Dissertations / Theses on the topic 'Core-shell Heterostructure'

Nanocrystal quantum dots (NQDs) show great promise in the advancement of the field of photovoltaics. While the maximum efficiency of conventional solar cell (SC) devices is limited to ∼ 31% (Shockley-Queisser limit), devices based on NQDs may attain a maximal thermodynamic efficiency of 42% through the exploitation of multiple exciton generation (MEG). In this process, several electron- hole pairs are created by the absorption of a single high energy photon, as opposed to the single excitons created in conventional solar cell devices. IV-VI semiconductor nanocrystals (PbS, PbSe) are of particular interest as candidates for the exploitation of MEG due to the narrow band gap, high confinement energies, and long radiative carrier lifetimes observed in these systems. In order to realise the full potential of MEG devices, full characterisation of the optoelectronic properties of the underlying nanoparticles is desirable. While the size-dependent properties of NQDs are well understood, the effects of NQD shape are less so. This thesis investigates the effect of ellipticity on the optoelectronic properties associated with spheroidal NQDs. To this end, a four-band, anisotropic, and radially variant k · p system Hamiltonian is expanded in a planewave basis in order to calculate single-particle eigenenergies and eigenfunctions of colloidal PbSe/PbS core/shell heterogeneous NQDs of varying ellipticity. Many-body effects are accounted for via a full configuration interaction (CI) Hamiltonian, the basis of which is comprised of the single-particle states. Exci- tonic and bi-excitonic corrections are then found by mixing of the basis states. In this manner, such diverse electronic and optical properties as quasi-particle binding energies, momentum matrix elements, and charge carrier lifetimes, both radiative and non-radiative, may be predicted.

With an increasing demand for alternative clean energy solutions, much effort is being invested in the progression of nanoscale semiconductor materials in hopes of better harnessing solar energy. ZnO and TiO2 remain the most prominent photocatalytically active materials. This thesis reports on a comparison between nanoscale core-shell and hierarchical Zn-Ti-O/ZnO heterostructures. After a seed layer thickness optimization, hydrothermally grown ZnO nanorods were coated with mixed concentrations of Ti and Zn within an oxygen rich sputtering environment at two distinct temperature zones. Core-shell structures resulted from low temperature (23°C) depositions while hierarchical branch structures grew at high temperature (800°C). Excluding deposition temperature and the strategic variation of Zn and Ti gun power, every fabrication process remained identical between the two resultant heterostructure groups. Amongst the variety of samples produced, one from each heterostructure group proved notably similar in structural dimension, composition, and crystallization, yet demonstrated distinct differences in photoluminescence and dye degradation via UV-visible light spectroscopy. While photoluminescence results indicated core-shell heterostructure more photocatalytically promising, hierarchical heterostructure prevailed as the more powerful photocatalyst. Increased surface area due to hierarchical branching in conjunction with enhanced light exposure was believed responsible for the improved photocatalytic effectiveness.

Core-shell nanowire LEDs are light emitting devices which, due to a high aspect ratio, have low substrate sensitivity, allowing the possibility of low defect density GaN light emitting diodes. Current growth techniques and physical non-idealities make the production of high conductivity p-type GaN for the shell region of these devices difficult. Due to the structure of core-shell nanowires and the difference in conductivity between ntype and p-type GaN, the full junction area of a core-shell nanowire is not used efficiently. To address this problem, a series of possible doping profiles are applied to the core of a simulated device to determine effects on current crowding and overall device efficiency. With a simplified model it is shown that current crowding has a possible dependence on the doping in the core in regions other than those directly in contact with the shell. The device efficiency is found to be improved through the use of non-constant doping profiles in the core region with particularly large efficiency increases related to profiles which modify portions of the core not in contact with the shell

Le contrôle optique des propriétés physiques d’un matériau suscite l’intérêt des scientifiques pour des enjeux aussi bien fondamentaux qu’appliqués. L’axe de recherche original que nous avons développé dans le cadre de ce travail de thèse visait à la réalisation et l’étude d’hétérostructures moléculaires photo-magnétiques dans des gammes de température susceptibles d’applications. L’approche proposée consistait à élaborer des hétérostructures de type multiferroïque constituées de deux phases, l’une piézomagnétique et l’autre photo-strictive. L’idée était d’optimiser le couplage, d’origine élastique, entre ces propriétés pour permettre l’observation d’effets photo-magnétiques à des températures plus élevées que celles rapportées pour les matériaux monophasés. La couche photo-strictive peut se déformer sous irradiation lumineuse, générant des contraintes biaxiales dans la couche magnétique. Si celle-ci présente une forte réponse piézomagnétique, son aimantation peut être fortement modifiée, notamment au voisinage du point de Curie, allant jusqu’à un éventuel décalage de la température critique sous contrainte. Les composés moléculaires analogues du Bleu de Prusse, de formule générique AxM[M’(CN)6]y . zH2O (où A est un alcalin et M,M’ des métaux de transition), semblaient particulièrement adaptés à l’élaboration de telles hétérostructures. Nous avons utilisé le composé Rb0,5Co[Fe(CN)6]0,8 . zH2O pour la phase photo-strictive, au coeur, et Rb0,2Ni[Cr(CN)6]0,7 . z’H2O ou K0,2Ni[Cr(CN)6]0,7 . z’H2O pour la phase magnétique, en coquille. Ces deux phases présentent un désaccord paramétrique de 5,3%.L’objectif principal de ce travail de thèse était de comprendre et de contrôler le couplage élastique entre le cœur et la coquille. Nous avons ainsi dans un premier temps mis en évidence l’existence de ce couplage, la présence de la coquille modifiant les propriétés de photo-commutation du cœur et la déformation du réseau cristallin du cœur étant partiellement transmise à la coquille, induisant des modifications structurales et magnétiques de la coquille. Nous nous sommes dans un second temps intéressés à différents paramètres pouvant influence le couplage. D’abord en étudiant des paramètres géométriques, en faisant varier la taille des particules de cœur, l’épaisseur de la coquille et la microstructure de la coquille. Nous avons à cette occasion mis en évidence les facteurs régissant la croissance des particules de cœur et de la coquille. Ces études ont révélé que le rapport volumique entre le cœur et la coquille contrôlait la qualité du couplage, et que des modifications de la microstructure avait une influence à la fois sur les propriétés de photo-commutation du cœur, mais aussi sur la réponse de la coquille. Enfin, nous avons étudié des coquilles de nature chimique différente pour changer le désaccord paramétrique entre le cœur et la coquille. Il en ressort qu’en diminuant le désaccord, on améliore le couplage, mais cela se traduit notamment par une rétroaction de la coquille plus forte. Si cette rétroaction devient trop importante, le réseau du cœur ne peut plus se déformer. Il s’agit donc de trouver un compromis entre force du couplage et force de la rétroaction de la coquille. Finalement, nous avons mis en évidence le fait que l’on ne peut pas simplement associer l’effet de la coquille à un effet de pression hydrostatique, mais que le couplage des réseaux cristallins joue un rôle important dans la synergie entre les deux phases
The optical control of the physical properties of a material has drawn considerable attention during the past few years for a fundamental point of view and for applications. The originality of the project developed during this thesis was based on the synthesis and the study of photo-magnetic heterostructures in a temperature range convenient for applications. The approach consisted of developing multiferroic-like heterostructures that associate a piezomagnetic phase and a photo-strictive phase. The idea was to exploit the coupling of elastic origin between these properties, to allow the observation of photo-magnetic effects at temperatures higher than those reported for single-phase materials. The photo-strictive phase can deform under light irradiation, generating biaxial strain in the magnetic phase. If the piezomagnetic response of the latter is high enough, its magnetization could be modulated, especially at the vicinity of the Curie temperature, with a possible shift of the critical temperature under stress. In this project, we focused on molecular solids based on polycyanometallates, namely Prussian blue analogues, whose generic formula is AxM[M’(CN)6]y . zH2O (where A is an alkali metal and M,M transition metals). We used the compound Rb0,5Co[Fe(CN)6]0,8 . zH2O for the photo-strictive phase and Rb0,2Ni[Cr(CN)6]0,7 . z’H2O or K0,2Ni[Cr(CN)6]0,7 . z’H2O for the magnetic phase. These two phases have a lattice mismatch of 5.3%The main objective of this work was to understand and to control the elastic coupling between the core and the shell. We first highlighted the existence of this coupling, the presence of the shell changing the photo-switching properties of the core, and the deformation of the crystalline lattice of the core inducing structural and magnetic modifications in the shell. Then, we focused on the study of different parameters which can have an impact on the behavior of the heterostructures under light irradiation. We showed that the volumic ratio between the core and the shell is a key factor to control the efficiency of the coupling. The microstructure of the shell can also play an important role, but is not as well understood. In the end, we studied other Prussian blue analogs shells in order to change the lattice mismatch between the core and the shell. We could evidence that by reducing the lattice mismatch we tend to increase the coupling, but if this coupling is to strong, the retroaction of the shell hinders completely the dilatation of the core lattice. The idea is also to find a compromise between the strength of the coupling and the strength of the shell retroaction. In the end, we proved that we cannot associate the effect of the shell to an hydrostatic pressure, but that the coupling of the crystalline lattices play an important role in the synergy between the two phases

Le développement d’architectures nanostructurées originales composées de matériaux abondants et non-toxiques fait l’objet d’un fort intérêt de la communauté scientifique pour la fabrication de dispositifs fonctionnels efficaces et à bas coût suivant des méthodes d’élaborations faciles à mettre en œuvre. Les réseaux de nanofils de ZnO élaborés par dépôt en bain chimique sont, à ce titre, extrêmement prometteurs. L’étude des propriétés de ces réseaux de nanofils et leur intégration efficace au sein de dispositifs nécessitent toutefois un contrôle avancé de leurs propriétés structurales et physiques, notamment en terme de polarité, à l’aide de techniques de lithographies avancées.Le dépôt en bain chimique des nanofils de ZnO est d’abord effectué sur des monocristaux de ZnO de polarité O et Zn préparés par lithographie assistée par faisceau d’électrons. Par cette approche de croissance localisée, un effet significatif de la polarité des nanofils de ZnO est mis en évidence sur le mécanisme de croissance des nanofils, ainsi que sur leurs propriétés électriques et optiques. La possibilité de former des nanofils de ZnO sur des monocristaux de ZnO semipolaires nous a de plus permis d’affiner la compréhension de leurs mécanismes de croissance sur les couches d’amorces polycristallines de ZnO. Par la suite, le dépôt des nanofils de ZnO en bain chimique est développé sur des couches d’amorces polycristallines de ZnO préparés à l’aide de la lithographie assistée par nano-impression. Suivant cette approche, des réseaux de nanofils de ZnO localisés sont formées sur de grandes surfaces, ce qui permet d’envisager leur intégration future au sein de dispositifs fonctionnels.Les nanofils de ZnO sont ensuite combinés avec des coquilles semiconductrices de type p par des méthodes de dépôt chimique en phase liquide ou en phase vapeur afin de fabriquer des hétérostructures cœurs-coquilles originales. Le dépôt de couches successives par adsorption et réaction (SILAR) d’une coquille absorbante de SnS de phase cubique est optimisé sur des nanofils de ZnO recouverts d’une fine couche protectrice de TiO2, ouvrant la voie à la fabrication de cellules solaires à absorbeur extrêmement mince. Enfin, un photo-détecteur UV autoalimenté prometteur, présentant d’excellentes performances en termes de réponse spectrale et de temps de réponse, est réalisé par le dépôt chimique en phase vapeur d’une coquille de CuCrO2 sur les nanofils de ZnO
Over the past decade, the development of novel nanostructured architectures has raised increasing interest within the scientific community in order to meet the demand for low-cost and efficient functional devices composed of abundant and non-toxic materials. A promising path is to use ZnO nanowires grown by chemical bath deposition as building blocks for these next generation functional devices. However, the precise control of the ZnO nanowires structural uniformity and the investigation of their physical properties, particularly in terms of polarity, remain key technological challenges for their efficient integration into functional devices.During this PhD, the chemical bath deposition of ZnO nanowires is combined with electron beam lithography prepared ZnO single crystal substrates of O- and Zn-polarity following the selective area growth approach. The significant effects of polarity on the growth mechanism of ZnO nanowires, as well as on their electrical and optical properties, are highlighted by precisely investigating the resulting well-ordered O- and Zn-polar ZnO nanowire arrays. An alternative nano-imprint lithography technique is subsequently used to grow well-ordered ZnO nanowire arrays over large areas on various polycrystalline ZnO seed layers, thus paving the way for their future integration into devices. We also demonstrate the possibility to form ZnO nanowires by chemical bath deposition on original semipolar ZnO single crystal substrates. These findings allowed a comprehensive understanding of the nucleation and growth mechanisms of ZnO nanowires on polycrystalline ZnO seed layers.In a device perspective, the ZnO nanowires are subsequently combined with p type semiconducting shells by liquid and vapor chemical deposition techniques to form original core-shell heterostructures. The formation of a cubic phase SnS absorbing shell is optimized by the successive ionic layer adsorption and reaction (SILAR) process on ZnO nanowire arrays coated with a thin protective TiO2 shell, which pave the way for their integration into extremely thin absorber solar cells. A self-powered UV photo-detector with fast response and state of the art performances is also achieved by the chemical vapor deposition of a CuCrO2 shell on ZnO nanowire arrays

Cette thèse porte sur l’étude du transport de charge et de spin dans les nanostructures 0D, 2D et les hétérostructures 2D-0D de Van der Waals (h-VdW). Les nanocristaux pérovskite de La0.67Sr0.33MnO3 ont révélé des magnétorésistances (MR) exceptionnelles à basse température résultant de l’aimantation de leur coquille indépendamment du coeur ferromagnétique. Les transistors à effet de champ à base de MoSe2 ont permis d’élucider les mécanismes d’injection de charge à l’interface metal/semiconducteur 2D. Une méthode de fabrication des h-VdW adaptés à l’électronique à un électron est rapportée et basée sur la croissance d’amas d’Al auto-organisés à la surface du graphene et du MoS2. La transparence des matériaux 2D au champ électrique permet de moduler efficacement l’état électrique des amas par la tension de grille arrière donnant lieu aux fonctionnalités de logique à un électron. Les dispositifs à base de graphene présentent des MR attribuées aux effets magnéto-Coulomb anisotropiques
This thesis investigates the charge and spin transport processes in 0D, 2D nanostructures and 2D-0D Van der Waals heterostructures (VdWh). The La0.67Sr0.33MnO3 perovskite nanocrystals reveal exceptional magnetoresistances (MR) at low temperature driven by their paramagnetic shell magnetization independently of their ferromagnetic core. A detailed study of MoSe2 field effect transistors enables to elucidate a complete map of the charge injection mechanisms at the metal/MoSe2 interface. An alternative approach is reported for fabricating 2D-0D VdWh suitable for single electron electronics involving the growth of self-assembled Al nanoclusters over the graphene and MoS2 surfaces. The transparency the 2D materials to the vertical electric field enables efficient modulation of the electric state of the supported Al clusters resulting to single electron logic functionalities. The devices consisting of graphene exhibit MR attributed to the magneto-Coulomb effect

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.

碩士
國立臺北科技大學
光電工程系研究所
97
The formation of heterostrucure in nanorods is essential for their potential applications in nanoelectronic and photonic devices. Here we demonstrate that vertically well-aligned ZnO nanorods and ZnO/MgZnO core-shell nanorods can be successfully synthesized via catalyst-free vapor phase transport combined with pulsed laser deposition (PLD) method. The thesis consists of two parts. First, the vertically well-aligned ZnO nanorods were grown on a PLD-predeposited ZnO thin film via a simple thermal evaporation and vapor transport process. These ZnO nanorods were quite uniform with a diameter of ~27 nm and length of ~1 μm. Room-temperature photoluminescence spectra of the samples showed only a strong band-edge emission, indicating the high crystalline quality. The well-aligned ZnO nanorods were used as a template for the synthesis of nanorod heterostructures. In the second part, the vertically well-aligned ZnO/MgZnO core-shell structures of the nanorods were synthesized by PLD of MgZnO onto the ZnO nanorod template. The core-shell heterostructure nanorods were examined by high-resolution transmission electron microscopy measurements. The optical properties of the heterostructure nanorods were analyzed by photoluminescence. The HRTEM images and the corresponding FFT patterns of the nanorods implied that the core/shell is wurtzite structured ZnO/MgZnO with well-defined epitaxial relationship. The positions of the MgxZn1-xO shells, obtained by pused laser ablating MgyZn1-yO targets with y=0.0909 and 0.25, were determained by the Vegard’s law to be x=0.14 and 0.30, respectively. Room-temperature PL spectrum from the ZnO/MgxZn1-xO core-shell nanorods exhibits strong emissions from ZnO core (located at 3.298eV) and MgxZn1-xO shell (located at 3.539eV for x=0.14 and 3.935eV for x=0.30). The core/shell relative emission intensity can be controlled by the shell thickness.

Semiconductor nanowire field-effect transistors (NWFET) have been recognized as a possible alternative to silicon-based CMOS technology as traditional scaling limits are neared. The core-shell nanowire structure, in particular, also allows for the enhancement of carrier mobility through radial band engineering. In this thesis, we have evaluated the possibility of electron confinement in strained Si-Si1-xGex core-shell nanowire heterostructures. Cylindrical strain distribution was calculated analytically for structures of various dimensions and shell compositions. The strain-induced conduction band edge shift of each region was found using k•p theory coupled with a coordinate system shift to account for strain. A positive conduction band offset of up to 200 meV was found for a Si-Si0.2Ge0.8 structure. We have also designed and characterized a modulation doping scheme for p-type, Ge-SiGe core-shell NWFETs. Finite element simulations of hole density versus radial position were done for different combinations of dopant position and concentration. Three modulation doped nanowire samples, each with a different boron doping density in the shell, were grown using a combined vapor-liquid-solid and chemical vapor deposition process. Low temperature current-voltage measurements of bottom- and top-gate samples indicate that hole mobility is limited by the proximity of charged impurities.
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The goal of this project is to fabricate a low cost chemical vapor deposition (CVD) setup and synthesize hybrid nanomaterials i.e. copper oxide (CuO)-CdX (X=Se, S) nanoparticles decorated core-shell heterostructure. The synthesized hybrid nanomaterials have been fabricated into a device (photodetector) for the measurement of current-voltage characteristics in dark and under UV illumination. Furthermore, the growth model for the formation of core-shell heterostructure has also been discussed in this project. Chapter-I narrates about the fundamentals of materials, nanomaterials and hybrid nanomaterials. In this chapter, the importance, properties, application of nanomaterials have been outlined. Moreover, the properties and morphology and corresponding application are highlydependent on the synthesis methods. Chemical vapor deposition (CVD) technique is found be one of versatile among all other preparation methods. The motivation by addressing the challenges have been discussed thoroughly. Chapter-II describes the fabrication of a low cost CVD setup. For the fabrication of CVD setup, a three-zone horizontal furnace, reaction tube, a rotary van pump and three mass flow meters have been procured. A liquid precursor handling system and a reaction chamber which has fitted with two couplings have been designed. All these subcomponents have been assembled and integrated into a single unit CVD setup. Chapter-III discusses about the detailed experimental procedure for the synthesis of CuO nanowires-CdX (X=Se, S) nanoparticles decorated core-shell heterostructure. For the synthesis of CuO-CdX (X= Se, S) heterostructure nanomaterials, CuO nanowires have been synthesizedfirst by using thermal oxidation of Cu foil in air at 5000C for 5 hours. These CuO nanowires grown on cu foils have been used for the synthesis of heterostructure by using the fabricated CVD. All these materials i.e. CuO nanowires, CuO-CdSe & CuO-CdS heterostructure have been characterized by field emission electron microscopy (FESEM) attached with energy dispersive spectroscopy (EDS), x-ray diffraction (XRD), transmission electron microscopy (TEM) attached with high resolution TEM (HRTEM) and selected area diffraction pattern (SAED), RAMAN spectroscopy & UV-Vis spectroscopy. Moreover, these materials have been fabricated intoa photodetector for the measurement of current-voltage characteristics in dark and under UV illumination. Chapter-IV describes the detailed material characterization of CuO-CdSe heterostructure nanomaterials. The FESEM image of CuO nanowires reveals the formation CuO nanowires stretching out of the surface. The surface of CuO nanowires is very much smooth and impurity free. Formation of beaded like structures of CdSe is found to be attached intermittently on the surface of CuO nanowires. The presence of Cd, Se elements in the materials has been confirmed by EDS. However, the formation of these bead structure is well confirmed TEM along with the formation of core-shell heterostructure. XRD, HRTEM, SAED pattern confirms the crystalline nature of the materials. Raman spectroscopy further confirms the presence of CdSe in the CVD synthesized materials. Using UV-Vis spectroscopy measurement the band gap is found to be ~2.2eV for CuO nanowires and 3.96eV for CuO-CdSe heterostructure. Chapter-V discusses about the material characterization of CuO-CdS nanomaterials. From FESEM image, the rough surface of CuO-CdS is found by FESEM observation which is attributed to the deposition of CdS nanoparticles thoroughly on to the surface of CuO nanowires during preparation of CuO-CdS core-shell structure by CVD process. The presence of Cd, S elements in the materials has been confirmed by EDS. The formation of core-shell heterostructure has been well verified by TEM. The crystalline natures of the materials have been confirmed by XRD, HRTEM, and SAED pattern. Raman spectroscopy further confirms the presence of CdS in the CVD synthesized materials. The band gap is found to be ~3.73eV for CuO-CdS heterostructure as measured by UV-Vis spectroscopy. Chapter-VI discusses about some general trends in growth mechanism of hybrid nanomaterials and a probable growth mechanism of the present research work has been suggested as deduced from experimental characterization. The probable growth mechanism for CuO-CdSe is found to be gas phase adsorption, whereas surface diffusion and gas phaseadsorption growth mechanism for CuO-CdS has been suggested. However, the exact growth mechanism is yet to be established that needs further investigation in detail. Furthermore, the current-voltage characteristics of the fabricated photodetector have been measured by Keithley source meter 2400. The measured current for the CuO is 1.4μA at bias voltage 3 Volt. Similarly, the dark current measured for the CuO-CdSe is 11 μA. However, the current increased to 33μA under UV illumination at the biasing 3V. For CuO-CdS, the current is found to be 10.8μA and increased to 23.8 under UV illumination at the biasing 5V. The increase in photocurrent attribute because of the effective charge separation in electron-hole in the heterojunction, which has been discussed thoroughly in the chapter by using band diagram.

碩士
國立東華大學
材料科學與工程學系
104
Abstract The Ge/Ge/GeOx nanowire core-shell nanowire heterostructures were synthesiz-ed on the sapphire substrates by using a non-toxic physical vapor deposition method. First, Ge vapor were transported onto the Au coated sapphire substrates to grow Ge nanowires. By controlling the ambient oxygen (0%, 1%, 5%, and 10%) during growth, Ge/GeO2/GeOx nanowire heterostructures with different shell thickness can be obtained. The field-emission scanning electron microscope (FE-SEM) and energy dispersive spectroscopy (EDS) were used to characterize samples. The sample consi-sts of nanowires with smooth surface and their diameters were less than 80 nm. EDS results shows only Ge and O were detected on samples, confirming the high purity of sample. The X-ray diffraction results confirm the crystal structures are cubic Ge and hexagonal GeO2 with high crystallinity. The scanning transmission electron micros-cope (STEM) was used to further characterize nanowire heterostructures. The results reveals that with increasing oxygen ambient, the thickness of oxide layer increases. For samples obtained under low oxygen ambient (0%, 1%, and 5%) were Ge/GeOx core-shell nanowire heterostructures. Sample grown under high oxygen ambient (10%) was Ge/GeO2/GeOx core-shell nanowire heterostructures. Their surface valence states were further confirmed by X-ray photoelectron spectroscopy (XPS). Macro-Raman spectroscopy indicates a red shift behavior due to size effect and phonon confinement. Photoluminescence (PL) results shows Ge/GeOx core-shell nanowire heterostructures are with strong blue-green emission due to various defect transitions such as oxygen vacancy and germanium-oxygen vacancy pairs.

In order to achieve high performance III-V nanowire heterostructure based devices, it is essential that the heterostructures are of good structural and optical quality. Hence controlling the morphology and crystal structure is a critical factor in the nanowire heterostructure growth. Au-nanoparticle assisted vapour-liquid-solid (V-L-S) technique is one of the superior methods used in recent years for nanowire growth. This thesis investigates InP-based core-shell nanowire growth by the metal organic chemical vapour deposition (MOCVD) on (111)B InP substrates via V-L-S growth mechanism. The microstructure of core-shell nanowires is characterised by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Micro-photoluminescence and micro-Raman measurements are carried out to study the optical properties. The growth of InP/InxGa{u2081}-xAs and InP/InxGa{u2081}-xP core-shell nanowires is investigated in this work to study the effect of shell composition on nanowire morphology, crystal structure and optical properties. The morphology of InP/InxGa{u2081}-xAs nanowires depends on the shell composition. It is found that along with the InGaAs shell growth, significant axial growth of the InGaAs also occurs and with the increase in In composition, this axial component increases proportionately. The InP nanowire core and the InGaAs shell have a wurtzite crystal structure but the axial section of InGaAs has a defect free zinc-blende phase underneath the Au particle. InP/InxGa{u2081}-xAs core-shell nanowires showed PL emissions from both InP core and the InxGa{u2081}-xAs shell at 4K with a 40meV redshift in the InP PL emission from nanowires with xv,In{u2265}0.51. In the case of InP/InxGa{u2081}-xP core-shell nanowires, with higher In composition (xv,In> 0.50) in the InxGa{u2081}-xP shell, uniform and smooth shell formation around the core is observed. Room temperature photoluminescence is observed from the nanowires (0 < xv,In < 0.90) which indicates the good crystal quality. Core-shell nanowires with xv,In < 0.50 show PL emission at {u223C}1.46eV at 300K, corresponding to the emission peak of wurtzite InP nanowires. A dramatic blueshift in the photoluminescence emission is observed from samples with xv,In> 0.50 , ranging from 1.48eV to 1.55eV, which is comparable with photoluminescence observed from InxGa{u2081}-xP epilayer grown with xv,In > 0.50. Raman scattering results showed no evidence of strain induced shift in the InP-like phonon modes.

We investigate the temperature and size distribution of Ag, Ge, CdSe and ZnS nanoparticles undergoing UV excimer laser pulses. A two laser pulse experiment is designed to monitor nanoparticle size before and after laser interaction. We study HRTEM images and measure the ablation and fluorescence spectra of particles before and after evaporation. Results show that the nanoparticle mean radius decreases from 3.4 ± 0.2 nm to 2.6 ± 0.2 nm, from 4.3 ± 0.1 nm to 3.5 ± 0.1 nm, and from 3.1 ± 0.2 nm to 2.6 ± 0.2 nm for Ag, Ge and CdSe, respectively. No ZnS nanoparticle size reduction was observed. Theoretical models for nanoparticles undergoing laser heating show that temperatures above the bulk and nanoparticle material melting point reduce the nanoparticles size by a factor of 0.3 and suggest recondensation before collection. For CdSe nanoparticles collected on dry substrates and solvents, blue shifted fluorescence (PL) peaks support the size reduction.
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碩士
國立臺北科技大學
機械工程系機電整合碩士班
106
At the beginning of this study, a horizontal furnace tube was used to deposit ZnS nanowires on P-type Silicon substrate (100) by chemical vapor deposition and through a VLS mechanism, and the working pressure was changed to find out the best crystallinity. And the crystal structure is wurtzite structure with the (002) plane as the preferred growth direction. The second step is annealing ZnS nanowires at different temperatures in an oxygen environment. From the results of the annealing, it can be found that the surface of the ZnS nanowire starts to transform to ZnO at 600 °C and formation ZnS-ZnO core-shell heterostructure. If the temperature increase higher than 600 °C will completely convert ZnS into ZnO. From the SEM, it was observed that after the ZnS nanowires formation ZnS-ZnO core-shell heterostructure, the wire-like surface changed from smooth to rough. And ZnS nanowires was converted into ZnO branch nanostructure at higher temperature. Finally, we used two different wavelengths of UV light to measurement our electronic component. By irradiating the UV light at wavelengths of 300 nm and 365 nm, respectively, the ZnS nanowires and the ZnS-ZnO core-shell heterostructure will generate currents and responses, but the ZnS nanowires shows selective for specific wavelengths, because of its wide band gap, making ZnS nanowires unresponsive to ultraviolet light at a wavelength of 365 nm. And forming a ZnS-ZnO core-shell heterostructure by annealing at oxygen environment increases the response wavelengths through the presence of ZnO. This photodetectors exhibit high spectral selectivity and wide range photoresponse for ultraviolet light. The current gain at the ultraviolet wavelength of 300 nm is increased from 1.57 in the original ZnS nanowires to 160 in hybrid structure, due to the generation of electron-hole pairs under light illumination, the electrons and holes move at the interface, facilitating the formation of a charge transfer state and the spatial separation of the photocatalytic carriers within the nanowires. And since the energy band arrangement of the ZnS-ZnO core-shell heterostructure contributes to the photocatalytic of organic substances, the reaction rate is the original ZnS nanowires 1.6 times.

碩士
國立清華大學
化學系
101
CHAPTER 1 One−Pot Synthesis of Silver Nanocubes and Their Morphological Transformation We have used one-pot reaction to synthesize cubic silver nanoparticles with average sizes of 62 to 80 nm in aqueous solution at 50 ºC for 6 hours. The reagents used here are AgNO3, cetyltrimethylammonium chloride (CTAC), ascorbic acid, and NaOH. In this method, silver nanoparticles are obtained by using ascorbic acid as reducing agent to reduce AgCl(s) in the presence of CTAC. NaOH is added to increase the reducing ability of ascorbic acid. Silver nanocubes with sizes varying from 62 to 80 nm are obtained by increasing the amount of AgNO3. Different amounts of NaOH also influence the morphology of silver nanoparticles. By increasing the amount of NaOH, the formation of {111} facets is enhanced and the shapes of silver nanoparticles change from cubes to truncated cubes and cuboctahedra due to the increased reaction rate. In addition, reaction rate is also increased when CTA+NO3─ serve as the surfactant. Furthermore, silver cuboctahedra are obtained and evolve into truncated octahedra by increasing the reaction temperature. CHAPTER 2 Synthesis and Plasmonic Properties of Au−Pd Core−Shell Heterostructures with Variable Shapes and Shell Thickness In this work, we report the investigation of plasmonic properties of Au−Pd core−shell heterostructures with different shapes, including cubes, cuboctahedra, truncated octahedra, and octahedra. Here, we have used a seed-mediated growth method to synthesize Au−Pd core−shell heterostructures with 35, 45, and 74 nm gold octahedra as cores. Au−Pd core−shell heterostructures with various shapes and sizes are prepared by mixing cetyltrimethylammonium bromide (CTAB), octahedral gold cores, H2PdCl4, and ascorbic acid at 50 ºC in less than 2 hours. The uniform shape of these nanocrystals and the ability to tune the shell thickness allow us to investigate their localized surface plasmon resonance (LSPR) properties. When the shell thickness become thin enough, blue shift of Au LSPR absorption band of Au−Pd core−shell heterostructures is observed. The Au LSPR absorption band red-shifts and become more obvious when the shape of core−shell nanostructures transforms from cubes to octahedra due to the shell thickness variation induced by change in the mole ratios of Pd : Au in a nanoparticle. Comparing to previous studies, this is the first time the plasmonic properties of bimetallic core−shell heterostructures with various shapes and tunable sizes is investigated. For Au−Pd core−shell octahedra, their Au LSPR absorption band is more pronounced and they may have the potential to be applied for plasmonic sensing such as hydrogen sensing.

博士
國立交通大學
材料科學與工程學系所
102
Due to the difference in band structure between the constituents, semiconductor heterostructures exhibit remarkable charge separation property which is beneficial to solar fuel generation. On the other hand, the advantages of quantum dots-based light emitting diodes (QD-LEDs) include color tunability, high color saturation and high color rendering index (CRI) white lighting. The performance of both photon-to-electron and electron-to-photon conversions is closely related to the intrinsic charge carrier dynamics of the constitutes. In this dissertation, the correlation between the charge carrier dynamics and the photoelectric conversion efficiency for semiconductor heterostructures and core/shell Cd1-xZnxSe QDs was investigated. Three individual yet relevant projects were included in the dissertation: First, we demonstrated that Au-decorated NaxH2-xTi3O7 nanobelts (NaxH2-xTi3O7-Au NBs) may exhibit remarkable photocatalytic performance under visible light illumination due to the remarkable charge separation property. In order to further enhance the photocatalytic efficiency, a thin layer of Cu2O was deposited on the Au surface of the Au-decorated NaxH2-xTi3O7 NBs to form Z-scheme NaxH2-xTi3O7-Au-Cu2O nanoheterostructures. Because of the relative band alignment of the constituents, Au may mediate the carrier transfer of NaxH2-xTi3O7-Au NBs to render them enhanced photocatalytic performance. Time-resolved photoluminescence (PL) spectra were measured to quantitatively analyze the electron transfer in the Z-scheme NaxH2-xTi3O7-Au-Cu2O NBs. The carrier utilization efficiency of the samples was evaluated and the result was correlated with that of the charge carrier dynamics measurement, which may provide insightful information when using Z-scheme heterostructures in photoconversion applications. Second, we investigated the plasmonic effect of noble metal nanocrystals on the photocatalytic properties of semiconductor nanostructures. Since the surface plasmon resonance (SPR) of metal (e.g. Ag and Au) energizes the conduction electrons and excites them from the outermost bands to higher energy states, there is a great probability that these electrons can participate in chemical reactions. We developed a Ag-decorated SiO2 NSs, which exhibited significantly red-shifted and relatively broad SPR absorption spanned from visible to near-infrared region. The photocatalytic activity of Ag-decorated SiO2 NSs was corresponded with the SPR absoption ability. On the other hand, by acting as an antenna that localizes the optical energy by SPR, plasmonic Au can sensitize TiO2 to light with energy below the band gap, generating additional charge carriers for water oxidation. The photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 nm to 800 nm, by manipulating the shape of the decorated Au nanostructures. The analysis results suggested that the enhanced photoactivity of Au NP-decorated TiO2 nanowires in UV region was attributed to effective surface passivation. Since the existence of surface states greatly affected the photoconversion performance of TiO2, we employed a facile precursor-treatment approach for effective surface passivation of rutile TiO2 nanowire photoanode to improve its performance in photoelectrochemical water oxidation. Last, we developed a single-step hot-injection process to synthesize core/shell Cd1-xZnxSe QDs with tunable emission wavelengths. Because of the higher reactivity of the Cd precursor, QDs whose composition was rich in CdSe were generated at the beginning of reaction. As the reaction proceeded, the later-formed ZnSe shell was simultaneously alloyed with the core, giving rise to a progressive alloying treatment for the grown QDs. During the reaction period, the continued blue shifiting emissioned Cd1-xZnxSe QDs were obtained. A LED composed of conducting polymer with Cd1-xZnxSe QDs was fabricated to test the electroluminescence properties, which show high color purity for the emissions from LED. The findings from this work also demonstrate the advantage of using the current single-step synthetic approach to obtain a batch of Cd1-xZnxSe QDs that may emit different colors in prototype LEDs.

碩士
國立清華大學
化學系
103
CHAPTER 1 Seed-Mediated Growth of Silver Nanocubes and Their Morphological Transformation Silver nanoparticles are often synthesized in organic solvents with the use of high reaction temperatures. If nanoparticles can be synthesized in aqueous solution, the method would be energy-saving and environmentally friendly. In the literature, long reaction time and high temperatures are still need to synthesize silver nanocubes. Here we present a facile and low temperature approach to prepare silver nanocubes in aqueous solution and investigat how the reaction rate controls the final product morphology. In this study, we have developed a seed-mediated growth method to synthesize Ag nanocrystals in aqueous solution. The method involves the addition of a small volume of a seed solution to an aqueous solution of silver nitrate (AgNO3), cetyltrimethylammonium chloriode (CTAC), and ascorbic acid (AA). We utilized AgNO3 as silver source, CTAC as surfactant, and AA as reducing agent. Silver nanocubes were generated in 2 hours at 60 ºC. Transmission electron microscopy (TEM), powder X–ray diffraction (PXRD) pattern, and scanning electron microscopy (SEM) have been employed to characterize the nanocubes enclosed by {100} facets. The edge length of cubes can also be tuned from 46 to 55 nm. Here, we also present the effects of NH3 solution on morphological transformation. The acceleration of the reaction rate by introducing NH3 solution promotes the formation of the {111} facets. The solution color at different time points during synthesis also proved that the reaction rate controlled the final particle morphology. CHAPTER 2 Synthesis of Au–Ag Core–Shell Heterostructures with Systematic Shape Evolution and Their Optical Properties In this study, we have utilized rhombic dodecahedral gold nanocrystals as the structure-directing cores for the growth of Ag shells in aqueous solution. Au–Ag core–shell heterostructures with different morphologies can be directly synthesized. The reagents we used are silver nitrate (AgNO3), cetyltrimethylammonium chloriode (CTAC), ascorbic acid (AA), and sodium hydroxide (NaOH). By simply varying the concentration of reducing agent or silver source, shape evolution from cubes, truncated cubes, cuboctahedra, truncated octahedra and octahedra were obtained. The reaction was finished within 50 minutes at 30 ºC. This is a time- and energy saving method. These monodisperse nanocrystals can readily form self-assembled structures. By monitoring the solution color at different time points during synthesis or changing the temperature, particle growth rates was found to be fastest for octahedra covered by {111} facets. On the other hand, a slower reaction rate favors the generation of cubes enclosed by {100} facets. The nanocube and nanooctahedra size can also be tuned within a range. UV–vis spectra were used to investigate their unique optical property and suggested that their optical responses are closely related to silver shell thickness and gold core size. Both spectral blue-shifts and red-shifts of the Au–Ag nanocrytals compared to Au cores have been observed. With very thin shell thickness, spectral blue-shift was recorded. As particle size increases, red-shift occur.