Semiconductor nanowires acts as an emerging class of materials with great potential for applications in future electronic devices. Small size, large surface to volume ratio and high carrier mobility of nanowires make them potentially useful for electronic applications with high integration density. In this thesis, the focus was on the growth of high quality ZnO nanowires, fabrication of field effect transistors and UV- photodetectros based on them. Intrinsic nanowire parameters such as carrier concentration, field effect mobility and resistivity were measured by configuring nanowires as field effect transistors. The main contribution of this thesis is the development of a high gain UV photodetector. A single ZnO nanowire functioning as a UV photodetector showed promising results with an extremely high spectral responsivity of 120 kA/W at wavelength of 370 nm. This corresponds to high photoconductive gain of 2150. To the best of our knowledge, this is the highest responsivity and gain reported so far, the previous values being responsivity=40 kA/W and gain=450. The enhanced photoconductive behavior is attributed to the presence of surface states that acts as hole traps which increase the life time of photogenerated electrons raising the photocurrent. This work provides the evidence of such solid states and preliminary results to modify the surface of ZnO nanowire is also produced.

The main objective of this Thesis is the synthesis, characterization and application in the PROX reaction of several series of catalysts synthesized using different methods, in order to find efficient materials with high catalytic activity. This work has been divided in six chapters. Chapter I presents an introduction about current energy problems and hydrogen (energy carrier) as a solution, with its limitations and advances on its use: storage, transport and security. Finally, its use as a fuel in PEMFC (Proton Exchange Membrane Fuel Cells) and the advantages comparing other existent fuel cells has been discussed. A brief summary was done about the catalytic systems studied in literature for PROX reaction and their characteristics. Following, Chapter 2 describes all the characterization techniques which were used to study the properties of the synthesized supports and catalysts, together with the experimental system employed for the catalytic tests. Most of these equipments belong to SSTTI of University of Alicante or are hand-made systems in the LMA group. DRIFTs and DTP were done at TUDelft, Catalysis Engineering Section. Next chapters contain the most important results obtained during the complete research work; they have been divided in two parts. On one hand, Chapter 3 describes the synthesis of pure TiO2 using five different methods, and its use as platinum support for PROX catalysts. On the other hand, taking into account the obtained results, three synthesis methods were chosen, and the study was centered in the addition of CeO2 to TiO2. Chapter 4 discusses the TixCe(1-x)O2 series synthesized using the sol-gel method. In Chapter 5 a solvothermal procedure was employed for the synthesis of supports with similar composition and, finally, impregnation of cerium over commercial P25 titania was compared in Chapter 6. Each chapter contains a complete study of characterization of both supports and catalysts. Synthesis parameters, pretreatment and reaction conditions were also modified for the best catalyst of the three series. Analyzing the catalysts’ behavior, a reaction mechanism was proposed. Finally, general conclusions summarize the most relevant results obtained in each chapter.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

The scope of this thesis was to study thermodynamics of lead oxide and cobalt oxide as the chemical looping agent for oxygen. Furthermore, the theoretical results were verified experimentally. Ellingham diagrams were constructed for the selected oxides. Then, detailed thermodynamic analysis was conducted for stability analysis at different temperatures and pressures. Equilibrium product compositions for various reactions involving these oxides were calculated via Gibbs free energy minimization analysis. Finally, it was shown that cobalt, lead and their oxide forms can be used for syngas production. In the experimental part PbOx, CoOx, Pt-doped CoOx and Pt-doped cobalt alumina, and mixed lead cobalt oxides were synthesized. In addition, technical grade cobalt oxide and lead rods were also used. XRD analysis indicated that Co3O4, Pb2O3 and &alpha / -PbO were the main crystal structures. Oxygen evolution from mixed oxides was monitored by TPD in a home built system. Re-oxidation of the reduced metals was successfully conducted using CO2 and H2O as oxidizing agents. Oxygen TPD studies indicated that oxygen evolution rates and amounts were higher and started at lower temperatures when two oxides were together. These observations were consistent with the predictions obtained from thermodynamics. In a series packed bed reactor, evolved oxygen from the mixed oxides were used to react with coal packed upstream of the oxides. It was found that coal oxidation can be achieved around 400oC and 600oC by using Pb/Co=3 (wt./wt.) looping media with almost no CO2 formation. It was also found that desired product selectivity (CO) can be increased by controlling reactive agent (O2) concentration in reaction environment.

Diamond cuboid-, rhombohedron- and hexagon-shaped nanocrystals as well as branched rods of the solid solution Co1-xMnxO have been synthesized via a solvothermal synthetic route from manganese formate and cobalt acetate at elevated temperature. Rhombohedra and hexagons have dimensions no larger than 50 nm on the longest axis, rods have branches up to 150 nm long and cuboids grow up to 250 nm on a side. X-ray and electron diffraction and transmission electron microscopy analyses show that these nanoparticles are single crystals of wurtzite-type and rock salt-type Co1-xMnxO. Varying the surfactant, water and precursor ratios allows control of particle size, morphology and stoichiometry. Extending growth time at high temperatures (>370°C) leads to the disappearance of the wurtzite phase due to Ostwald ripening. Longer reaction times at temperatures between 345-365°C lead to more crystalline wurtzite-lattice particles. These results show that nanoparticle morphologies and crystal lattices arise from crystal growth and Ostwald ripening at different rates selecting for either small, smooth-surfaced wurtzite lattice particles or large, dendritically-grown rock salt lattice particles.

One-dimensional and hierarchical nanostructures have acquired tremendous attention in the past decades due to their possible application. In spite of the rapid emergence of new morphologies, the underlying growth mechanism is still not well understood. The lack of effective p-type or n-type doping is another obstacle for many semiconducting nanomaterials. A deeper investigation into these structures and new methods to fabricate devices are of significant impact for nanoscience and nanotechnology. Motivated by a desire to understand the growth mechanism of nanostructures and investigate novel device fabrication method, the research described in this thesis carried out on the synthesis, characterization, and device fabrication of semiconducting nanostructures. The main focus of the research was on ZnO, SnO2, and Zn3P2 for their great capability for fundamental phenomena studying, promising applications in sensors and optoelectronics, and the potential generalization of results to other materials. Within this study the following goals have been achieved: 1) Improved understanding of polar-surface-induced growth mechanism in wurtzite-structured ZnO and generalization of this growth mechanism with the discovery and analysis of rutile ¨Cstructured SnO2, 2) observation of the significance of the transversal growth, which is usually ignored, in interpenetrative ZnO nanowires, 3) rational design and growth control over versatile nanostructures of ZnO and Zn3P2, and 4) conjunction of p-type Zn3P2 and n-type ZnO semiconducting nanostructures for device fabrications. The framework for the research is reviewed first in chapter 1. Chapter 2 gives the detailed experimental setup, synthesis procedure, and common growth mechanism for nanostructure growth. A detailed discussion on the growth of ZnO nanostructures in chapter 3 provides more insight into the polar-surface-induced growth, transversal growth, vapor-solid growth, and vapor-liquid-solid growth during the formation of nanostructures. Polar-surface-induced growth is also confirmed in the growth of SnO2 nanostructures, which is also included in chapter 2. Chapter 3 presents Zn3P2 nanostructures from the newly designed experiment setup and the device fabrication from ZnO and Zn3P2 crossed nanowires.

A novel high-k gate dielectric material, i.e., hafnium-doped tantalum oxide (Hf-doped TaOx), has been studied for the application of the future generation metal-oxidesemiconductor field effect transistor (MOSFET). The film's electrical, chemical, and structural properties were investigated experimentally. The incorporation of Hf into TaOx impacted the electrical properties. The doping process improved the effective dielectric constant, reduced the fixed charge density, and increased the dielectric strength. The leakage current density also decreased with the Hf doping concentration. MOS capacitors with sub-2.0 nm equivalent oxide thickness (EOT) have been achieved with the lightly Hf-doped TaOx. The low leakage currents and high dielectric constants of the doped films were explained by their compositions and bond structures. The Hf-doped TaOx film is a potential high-k gate dielectric for future MOS transistors. A 5 àtantalum nitride (TaNx) interface layer has been inserted between the Hf-doped TaOx films and the Si substrate to engineer the high-k/Si interface layer formation and properties. The electrical characterization result shows that the insertion of a 5 àTaNx between the doped TaOx films and the Si substrate decreased the film's leakage current density and improved the effective dielectric constant (keffective) value. The improvement of these dielectric properties can be attributed to the formation of the TaOxNy interfacial layer after high temperature O2 annealing. The main drawback of the TaNx interface layer is the high interface density of states and hysteresis, which needs to be decreased. Advanced metal nitride gate electrodes, e.g., tantalum nitride, molybdenum nitride, and tungsten nitride, were investigated as the gate electrodes for atomic layer deposition (ALD) HfO2 high-k dielectric material. Their physical and electrical properties were affected by the post metallization annealing (PMA) treatment conditions. Work functions of these three gate electrodes are suitable for NMOS applications after 800°C PMA. Metal nitrides can be used as the gate electrode materials for the HfO2 high-k film. The novel high-k gate stack structures studied in this study are promising candidates to replace the traditional poly-Si-SiO2 gate stack structure for the future CMOS technology node.

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