Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references. Also issued in print.

Thesis (Ph.D.)--Materials Science and Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Snyder, Robert; Committee Co-Chair: Wang, Zhong Lin; Committee Member: El-Sayed, Mostafa; Committee Member: Milam, Valeria; Committee Member: Summers, Christopher; Committee Member: Wong, C. P.

Thesis (Ph.D.)--Texas Christian University, 2009. / Title from dissertation title page (viewed Apr. 19, 2010). Includes abstract. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

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

Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on September 14, 2007) Vita. Includes bibliographical references.

La nanostructuration des matériaux thermoélectriques semble être la solution pour palier au faible rendement généralement rencontré chez ces matériaux. Bien qu’il y ait eu de nets progrès en manipulant de nouvelles formulations et en nano-structurant les composés existants, peu de travaux ont été entrepris sur la fabrication de dispositifs permettant la micro-génération ou le micro-refroidissement thermoélectrique. Dans ce contexte, les travaux de recherche ont été en premier lieu l’optimisation et la maitrise de la croissance par voie électrochimique de réseaux auto-ordonnés de nanofils thermoélectriques à base de tellurure de bismuth de type n (Bi2Te3-xSex) et de type p (BixSb2-xTe3) dans des membranes d’alumine nanoporeuses. Les propriétés structurales et thermoélectriques de ces nanofils ont été investigués. Concernant les caractérisations électriques, une nouvelle méthode de mesure sur nanofil unique dans sa membrane a été développée. Elle permet de s’affranchir des problèmes d’oxydation quand le fils est libéré de sa membrane dans les méthodes de mesure classiques. La conductivité thermique a aussi été estimée sur des fils dans leurs membranes avec la méthode 3-omega. La deuxième partie de la thèse a été consacré à des premiers essais d'assemblage des deux types pour évaluer la performance d’un prototype de thermogénérateur (TEG) fonctionnant à une température proche de l’ambiante. Et en parallèle une étude des problèmes de résistance de contact entre le métal et le thermoélectrique a été réalisée. / Nanostructuration of thermoelectric materials seems to be the solution for their low efficiency generally encountered. Even if fair progress have been realized in manipulation of new formulations and nanostructuration of existing materials, few studies have been undertaken to develop devices for thermoelectric micro-generation or micro-cooling. In this context, bismuth antimony telluride (BixSb2-xTe3) and bismuth tellurium selenide (Bi2Te3-xSex) nanowires, have been potentiostatically electrodeposited using anodic alumina membrane. Nanowires structure, stoichiometry and thermoelectric properties have been investigated. A new method for electrical characterization have been developed, consisting in direct electrical transport measurement on a single nanowire embedded in his matrix. This method avoid oxidation effects and time-consuming processing as nanowires need no preliminary preparation and remain in their template during measurement. Thermal conductivity was also investigated using 3-omega method. The second part of this work was dedicated to first tests on n-type and p-type assembly in order to study the efficiency of TEG prototype working at ambient temperature. In parallel, a study of contact resistance (thermoelectric / metal) have been conducted.

abstract: Raman scattering from Ge-Si core-shell nanowires is investigated theoretically and experimentally. A theoretical model that makes it possible to extract quantitative strain information from the measured Raman spectra is presented for the first time. Geometrical and elastic simplifications are introduced to keep the model analytical, which facilitates comparison with experimental results. In particular, the nanowires are assumed to be cylindrical, and their elastic constants isotropic. The simple analytical model is subsequently validated by performing numerical calculations using realistic nanowire geometries and cubic, anisotropic elastic constants. The comparison confirms that the analytic model is an excellent approximation that greatly facilitates quantitative Raman work, with expected errors in the strain determination that do not exceed 10%. Experimental Raman spectra of a variety of core-shell nanowires are presented, and the strain in the nanowires is assessed using the models described above. It is found that all structures present a significant degree of strain relaxation relative to ideal, fully strained Ge-Si core-shell structures. The analytical models are modified to quantify this strain relaxation. / Dissertation/Thesis / Ph.D. Physics 2011

L’oxyde de zinc est un semiconducteur pressenti pour une large variété d’applications optoélectroniques, biologiques ou encore détecteurs de gaz. En effet, en plus d’être un matériau abondant, il possède plusieurs propriétés remarquables comme sa large bande interdite de 3,33 eV, sa grande mobilité électronique de 200 cm²/(V.s) mais également sa capacité à croitre sous plusieurs formes nanométriques par des techniques de dépôt bas coût et facilement adaptables au milieu industriel. Les nanofils de ZnO élaborés par la technique de dépôt en bain chimique seront utilisés dans cette thèse pour leur intégration dans des cellules solaires de 3ème génération. Dans ces cellules, la morphologie des nanofils ainsi que leur dopage est primordial pour obtenir des rendements intéressants. Ce dernier aspect en particulier n’a cependant pas été étudié en détails dans la littérature concernant le dépôt en bain chimique. Ce travail présente donc une façon innovante de contrôler simultanément la morphologie et le dopage des nanofils par cette technique de dépôt. Un mécanisme de croissance et de dopage a été déterminé grâce à des simulations thermodynamiques, des mesures de pH in-situ et plusieurs méthodes de caractérisation telles que la microscopie électronique à balayage et à transmission, la diffraction des rayons X, la spectroscopie Raman en température et la microscopie à force atomique en mode électrique. Les nanofils de ZnO réalisés sont ensuite intégrés dans des cellules solaires à colorant pour étudier l’intérêt de l’optimisation des nanofils sur les performances des cellules solaires. Finalement, ces nanofils de ZnO combinés à une électrode en nanofils d’argent peuvent être intégrés sur substrat flexible pour réaliser une cellule à colorant plus légère et maniable et donc visant davantage d’applications. / Zinc oxide is a semiconductor considered for a wide range of optoelectronic, biological, or gas sensor applications. Indeed, apart from being an abundant material, it has several remarkable properties as its electronic mobility (200 cm²/(V.s)) and his ability to grow under different nano-scale shapes thanks to low cost and easily scalable deposition techniques. ZnO nanowires grown by the chemical bath deposition technique are used in this thesis for their integration in 3rd generation solar cells. In these devices as in many other applications, the nanowire mophology and electrical property are essential to get interesting efficiency. This last aspect, in particular, has never been studied in details in the literature concerning chemical bath deposition. This work introduces a new way to control simultaneously the morphology and the doping level of ZnO nanowires by this deposition technique. A growth and doping mechanism has been identified thanks to thermodynamics simulations, in-situ pH measurements, and several characterization techniques like scanning electron microscope, X-ray diffraction, temperature dependant Raman spectroscopy, and atomic force microscopy. The ZnO nanowires were also embedded in dye sensitized solar cells to study the effect of morphology and doping level on the efficiency. ZnO nanowires combined with an Ag nanowire electrode can be deposited on a flexible substrate to make a flexible dye sensitized solar cell.

abstract: Nanowires (NWs) have attracted many interests due to their advance in synthesis and their unique structural, electrical and optical properties. NWs have been realized as promising candidates for future photonic platforms. In this work, erbium chloride silicate (ECS), CdS and CdSSe NWs growth by vapor-liquid-solid mechanism and their characterization were demonstrated. In the ECS NWs part, systematic experiments were performed to investigate the relation between growth temperature and NWs structure. Scanning electron microscopy, Raman spectroscopy, X-ray diffraction and photoluminescence characterization were used to study the NWs morphology, crystal quality and optical properties. At low growth temperature, there was strong Si Raman signal observed indicating ECS NWs have Si core. At high growth temperature, the excess Si signal was disappeared and the NWs showed better crystal quality and optical properties. The growth temperature is the key parameter that will induce the transition from Si/ECS core-shell NWs structure to solid ECS NWs. With the merits of high Er concentration and long PL lifetime, ECS NWs can serve as optical gain material with emission at 1.5 μm for communications and amplifiers. In the CdS, CdSSe NWs part, the band gap engineering of CdSSe NWs with spatial composition tuning along single NWs were demonstrated. The first step of realizing CdSSe NWs was the controlled growth of CdS NWs. It showed that overall pressure would largely affect the lengths of the CdS NWs. NWs with longer length can be obtained at higher pressure. Then, based on CdS NWs growth and by adding CdSe step by step, composition graded CdSSe alloy NWs were successfully synthesized. The temperature control over the source vapor concentration plays the key role for the growth. / Dissertation/Thesis / M.S. Electrical Engineering 2012