Rozprawy doktorskie na temat „Semiconductor-Semiconductor Core Shell Nanomaterials”

This thesis explores how the carrier dynamics within semiconductor nanocrystals can be directly engineered through specific core-shell design. Emphasis is placed on how material characteristics, such as strain or alloying at a core-shell interface, can influence the exciton energies and the recombination dynamics within semiconductor nanocrystals. This study synthesises type-II heterojunction ZnTe/ZnSe core-shell nanocrystals via a diethyl zinc-free synthesis method, producing small size distributions and quantum yields as high as 12%. It was found that the 7% lattice mismatch between the core and shell materials places limitations on the range of structures in which coherent growth is achieved. By developing compositional and strained atomistic core-shell models a variety of physical and optical properties could be simulated and has led to a clear picture of the core-shell architecture to be built. This characterisation provides evidence that the low bulk modulus ZnTe cores are compressed by the higher bulk modulus smaller lattice constant ZnSe shells. Further studies show how strain is manifested in structures with 'sharp' core-shell interfaces and how intentional alloying the interface can influence the growth and exciton energies. A (2-6)-band effective mass model was able to distinguish between the as-grown 'sharp' and 'alloyed' interfaces which indicated that strain accentuates the redshift of the excitonic state whilst reduced strain within an alloyed interface sees a reduced redshift. Single nanocrystal spectroscopy investigations of brightly emitting single graded alloyed nanocrystals and of a size series of commercially available CdSe/ZnS nanocrystals showed almost no fluorescence intermittency (nearly 'non-blinking'). These investigations also identified trion recombination as the main mechanism within the blinking 'off' state. Ultimately this thesis adds to the growing understanding of how specific core-shell architectures manipulate the electronic structure and develops techniques to identify specific material characteristics and how these characteristics influence the physical and optical properties within semiconductor nanocrystals.

Piezotronics is a new field, as first explored by Professor Zhong Lin Wang (Georgia Institute of Technology, Atlanta, USA), which describes the exploitation of the piezoelectric polarization and internal electric field inside semiconductor nanostructures by applying strain, to develop electronic devices with new functionality. Such concepts find applications in both III-V and II-VI semiconductor compounds, in optics, optoelectronics, catalysis, and piezoelectricity, sensors, piezoelectric transducers, transparent conductor and nanogenerators. In this work I explore the strain dependence of the piezoelectric effect in wurtzite ZnO crystals. The Linear and quadratic piezoelectric coefficients of III-V (GaP, InP, GaAs and InAs) wurtzite semiconductors are also calculated using ab-initio density functional theory. The polarization in terms of the internal anion–cation displacement, the ionic and dipole charges is written and the ab initio Density Functional Theory is used to evaluate the dependence of all quantities on the strain tensor. The piezoelectric effect of III–V semiconductors are nonlinear in the strain tensor. The quadratic piezoelectric coefficients and a revised value of the spontaneous polarization are reported. Furthermore, the ZnO nanowires is found to be non-linear piezoelectric effect and leads to predictions in some cases opposite to those obtained using the widely used linear model. The predicted magnitude of such coefficients are much larger than previously reported and of the same order of magnitude as those of III-N semiconductors. We also model the bending distortion created on a III-V wurtzite nanowire by an atomic force microscope tip induced deflection to calculate the piezoelectric properties of both homogenous and core shell structures. A number of combinations of III-V materials for the core and the shell of the nanowires, are shown a favour much increased voltage generation. The largest core voltages in core/shell combinations of InAs/GaP, InP/GaP, GaP/ InAs and GaP/InP are observed which can be theoretically 3 orders of magnitude larger than the typical values of ±3V in homogenous nanowires. Also considering properties such as bandgap discontinuity and mobility, III-V wurtzite core shell nanowires are candidates for high performance components in piezotronics and nanogeneration.

L’objectif de cette thèse est de développer un réseau de nanofils GaAs (coeur) / oxyde (coquille) pour la photoélectrolyse de l'eau. Pour cela, la géométrie des nanofils GaAs a été d’abord optimisée en ajustant différents paramètres expérimentaux de la croissance auto-catalysée de ces nanofils par Épitaxie par Jets Moléculaires. Nous avons ensuite étudié systématiquement l'oxydation de surface des nanofils GaAs et son effet négatif sur la croissance de la coquille. Nous avons donc développé une méthode dite d'encapsulation / désencapsulation d'une couche d'arsenic (As) amorphe qui protège les facettes des NFs de l'oxydation. Une étude physico-chimique a montré l'effet bénéfique d'une telle méthode sur la croissance de la coquille. La croissance d'une coquille de SrTiO3 sur des nanofils de GaAs a ensuite été réalisée. Des caractérisations approfondies de la croissance de la coquille de SrTiO3 sur les NFs de GaAs ont été réalisées. La plus grande partie de la structure pérovskite SrTiO3 était en relation d'épitaxie avec le réseau cristallin de GaAs. La dernière partie de cette thèse concerne l’utilisation de tels réseaux de nanofil GaAs / oxyde pour les dispositifs PEC où l'oxyde sert de couche de passivation. L'influence du dopage et de la morphologie des nanofils GaAs a d'abord été étudiée. Les propriétés des réseaux de nanofils de GaAs / SrTiO3 et de GaAs / TiO2 servant de photoélectrodes dans des dispositifs PEC sont étudiées
The objective of this PhD is to develop the network of GaAs (core) / oxide (shell) nanowires for solar water splitting. The geometry of the GaAs nanowires was firstly optimized by adjusting different experimental parameters of the self-catalyzed growth of these nanowires by molecular beam epitaxy. We then systematically studied the surface oxidation of the GaAs nanowires and its negative effect on the growth of the shell. We have therefore developed a method called the arsenic (As) capping / decapping method that protects the facets of nanowires from the oxidation. A physico-chemical study has shown the beneficial effect of such a method on the growth of the shell. The growth of a SrTiO3 shell on GaAs nanowires was then performed. In-depth characterizations of SrTiO3 shell growth on GaAs nanowires were carried out. Most of the SrTiO3 perovskite structure was in epitaxial relationship with the GaAs crystalline lattice. The last part of this thesis concerns the application of such GaAs / oxide nanowire networks to PEC devices where the oxide serves as a passivation layer. The influence of the doping and the morphology of GaAs nanowires was first studied. The properties of GaAs / SrTiO3 and GaAs / TiO2 nanowire networks used as photoelectrodes in PEC devices are finally studied

A great deal of interest has been dawn on the colloidal chemistry based semiconductor nanocrysallites, also known as quantum dots (QDs). Because of the strong quantum confinement, quantum dots have unique size-dependent optical properties, which are much more superior to the conventional organic fluorescence materials. In addition, strong chemical resistant makes inorganic semiconductor QDs ideal candidate for next-generation of bio-labels and drug delivery vehicles. This report presents a user friendly approach to synthesize high quality biocompatible CdSe QDs in aqueous solution. Size of the dots can be controlled by adjusting the temperature, pH of the solution, and ratio of the precursors. A thin CdS layer was grown on CdSe QDs, forming a CdSe/CdS core/shell structure, to improve the photoluminescence. In order to use these QDs in-vivo, a more chemically robust coating, silica, was grown on the core/shell structure QD. The optical properties of the QDs were characterized by absorption and photoluminescence spectra. X-ray diffraction and transmission electron microscopy were conducted to verify the QDs composition and structure.
Ph. D.

Group III-V semiconductor nanowires, showing novel properties, are promising building blocks for future applications. However, characterisation of individual NWs at the atomic level, which remains challenging, is critical to understand the detail growth mechanism. Therefore, this doctoral research aims, to develop a characterisation technique amenable to the specific configuration of a single nanowire, and to apply this technique to understand III-V nanowire growth. First, we developed a unique atom probe microscopy technique for directly measuring pristine nanowires on the growth substrate, which allows avoiding any damage introduced during sample preparation. Both experimental results and electrostatic simulations demonstrate the viability of this measuring approach to image the entire cross-section of a specimen, termed full tip imaging. Second, we characterised the distribution and relative concentration of dopants. The reconstruction parameters were determined based on crystallographic features. Background noises were removed based on multiple detector events. The concentration profile shows the dopants distributing radially inhomogeneous. Third, we investigated the vapour-liquid-solid and vapour-solid growth components in individual ternary InGaAs nanowires. A Ga-enriched core and In-enriched shell structure in the InGaAs nanowire has been found, which can essentially be related to the different growth mechanisms. This doctoral research advanced the atom probe microscopy characterisation of III-V nanowires. It developed a way to measure the pristine nanowire and increased the field-of-view by using the unique specimen preparation method. The atom probe microscopy results offer important compositional and structural information for the understanding of nanowire growth in order to realize applications in the future.

We have performed a first principles density functional theory (DFT) calculations to study the thermal conductivity in ZnO nanotubes, ZnO nanowires, and Si/Ge shell-core nanowires. We found the equilibrium configuration and the electric band structure of each nanostructure using DFT, the interatomic force constants and the phonon dispersion relations were calculated using DFPT as implemented in Quantum Espresso. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach using a Green's function method. All ZnO nanostructures studied exhibit semiconducting behavior, with direct bandgap at the Gamma point. The calculated values for the bandgaps were larger than the value of the bandgap of the bulk ZnO. We were able to identify phonon modes in which the motion of Zn atoms is significant when it is compared with the motion of oxygen atoms. The thermal conductivity depends on the diameter of the nanowires and nanotubes and it is dramatically affected when the nanowire or nanotube is doped with Ga. For Si/Ge nanowires, the slope and the curvature of acoustic modes in the phonon dispersion relation increases when the diameter increases. For nanowires with the same number of atoms, the slope and curvature of acoustic modes depends on the concentration of Si atoms. We were able to identify phonon modes in which the motion of core atoms is significant when it is compared with motion of atoms on the nanowire's shell. The thermal conductivity in these nanostructures depends on the nanowire's diameter and on the Si atoms concentration.

As the feature size of MOSFET is scaling down to nano-size, series of problems need to be overcome to continue Moore’ Law, which seems an impossible task with traditional bulk Si technology due to the physical limitation and various negative effects being subject to small feature size. The critical issues for both further improving the devices’ performance and lowering their cost lie in the exploration of substitutions for Si and the control of morphological and compositional properties of materials. group III-V nanowires due to its unique properties are considered as the building block for next-generation electronic devices. To fulfill these commercial applications with group III-V nanowires, a fundamental and quantitative understanding of growth-structure-property relationships is central to applications where nanowires exhibit clear advantages. Therefore, this doctoral research systematically investigates three different semiconductor nanowires: Au-seeded, self-seed and planar nanowires, in terms of elemental, morphological and structural aspects by taking advantage of cutting-edge technique atom probe tomography, and endeavor to unveil the correlation between nanowires’ intrinsic properties and performance. Based on the atom probe findings, the growth mechanism of Au-seeded and self-seeded nanowire have been systematically discussed, and new model has been proposed to explain the phenomena on the basis of density functional calculation. Moreover, the doping distribution in planar nanowires has also been carefully investigated, and the results demonstrate that the dopants can diffuse into the substrate which subsequently degrade the device performance due to parasitic channel effect, and accordingly, suggestions have been given to optimize the planar nanowire growth for improved dopant distribution. The outcomes of this project are expected to theoretically support high-quality nanowire synthesis for specific applications.

Gallium nitride (GaN) and its related semiconductor alloys are attracting tremendous interest for their wide range of applications in blue and green LEDs, diode lasers, high-temperature and high-power electronics. Nanomaterials such as InGaN/GaN core-shell three-dimensional nanostructures are seen as a breakthrough technology for future solid-state lighting and nano-electronics devices. In a core-shell LED, the active semiconductor layers grown around a GaN core enable control over a wide range of wavelengths and applications. In this thesis the capability for the heteroepitaxial growth of a proof-of-principle core-shell LED is advanced. A design that can be applied at the wafer scale using metalorganic vapor phase epitaxy (MOVPE) crystal growth on highly uniform GaN nanorod (NR) structures is proposed. This project demonstrates understanding over the growth constraints of active layers and dopant layers. The impact of reactor pressure and temperature on the morphology and on the incorporated InN mole fraction was studied for thick InGaN shells on the different GaN crystal facets. Mg doping and effectiveness of the p-n junction for a core-shell structure was studied by extensive growth experiments and characterization. Sapphire and Si substrates were used, and at all the stages of growth and fabrication. The structures were optimized to achieve geometry homogeneity, high-aspect-ratio, incorporation homogeneity for InN and Mg dopant. The three-dimensional nature of NRs and their light emission provided ample challenges which required adaptation of characterization and fabrication techniques for a core-shell device. Finally, an electrically contacted core-shell LED is demonstrated and characterized. Achieving a proof-of-principle core-shell device could be the starting point in the development of nanostructure-based devices and new physics, or in solving technical problems in planar LEDs, such as the polarization of emitted light, the quantum-confined Stark effect, efficiency droop, or the green gap.

Quantum dots are semiconductor nanocrystals that have tunable emission through changes in their size. Producing bright, efficient quantum dots with stable fluorescence is important for using them in applications in lighting, photovoltaics, and biological imaging. This study aimed to optimize the process for coating CdSe quantum dots (which are colloidally suspended in octadecene) with a ZnS shell through the pyrolysis of organometallic precursors to increase their fluorescence and stability. This process was optimized by determining the ZnS shell thickness between 0.53 and 5.47 monolayers and the Zn:S ratio in the precursor solution between 0.23:1 and 1.6:1 that maximized the relative photoluminescence quantum yield (PLQY) while maintaining a small size dispersion and minimizing the shift in the center wavelength (CWL) of the fluorescence curve. The process that was developed introduced a greater amount of control in the coating procedure than previously available at Cal Poly. Quantum yield was observed to increase with increasing shell thickness until 3 monolayers, after which quantum yield decreased and the likelihood of flocculation of the colloid increased. The quantum yield also increased with increasing Zn:S ratio, possibly indicating that zinc atoms may substitute for missing cadmium atoms at the CdSe surface. The full-width at half-maximum (FWHM) of the fluorescence spectrum did not change more than ±5 nm due to the coating process, indicating that a small size dispersion was maintained. The center wavelength (CWL) of the fluorescence spectrum red shifted less than 35 nm on average, with CWL shifts tending to decrease with increasing Zn:S ratio and larger CdSe particle size. The highest quantum yield was achieved by using a Zn:S ratio of 1.37:1 in the precursor solution and a ZnS shell thickness of approximately 3 monolayers, which had a red shift of less than 30 nm and a change in FWHM of ±3 nm. Photostability increased with ZnS coating as well. Intense UV irradiation over 12 hours caused dissolution of CdSe samples, while ZnS coated samples flocculated but remained fluorescent. Atomic absorption spectroscopy was investigated as a method for determining the thickness of the ZnS shell, and it was concluded that improved sample preparation techniques, such as further purification and complete removal of unreacted precursors, could make this testing method viable for obtaining quantitative results in conjunction with other methods. However, the ZnS coating process is subject to variations due to factors that were not controlled, such as slight variations in temperature, injection speed, and rate and degree of precursor decomposition, resulting in standard deviations in quantum yield of up to half of the mean and flocculation of some samples, indicating a need for as much process control as possible.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1999.
Includes bibliographical references.
This thesis describes the characterization and applications of nanometer sized semiconductor (or quantum dot) colloids produced by chemical means. The nanocrystals are synthesized by pyrolysis of organometallic precursors in the coordinating solvent trioctylphosphine oxide (TOPO). The important developments that have contributed to this method are discussed. Manganese doped CdSe nanocrystals are synthesized using a manganese and selenium containing organometallic compound. Chemical etching and electron paramagnetic resonance (EPR) experiments reveal that most of the dopant atoms lie near the surface within the inorganic lattice. Results from fluorescence line narrowing (FLN) and photoluminescence excitation (PLE) spectroscopies show that doped nanocrystals behave as if they were undoped nanocrystals in an external magnetic field. The nanocrystal surface is initially passivated by dative organic ligands. Better passivation and optical properties are achieved by growth of a large band gap semiconductor shell that provides both a physical and an energetic barrier between the exciton and the surface. (CdSe)ZnS (core)shell are prepared with control over both core and shell sizes. The composite nanocrystals are characterized by absorption, emission, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (W AXS). The maximum quantum yield is achieved when the core is protected from oxidation by a complete shell; thicker shells show no further increase in quantum yield values, due to defects caused by the large lattice mismatch. Exchange of surface TOPO ligands for mercaptocarboxylic acids produces (core)shell nanocrystals that, when treated with base, are soluble in water and remain fluorescent. Established protocols are used to link these water-soluble nanocrystals to the biomolecules avidin or biotin, producing useful fluorescent labels. Stable phosphine tellurides are prepared using hexapropylphosphorus triamide (HPPT). This precursor is used to prepare CdTe nanocrystals that display room temperature quantum yields up to 70%. The CdTe growth is investigated by absorption and emission spectroscopy. CdTe nanocrystals are characterized by TEM and WAXS.
by Frederic Victor Mikulec.
Ph.D.

Les recherches sur les nanocristaux semiconducteur (NCs) ont conduit à des résultats scientifiques fascinants, spécialement pour l'application en dispositifs optoelectroniques. Afin de répondre à certaines exigences comme des coûts mineurs, des gains d'efficacité, des composants respectueux de l'environnement, etc., des nouvelles méthodes sont explorées: dans les procédés en solution, dans l'ingénierie de bande et des niveaux d'énergie. En particulier, la méthode de synthèse peut influencer les propriétés optoélectroniques. Par conséquent, une meilleure compréhension des facteurs complexes pendant la synthèse entraînera une amélioration des performances.La microscopie électronique avancée fournit un moyen précis de recueillir des informations sur la morphologie, la structure cristalline et la composition chimique des matériaux avec une résolution spatiale au niveau atomique. La première partie de cette thèse traite de la synthèse et de la préparation des échantillons pour la microscopie électronique à transmission en haute résolution (HRTEM).La deuxième partie traite du mécanisme de croissance des NCs Cu2ZnSnS4 synthétisés par une méthode colloïdale. La morphologie et la stoechiométrie des intermédiaires de réaction extraits après différents intervalles de temps sont déterminés par HRTEM et analyse dispersive en énergie (EDS).Deux méthodes complémentaires, la diffraction par nanofaisceau d’électrons en précession (NPED) et la microscope électronique en transmission par balayage à haute résolution avec imagerie en champ sombre avec détecteur annulaire à grand angle (HRSTEM-HAADF) permettent une profonde caractérisation de la structure cristalline.En outre, la structure cristalline de NCs CsPbBr3 est résolue avec simulations de STEM-HAADF. Cet approche peut différencier entre structures cristallines cubiques et orthorhombiques, impossible avec techniques de diffraction traditionnelles. Enfin, l'influence des méthodes de synthèse sur la morphologie et sur la structure cristalline de NCs CuFeS2 pour applications dans le domaine de la thermoélectricité est analysée par HRTEM
The investigations of semiconductor nanocrystals (NCs) led to fascinating scientific results in optoelectronic devices. In order to fulfill certain requirements, i.e. cheaper costs, higher efficiencies, environmental friendly components etc., new methods are explored in solution-processing, band gap and energy level engineering. Particularly, the method of synthesis can alter the optoelectronic properties. Therefore, a better understanding of the intricate factors during synthesis will lead to improved performances. Advanced electron microscopy provides a precise way to gather information about morphology, crystal structure and chemical composition of materials with a spatial resolution down to the atomic level. The first part of this thesis deals with the optimization of the synthesis and sample preparation for high resolution transmission electron microscopy (HRTEM).The second part deals with the growth mechanism of Cu2ZnSnS4 NCs synthesized by a colloidal method. The morphology and stoichiometry of the samples extracted after different time intervals are characterized by HRTEM and electron dispersion spectroscopy (EDS). Two complementary methods, Nanobeam Precession Electron Diffraction (NPED) and High Resolution Scanning Transmission Electron Microscopy by High Angle Annular Dark-Field Imaging (HRSTEM-HAADF), provide an in-depth crystal structure characterization.Moreover, the crystal structure of CsPbBr3 NCs is solved by probing STEM-HAADF simulations. This approach is able to differentiate cubic and orthorhombic crystal structures, which is otherwise impossible by diffraction techniques. Finally, the influence of synthesis methods on the morphology and crystal structure of CuFeS2 NCs is investigated by HRTEM for thermoelectric applications

The first part of my PhD work is about the lattice vibrations in silicon nanowires. First-principles calculations based on the linear response are performed to investigate the quantum confinement effect in lattice vibrations of silicon nanowires (SiNW). The radial breathing modes (RBM) are found in our calculations, which have a different size-dependent frequency shift compared with the optical modes. They are well explained by the elastic model. Finally, the relative activity of the Raman scattering in the smallest SiNW is calculated. The RBM can be clearly identified in the Raman spectrum, which can be used to estimate the size of nanowires in experiment. In the second part of my PhD work, we focus on the electron-hole pair (exciton) in semiconductor nanowires and its influence on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire with a diameter of 1.2 nm. Using plane wave and pseudopotentials, the quasiparticle states are calculated within the so-called GW approximation, and the electron-hole interaction is evaluated with the Bethe-Salpeter Equation (BSE). The enhanced excitonic effect is found in the absorption spectrum. The third part of my work is about the electronic structure in Si/Ge core-shell nanowires. The electronic band structure is studied with first-principles methods. Individual conduction and valence bands are found in the core part and the shell part, respectively. The band offsets are determined, which give rise to the spatial separation of electron and hole charge carriers in different regions of the nanowires. This allows for a novel-doping scheme that supplies the carriers into a separate region in order to avoid the scattering problem. This is the key factor to create high-speed devices. With the confinement effect, our results show important correction in the band offset compared with the bulk heterostructure. Finally, an optimum doping strategy is proposed based on our band-offset data.

Durable, cheap, and lightweight polymer based solar cells are needed, if simply to meet the demand for decentralized electrical power production in traditionally “off-grid” areas. Using a blend of Poly(3-hexylthiophene-2,5-diyl) (P3HT), Phenyl-C61-butyric acid methyl ester (PCBM), and the low band-gap polymer Poly[2,6-(4,4-bis-(2- ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), we have fabricated devices with a wide spectral response and 3% power conversion efficiency in AM 1.5 conditions; however, this thin film system exhibits only 0.43 optical density at 500 nm. To improve the performance of this polymer blend photovoltaic, we aim to increase absorption by adding CdSe(ZnS) core (shell) quantum dots. Four groups of devices are fabricated: a control group with an active polymer layer of 16 mg/mL P3HT, 16 mg/mL PCBM, and 4 mg/mL PCPDTBT; and three groups with dispersed quantum dots at 4 mg/ml, 1 mg/mL, and 0.25 mg/mL. The (CdSe)ZnS quantum dots are coated with octadecylamine ligands and have a peak absorbance at 560 nm and peak emission at 577 nm. The active layer was dissolved in chlorobenzene solvent and spun on glass substrates, patterned with indium tin oxide. The devices were then annealed for fifteen minutes at 110° C, 140° C, and 170° C. Current-voltage characteristic curves v and optical density data were taken before and after the anneal step. Finally, surface characterization was conducted with atomic force microscopy and electrostatic force microscopy. When compared to the control, the sensitized devices exhibited increased absorption and depressed electrical performance with increasing quantum dot loading. The surface morphology, both electrical and physical, showed deviation from the typical values and patterns shown by the control that increased with quantum dot loading. When the degrading electrical characteristics, increasing optical absorbance, and surface changes, are considered together, it becomes likely that the quantum dots interact in a significant manner with the morphology of the P3HT phase, which leads to an overall decrease in performance.

A straightforward, rapid method to create colloidally stable and brightly luminescent core/shell CdSe-based nanoplatelets (NPLs) with fluorescence quantum yields (QYs) up to 50% is demonstrated. A layer-by-layer deposition technique based on a two-phase mixture ‒ consisting of a nonpolar phase which includes the NPLs, and a saturated ionic polar phase ‒ to separate the reagents and hinder the nucleation of the shell material is used. The deposition of the first sulfur layer leads to a significant red-shift (by more than 100 nm) of the optical absorption and emission of the NPLs. Hence, by varying either the sulfur precursor content or the reaction time one can precisely and continuously tune the absorption and emission maxima from 520 to 630 nm. This evolution of the absorption onset during the shell growth is explained quantitatively using density-functional theory and atomistic statistical simulations. The emission can be further enhanced by exposure of the NPL solution to ambient sunlight. Finally, it is demonstrated that the core/shell NPLs can be transferred from the organic solution to aqueous media with no reduction of their QY that opens the door to a broad range of practical applications.

The aim of this thesis is to understand the dynamics of the nucleation and growth of III-V semiconductor nanowires and associated heterostructures grown by chemical beam epitaxy. These nanowires represent well-controlled and high quality materials suitable for both fundamental physics and applications in optical and electronic devices. The first part of the thesis investigates growth recipes to obtain Au-catalyzed InAs NWs with controlled morphology. Good control of NW length and diameter distributions has been achieved by a systematic study of two different Au deposition techniques: Au thin film deposition and colloidal dispersion. Triggered by the issues of Au contamination and CMOS compatibility, the second part of the thesis is dedicated to the investigation of the nucleation and growth mechanisms of Au-free InAs NWs on Si (111) substrates. A thorough analysis of the silicon substrate preparation is conducted and an optimized silicon surface for the nucleation of Au-free nanowires is identified. We show that the silicon surface can be modified by in situ and ex situ parameters allowing us to control the density of NWs. Growth conditions were established for growing InAs NWs either by catalyst-free or self-catalyzed mechanisms on Si (111). The catalyst-free growth proceeds in the vapor-solid growth mechanism without the use of any catalyst particle while the self-catalyzed growth proceeds in the vapor-liquid-solid mechanism involving a liquid In droplet. Growth models are proposed in order to interpret the experimental findings. The third part of the thesis concerns the growth of axial and radial (core-shell) heterostructured NWs. Nanowire heterostructures combining either highly lattice mismatched materials (GaAs and InAs) or almost lattice matched materials (InAs and GaSb) are investigated. GaAs/InAs and InAs/GaAs axial heterostructures are grown by Au-catalyzed method. Here, it is demonstrated that the catalyst composition, rather than other growth parameters, as postulated so far, controls the growth mode and the resulting NW morphology. We have also explored the growth of core-shell InAs/GaSb heterostructures by catalyst-free mechanism. The morphology and structural properties of InAs/GaSb core-shell heterostructures are optimized to fabricate Esaki tunnel diodes exploiting their broken-gap band alignment.

Le contrôle et l'optimisation des propriétés d'émission des nanomatériaux peuvent être obtenus par un couplage efficace entre nanoémetteurs et nanostructures permettant d’obtenir une directivité plus élevée, une dynamique d’émission plus rapide. Il est pour cela nécessaire d’obtenir l'accord spectral de l'émetteur avec les modes de nanostructures, le positionnement spatial du nanoémetteur à l'endroit où l'intensité du mode résonant de la nanostructure est maximale, et une orientation du dipôle nanoémetteur parallèle au champ électrique résonant. En plasmonique les résonances larges des modes permettent un accord spectral facile. L’accord spatial est plus difficile, mais des stratégies ont été mises en œuvre avec succès. Le contrôle de l'orientation du dipôle reste lui un défi. En plasmonique, par exemple, une interaction efficace ne peut être obtenue que pour des dipôles orthogonaux à la surface métallique. La détermination de l'orientation du dipôle émetteur est donc cruciale pour les dispositifs plasmoniques tels que les nano-antennes.Dans ma thèse, j'ai contribué au développement d'une méthode polarimétrie visant à analyser le dipôle émetteur d'un nanoémetteur et son orientation. J'ai effectué des expériences et les ai analysées. Le modèle décrit l'émission d'un dipôle proche d'une interface plane dans un large éventail de conditions expérimentales réalistes, en particulier le cas où le nanoémetteur se trouve à proximité d’un film d'or. Dans cette situation, pour des nanocristaux de CdSe/CdS assimilable à deux dipôles orthogonaux dégénérés, l'imagerie défocalisée n'est pas suffisamment sensible pour fournir des informations quantitatives fiables sur l'orientation de l'émetteur. A contrario, la polarimétrie permet de répondre à cette question. Avec le même modèle, le diagramme d'émission correspondant à l'émission dipolaire en champ lointain pour toutes ces conditions expérimentales a été calculé. En combinant la polarimétrie et l’étude des diagrammes de rayonnement, on peut obtenir des informations sur la structure dipolaire et l’orientation des dipôles. J'ai appliqué cette méthode pour étudier les nanoplaquettes semi-conductrices colloïdales de CdSe/CdS avec différentes formes géométriques : plaquettes carrées minces, plaquettes rectangulaires minces et plaquettes cubiques. J'ai établi une relation entre les structures géométriques des plaquettes et la nature et l'orientation de leurs dipôles émetteurs associés
Control and optimization of nanomaterial emission properties, can be obtained thanks to efficient coupling between nanoemitters and nanostructures for achieving higher directivity, quicker dynamics. The requirements are the spectral tuning of the emitter to the nanostructures modes, the spatial positioning of the nanoemitter at the location of maximum intensity of the resonant nanostructure mode, and a proper orientation of the dipole nanoemitter. In plasmonics, the spectrally broad resonances make the spectral tuning easy. Whereas for spatial tuning, many strategies have been implemented successfully, the control of the dipole orientation remains a challenge. In plasmonics, for example, efficient interaction can only be achieved for dipoles orthogonal to the metallic surface. The determination of the orientation of the emitting dipole is thus very crucial for plasmonic devices such as nanoantennas. In my thesis, I contributed to the development of a polarimetric method aiming at the analysis of a nanoemitter’s emitting dipole and its orientation. I performed experiments and analyzed them. The model I used describes the emission of a dipole close to a plane interface in a wide range of realistic experimental conditions, including a very common case in plasmonics when the nanoemitter lies close a gold film. In this situation for CdSe nanocrystals which can be considered as two orthogonal degenerated emitting dipoles, the more standard defocused imaging method is not sufficiently sensitive to provide reliable quantitative information on the emitter’s orientation. With the same model, I also computed the emission diagram corresponding to the dipolar emission in far field for all these experimental conditions. By analyzing the emission’s polarization together with the emission pattern, I could determine the three-dimensional orientation of an emitting dipole. I applied this method to investigate the dipolar structure and orientation of colloidal semiconducting CdSe/CdS nanoplatelets with different geometries: thin square platelets, thin rectangular platelets, and cubic platelets. I established a relationship between the geometric structures of the platelets and the dimensionality and orientation of their associated emitting dipoles

Ce travail a pour objectif la fabrication, en épitaxie par jets moléculaires, de nanofils coeurcoquilleà base de GaAs et AlGaAs déposés sur des substrats Si(111), en vue de réaliser desréseaux de fils pour de nouvelles cellules solaires, et pour des fils photoniques permettant uneapproche bottom-up d'émetteurs de photons uniques.La première partie de ce travail est une étude systématique des paramètres clés qui contrôlent lacroissance uni-dimensionnelle de fils GaAs élaborés par un mécanisme vapeur-liquide-solideauto-catalysé, à savoir le rapport des flux As/Ga, la température du substrat, et la vitesse decroissance.La seconde partie se concentre sur la croissance et la caractérisation de fils GaAs recouvertsd'une coquille d'alliages AlGaAs (35% Al) afin de s'affranchir des recombinaisons de surface.Ces coquillesde AlGaAs sont fabriquées en conditions riche-As (rapport As/Ga > 10) afin deconsommer les gouttes de catalyseur gallium et de promouvoir une croissance radiale (le taux decroissance maximal axial/radial est égal à 6). Diverses caractérisations optiques sont réalisées àbasse température sur ces ensembles de fils : cathodoluminescence, photoluminescence etspectroscopie résolue en temps. L'intensité de luminescence et la durée de vie des porteursaugmentent fortement avec la présence de la coquille : une épaisseur de 7 nm de cette dernièreest suffisante pour optimiser la passivation des nanofils et supprimer les recombinaisons liéesaux états de surface. Une fine couche extérieure de GaAs est nécessaire pour éviter touteoxydation de la coquille d'alliage AlGaAs.De plus, grâce à des mesures de CL résolues spatialement, les longueurs de diffusion desexcitons dans ces fils ont été obtenues, allant de 0.7 μm à 1.5 μm pour des épaisseurs decoquilles comprises entre 20 et 50 nm. Des valeurs plus petites sont mesurées pour des coquillesplus épaisses, ce qui tend à montrer l'introduction de défauts dans l'alliage qui pourraientlimiter la qualité de l'interface. Le décalage en énergie de l'émission fournit des informationssur la génération de contraintes dans ces fils coeur-coquille et sur le champ piézo-électrique quien découle.

Au cours des vingt dernières années, la diffusion inélastique de la lumière par les modes propres de vibration des nanoparticules, appelés modes de Lamb, s’est avérée être une méthode très efficace pour caractériser la taille et les propriétés mécaniques des nano-objets. La fréquence de résonance d’une nano-sphère, dans la gamme du gigahertz, est donnée, en première approximation, par le ratio de la vitesse acoustique du matériau massif et la taille du confinement. Les raffinements du modèle théorique permettent d’obtenir, à partir de ces modes de vibration, des informations essentielles sur la géométrie et l’environnement local des nano-objets. L’objectif de cette thèse est de sonder le domaine de validité du modèle de Lamb, d’analyser les différents impacts de l’environnement sur ces modes de vibration et de développer de nouvelles méthodes pour les mesurer. Plusieurs aspects de l’interaction avec le milieu extérieur peuvent ainsi être pris en considération selon le type de système étudié. D’une part, la délocalisation de l’onde acoustique dans le cas de systèmes cœur-coquille, qui est gouvernée par les impédances acoustiques respectives du cœur et de la coquille, et qui se traduit par un couplage mécanique. D’autre part, l’effet de masse inertielle induite par la présence de ligands organiques à la surface de la particule qui modifie la fréquence de résonance. La validité de ces deux approches est ainsi discutée en fonction de la configuration des objets considérés, puis ces modèles théoriques sont appliqués à des cas réels tels que des nanoparticules cœur-coquille et des nano-plaquettes de semi-conducteurs ou des agrégats métalliques colloïdaux. L’effet de masse inertielle s’avère non négligeable pour des objets de petites tailles et il est ainsi montré la faisabilité de réaliser des nano-balances ultra-sensibles capable de sonder l’environnement proche des nano-objets. Par ailleurs, dans le cas des agrégats d’or, cette approche permet de discuter les limites du modèle de Lamb, basé sur la théorie des milieux continus, sur des vibrations n’impliquant que six atomes. Ainsi, grâce à la spectroscopie Raman basses fréquences, il apparait que les résultats expérimentaux des vibrations de ces objets s’accordent à la fois avec l’approche des milieux continus en considérant l’effet de masse inertielle et aussi avec les calculs de dynamique moléculaire. Finalement, le développement expérimental d’un montage optique capable de mesurer ces modes Raman basses fréquences sur une particule unique en milieu liquide est présenté. Cette approche nécessite de localiser une particule en milieu liquide à l’aide de nano-pinces plasmoniques puis d’exalter le signal Raman basses fréquences en stimulant les modes de vibration par électrostriction. Les perspectives étant d’appliquer cette méthode à l’étude de la dynamique vibrationnelle de nano-objet unique tel que des virus ou des protéines
Over the past twenty years, inelastic light scattering by vibrational eigenmodes of nanoparticles, called Lamb modes, has proven to be an effective method for characterizing the size and mechanical properties of nano-objects. The resonant frequency of a nano-sphere, in the gigahertz range, is given, as a first approximation, by the ratio of the acoustic velocity of the bulk material and the size of confinement. The refinements of the theoretical model allow to obtain, from these eigenmodes, information on the shape and local environment of nano-objects.The objective of this thesis is to probe the domain of validity of the Lamb model, to analyze the different impacts of the environment on eigenmodes and to develop a new strategy to measure them. Several aspects of interaction with the external medium can be considered depending on the system studied. On the one hand, the delocalization of the acoustic wave in the case of core shell systems is ruled by the acoustic impedance of the core and the shell and leads to a mechanical coupling. On the other hand, the inertial mass effect induced by the presence of organic ligands on the surface of the particle modifies the resonant frequency. The validity of both approaches is discussed according to the configuration and these models are applied to real cases, such as semiconductor core shell nanoparticles and nanoplatelets, or gold colloidal clusters. The inertial mass effect is significant for small objects and it is shown the feasibility to realize ultra-sensitive nano-balance capable of probing the local environment of nano-objects. Furthermore, in the case of gold clusters, this approach makes it possible to discuss the limit of the Lamb model, based on continuum mechanics, to interpret vibrations involving only six atoms. Thanks to low frequency Raman spectroscopy, it appears that the experimental results are in good agreement with both the continuum mechanics approach, by considering the inertial mass effect, and also with density functional theory (DFT) calculations. Finally, the experimental development of an optical set-up capable of measuring low frequency Raman modes on a single nanoparticle in a liquid medium is presented. This technic requires to localize a nanoparticle in a liquid medium with plasmonic tweezers and to enhance the low frequency Raman signal by stimulating vibrational modes with electrostriction. The perspectives are to apply this method to the dynamical study of a single object such as viruses or proteins

Im Folgenden werden Transferprozesse in Nanohybridsystemen theoretisch untersucht. Diese Hybridsysteme sind vielversprechende Kandidaten für neue optoelektronische Anwendungen und erfahren daher ein erhebliches Forschungsinteresse. Jedoch beschränken sich Arbeiten darüber hauptsächlich auf experimentelle Untersuchungen und kaum auf die dazugehörige theoretische Beschreibung. Bei den theoretischen Betrachtungen treten entscheidende Limitierungen auf. Es werden entweder Details auf der atomaren Ebene vernachlässigt oder Systemgrößen betrachtet, die wesentlich kleiner als im Experiment sind. Diese Thesis zeigt, wie die bestehenden Theorien verbessert werden können und erweitert die bisherigen Untersuchungen durch die Betrachtung von vier neuen hoch relevanten Nanohybridsystemen. Das erste System ist eine Nanostruktur, die aus einem Au-Kern und einer CdS-Schale besteht. Beim zweiten System wurde eine ZnO/Para-Sexiphenyl Nanogrenzfläche untersucht. Die zwei anderen Systeme beinhalten jeweils einen CdSe-Nanokristall, der entweder mit einem Pheophorbide-a-Molekül oder mit einem röhrenförmigen Farbstoffaggregat wechselwirkt. In allen Systemen ist der Anregungsenergie-Transfer ein entscheidender Transfermechanismus und steht im Fokus dieser Arbeit. Die betrachteten Hybridsysteme bestehen aus zehntausenden Atomen und machen daher eine individuelle Berechnung der einzelnen Subsysteme sowie deren gegenseitiger Wechselwirkung notwendig. Die Halbleiter-Nanostrukturen werden mit der Tight-Binding-Methode und der Methode der Konfigurationswechselwirkung beschrieben. Für das molekulare System wird die Dichtefunktionaltheorie verwendet. Die dazugehörigen Rechnungen wurden von T. Plehn ausgeführt. Das metallische Nanoteilchen wird durch quantisierte Plasmon-Moden beschrieben. Die verwendeten Theorien ermöglichen eine Berechnung von Anregungsenergietransfer in Nanohybridsystemen von bisher nicht gekannter Systemgröße und Detailgrad.
In the following, transfer phenomena in nanohybrid systems are investigated theoretically. Such hybrid systems are promising candidates for novel optoelectronic devices and have attracted considerable interest. Despite a vast amount of experimental studies, only a small number of theoretical investigations exist so far. Furthermore, most of the theoretical work shows substantial limitations by either neglecting the atomistic details of the structure or drastically reducing the system size far below the typical device extension. The present thesis shows how existing theories can be improved. This thesis also expands previous theoretical investigations by developing models for four new and highly relevant nanohybrid systems. The first system is a spherical nanostructure consisting of an Au core and a CdS shell. By contrast, the second system resembles a finite nanointerface built up by a ZnO nanocrystal and a para-sexiphenyl aggregate. For the last two systems, a CdSe nanocrystal couples either to a pheophorbide-a molecule or to a tubular dye aggregate. In all of these systems, excitation energy transfer is an essential transfer mechanism and is, therefore, in the focus of this work. The considered hybrid systems consist of tens of thousands of atoms and, consequently, require an individual modeling of the constituents and their mutual coupling. For each material class, suitable methods are applied. The modeling of semiconductor nanocrystals is done by the tight-binding method, combined with a configuration interaction scheme. For the simulation of the molecular systems, the density functional theory is applied. T. Plehn performed the corresponding calculations. For the metal nanoparticle, a model based on quantized plasmon modes is utilized. As a consequence of these theories, excitation energy transfer calculations in hybrid systems are possible with unprecedented system size and complexity.

Le travail illustré par ce manuscrit de thèse présente l'étude structurale d'hétéro-structures semi-conductrices à base de nitrures d'éléments III avec l'un des outils les plus puissants de la recherche scientifique: le rayonnement synchrotron. La cartographie haute résolution de l'espace réciproque, la diffraction anomale multi-longueur d'onde, la spectroscopie d'absorption X et la spectroscopie en condition de diffraction nous ont permis de caractériser la structure à l'échelle atomique de différentes régions d'un même système. Dans un premier temps, nous montrons que les nanofils GaN sur Si(111) ont une polarité N et proposons un mécanisme de nucléation. Dans un second temps, nous mettons en évidence un phénomène d'inter-diffusion stimulée par la contrainte dans les boîtes quantiques GaN / AlN recuites à haute température. Enfin, nous observons la présence d'un ordre local à courte distance dans les nanofils coeur-coquille InGaN / GaN. Cette organisation atomique pourrait être induite par la présence de contrainte, nous avons initié une étude de l'anisotropie de l'ordre à courte distance pour explorer cette hypothèse.

博士
逢甲大學
化學工程學系
103
In this study, hierarchical ZnO photocatalyst and stainless steel wire mesh-based hierarchical ZnO photocatalyst were synthesized by a hydrothermal route and used for photocatalytic degradation and hydrogen production applications. By changing the reaction conditions and dopant concentration, nanostructure, chemical and crystalline properties of the photocatalysts can be adjusted to optimize the photocatalytic degradation and hydrogen production efficiency. Effects of reaction conditions on the nanostructure, surface chemistry, crystalline properties, photocatalytic degradation activity and hydrogen production activity were investigated by FESEM, TEM, XRD, XPS, DRS, GC and SECM. The energy gap and light absorption characteristics of the photocatalyst can be tuned by doping. It helps the separation of photogenerated electron-hole pairs. The UV-Visible DRS spectra indicates that the direct energy gap (Eg) decreases with increasing dopant concentration. The PEC-SECM was used to evaluate the photocurrent response and the photocatalytic activity. This study can be divided into two parts, including photocatalytic degradation and hydrogen production. In photocatalytic degradation section, the methyl orange dye can be completely degraded within 50 min by sliver decorated hierarchical ZnO photocatalyst under UV light irradiation. For the sliver decorated hierarchical ZnO photocatalysts, the effects of nanostructure, chemical and crystalline properties on the photocatalytic degradation activity were investigated. For the silver doped stainless steel wire mesh-based hierarchical ZnO photocatalyst, degradation of methyl orange can be achieved within 60 min under visible light irradiation. For the silver doped ZnO decorated stainless steel wire mesh-based hierarchical photocatalysts, the effects of nanostructure, chemical and crystalline properties on the photocatalytic degradation activity were investigated. The photocatalytic hydrogen production performances of sulfur doped stainless steel wire mesh-based hierarchical ZnO phtotcatalysts reached 3400 μmol g-1h-1 when the Na2S precursor concentration is 15 mM. The photocatalytic hydrogen production performances of core-shell stainless steel wire mesh-based hierarchical ZnO/ZnS phtotcatalysts reached 55.67 μmol g-1h-1 when the Na2S precursor concentration is 30 mM under visible irradiation.

通過與具有表面等離子體激元特性的金屬納米晶複合，半導體納米材料的捕光能力可以得到很大地提高。銀、金納米晶因其獨特的表面等離子體特性，已被廣泛應用于半導體複合物的製備。其中通過沉積或者粘合方式得到的複合物存在一定弊端，比如：金屬納米晶暴露在實驗環境中，導致其團聚、變形、脫落、或者長大，使原有的獨特表面等離子特性改變或消失。核/殼結構納米材料可以有效地避免以上問題，因而表現出優越的光活性。爲了進一步拓寬金屬/半導體核/殼結構在光能捕獲方面的應用，我们需要深入理解製備過程中表面等離子體激元特性及材料結構的變化、設計合成新的納米材料。在這篇畢業論文中，我研究了在製備Ag/Ag2S核/殼結構過程中的表面等離子體特性及材料結構的變化，制備了Au/TiO₂核/殼結構，并對他們的應用及表面等離子體共振激元特性進行了研究。
理解硫化過程有助於更好的控制其表面等離子體特性和結構組成。因此，我分別從實驗和數值模擬兩方面研究了銀納米立方塊在硫化過程中表面等離子體特性及其相應Ag/Ag₂S 核/殼的組成及結構的變化。硫化反應分別在溶液及單顆粒環境下進行。同時，我們應用數值模擬計算揭示硫化過程中表面等離子體特性及模式變化。實驗和數值計算均表明硫化反應首先發生在銀納米立方塊的棱角和頂點。隨著反應的進行，銀立方塊被逐步鈍化為球狀銀納米顆粒。與此同時，納米立方結構的尺寸也隨之小幅增加。
二氧化鈦是一種重要的被應用於光能捕獲的半導體納米材料。因其低毒性、生物兼容性、化學及熱穩定性、耐光腐蝕性以及資源豐富等特點，TiO₂ 已經被廣泛研究。但是TiO₂僅在紫外光區具有光化學活性，這大大限制了其在光能捕獲方面的應用。儘管Au/TiO₂核/殼結構複合物可以提高TiO₂在可見區的光催化活性，但是對於該核/殼結構的合成鮮為報導，而且已報導的工作也是限制在以金納米球作為核層。與金納米球相比，金納米棒具有更引人關注的表面等離子體特性，例如金納米棒具有更高的電場增強，而且金棒的縱向共振波長可以從可見區調控到近紅外區。因此金納米棒/二氧化鈦核/殼結構可以更有效的提高二氧化鈦的光捕獲能力。在此論文中，我發展了一種合成Au/TiO₂核/殼結構的方法，并研究其在光能捕獲方面的應用。在該方法中，我選擇三價鈦作為鈦源，可控合成了Au/TiO₂ 核/殼結構。通過對核的尺寸及殼層厚度的調節，實現了對核/殼結構的共振波長的調控。另外這種方法也適用于其他單組份或者雙組份的鉑、鈀、金納米晶。爲了驗證在光能捕獲方面的應用潛能，Au/TiO₂核/殼結構納米材料被作為散射層而應用於染料敏化太陽能電池中，結果發現這種電池具有較高光電轉化效率。另外，我們還研究了表面等離子體共振激元增強下的活性氧化物的生成。再者，具有較高介電常數的二氧化鈦殼層可以與金納米晶核耦合產生法諾共振效應。結果表明金納米棒的橫向、縱向共振峰均能和殼層材料發生共振耦合而產生對應的法諾效應。納米棒的縱向共振峰的可調性實現了對應的法諾共振峰的可調性。同時，包覆二氧化鈦殼層后，金納米棒的橫向共振模式被大幅放大。
本論文的研究有利於人們了解金屬/半導體納米結構的設計及應用。硫化過程中表面等離子體共振激元特性及結構變化的研究，對具有特定組分及共振特性的複合物的設計合成具有指導意義。對貴金屬/半導體核殼結構製備、共振特性及應用的研究也擴展了其在光能捕獲方面的應用。
Over the past several years, integration of metal nanocrystals that can support localized surface plasmon has been demonstrated as one of the most promising methods to the improvement of the light-harvesting efficiency of semiconductors. Ag and Au nanocrystals have been extensively hybridized with semiconductors by either deposition or anchoring. However, metal nanocrystals tend to aggregate, reshape, detach, or grow into large nanocrystals, leading to a loss of the unique properties seen in the original nanocrystals. Fortunately, core/shell nanostructures, circumventing the aforementioned problems, have been demonstrated to exhibit superior photoactivities.To further improve the light-harvesting applications of (plasmonic metalcore)/(semiconductor shell) nanostructures, it is vital to understand the plasmonic and structural evolutions during the preparation processes, design novel hybridnanostructures, and improve their light-harvesting performances. In this thesis, I therefore studied the plasmonic and structural evolutions during the formation of (Ag core)/(Ag₂S shell) nanostructures. Moreover, I also prepared (noble metal core)/(TiO₂shell) nanostructures and investigated their plasmonic properties and photon-harvesting applications.
Clear understanding of the sulfidation process can enable fine control of the plasmonic properties as well as the structural composition of Ag/Ag₂S nanomaterials.Therefore, I investigated the plasmonic and structural variations during the sulfidation process of Ag nanocubes both experimentally and numerically. The sulfidation reactions were carried out at both the ensemble and single-particle levels.Electrodynamic simulations were also employed to study the variations of theplasmonic properties and plasmon modes. Both experiment and simulation results revealed that sulfidation initiates at the vertices of Ag nanocubes. Ag nanocubes arethen gradually truncated and each nanocube becomes a nanosphere eventually. The cubic shape is maintained throughout the sulfidation process, with the edge lengthii being increased gradually.
TiO₂ is one of the most important semiconductors that are employed inlight-harvesting applications. It has been extensively studied for a variety of applications by virtue of its low toxicity, biological compatibility, chemical and thermal stability, resistance to photocorrosion, and relative abundance. However, the photocatalytic activity of TiO₂ is limited to the UV region because of its wide bandgap, which limits its applications in light harvesting. Although (Au core)/(TiO₂ shell)nanostructures can improve the photocatalytic activities of TiO₂ in visible light, it hasonly been demonstrated in a few experiments and has been limited with Au nanospheres. Compared with Au nanospheres, Au nanorods offer more attractive plasmonic features, including stronger electric field enhancements and synthetically tunable longitudinal plasmon wavelengths over the visible to near-infrared region. The coating of Au nanorod therefore can largely improve light harvesting capabilityof TiO₂. In this thesis, I developed a facile and versatile method for the preparation of(Au nanocrystal core)/(TiO₂ shell) nanostructures by using a Ti(III) compound as thetitania precursor. By employing Au nanorods with different sizes and varying the shellthickness, the plasmonic bands of the core/shell nanostructures can be tailored. TiO₂can also be grown on other monometallic and bimetallic Pd, Pt, Au nanocrystals. As aproof-of-concept application, (Au nanorod core)/(TiO₂ shell) nanostructures wereutilized in dye-sensitized solar cells to function as a scattering layer. The resultantsolar cells exhibited higher power conversion efficiencies with a thinner thickness compared to the traditional TiO₂ solar cells. In addition, I also examined the property of plasmon-enhanced reactive oxygen species generation. Moreover, the TiO₂ shell with a high refractive index can efficiently couple with the plasmon resonance modesof the Au nanorod core, leading to Fano resonances. Fano resonances for both the transverse and longitudinal plasmon modes were simultaneously observed. The longitudinal Fano resonance is tunable by changing the plasmon energy of thenanorod core. In addition, coating with TiO₂ intensifies the transverse plasmon modeof the Au nanorod core.
I believe that my research study will be very helpful for the design and applications of metal/semiconductor nanostructures. The full understanding of the plasmonic and structural evolutions during the preparation processes will be useful for designing metal/semiconductor hybrid nanomaterials with desired compositions and plasmonic properties. The efforts towards the investigations of the preparation, plasmonic properties, and applications of (noble metal core)/(semiconductor shell) nanostructures are important for widening their light-harvesting applications.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Fang, Caihong = 具有表面等離子體激元特性的金屬/半導體核/殼納米結構 / 房彩虹.
Thesis (Ph.D.) Chinese University of Hong Kong, 2014.
Includes bibliographical references.
Abstracts also in Chinese.
Fang, Caihong = Ju you biao mian deng li zi ti ji yuan te xing de jin shu/ban dao ti he/qiao na mi jie gou / Fang Caihong.

碩士
國立交通大學
材料科學與工程學系
101
A facile and reproducible synthetic approach for preparation of core-shell Au-CdS nanocrystals with controllable shell thickness was reported. Due to the difference in band structures between Au and CdS, a pronounced photoinduced charge separation took place at the interface of Au and CdS, resulting in the electron-charged Au core and the hole-enriched CdS shell. The electron-charging of Au core in Au-CdS nanocrystals was revealed with the charge carrier dynamics measurement. An increase in the electron-transfer rate constant was observed for Au-CdS nanocrystals with increasing shell thickness, probably due to the less pronounced electron-hole interaction of thicker CdS that enabled a fuller extent of participation of photoexcited electrons in the charge separation process. On the other hand, the hole-enriched CdS shell of Au-CdS nanocrystals upon light illumination was characterized with a photocatalytic process. The photocatalytic activity of Au-CdS nanocrystals was found to increase with increasing shell thickness, attributable to the greater capability of light absorption achieved by the extensive growth of the CdS shell. The correlation of photocatalytic activity with the shell thickness of Au-CdS nanocrystals corresponded well with that of the electron-transfer rate constant. As compared to the relevant commercial products like N-doped P-25 TiO2 and CdS powders, the as-synthesized Au-CdS nanocrystals exhibited superior photocatalytic performance under visible light illumination, demonstrating their potential as an effective visible-light-driven photocatalyst. To further enhance the durability performance, Zn was introduced into the shell of Au-CdS, producing Au-Cd1-xZnxS nanocrystals that showed relatively high stability in photocatalysis. The present synthetic route can be readily extended to obtain other sulfide-semiconductor-coated Au nanocrystals, such as Au-ZnS, which may find potential use for methanol oxidation in the photoelectrochemical cell.

博士
臺灣大學
物理研究所
98
Because of their potential applications in making electronic and opto-electronic devices, low-dimensional semiconductor systems such as nanowires, nanorods and nanotubes have attracted great attention recently. In this thesis we report our studies on the growth and characterization of II-VI semiconductor nanowires and the opto-electronic properties of these nanowires. ZnO/ZnTe core-shell nanowires were successfully fabricated by first growing the ZnO core then ZnTe shell by CVD and MOCVD, respectively. Structure characterization of the core-shell nanowires were carried out by using X-ray diffraction, transmission electron microscope and scanning electron microscope and the core-shell nanowires were found to have good crystalline quality. Optical properties of nanowires were studies by using Raman scattering, confocal laser scanning microscope and the transmission measurements and the results also indicate the nanowires have good optical properties. The nanowire is then made into a single nanowire transistor and its electric properties were studied and the result indicates that it has proper biasing properties and can be use as a functional transistor. In addition to the ZnO/ZnTe system, the ZnO/ZnSe core-shell nanowires were also grown successfully by using the same growth methods. The ZnO/ZnSe nanowires were also found to have good crystal structure and optical properties.

碩士
中原大學
化學研究所
94
[Abstract] The investigations towards CdSe based semiconductor quantum dots are conducted by using CdO and Se as precursors in the TOPO-HAD-TOP solvent system via thermal pyrolysis. Similarly, CdSe and zinc stearate were used as precursors to prepare CdSe/ZnSe based semiconductor quantum dots. Preparations of the semiconductor quantum dots were achieved by changing the molar ratios between Cd and Se. The particle size distribution of the semiconductor quantum dots was found to be reaction time dependent. CdSe were blended with luminescence materials such as Tb(TMHD)3-DPSO and OC1C8-PPV, and the resulting optical properties thus were studied. The structure properties were obtained by UV-Vis absorption and fluorescence spectroscopy. The properties include the optical analysis, quantum yield, and energy gap. Field-emission scanning electron microscope and dynamic light scattering particle size analyzer were employed to measure the particle size distributions. The elemental compositions and crystalline structures were demonstrated by X-ray energy dispersive spectrometer and X-ray powder diffractometer. The semiconductor quantum dots were found to be a wurtzite structure (hexagonal closet packing). The particle size distribution was determined to be 4~9nm with a green to red emission color. The particle of the semiconductor quantum dots gets bigger when the reaction time is elongated. A red shift of the emission wavelength was also observed. However, the particle of the semiconductor quantum dots gets smaller when the molar ratio between Cd and Se gets smaller. A blue shift of the emission wavelength and higher quantum yield are observed. CdSe with a ZnSe core-shell leads to a higher quantum yield. When CdSe was blended with Tb(TMHD)3-DPSO luminescence material, the emission at the short wavelength decreased and the strength at the emission wavelength gets more focused. The emission intensity is reduced with the blending of OC1C8-PPV luminescence material, and the effects on the corresponding PLED devices still need further studies.

M.Sc.
ZnO nanostructures with different morphologies have been prepared by using microwave and conventional heating methods. The effect of solvent, zinc precursor, time and the concentration of sodium hydroxide on the morphology of zinc oxide were investigated, when different heating methods were employed. ZnO nanoparticles were prepared using the solvothermal method. Zinc precursor impact on the shape of zinc oxide nanoparticles formed depends on the solvent used during the synthesis. Different morphologies such as spheres, rods, hexagonal prisms, hexagonal plates, diamond-like and multipods were formed by a simple solution based method. The optical features for most of the formed shapes were typical of ZnO nanoparticles. The XRD patterns of the particles showed the most stable hexagonal phase with a high degree of crystallinity. A capping molecule has an impact on the shape of the nanoparticle. In this work, we also present the results from the study of the effect of the stabilizing molecule on the shape and formation of the core shell nanoparticles of CdSe/PbS. The capping molecules used were hexadecylamine (HDA), tri-n-octylphosphine oxide (TOPO) and stearic acid. The core shell nanomaterials were synthesized by using a method in which selenium powder was converted to TOPSe.Transmission electron microscopy was used to determine the morphology and the size of the ZnO and coreshell nanomaterials. Spherical particles were obtained when TOPO was used whereas the use of HDA induced the formation of non-spherical shapes. With both capping molecules, epitaxial shell growth was not achieved. The particles formed from both capping groups (HDA and TOPO) were large due to the long reaction time that instigates lager particle sizes. However, when stearic acid was used as a capping molecule, a perfect core shell arrangement was formed. The phase and the crystallinity of the formed particles were determined by the XRD.

III-V semiconductor nanowires have been investigated as key components for future electronic and optoelectronic devices and systems due to their direct band gap and high electron mobility. Amongst the III-V semiconductors, the planar GaAs material system has been extensively studied and used in industries. Accordingly, GaAs nanowires are the prime candidates for nano-scale devices. However, the electronic performance of GaAs nanowires has yet to match that of state-of-the-art planar GaAs devices. The present deficiency of GaAs nanowires is typically attributed to the large surface-to- volume ratio and the tendency for non-radiative recombination centres to form at the surface. The favoured solution of this problem is by coating GaAs nanowires with AlGaAs shells, which replaces the GaAs surface with GaAs/AlGaAs interface. This thesis presents a systematic study of GaAs/AlGaAs core-shell nanowires grown by metal organic chemical vapour deposition (MOCVD), including understanding the growth, and characterisation of their structural and optical properties. The structures of the nanowires were mainly studied by scanning electron microscopy and transmis- sion electron microscopy (TEM). A procedure of microtomy was developed to prepare the cross-sectional samples for the TEM studies. The optical properties were charac- terised by photoluminescence (PL) spectroscopy. Carrier lifetimes were measured by time-resolved PL. The growth of AlGaAs shell was optimised to obtain the best optical properties, e.g. the strongest PL emission and the longest minority carrier lifetimes. The sidewalls of the vapour-liquid-solid (VLS) grown GaAs nanowires were investi- gated. It was found that a Reuleaux triangle with 3 {112}A curved surfaces is the actual shape of the nanowire at the growth interface. This Reuleaux triangle changes into well defined {112} facets as a result of the simultaneous radial growth. A theoretical model was developed to explain the orientations of nanowire sidewall facets. The sidewalls of GaAs nanowires were found to transform to {110} facets at high temperature as a result of surface atom migration. The rate of the facet transformation was found to be controlled by temperature and the difference in the surface energies, which leads to different faceting behaviour along the length of the nanowire. While the sidewalls of the top segment were fully transformed into {110} facets, the sidewalls of the bottom of the nanowires were a mixture of {110} and {112} facets. This facet-change along the length of the nanowire directly affected the subsequent growth of AlGaAs shell. This was relevant to the non-uniform PL emission and the minority carrier lifetimes (tmc) along the GaAs/AlGaAs core-shell nanowires. The strongest PL emission and longest tmc was observed where the GaAs core had six {110} facets. PL intensity and tmc decreased towards the bottom of the nanowire where the sidewall facets of the GaAs core consisted of both {110} and {112} facets. The effect of AlGaAs shell growth parameters (including V/III ratio, temperature and time) on the optical properties of GaAs/AlGaAs core-shell nanowires was investigated on nanowires catalysed by Au particles with a diameter of 50 nm. The V/III ratio and shell growth temperature were found to profoundly affect the optical properties. A high V/III ratio and/or a high growth temperature dramatically increased tmc. Further increasing the V/III ratio and/or growth temperature resulted in drop of tmc. Interme- diate V/III ratio and shell growth temperature were chosen as a compromise to achieve long tPL. The AlGaAs shell growth time also showed a significant effect on tmc. tmc increased with shell growth time to a maximum, followed by a further drop with longer shell growth time. With the optimised AlGaAs shell growth, an average carrier life- time of (1.02 ± 0.4) ns was achieved from single GaAs/AlGaAs core-shell nanowires at room temperature. This is comparable to self-assisted nanowires grown by molec- ular beam epitaxy and also proved that Au catalyst is not detrimental to the optical properties in VLS-grown GaAs nanowires. The long lifetimes are mainly attributed to the improvement of the GaAs/AlGaAs interface quality that is comparable with planar heterostructures. The effect of AlGaAs shell growth time and shell thickness on tPL were investigated. It was found that both the shell thickness and shell growth time affected tmc. A certain shell thickness is required to prevent the carriers generated in GaAs core from tunnelling through the AlGaAs shell and recombining at the free surface of GaAs cap layer. Beyond this thickness, the shell growth time, which is related to the diffusion at the heterointer- face, becomes the primary parameter controlling the carrier lifetimes. Lifetimes as long as 1.9 ns were achieved by reducing the effect of diffusion. This work presents an in-depth understanding of the geometry of GaAs nanowires, demonstrates GaAs/AlGaAs core-shell nanowires with optical quality comparable with planar heterostructures and reveals intriguing structural/optical behaviour of the nano- wires. These findings will greatly assist the fabrication of efficient nanowire devices and show a strong future for nano-optoelectronic devices based on nanowires.

Nanostructured ZnO films were obtained via thermal oxidation of thin films formed with metallic Zn-nanoparticle dispersions. Commercial zinc nanoparticles used for this work were characterized by microscopic and thermal analysis methods to analyze the Zn-ZnO core shell structure, surface morphology and oxidation characteristics. These dispersions were spin-coated on SiO₂/Si substrates and then annealed in air between 100 and 600 °C. Significant nanostructural changes were observed for the resulting films, particularly those from larger Zn nanoparticles. These nanostructures, including nanoneedles and nanorods, were likely formed due to fracturing of ZnO outer shell due to differential thermal expansion between the Zn core and the ZnO shell. At temperatures above 227 °C, the metallic Zn has a high vapor pressure leading to high mass transport through these defects. Ultimately the Zn vapor rapidly oxidizes in air to form the ZnO nanostructures. We have found that the resulting films annealed above 400 °C had high electrical resistivity. The zinc nanoparticles were incorporated into zinc indium oxide solution and spin-coated to form thin film transistor (TFT) test structures to evaluate the potential of forming nanostructured field effect sensors using simple solution processing. The functionalization of zinc tin oxide (ZTO) films with self-assembled monolayers (SAMs) of n-hexylphosphonic acid (n-HPA) was investigated. The n-HPA modified ZTO surfaces were characterized using contact angle measurement, x-ray photoelectron spectroscopy (XPS) and electrical measurements. High contact angles were obtained suggesting high surface coverage of n-HPA on the ZTO films, which was also confirmed using XPS. The impact of n-HPA functionalization on the stability of ZTO TFTs was investigated. The n-HPA functionalized ZTO TFTs were either measured directly after drying or after post-annealing at 140 °C for 48 hours in flowing nitrogen. Their electrical characteristics were compared with that of non-functionalized ZTO reference TFTs fabricated using identical conditions. We found that the non-functionalized devices had a significant turn-on voltage (V[subscript ON]) shift of ~0.9 V and ~1.5 V for the non-annealed and the post-annealed conditions under positive gate bias stress for 10,000 seconds. The n-HPA modified devices showed very minimal shift in V[subscript ON] (0.1 V), regardless of post-thermal treatment. The VON instabilities were attributed to the interaction of species from the ambient atmosphere with the exposed ZTO back channel during gate voltage stress. These species can either accept or donate electrons resulting in changes in the channel conductance with respect to the applied stress.
Graduation date: 2012
Access restricted to the OSU Community at author's request from Jan. 6, 2012 - Jan. 6, 2013