Rozprawy doktorskie na temat „Semiconducting Nanostructures”

This diploma thesis discusses the gas sensor preparation via anodic oxidation. It names sensor types, deals with the sensing principle of electrochemical sensors in detail and submits sensor parameters. It describes preparation technology and characterization technology methods. In the experimental part, it focuses on both the measurement methodology and the electrochemical oxygen sensor covered with titanium dioxide nanocolumns fabrication. Not the least it discusses acquired research results.

La présente thèse étudie les nanoparticules de ZnO incorporées dans une matrice d'acide polyacrylique (PAA) mésosphérique synthétisée via un protocole d'hydrolyse. La structure hybride mésosphérique de ZnO / PAA a précédemment démontré son efficacité pour émettre de la lumière visible dans une large gamme, qui résulte des défauts intrinsèques de niveaux profonds dans les nanocristaux de ZnO. Pour modifier davantage le spectre de photoluminescence (PL) et améliorer le rendement quantique de PL (PL QY) du matériau, le ZnO dopé au métal et le ZnO / PAA revêtu de silice sont fabriqués indépendamment. Au niveau du ZnO dopé avec des éléments métalliques, la nature, la concentration, la taille et la valence du dopant affectent la formation des mésosphères et par conséquent la PL et le PL QY. Les ions plus grands que Zn2+ avec une valence plus élevée ont tendance à induire des mésosphères plus grandes et des nanoparticules de ZnO non incorporées. Le dopage conduit généralement à l'extinction de la PL, mais le spectre PL peut toujours être ajusté dans une large plage (entre 2,46 eV et 2,17 eV) sans dégrader le PL QY en dopant avec de petits ions à une faible concentration de dopage (0,1 %). Concernant le ZnO / PAA revêtu de silice, un revêtement optimal est obtenu, qui dépend corrélativement de la quantité de TEOS et d'ammoniac dans le processus de revêtement. La quantité de TEOS n'affecte pas la structure cristalline de ZnO ou le spectre PL du matériau, mais une concentration élevée d'ammoniac peut dégrader les mésosphères de PAA et épaissir la couche de silice. Une fine couche de silice qui n'absorbe pas trop de lumière d'excitation mais recouvre complètement les mésosphères s'avère être la plus efficace, avec une amélioration drastique du PL QY d’un facteur six. En ce qui concerne l'application, les matériaux souffrent d’une dégradation thermique à des températures élevées jusqu'à 100 °C, auxquelles les diodes électroluminescentes blanches (WLEDs) fonctionnent généralement. Cependant, le ZnO / PAA revêtu de silice induit une intensité d'émission plus élevée à température ambiante pour compenser la dégradation thermique<br>The present thesis studies ZnO nanoparticles embedded in a mesospheric polyacrylic acid (PAA) matrix synthesized via a hydrolysis protocol. The mesospheric ZnO/PAA hybrid structure was previously proved efficient in emitting visible light in a broad range, which results from the deep-level intrinsic defects in ZnO nanocrystals. To further tune the photoluminescence (PL) spectrum and improve the PL quantum yield (PL QY) of the material, metal-doped ZnO and silica-coated ZnO/PAA are fabricated independently. For ZnO doped with metallic elements, the nature, concentration, size and valence of the dopant are found to affect the formation of the mesospheres and consequently the PL and PL QY. Ions larger than Zn2+ with a higher valence tend to induce larger mesospheres and unembedded ZnO nanoparticles. Doping generally leads to the quenching of PL, but the PL spectrum can still be tuned in a wide range (between 2.46 eV and 2.17 eV) without degrading the PL QY by doping small ions at a low doping concentration (0.1 %). For silica-coated ZnO/PAA, an optimal coating correlatively depends on the amount of TEOS and ammonia in the coating process. The amount of TEOS does not affect the crystal structure of ZnO or the PL spectrum of the material, but high concentration of ammonia can degrade the PAA mesospheres and thicken the silica shell. A thin layer of silica that does not absorb too much excitation light but completely covers the mesospheres proves to be the most efficient, with a drastic PL QY improvement of six times. Regarding the application, the materials suffer from thermal quenching at temperatures high up to 100°C, at which white light emitting diodes (WLEDs) generally operates. However, silica-coated ZnO/PAA induces higher emission intensity at room temperature to make up for the thermal quenching

Made available in DSpace on 2014-06-11T19:31:04Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-11-16Bitstream added on 2014-06-13T19:01:19Z : No. of bitstreams: 1 savu_r_dr_bauru.pdf: 10688901 bytes, checksum: 4c1846c73d88b2e598b43e7a14ea1b7c (MD5)<br>Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)<br>A presente pesquisa teve como principal objetivo a obtenção de estruturas nanométricas de óxido de índio, óxido de estanho e óxido de zinco por evaporação térmica e síntese hidrotérmica e a construção e teste de sensores de gases e de fotodetectores de ultravioleta baseados nessas nanoestruturas. Foram realizados estudos da influência dos parâmetros experimentais das duas rotas de síntese usadas sobre as morfologias e as propriedades das estruturas. Para a obtenção das camadas nanoestruturadas por evaporação térmica foi especialmente construído um forno tubular que permitiu o controle da temperatura de deposição independente da temperatura de evaporação e da distância entre a fonte de evaporação e o substrato. Esses parâmetros, pouco explorados nas pesquisas reportadas na literatura, exerceram uma grande influência sobre a morfologia e as propriedades dos nanofios obtidos. O equipamento permitiu ainda um controle preciso da composição da atmosfera e da pressão de síntese. Na síntese química em solução, a construção de um reator hidrotérmico permitiu o estudo da influência da taxa de resfriamento sobre as dimensões, cristalinidade, morfologia e propriedades das nanoestruturas. Esse estudo, o primeiro do gênero na literatura, ressaltou a importância no controle deste parâmetro para sintetizar estruturas com propriedades melhoradas. As demais variáveis estudadas foram: a concentração das soluções, as camadas catalisadoras, a temperatura e o tempo de síntese. Foram testadas duas estratégias para a obtenção dos filmes nanoestruturados: spin-coating de suspensões de nanoestruturas sobre substratos de silício oxidado ou o crescimento das mesmas, durante a síntese, sobre substratos com camadas catalisadoras de zinco. Os nanofios e as camadas funcionais foram caracterizados por Difração de Raios-X (DRX), Microscopia Eletrônica de Varredura...<br>The subject of this thesis covers the synthesis and growth of indium, tin and zinc oxide nanostructures by thermal evaporation and hydrothermal synthesis and the fabrication and testing of gas sensors and ultraviolet photodetectors based on these nanosized structures. For both chemical and physical routes, the influence of processing conditions over the morphology, dimensions and electrical properties of the nanowires was investigated. In order to obtain nanostructured layers by thermal evaporation a tubular furnace was specifically builti, allowed the control of the source-substrate distance and the deposition temperature independently of the evaporation one. These parameters, slightly explored in the literature, granted a big influence over the nanowires morphology and properties. Moreover, the equipment permitted the control of deposition atmosphere and pressure. The design and assembly of a hydrothermal reactor allowed studying the influence of the cooling rate over the dimension, morphology, cristallinity and, consequently, the properties of the nanostructures. This study highlighted the importance of controlling this particular parameter in the hydrothermal process, yielding nanostructured materials with enhanced properties. Variables such as solution concentration, synthesis temperature and time, surfanctants and precursors were also explored in the hydrothermal process. In order to obtain nanostructured thin films using the chemical bath deposition, two processing techniques were employed: spin-coating of powder suspensions over oxidized silicon substrates and nanostructured anisotropic growth directly from solution using zinc coated substrates. The nanowires and the functional nanostructured layers were characterized by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE - SEM), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS)... (Complete abstract click electronic access below)

Resumo: A presente pesquisa teve como principal objetivo a obtenção de estruturas nanométricas de óxido de índio, óxido de estanho e óxido de zinco por evaporação térmica e síntese hidrotérmica e a construção e teste de sensores de gases e de fotodetectores de ultravioleta baseados nessas nanoestruturas. Foram realizados estudos da influência dos parâmetros experimentais das duas rotas de síntese usadas sobre as morfologias e as propriedades das estruturas. Para a obtenção das camadas nanoestruturadas por evaporação térmica foi especialmente construído um forno tubular que permitiu o controle da temperatura de deposição independente da temperatura de evaporação e da distância entre a fonte de evaporação e o substrato. Esses parâmetros, pouco explorados nas pesquisas reportadas na literatura, exerceram uma grande influência sobre a morfologia e as propriedades dos nanofios obtidos. O equipamento permitiu ainda um controle preciso da composição da atmosfera e da pressão de síntese. Na síntese química em solução, a construção de um reator hidrotérmico permitiu o estudo da influência da taxa de resfriamento sobre as dimensões, cristalinidade, morfologia e propriedades das nanoestruturas. Esse estudo, o primeiro do gênero na literatura, ressaltou a importância no controle deste parâmetro para sintetizar estruturas com propriedades melhoradas. As demais variáveis estudadas foram: a concentração das soluções, as camadas catalisadoras, a temperatura e o tempo de síntese. Foram testadas duas estratégias para a obtenção dos filmes nanoestruturados: spin-coating de suspensões de nanoestruturas sobre substratos de silício oxidado ou o crescimento das mesmas, durante a síntese, sobre substratos com camadas catalisadoras de zinco. Os nanofios e as camadas funcionais foram caracterizados por Difração de Raios-X (DRX), Microscopia Eletrônica de Varredura... (Resumo completo, clicar acesso eletrônico abaixo)<br>Abstract: The subject of this thesis covers the synthesis and growth of indium, tin and zinc oxide nanostructures by thermal evaporation and hydrothermal synthesis and the fabrication and testing of gas sensors and ultraviolet photodetectors based on these nanosized structures. For both chemical and physical routes, the influence of processing conditions over the morphology, dimensions and electrical properties of the nanowires was investigated. In order to obtain nanostructured layers by thermal evaporation a tubular furnace was specifically builti, allowed the control of the source-substrate distance and the deposition temperature independently of the evaporation one. These parameters, slightly explored in the literature, granted a big influence over the nanowires morphology and properties. Moreover, the equipment permitted the control of deposition atmosphere and pressure. The design and assembly of a hydrothermal reactor allowed studying the influence of the cooling rate over the dimension, morphology, cristallinity and, consequently, the properties of the nanostructures. This study highlighted the importance of controlling this particular parameter in the hydrothermal process, yielding nanostructured materials with enhanced properties. Variables such as solution concentration, synthesis temperature and time, surfanctants and precursors were also explored in the hydrothermal process. In order to obtain nanostructured thin films using the chemical bath deposition, two processing techniques were employed: spin-coating of powder suspensions over oxidized silicon substrates and nanostructured anisotropic growth directly from solution using zinc coated substrates. The nanowires and the functional nanostructured layers were characterized by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE - SEM), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS)... (Complete abstract click electronic access below)<br>Orientador: Maria Aparecida Zaghete Bertochi<br>Coorientador: Elson Longo<br>Banca: Antonio Ricardo Zanatta<br>Banca: Mônica Alonso Cotta<br>Banca: Talita Mazon Anselmo<br>Banca: Sidney José Lima Ribeiro<br>O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp<br>Doutor

Structures semi-conductrices à base de NiTiO3, et l'étude de leurs propriétés dans le but de les appliquer en photocatalyse. Une étude théorique et des simulations numériques ont été effectuées pour analyser les propriétés électroniques, vibrationnelles et optiques de NiTiO3 massif ou sous forme de clusters nanométriques. Les poudres NiTiO3 ont été synthétisées par sol-gel par réaction en phase solide, tandis que les films minces ont été obtenus par pulvérisation cathodique rf-magnétron. Les caractérisations de leurs propriétés physiques confirment l'obtention de NiTiO3 polycristallin dans sa phase ilménite. La détermination du gap électronique à 2,25 eV suggère la faisabilité de mise en oeuvre des matériaux synthétisés comme photocatalyseurs actifs sous irradiation en lumière visible. Cette fonctionnalité a été testée par la dégradation du bleu de méthylène en solution aqueuse en utilisant les couches minces de NiTiO3 sous irradiation visible, atteignant la dégradation de 60% de la concentration initiale du colorant en 300 minutes. En outre, l'électro-oxydation du méthanol a été réalisée en appliquant une tension externe sur une électrode contenant des poudres NiTiO3 dans des milieux alcalins. Les ions de Ni présents dans le catalyseur ont été identifiés comme des espèces actives et que l'oxydation des molécules organiques se produit sur la surface des sites de Ni3+. En résumé, NiTiO3 a été synthétisé sous forme de poudres et de films minces ayant des caractéristiques appropriées pour la photocatalyse hétérogène efficace et les capacités catalytiques de NiTiO3 ont été démontrées sur la photodégradation du bleu de méthylène et l'électrooxydation de méthanol<br>The thesis work is devoted to the synthesis of NiTiO3-based semiconductive structures, i.e. powders and thin films, and the investigation of their related properties with the aim of their applications in photocatalysis. Theoretical approach and numerical simulations of the electronic, vibrational and optical properties of bulk and nanosized NiTiO3 structures have been carried out in order to deepen the understanding of the experimental results. The synthesis of NiTiO3 powders has been achieved by sol-gel and solid state reaction, while NiTiO3 thin films have been grown by rf-sputtering.Characterizations on their structural, vibrational and optical properties confirm the stabilization of polycrystalline NiTiO3 in its ilmenite phase in both powders and thin films as well. The determination of a band gap at 2.25 eV suggests the feasibility to implement the synthesized materials as visible-light-active photocatalysts. This feature has been tested in thedegradation of methylene blue in aqueous solution using rf-sputtered NiTiO3 thin films irradiated with visible light,achieving the degradation of 60% of the initial concentration of the colorant in 300 minutes. In addition, the electro-oxidation of methanol has been accomplished by applying an external voltage on an electrode containing NiTiO3 powders in alkaline media. The Ni ions present in the catalyst have been identified as the active species with the oxidation of the organic molecules on the surface of Ni3+ sites. As a main achievement, NiTiO3 has been synthesized as powders and thin films with suitable characteristics for efficient heterogeneous photocatalysis and the catalytic capabilities of NiTiO3 have beendemonstrated on the photodegradation of Methylene Blue and the electro-oxidation of methanol

Preparing MoS2 films in mild conditions, using deposition of suspended MoS2 nanoplatelets onto the substrate is described. For this purpose, the nanosized MoS2 particles were obtained via restacking of MoS2 single layers produced by chemical exfoliation of bulk MoS2 crystals in liquid media. X-Ray diffraction study of the films showed that the basal planes of MoS2 crystallites are mainly oriented in the plane paral-lel to the substrate. Atomic force microscopy examination revealed the dependence of the film surface to-pography, as well as the roughness characteristics on the film thickness, which varied in the range of 0.03-2.2 m. Optical absorption spectra of the obtained MoS2 films were found to contain the same absorption bands as the spectra of thin natural MoS2 single crystals. Dark conductivity of the films was determined to be ~ 10–3 S∙сm–1 at 300 K. The present MoS2 films were found to be photosensitive in the range of 300-800 nm, providing the maximum value of photocurrent under photoexcitation at ~ 440 nm. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35055

This thesis is concerned with the influence of nanoscale boundaries and interfaces upon the electronic processes that occur within both organic and inorganic semiconductors. Photoluminescent polymers, highly conducting polymers and nanoscale inorganic semiconductors have been investigated using state-of-the-art ultrafast optical techniques, to provide information on the sub-picosecond photoexcitation dynamics in these systems. The influence of dimensionality on the excitation transfer dynamics in a conjugated polymer blend is studied. Using time-resolved photoluminescence spectroscopy, the transfer transients both for a three-dimensional blend film, and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of a SnS2 matrix have been measured. A comparison of the experimental data with a simple, dimensionality-dependent model is presented, based on point dipole electronic coupling between electronic transition moments. Within this approximation, the energy transfer dynamics are found to adopt a three-dimensional character in the solid film, and a two-dimensional nature in the monolayers present in the SnS2 -polymer nanocomposite. The time-resolved conductivity of isolated GaAs nanowires has been investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs, before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility has been extracted using the Drude model for a plasmon and is found to be remarkably high, being roughly one third of that typical for bulk GaAs at room-temperature and indicating the high quality and low bulk defect density in the nanowires studied. Finally, the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers, P3HT and MDMO-PPV, blended with PCBM are presented. The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the π−π* absorption peak (560 nm for P3HT), and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which is attributed to the presence of a charge transfer complex. In addition, the excitation-fluence dependence of the photoconductivity is studied over more than four orders of magnitude. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived positive polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times (~ ps) the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ~0.1cm2 V−1 s−1. At later times, a sub-linear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer timescale (> 1 μs).

Thesis (Ph. D.)--University of California, Riverside, 2010.<br>Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed May 14, 2010). Includes bibliographical references. Also issued in print.

Les cellules photovoltaiques proposent une solution au problème énergétique en raison de leur source inépuisable: le soleil. Plusieurs types de cellules, qu'elle soient inorganiques ou organiques, sont étudiées, avec comme objectif d'obtenir de hauts rendements pour de faibles coûts. Dans ce contexte, ce travail de thèse se propose d'étudier des cellules solaires hybrides nanostructurées à base de nanofils de silicium et de matériaux organiques afin de bénéficier des avantages de ces différents matériaux. La morphologie controlée de la croissance des nanofils de silicium par dépôt chimique en phase vapeur assisté par plasma (PECVD) via un procédé Vapeur-Liquide-Solide est présentée. Le mélange de matériaux organiques est ensuite déposé sur les nanofils de silicium par un procédé d'enduction par centrifugation. Dans ce type de cellules hybrides, les nanofils de silicium jouent le rôles de matériaux accepteurs ou aident à l'absorption de la lumière. Pour améliorer les performance de ces cellules, il est nécessaire d'optimiser la qualité du réseau de nanofils par une gravure chimique visant à éliminer les traces de catalyseur résiduelles ainsi que l'oxyde natif du silicium. Cet effet de la gravure a été largement étudié et discuté. De plus les propriétés d'accepteur d'électrons des nanofils de silicium à base de catalyseurs de Bismuth ont été étudiées. Les résultats montrent clairement le potentiel de ce type de cellules, notamment 1) l'augmentation de la conversion de lumière par l'amélioration de l'efficacité du rendement quantique pour les grandes longueurs d'onde, 2) l'utilisation d'une grande variété de nanofils avec des morphologies et propriétés électriques finement controlées<br>Solar cells are an exciting alternative energy technology due to the infinite energy source, the Sun. Many types of solar cells based on inorganic or organic materials are currently developed with the objective of higher efficiency and lower cost. In this context, this thesis suggests to study nano-structured hybrid solar cells based on silicon nanowires (SiNWs) and organic active materials to benefit advantages of both materials. SiNWs are grown by PECVD on transparent conducting oxide via Vapor-Liquid-Solid (VLS) mechanism with careful control of their nano-morphology. The organic materials made of polymers or blend polymers are then deposited by spin-coating on top of SiNWs. In these hybrid solar cells the SiNWs are used as light-trapping medium and/or electron acceptor material. For better solar cell performance, the optimization of SiNWs array is carried out by removing residual catalyst and etching parasitic hydrogenated amorphous silicon. Their effects on hybrid solar cells have been fully analyzed and discussed. Furthermore, the electron-acceptor properties of the nano-structured SiNWs have been estimated with Bismuth-doped n-type SiNWs. The results clearly reveal the potential of this type of hybrid solar cells, namely, 1) power conversion efficiency improvement by enhancing external quantum efficiency in longer wavelength regime and 2) variety uses of SiNWs by tuning their electrical property and morphology

Cette thèse présente une étude des propriétés électroniques de systèmes de basse dimension à base de métaux et de semiconducteurs. La première partie de l'étude traite le confinement de l'état de Shockley dans des nanostructures tridimensionnelles d'Ag(111), par des mesures STM/STS à très basse température (5 K). Nous avons d'abord analysé en détail la structure en énergie et la distribution spatiale des modes confinés. Nous avons ensuite mis à profit la nature discrète du spectre en énergie pour étudier le temps de vie des quasiparticules. Un comportement typique de liquide de Fermi est mis en évidence, et nous montrons que le mécanisme de diffusion dominant est associé au couplage électron-phonon. La contribution extrinsèque provenant du confinement partiel de l'onde électronique a également été obtenue. Une loi d'échelle est observée avec la taille des nanostructures, ce qui permet d'extraire un coefficient de réflexion plus important que dans de simples ilôts monoatomiques. La seconde partie de l'étude est consacrée aux couches ultra-minces semiconductrices obtenues par dépôts d'alcalins (K, Rb, Cs) sur la surface Si(111):B-[racine]3. Ce travail résout la controverse concernant la nature de l'état fondamental de ce système, et notamment l'origine de la reconstruction 2[racine]3 obtenue à la saturation du taux de couverture. La compréhension en amont de la structure cristallographique permet d'élucider les propriétés électroniques. Nous montrons qu'une approche à un électron, conduisant à un isolant de bandes, décrit le système de manière convaincante, malgré l'indication de forts effets polaroniques. Ce résultat est le fruit d'une étude approfondie combinant des techniques diverses et complémentaires (LEED, ARPES, XPS, STM/STS et calcul DFT)<br>This thesis is devoted to the electronic properties of low-dimensional systems based on metal and semiconducting materials. The first part deals with the Shockley state confinement in Ag(111) nanostructures, by means of very-low temperature (5 K) STM/STS measurements. We study the electronic structure and spatial distribution of the confined modes. Then the discrete nature of the electronic spectrum allows one to yield the quasiparticule lifetime. A Fermi-liquid behaviour is evidenced and we show that the dominant decay mechanism is attributed to the electron-phonon coupling. The extrinsic contribution arising from the partial confinement of the electronic wave is obtained as well. A scaling law with the nanostructure width is demonstrated, from which we deduce a higher reflection amplitude than in monoatomic islands. In the second part of the thesis, we study semiconducting ultra-thin films produced by alkali (K, Rb, Cs) deposition on the Si(111):B-[root of]3 surface. This work solves the controversy concerning the ground state of this system, and especially the nature of the 2[root of]3 surface recontruction obtained at saturation coverage. Prior understanding of the crystallographic structure allows to elucidate the electronic properties. We show that a one-electron picture, leading to a band insulator scenario, gives a good description of the system, in spite of strong polaronic effects. This conclusion results from an in-depth, combined study of complementary techniques (LEED, ARPES, XPS, STM/STS and DFT calculations)

The push for electronic devices on smaller and smaller scales has driven research in the direction of transition metal dichalcogenides (TMD) as new ultra-thin semiconducting materials. These ‘two-dimensional' (2D) materials are typically on the order of a few nanometers in thickness with a minimum all the way down to monolayer. These materials have several layer-dependent properties such as a transition to direct band gap at single-layer. In addition, their lack of dangling bonding and remarkable response to electric fields makes them promising candidates for future electronic devices. For the purposes of this work, two 2D TMDs were studied, MoS2 and MoTe2. This dissertation comprises of three sections, which report on exploration of charge lifetimes, investigation environmental stability at elevated temperatures in air, and establishing feasibility of UV laser annealing for large area processing of 2D TMDs, providing a necessary knowledge needed for practical use of these 2D TMDs in optoelectronic and electronic devices. (1) A study investigating the layer-dependence on the lifetime of photo-generated electrons in exfoliated 2D MoTe2 was performed. The photo-generated lifetimes of excited electrons were found to be strongly surface dependent, implying recombination events are dominated by Shockley-Read-Hall effects (SRH). Given this, the measured lifetime was shown to increase with the thickness of exfoliated MoTe¬2; in agreement with SRH recombination. Lifetimes were also measured with an applied potential bias and demonstrated to exhibit a unique voltage dependence. Shockley-Read-Hall recombination effects, driven by surface states were attributed to this result. The applied electric field was also shown to control the surface recombination velocity, which lead to an unexpected rise and fall of measured lifetimes as the potential bias was increased from 0 to 0.5 volts. (2) An investigation into the environmental stability of exfoliated 2D MoTe2 was conducted using a passivation layer of amorphous boron nitride as a capping layer for back-gated MoTe2 field effect transistor (FET) devices. A systematic approach was taken to understand the effects of heat treatment in air on the performance of FET devices. Atmospheric oxygen was shown to negatively affect uncoated MoTe2 devices while BN-covered FETs showed remarkable chemical and electronic characteristic stability. Uncapped MoTe2 FET devices, which were heated in air for one minute, showed a polarity switch from n- to p-type at 150 °C, while BN-MoTe2 devices switched only after 200 °C of heat treatment. Time-dependent experiments at 100 °C showed that uncapped MoTe2 samples exhibited the polarity switch after 15 min of heat treatment while the BN-capped device maintained its n-type conductivity. X-ray photoelectron spectroscopy (XPS) analysis suggests that oxygen incorporation into MoTe2 was the primary doping mechanism for the polarity switch. (3) The feasibility of UV laser annealing as a post-process technique to sinter 2D crystal structures from sputtered amorphous MoS2 was explored. Highly crystalline materials are sought after for their use in electron and opto-electronic devices. Sputtered MoS2 has the advantage of potential for large area deposition and high scalability, however, it requires high temperatures (>350 °C) for their crystalline growth. Which creates difficulty for devices grown on polymer substrates. Low-temperature and room temperature deposition results in amorphous films which is detrimental for electric devices. A one-step lase annealing procedure was developed to provide amorphous to crystalline conversion of nanometer thin MoS2 films. Samples were annealed using an unfocused laser beam from a KrF (248 nm) excimer source. The power density was found to be 1.04 mJ/mm2. Raman analysis of laser annealed MoS2 was shown to exhibit a significant improvement of the 2D MoS2 crystallinity compared to as-deposited films on both SiO2/Si, as well as polydimethylsiloxane (PDMS) substrates. Annealed samples showed improvement of their conductivity on an order of magnitude. A top-gated FET device was fabricated on flexible PDMS substrates using Al2O3 as a gate oxide. Measured field effect mobility of annealed samples showed significant improvement over as-deposited devices.

Les spectroscopies de diffusion inélastique de la lumière (Raman/Brillouin) sont un outil versatile qui permet d'étudier les phonons thermiques de la matière à différentes échelles. Dans les milieux nano-granulaires, l'étude des phonons acoustiques dont la longueur d'onde est grande devant le diamètre D des grains (?/D &gt;&gt; 1) permet de caractériser l'élasticité macroscopique gouvernée par la loi du contact de Hertz. La validité de la loi de contact est étudiée pour des poudres d'oxyde constituées de nanoparticules sphériques d'une taille de quelques nanomètres. Lorsque la demi-longueur d'onde des phonons acoustiques devient égale à la dimension du confinement (diamètre D pour les sphères, épaisseur e pour une plaquette), la propagation n'est plus possible et un phénomène de résonance mécanique apparaît. La spectroscopie Raman basse fréquence a été utilisée pour caractériser les modes de vibration acoustique de nanoplaquettes semiconductrices habillées d'un « manteau » organique. Lorsque l'épaisseur est suffisamment faible (e ~1 nm) une forte déviation de la fréquence de résonance est observée par rapport au modèle de la plaquette libre, attribuée à la présence des molécules organiques et est interprétée par un effet nano-balance. Lorsque l'objet confinant est un nano-dimère métallique, une hybridation plasmonique et acoustique des nanoparticules ont lieu conjointement. L'excitation résonante du plasmon dimèrique permet d'observer à l'échelle d'un dimère unique la diffusion par les modes de vibration dipolaire hybridé l=1 ainsi que les modes non hybridés de moment angulaire l &gt;2, interdits par les règles de sélection précédemment établies pour ce régime de taille<br>Inelastic light scattering spectroscopies (Raman/Brillouin) are a versatile tool to study thermal phonons at various scales. In nano-granular media, the study of acoustic phonons with a wavelength much greater than the grain diameter D (?/D &gt;&gt; 1) allows one to characterize the macroscopic elasticity governed by Hertz law of the contact. The validity of Hertz law is studied for powders made of oxide nanoparticles a few nanometers in diameter. When the phonon half-wavelength reaches the confinement dimension (diameter D for spheres, thickness e for plates) propagation is forbidden and mechanical resonances occur. Low frequency Raman spectroscopy has been used to characterize the acoustic resonances of semiconducting nanoplatelets “dressed” with an organic surfactant layer. When the thickness becomes thin enough (e ~ 1 nm), the resonance frequency is significantly downshifted compared to a free platelet, attributed to a mass load effect due to the organic molecules. When the confining object is a metallic nano-dimer, both plasmonic and acoustic hybridization occur at the same time. The resonant excitation of the dimeric plasmon allows one to observe down to single nano-object scale the inelastic scattering by dimer hybridized dipolar vibration modes l=1 as well as non-hybridized modes with higher angular momentum l &gt;2, known to be Raman inactive in this size range according to previously established selection rules. Possibilities for a new plasmon-vibration coupling mechanism are discussed

As solar hydrogen is a sustainable and environmental friendly energy carrier, it is considered to take the place of fossil fuels in the near future. Solar hydrogen can be generated by splitting of water under solar light illumination. In this study, the use of nanostructured hematite thin-film electrodes in photocatalytic water splitting was investigated. Hematite (á-Fe2O3) has a narrow band-gap of 2.2 eV, which is able to utilise approximately 40% of solar radiation. However, poor photoelectrochemical performance is observed for hematite due to low electrical conductivity and a high rate of electron-hole recombination. An extensive review of useful measures taken to overcoming the disadvantages of hematite so as to enhance its performance was presented including thin-film structure, nanostructuring, doping, etc. Since semiconductoring materials which exhibit an inverse opal structure are expected to have a high surface-volume ratio, unique optical characteristics and a shorter distance for photogenerated holes to travel to the electrode/electrolyte interface, inverse opals of hematite thin films deposited on FTO glass substrate were successfully prepared by doctor blading using PMMA as a template. However, due to the poor adhesion of the films, an acidic medium (i.e., 2 M HCl) was employed to significantly enhance the adhesion of the films, which completely destroyed the inverse opal structure. Therefore, undoped, Ti and Zn-doped hematite thin films deposied on FTO glass substrate without an inverse opal structure were prepared by doctor blading and spray pyrolysis and characterised using SEM, EDX, XRD, TGA, UV-Vis spectroscopy and photoelectrochemical measurements. Regarding the doped hematite thin films prepared by doctor blading, the photoelectrochemical activity of the hematite photoelectrodes was improved by incorporation of Ti, most likely owing to the increased electrical conductivity of the films, the stabilisation of oxygen vacancies by Ti4+ ions and the increased electric field of the space charge layer. A highest photoresponse was recorded in case of 2.5 at.% Ti which seemed to be an optimal concentration. The effect of doping content, thickness, and calcination temperature on the performance of the Ti-doped photoelectrodes was investigated. Also, the photoactivity of the 2.5 at.% Ti-doped samples was examined in two different types of electrochemical cells. Zn doping did not enhance the photoactivity of the hematite thin films though Zn seemed to enhance the hole transport due to the slow hole mobility of hematite which could not be overcome by the enhancement. The poor performance was also obtained for the Ti-doped samples prepared by spray pyrolysis, which appeared to be a result of introduction of impurities from the metallic parts of the spray gun in an acidic medium. Further characterisation of the thin-film electrodes is required to explain the mechanism by which enhanced performance was obtained for Ti-doped electrodes (doctor blading) and poor photoactivity for Zn and Ti-doped samples which were synthesised by doctor blading and spray pyrolysis, respectively. Ti-doped hematite thin films will be synthesised in another way, such as dip coating so as to maintain an inverse opal structure as well as well adhesion. Also, a comparative study of the films will be carried out.

博士<br>國立臺灣科技大學<br>電子工程系<br>101<br>In this dissertation, a detailed characterization focusing on the surface morphology, stoichiometry, structural, orientations and vibration modes of the metal-oxide thin film nanostructures of alpha-Bi2O3, c-In2O3 and NiO with one-dimensional morphology have been carried out by means of field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), and micro-Raman scattering. The bismuth oxide and indium oxide thin-film nanostructures have been grown by vapor transport method driven with vapor-liquid-solid mechanism on Si(100) substrate. Nickel oxide thin-film nanostructures were grown by hot-filament chemical vapor deposition method on sapphire(100) substrates. The optical properties of these metal-oxide thin film nanostructures have been investigated. Several optical characterization techniques, such as thermoreflectance (TR) and photoluminescence (PL) were performed to characterize the optical properties of these samples. The near-band-edge and above-band-edge transitions, such as excitonic transitions, direct band gap and direct band gap with Burstein-Moss shift (BMS) effect of degenerate semiconductor have been determined by TR. PL measurements revealed the near-band-edge emissions and defect emissions of these metal-oxide thin film nanostructures. The visible active semiconducting alpha-Bi2O3 and c-In2O3 may be applied in solid-state white lighting optoelectronics. Furthermore, the excitonic emissions of c-In2O3 and NiO revealed similar characters, which are also similar to other transparent conducting oxide (i.e. ZnO).

博士<br>國立臺灣大學<br>物理研究所<br>97<br>Motivated by a desire to understand the basic concepts of one-dimensional nanostructure growth, the research described in this thesis aims at understanding the basic mechanisms controlling the synthesis and formation of a specific group of II-VI semiconducting nanostructures. In particular, this thesis examines one-dimensional nanostructures and different morphologies of semiconductors that lead to the novel properties of the materials at the nanoscale. In order to understand how to manipulate the properties of the grown nanostructures, this thesis focuses on having an understanding of the growth mechanism that dictates the morphology and structure. In addition, we also try to understand the impact changes on the nanoscopic scale of the nanomaterials due to the macroscopic setup in the experiment. Having a better understanding and exerting more precise control over the growth of nanomaterials will allow a higher level of selectivity, more control over dimensionality and the type of morphology, easier manipulation, and the simpler incorporation of these structures into a nanotechnological device. In general, this thesis covers the synthesis and characterization of the following nanomaterials: (1) ZnO nanodendrites and nanotowers, (2) ZnSe nanowires, (3) ZnO nanobottles decorated with ZnO nanotips, (4) metal (Zn, Sn) nanotubes and metal (Cu, Ag) nanotube/nanowire junctions, and (5) ZnO nanotubes. Efforts have been made to pinpoint the underlying science and to exploit their possible engineering applications.

博士<br>國立清華大學<br>材料科學工程學系<br>101<br>Synthesis, characterizations, and physical properties of CrSi2 and CdTe-based nanostructures have been investigated. Both materials have attracted intensive research effort in recent years, owing to their semiconducting properties and various practical applications. The design and development in functional nanodevices based on these two materials are of current interest. Furthermore, the possible further advanced applications of these structures are also proposed. Free-standing CrSi2/SiO2 nanocables have been synthesized via a simple atmospheric pressure chemical vapor deposition (APCVD) method. High quality nanocables were produced by a chemical vapor transport based method without using metal catalysts. The nanocables were formed by a direct reaction of CrCl2 vapor and a Si substrate in one single step. Structural characterization confirms the core of these nanocables to be hexagonal CrSi2, grown in the [001] direction. The room temperature ferromagnetism in CrSi2/SiO2 nanocables is discovered for the first time and the hysteresis loops show shape anisotropy effect when the applied magnetic field is perpendicular or parallel to the substrate. Through first-principles calculation, we found that the Cr atoms at the interface between CrSi2 and SiO2 layers possess a significant high saturation magnetization up to 2.202 B, while the Cr atoms in the middle of CrSi2 layer have a negligible magnetic moment (0.002 B). The calculated total saturation magnetization of the measured sample is close to the measured value obtained by superconducting quantum interference device (SQUID). The room temperature ferromagnetism in CrSi2/SiO2 nanocables is attributed to unpaired Cr atoms at the interface and high surface-to-volume ratio of these 1D nanostructures. Surface spins of pure CrSi2 nanowires after the removal of outer SiO2 layer also contribute large magnetic moment. Moreover, a comparison of measured values for CrSi2/SiO2 nanocables with different aspect ratios indicates that the magnetization is indeed proportional to interface area. The results obtained from the present fundamental studies shall lend substantial support to the development of future dilute magnetic semiconductor devices. Thermoelectric and mechanical properties of an individual CrSi2 nanowiwre were investigated. ZT value ~0.30+/-0.01 for a single CrSi2 NW with ~70 nm in diameter can be directly assessed by employing Harmon method. A remarkable enhancement of ~15 % for ZT value up to ~0.35+/-0.01 can be achieved due to reduced thermal conductivity by roughening the surface of NW because of higher surface-to-volume ratio. ZT value was found to increase with decreasing diameter. On the other hand, elastic modulus of CrSi2 NWs is first investigated in the present study. The elastic modulus was found to be independent of diameter and the averaged modulus value of ~225 GPa is obtained. The enhancement of the thermoelectric properties of CrSi2 nanowires with robust mechanical properties may lead to their practical applications in advanced thermoelectric devices in the future. A zinc blende structure based nanogenerator were developed. Here, free-standing CdTe microwires/nanowires (MWs/NWs) have been synthesized with a facile one-step hydrothermal method. The structural analysis shows that the synthesized materials (which consist of wires and particles) are composed of multiple phases with much more zinc blende CdTe than wurtzite CdTe. This coexistence of two phases was also confirmed using high resolution transmission electron microscopy (HRTEM). A laterally packaged nanogenerator (NG) can generate up to 0.3 V and 40 nA when strain is applied on the individual MW. Due to the high stability, the MW can be used in piezoelectric applications under various circumstances. Through these combined properties, zinc blende-based CdTe material appears to be promising for application in self-powered system in energy harvesting.

博士<br>國立交通大學<br>電子物理系所<br>98<br>Various semiconductor nanowires have been synthesized by using either physical or chemical growth methods in the past decades. These nanowires applied to single electron transistors, field-effect transistors, optoelectronics, and nanoelectronics through top-down or bottom-up assembling approaches has been immediately demonstrated. Because of most nanoscale electronics having two-probe configuration as the source and drain electrodes, identification and determination of intrinsic electrical properties of nanowires and the contribution of nanocontact through a systematic procedure of this two-probe approach become very important. Although a lot of two-probe nanoelectronics and applications have been demonstrated in previous reports, the electrical properties bringing either from the nanocontacts or from the nanowires have not been uncovered clear yet. In this work, a two-probe technique was adopted to explore electrical properties of ZnO, InP, GaP nanowires and polyaniline nanofibers. The first portion of this work is to utilize high quality ZnO nanowires to fabricate two-probe nanodevices and to survey the impact of nanocontact on nanowire based nanoelectronics. According to temperature behaviors of current-voltage curves and resistances, the devices could be grouped into three types, including two Ohmic contacts, one Ohmic and one Schottky contacts, and two back-to-back Schottky contacts. The nanocontact could be treated as disordered system and be explained by Mott variable range hopping model for electrons of the form . The exponential parameters of Mott variable range hopping theory rises from 2 to 4 with an increase of specific contact resistivity at room temperature, implying a change from one- to three-dimensional hopping. Moreover, after understanding how to distinguish the nanowire- and contact-dominated nanodevices, we demonstrate that the two-probe measurement can be applied to the exploration of the intrinsic properties of semiconductor nanowires. This two-probe measurement approach also works on highly resistive nanowires without an Ohmic contact issue. By using this method, electron transport behaviors, resistivity, and carrier concentrations of ZnO, InP, and GaP semiconductor nanowires have been investigated. The interface problems in nanowire-based electronics play important roles due to the reason that the reduced contact area in nanoelectronics multiplies enormously the contribution of electrical contact properties. The second portion is to illustrate the sensitivity difference in response to light and oxygen gas between the nanowire- and contact-dominated InP nanowire devices. By using a standard electron-beam lithography technique, two-probe InP nanowire devices were fabricated. Although the InP were picked up from the same source sample and the dimensions of the nanowires and nanodevices were also kept the same, the room-temperature resistance of these devices varied considerably. It was conjectured the difference of room-temperature comes from the contribution of contact resistance. According to the temperature behaviors, the nanowire devices can be categorized into nanowire- and contact-dominated ones. The temperature dependent resistances follow the thermally activated and three-dimensional Mott variable range hopping transport at high and low temperatures, respectively. Both nanowire- and contact-dominated devices were exposed to light and oxygen gas to see any difference. In comparison with the nanowire-dominated devices, the contact-dominated InP nanowire devices always exhibit a much higher ratio of resistance changes in response to either light or oxygen gas exposures. The last portion of this work is to study the electrical transport of polyaniline nanofibers. Polyaniline nanofibers were synthesized by using polymerization at the interface of immiscible solvents. They exhibit a uniform nanoscale morphology rather than agglomeration with granular structures as that produced via conventional chemical oxidation. The as-synthesized polyaniline nanofibers are doped (dedoped) with an HCl acid (NH3 base) and their temperature behaviors of resistances all follow an exponential function with an exponent of T-1/2. To achieve the measurement of conduction mechanism in a single nanofiber, the dielectrophoresis technique is implemented to position nanofibers on top of and across two Ti/Au electrodes patterned by electron-beam with a nanogap of 100-200 nm. Their temperature behaviors and electric field dependences are unveiled and the experimental results agree well with the theoretical model of charge-energy-limited-tunneling. Through fitting to this transport model, the size of conductive grain, the separation distance between two-grains, and the charging energy per grain in a single polyaniline nanofiber are estimated to be about 5 nm, 3 nm, and 78 meV, respectively. This nanotechnological approach has been applied to determination of mesoscopic charge transport in the polyaniline conducting polymer.

博士<br>國立臺灣大學<br>化學研究所<br>94<br>As the improvement in the nanoscience and nanotechnology, the synthesis and technology for the fabrication of nanosized materials have great development and become more complete than the past few years. In addition, it also has more structural changes in its dimensionality and size. In this thesis, four kinds of different one-dimensional semiconducting nanomaterials have been successfully fabricated using different physical or chemical synthetic methods and these nanomaterials are single-crystalline silicon nanowires, amorphous silicon dioxide nanotubes, single-crystalline tin dioxide nanobelts and single-crystalline copper indium diselenides nanorods. By mixing the pure silicon powders and the catalysts (including metal or silicon dioxide powders) and with assistance for the laser ablation technology, the single-crystalline silicon nanowires (SiNWs) can be fabricated with the diameters reach 5-40 nm and the lengths extend to tens of micrometers. While the metal powders (like Fe, Ru and Pr) are used as the catalysts for the syntheses of SiNWs, the most stable Si {111} facets are grown and the growth direction for the SiNWs is parallel to the facets growth direction, i.e., the wire growth direction is <111> for the metal-catalyzed SiNWs. Such SiNWs are grown via the typical vapor-liquid-solid (VLS) growth mechanism that existing the eutectic liquid droplet formation during the synthesis. On the other hand, as the silicon dioxide (SiO2) used as the catalysts for the fabrication of SiNWs, the Si {111} facets also grow; however, the wire growth direction, as the <112> direction, is perpendicular to the lattice plane growth direction. The SiNWs catalyzed by SiO2 follow an oxide-assisted (OA) growth mechanism during their growths. Furthermore, based on the different chamber pressure used during the experiments, it can be found that with the increasing of the pressure, the diameters for SiNWs enlarge and the lengths for SiNWs shorten. The Raman spectra for the different diameter SiNWs are measured and the most intense F2g phonon mode, which is located ~ 520 cm-1, can be found that with decreasing for the diameter of SiNWs, the red-shifted behavior of the F2g mode is clearly seen from the corresponding Raman spectra. By using the chemical vapor deposition method, the one-dimensional silicon dioxide nanotubes (SiO2NTs) are produced on the silicon substrate coated with Au nanoparticles which are preannealed at high temperature. The SiO2NTs can reach to 40-100 nm in diameters and extend to few micrometers in lengths. According to the electron diffraction (ED) pattern for the SiO2NTs, it can be confirmed that these nanotubes are amorphous. Besides this, the nanotubes can be separated into two groups, as thick- and thin-walled SiO2NTs, based on their different synthesis temperatures. Moreover, with the different reaction temperatures, the different shapes of Au nanoparticles are grown and this causes the different thicknesses of the SiO2NTs. With detailed analysis on the SiO2NTs, it can be figured out that the SiO2 species are diffused from the Au {111} facets and the walls of SiO2NTs are along the <022> direction of the thick-walled SiO2NTs while the <200> direction of the thin-walled SiO2NTs. Moreover, with the higher reaction temperature, the amorphous silicon dioxide nanowires (SiO2NWs) are synthesized on the silicon substrate. The Raman spectra of SiO2NTs and micro-crystallite SiO2 powders are taken and used for the characterization and both have the intense Raman peak (Si-O phonon mode) at ~ 467 cm-1. With the thermal evaporation-condensation method, the high purity single-crystalline tin dioxide nanobelts (SnO2NBs) are fabricated via the thermal heating of tin monoxide (SnO) powders in high temperature. The SnO2NBs have their belt width for 30-90 nm, the belt thickness for 20-30 nm and the belt length for tens of micrometers. Based on the X-ray diffraction (XRD) measurements of the SnO2NBs, it can be confirmed that the nanobelts are the pure rutile tetragonal structures. From the high-resolved transmission electron microscopic images, the {111} lattice planes are clearly seen and the SnO2 nanobelt grows along the <130> direction indexed from the corresponding ED pattern. The growth of the SnO2NBs can be attributed to the self-disproportion reaction of SnO bulk powders via the vapor-solid (VS) growth mechanism. In the Raman spectrum measurement, the rutile SnO2NBs have good signal to noise ratio and the peaks at 475.9, 635.5, and 777.2 cm-1 are resolved which are corresponding to the Eg, A1g, and B2g phonon modes, respectively. The last part in this thesis is the fabrication of the copper indium diselenide nanorods (CuInSe2NRs) which are commonly used in the solar cell technology. During the synthesis works, two ways are used for producing the CuInSe2 nanostructures, including the laser ablation/anodic aluminum oxide (AAO) membranes and the solvothermal methods. In the laser ablation/AAO membranes method, the AAO membranes have the highly uniform pore distribution and the CuInSe2 species can diffuse into the hollow channels to form the eutectic composites with the metal catalysts coated on the membranes and grow the one-dimensional CuInSe2 nanorods. The diameters of the CuInSe2NRs can reach 150-200 nm, however, the lengths of the can only extend ~2 micrometers. On the other hand, the CuInSe2NRs can also be synthesized by the solvothermal method, but the total reaction time is needed for at least 36 hours. Due to the long reaction time, the better aspect ratio and the product yield for the CuInSe2NRs can be acquired. The CuInSe2NRs fabricated by the solvothermal method have the diameter size of 50-100 nm and the lengths can extend to tens of micrometers. Moreover, from the high resolution image, the {112} lattice planes are found in the nanorods and can be indexed that the nanorods grow along the <331> direction. The Raman spectrum for CuInSe2NRs is taken and can be found that the most intense A1 phonon mode located at 175.1 cm-1 is clearly verified. This is another evidence tells us that the nanorods are purely with the CuInSe2 structures for the chemical composition.

博士<br>國立臺灣大學<br>電機工程學研究所<br>100<br>In this thesis, we studied optoelectronic properties and electronic structures of one-dimensional (1-D) semiconducting/piezoelectric nanostructures with sizes beyond the quantum confinement regime and discussed their superior optoelectronic/photonic features as compared to their thin film of bulk counterpart. First of all, since 1-D nanostructures have subwavelength diameters and large aspect ratios, which combined with the high permittivity of semiconductors lead to a strong optical anisotropy, we report a novel optically anisotropic metamaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75o–85o), exhibiting giant in-plane birefringence and optical polarization degree in photoluminescence emission. The in-plane birefringence ( 0.11) of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of ZnO bulk. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission. Second, in 1-D nanostructures, with large surface-to-volume ratios and Debye lengths comparable to their diameters, their electronic and optoelectronic properties are strongly affected by the electronic structures at their surfaces. Here we systematically and in-depth investigated the correlation between electronic structures (especially at the surface) of 1-D (Er-doped) ZnO nanostrucrures and their optoelectronic properties through the following four subjects: (1) in situ probing the surface band bending (SBB) of the ZnO NWs using photoelectron spectroscopy in conjunction with the field-effect transistor measurements; (2) correlation between electronic structures of Er-Doped ZnO nanorod arrays and efficiency of 1.54 μm emission by studied by X-ray absorption spectroscopy; (3) enhanced near-band-edge emission of ZnO nanorods via the surface passivation; (4) correlation between photoresponse of ZnO nanobelts and the surface/interface effects. These studies are greatly beneficial for the 1-D nanostructure based device design of sensor and optoelectronic applications. Finally, since ZnO is the wurtzite polar semiconductor and has the electromechanical coupling effect, piezoelectric characteristics of well-aligned ZnO NWAs were investigated for energy-harvesting nanodevices via its piezoelectricity. Besides, lead zirconate titanate [PbZr1−xTixO3 (PZT)] is a typical piezoelectric material, so the PbZr02Ti0.8O3 NWAs were also studied. This study is useful for optimizing the performance for nanogenerator applications.

abstract: A distinct characteristic of ferroelectric materials is the existence of a reversible spontaneous polarization with the application of an electric field. The relevant properties ferroelectric lithium niobate surfaces include a low density of defects and external screening of the bound polarization charge. These properties result in unique surface electric field distribution with a strong electric field in the vicinity of domain boundaries, while away from the boundaries, the field decreases rapidly. In this work, ferroelectric lithium niobate (LN) is used as a template to direct the assembly of metallic nanostructures via photo-induced reduction and a substrate for deposition of ZnO semiconducting thin films via plasma enhanced atomic layer deposition (PE-ALD). To understand the mechanism the photo-induced deposition process the following effects were considered: the illumination photon energy and intensity, the polarization screening mechanism of the lithium niobate template and the chemical concentration. Depending on the UV wavelength, variation of Ag deposition rate and boundary nanowire formation are observed and attributed to the unique surface electric field distribution of the polarity patterned template and the penetration depth of UV light. Oxygen implantation is employed to transition the surface from external screening to internal screening, which results in depressed boundary nanowire formation. The ratio of the photon flux and Ag ion flux to the surface determine the deposition pattern. Domain boundary deposition is enhanced with a high photon/Ag ion flux ratio while domain boundary deposition is depressed with a low photon/Ag ion flux ratio. These results also support the photo-induced deposition model where the process is limited by carrier generation, and the cation reduction occurs at the surface. These findings will provide a foundational understanding to employ ferroelectric templates for assembly and patterning of inorganic, organic, biological, and integrated structures. ZnO films deposited on positive and negative domain surfaces of LN demonstrate different I-V curve behavior at different temperatures. At room temperature, ZnO deposited on positive domains exhibits almost two orders of magnitude greater conductance than on negative domains. The conductance of ZnO on positive domains decreases with increasing temperature while the conductance of ZnO on negative domains increases with increasing temperature. The observations are interpreted in terms of the downward or upward band bending at the ZnO/LN interface which is induced by the ferroelectric polarization charge. Possible application of this effect in non-volatile memory devices is proposed for future work.<br>Dissertation/Thesis<br>Ph.D. Physics 2011

The next generation of optoelectronic applications like stimuli-responsive sensors, functional displays or nanophotonic circuits demands a basic understanding of lightmatter interactions on the nanoscale. Top-down fabrication has been employed in the past to demonstrate coherent energy transfer in functional nanostructures, yet these fabrication methods are problematic due to their limited scalability and high costs as well as the high optical losses. This work adapted physical principles like radiation properties of metallic nanoantennas and Bragg diffraction in periodic nanostructures and realized these concepts using bottom-up self-assembly methods based on colloidal chemistry. With this approach, single plasmonic nanoparticles and semiconductor quantum emitters were co-assembled into complex structures. This work took the colloidal concept from plasmonics and introduced quantum dots in order to characterize the radiative and non-radiative decay processes as well as the arising light-matter interactions. Due to electromagnetic coupling between the components, hybridized modes were detected instead of the single particle resonances observed in the isolated case. It was furthermore shown that these colloidal building blocks can be assembled into functional optical grids on a large scale using template-assisted self-assembly. Thus, this work established spectroscopic principles for self-assembled colloidal building blocks that can be integrated in parallelized processes in the future.

The thesis consists of eight chapters of which the first chapter presents a brief overview of inorganic nanostructures. Synthesis and magnetic properties of MnO and NiO nanocrystals are described in Chapter 2, with emphasis on the low-temperature ferromagnetic interactions in these antiferromagnetic oxides. Chapter 3 deals with the synthesis and characterizations of nanocrystals of ReO3, RuO2 and IrO2 which are oxides with metallic properties. Pressure-induced phase transitions of ReO3 nanocrystals and the use of the nanocrystals for carrying out surface-enhanced Raman spectroscopy of the molecules form Chapter 4. Use of ionic liquids to synthesize different nanostructures of semiconducting metal sulfides and selenides is described in Chapter 5. Synthesis of Mn-doped GaN nanocrystals and their magnetic properties are described in Chapter 6. A detailed investigation has been carried out on the growth kinetics of nanostructures of a few inorganic materials by using small-angle X-ray scattering and other techniques (Chapter 7). The study includes the growth kinetics of nanocrystals of Au, CdS and CdSe as well as of nanorods of ZnO. Results of a synchrotron X-ray study of the formation of nanocrystalline gold films at the organic-aqueous interface are also included in this chapter. Chapter 8 discuses the use of the organic-aqueous interface to generate Janus nanocrystalline films of inorganic materials where one side of the film is hydrophobic and other side is hydrophilic. This chapter also includes the formation of nanostructured peptide fibrils at the organic-aqueous interface and their use as templates to prepare inorganic nanotubes.

The thesis consists of eight chapters of which the first chapter presents a brief overview of inorganic nanostructures. Synthesis and magnetic properties of MnO and NiO nanocrystals are described in Chapter 2, with emphasis on the low-temperature ferromagnetic interactions in these antiferromagnetic oxides. Chapter 3 deals with the synthesis and characterizations of nanocrystals of ReO3, RuO2 and IrO2 which are oxides with metallic properties. Pressure-induced phase transitions of ReO3 nanocrystals and the use of the nanocrystals for carrying out surface-enhanced Raman spectroscopy of the molecules form Chapter 4. Use of ionic liquids to synthesize different nanostructures of semiconducting metal sulfides and selenides is described in Chapter 5. Synthesis of Mn-doped GaN nanocrystals and their magnetic properties are described in Chapter 6. A detailed investigation has been carried out on the growth kinetics of nanostructures of a few inorganic materials by using small-angle X-ray scattering and other techniques (Chapter 7). The study includes the growth kinetics of nanocrystals of Au, CdS and CdSe as well as of nanorods of ZnO. Results of a synchrotron X-ray study of the formation of nanocrystalline gold films at the organic-aqueous interface are also included in this chapter. Chapter 8 discuses the use of the organic-aqueous interface to generate Janus nanocrystalline films of inorganic materials where one side of the film is hydrophobic and other side is hydrophilic. This chapter also includes the formation of nanostructured peptide fibrils at the organic-aqueous interface and their use as templates to prepare inorganic nanotubes.