In this dissertation, I present that at a vacuum of 3×10-7 Torr, residual O2, CO2, H2 and Ar exposure do not significantly degrade the field emission (FE) properties of ZnO nanorods, but N2 exposure significantly does. I propose that this could be due to the dissociation of N2 into atomic nitrogen species and the reaction of such species with ZnO. I also present the effects of O2, CO2, H2O, N2, H2, and Ar residual gas exposure on the FE properties of GaN and ZnS nanostructure. A brief review of growth of ZnO, GaN and ZnS is provided. In addition, Cs deposition on GaN nanostructures at ultra-high vacuum results in 30% decrease in turn-on voltage and 60% in work function. The improvement in FE properties could be due to a Cs-induced space-charge layer at the surface that reduces the barrier for FE and lowers the work function. I describe a new phenomenon, in which the resistivity of CVD-grown graphene increases to a higher saturated value under light exposure, and depends on the wavelength of the light—the shorter the wavelength, the higher the resistivity. First-principle calculations and theoretical analysis based on density functional theory show that (1) a water molecule close to a graphene defect is easier to be split than that of the case of no defect existing and (2) there are a series of meta-stable partially disassociated states for an interfacial water molecule. Calculated disassociation energies are from 2.5 eV to 4.6 eV, that match the experimental observation range of light wavelength from visible to 254 nm UV light under which the resistivity of CVD-grown graphene is increased.

This thesis reports a comprehensive study of quaternary BeMgZnO alloy and BeMgZnO/ZnO heterostructures for UV-optoelectronics electronic applications. It was shown that by tuning Be and Mg contents in the heterostructures, high carrier densities of two-dimensional electron gas (2DEG) are achievable and makes its use possible for high power RF applications. Additionally, optical bandgaps as high as 5.1 eV were achieved for single crystal wurtzite material which allows the use of the alloy for solar blind optoelectronics (Eg>4.5eV) or intersubband devices. A systematic experimental and theoretical study of lattice parameters and bandgaps of quaternary BeMgZnO alloy was performed for the whole range of compositions. Composition independent bowing parameters were determined which allows accurate predictions of experimentally measured values. The BeMgZnO thin films were grown by plasma assisted molecular beam epitaxy (P-MBE) in a wide range of compositions. The optimization of the growth conditions and its effects on the material properties were explored. The surface morphology and electrical characteristics of the films grown on (0001) sapphire were found to critically depend on the metal-to-oxygen ratio. Samples grown under slightly oxygen-rich conditions exhibited the lowest RMS surface roughness (as low as 0.5 nm). Additionally, the films grown under oxygen-rich conditions were semi-insulating (>105 Ω∙cm), while the films grown under metal-rich conditions were semiconducting (~102 Ω∙cm). Additionally, with increasing bandgap Stokes shift increases, reaching ~0.5 eV for the films with 4.6 eV absorption edge suggests the presence of band tail states introduced by potential fluctuations and alloying. From spectrally resolved PL transients, BeMgZnO films grown on a GaN/sapphire template having higher Mg/Be content ratio exhibit smaller localization depth and brighter photoluminescence at low temperatures. The optimum content ratio for better room temperature optical performance was found to be ~2.5. The BeMgZnO material system and heterostructures are promising candidates for the device fabrication. 2DEG densities of MgZnO/ZnO heterostructures were shown to improve significantly (above 1013 cm-2) by adding even a small amount of Be (1-5%). As an essential step toward device fabrication, reliable ohmic contacts to ZnO were established with remarkably low specific contact resistivities below 10-6 Ohm-cm2 for films with 1018 cm-3 carrier density.

Neste trabalho, são discutidas as características de transistores de filmes finos (TFTs) nos quais nanopartículas de óxido de zinco (ZnO) são empregadas como material ativo na camada semicondutora. O crescimento contínuo do interesse por este componente está associado à busca pelo desenvolvimento da tecnologia de dispositivos eletrônicos flexíveis, transparentes e de baixo custo. TFTs integrados com nanopartículas de ZnO são apresentados, e uma extensa rotina de caracterização elétrica transiente é realizada para avaliar como estes dispositivos se comportam e degradam ao longo do tempo. Foram medidas, ao total, 80 amostras de transistores integrados em duas configurações distintas: inverted staggered e inverted coplanar. A partir das medidas analisadas foram identificados dois grupos de comportamentos elétricos dominantes, os quais foram classificados em: efeitos abruptos e efeitos de memória. A partir dos dados coletados, foram formuladas hipóteses para modelar o comportamento típico observado. Para tanto, utiliza-se dos mecanismos de atividade de traps, de interação da camada semicondutora com o meio ambiente, de polarização de dipolos e difusão de cargas móveis no dielétrico, de formação de caminhos percolados paralelos pelas nanopartículas e de difusão de vacâncias de oxigênio e íons metálicos que podem estar associados ao comportamento elétrico observado. / In this work, the characteristics of thin-film transistors (TFTs) employing nanoparticulated zinc oxide (ZnO) as the active semiconductor channel layer are discussed. The growing interest in this component is associated to the development of low-cost, flexible and transparent electronic devices. The TFTs integrated with ZnO nanoparticles are presented and an extensive transient electrical characterization campaign was performed in order to evaluate how these devices behave and degrade over time. The measurement was performed for 80 samples of two different integration setups: inverted staggered and inverted coplanar. In the performed tests two main disturbances were identified, which were classified as abrupt and memory effects. From the collected data, hypothesis to model the observed typical behavior are formulated. Trapping activity, ambient interaction, dielectric dipoles, mobile charges, formed parallel-paths, oxygen vacancies and metallic ions diffusion are mechanisms that may be associated to the observed behavior.

<p>The principles of dye-sensitized solar cells were studied and are outlined in this thesis. An overview of the basic steps needed to create a DSC isfollowed by detailed experimental information on how to assemble the solar cells that were fabricated in this project. They were based on synthesizednanocrystalline ZnO thin films sensitized by chlorophyll a as well as isolated photopigments from spinach leaves. The nanocrystals werestudied using XRD, and it was confirmed that three different methods of synthesis resulted in ZnO crystals of a few nanometers. The solar cellswere assembled with Au electrodes in a sandwich configuration and their photovoltaic properties were measured. Overall light-to-electricity conversionwas low with the highest efficiency being 0.21 %. An astonishingly low efficiency of 0.0003 % was noted for a thin film which was not thermallytreated, and it is suggested that heat-treatment is of great importance. It was also found that photopigments from spinach can be extractedeasily and used as molecular sensitizer without any demanding purification steps.</p>

In this work we investigated fabrication of semiconductor nanomaterials and evaluated their potential for photo-chemical and photovoltaic applications. We investigated different II-VI semiconductor nanomaterial systems; (i) ZnO oriented nanowire arrays non-epitaxially grown from a substrate; and (ii) colloidal CdE (E=Te,Se,S) quantum structures synthesized by solution-based thermal decomposition of organo-metallic precursors. We have studied the synthesis of vertically aligned ZnO nanowire arrays (NWA), by a wet chemical process on various substrates. We have extended this method wherein nanofibers of poly-L-lactide act as a substrate for the radially oriented growth of ZnO nanowires. By combining the large surface area and the flexibility of the PLLA-ZnO hierarchical nanostructure we have shown the proof-of-principle demonstration of a ‘continuous-flow’ water treatment system to decompose known organic pollutants in water, as well as render common waterborne bacteria non-viable. We have studied synthesis of colloidal quantum dots (QD), and show size, morphology and composition tailored nanocrystals for CdE (E=S, Se, Te) compositions. We have studied the influence of crystal growth habits of the nanocrtsyals on the final morphology. Furthermore we have synthesized core-shell, CdSe-CdS QDs with spherical and tetrahedral morphologies by varying the reaction conditions. We show that these core-shell quantum dots show quasi-type II characteristics, and demonstrate with I-V measurements, the spatial localization of the charge carriers in these hetero-nanocrystals. For this purpose, we developed hybrid materials consisting of the core-shell quantum dots with electron acceptors (ZnO nanowires) and hole acceptors (polymeric P3HT nanofibers). In addition we have also compared the synthesis reaction when carried out with conventional heating and microwave-mediated heating. We find that the reaction is enhanced, and the yield is qualitatively better when using microwave induced heating. / QC 20120525

Parmi les biocarburants de deuxième génération, la chaîne "biomass to liquid" vise à convertir les résidus agricoles en carburant. Cette voie suppose une première étape de gazéification de la biomasse en un gaz de synthèse, mélange de CO et de H2. Ce gaz doit ensuite être désulfuré, afin de ne pas endommager le catalyseur Fischer-Tropsch, à l'aide d'oxydes métalliques tel que l'oxyde de zinc, qui se sulfure selon la réaction suivante : ZnO(s) + H2S(g) → ZnS(s) + H2O(g) a l'heure actuelle, aucune étude ne décrit les mécanismes de la réaction. le but de ce travail a donc été d'identifier les différentes étapes du mécanisme de sulfuration puis d'établir une loi de vitesse de réaction. L'étude cinétique a été réalisée par thermogravimétrie. La formation de cavités au cœur des particules sulfurées a été observée par MEB et MET, mettant en évidence une croissance externe de la phase de ZnS. Un mécanisme de sulfuration en huit étapes élémentaires a été proposé avec diffusion des atomes de zinc et d'oxygène de l'interface interne ZnO/ZnS vers la surface externe du ZnS formé. Le régime limitant de la réaction a été déterminé via l'étude de l'influence des pressions partielles de H2S et de H2O sur la vitesse de réaction. Un régime mixte sur la base de deux étapes élémentaires a été envisagé : une réaction d'interface externe avec désorption des molécules d'eau et la diffusion des atomes d'oxygène. la formation de cavités à l'interface interne ZnO/ZnS entraînant une diminution de la surface de contact entre les phases ZnO et ZnS a été mise en évidence, phénomène probablement à l'origine du ralentissement de la réaction observé sur les courbes cinétiques.

La present tesi doctoral tracta la modelitzacio de diferents estats de la materia: clusters o agregats, superficies i cristalls d'oxids metal.lics. Els metodes disponibles per abordar cada sistema han estat breument descrits, per justificar la seua utilitzacio en diverses aplicacions a materials. Primer, s'han proposat geometries per als agregats de V2O5 cations i neutres en fase gas, i s'han caracteritzat els estats electronics, estructura, espectre de vibracio i enllac quimic. Segon, s'han simulat superficies d'oxids metal.lics i processos d'adsorcio de molecules o atoms metal.lics: CH3OH i O2 sobre SnO2, Cu sobre ZnO. La interaccio adsorbat-superficie te lloc per un mecanisme acid/base, i s'ha caracteritzat la geometria i energia de cada supersistema (mode d'adsorcio, paper dels defectes de superficie). Finalment, s'ha estudiat el cristall de TiO2 en fase anatasa des de multiples punts de vista: descripcio de l'estructura en poliedres, estructura electronica (bandes i densitat d'estats), termodinamic (equacio d'estat) i d'enllac quimic ("Atoms In Molecules"). Les superficies mes estables han estat construides i els efectes de relaxacio avaluats.

The principles of dye-sensitized solar cells were studied and are outlined in this thesis. An overview of the basic steps needed to create a DSC isfollowed by detailed experimental information on how to assemble the solar cells that were fabricated in this project. They were based on synthesizednanocrystalline ZnO thin films sensitized by chlorophyll a as well as isolated photopigments from spinach leaves. The nanocrystals werestudied using XRD, and it was confirmed that three different methods of synthesis resulted in ZnO crystals of a few nanometers. The solar cellswere assembled with Au electrodes in a sandwich configuration and their photovoltaic properties were measured. Overall light-to-electricity conversionwas low with the highest efficiency being 0.21 %. An astonishingly low efficiency of 0.0003 % was noted for a thin film which was not thermallytreated, and it is suggested that heat-treatment is of great importance. It was also found that photopigments from spinach can be extractedeasily and used as molecular sensitizer without any demanding purification steps.

Nanotechnology is experiencing a flourishing development in a variety of fields covering all of the areas from science to engineering and to biology. As an active field in nanotechnology, the work presented in this dissertation is mostly focused on the fundamental study about the fabrication and assembly of functional oxide nanostructures. In particular, Zinc Oxide, one of the most important functional semiconducting materials, is the core objective of this research, from the controlled growth of nanoscale building blocks to understanding their properties and to how to organize these building blocks. Thermal evaporation process based on a single-zone tube furnace has been employed for synthesizing a range of 1D nanostructures. By controlling the experimental conditions, different morphologies, such as ultra-small ZnO nanobelts, mesoporous ZnO nanowires and core-shell nanowire were achieved. In order to pattern the nanostructures, a large-scale highly-ordered nanobowl structure based on the self-assembly of submicron spheres was created and utilized as patterning template. The growth and patterning techniques were thereafter integrated for aligning and patterning of ZnO nanowires. The aligning mechanisms and growth conditions were thoroughly studied so as to achieve a systematic control over the morphology, distribution and density. The related electronic and electromechanical properties of the aligned ZnO nanowires were investigated. The feasibility of some potential applications, such as photonic crystals, solar cells and sensor arrays, has also been studied. This research may set a foundation for many industrial applications from controlled synthesis to nanomanufacturing.

Union between top-down and bottom-up assembly is inevitable when scaling down physical, chemical, and biological sensors and probes. Current sensor/probe-based technologies are firmly founded on top-down manufacturing, with limitations in cost of production, manufacturing methods, and material constraints. As an alternative to such limitations, contemporary synthesis techniques for one-dimensional nanostructures have been combined with established methods of micro-fabrication for the development of novel tools and techniques for nanotechnology. More specifically, this dissertation is a systematic study of the synthesis and characterization of ZnO nanostructures for piezoelectric applications. Within this study the following goals have been achieved: 1) rational design and control of a diversity of novel ZnO nanostructures, 2) improved understanding of polar-surface-dominated (PSD) phenomena among Wurtzite crystal structures, 3) confirmation of Taskers Rule via the synthesis, characterization, and modeling of polar-surface-dominated nanostructures, 4) measurement of the surface-charge density for real polar surfaces of ZnO, 5) confirmation of the electrostatic polar-charge model used to describe polar-surface-dominated phenomena, 6) dispersion of ZnO nanobelts onto the selective layers of surface acoustic wave (SAW) devices for gas sensing applications, 7) manipulation of ZnO nanostructures using an atomic force microscope (AFM) for the development of piezoelectric devices, 8) fabrication of bulk acoustic resonator (BAR) and film bulk acoustic resonator (FBAR) devices based on the integrity of individual ZnO belts, 9) electrical characterization of a ZnO belt BAR device, 10) prediction and confirmation of the electrical response from a BAR device using a one-dimensional Krimholt-Leedom-Matthaei (KLM) model, and 11) development of a finite element model (FEM) to accurately predict the electrical response from ZnO belt BAR and FBAR devices in 3D.