combining the selenium heat treatment and the ZnS buffer layer