Dissertations / Theses on the topic 'Nanocrystals'

This thesis will mainly focus on the synthesis and characterization of colloidal lead halide perovskite (LHP) nanocrystals (NCs). It will also shed light on many synthetic aspects of these NCs, such as their size, shape and compositional control, as well as on how to increase their stability and processability, allowing the use of LHP NCs in devices. Furthermore, this thesis will present several nano-scaled reactions using lead halide NCs, which result in so-called “nanocrystal transformations”. To bridge the gap between the fundamental synthetic work and the application of these NCs, several proof-of-principle devices have been made with LHP NCs, including solar cells and LEDs and will all be demonstrated herein. Finally, several new halide based NCs have been synthesized and will be discusses, broadening the scope of this work not only to LHP NCs, but also to halide based NCs in general.

Optical imaging based on fluorescence has yet to be introduced as a clinical diagnostic tool due to the lack of reliable, photostable, and highly luminescent fluorophores. Fluorescent nanocrystals, or quantum dots (QDs), are promising alternatives to organic dyes, since QDs are small in size, resistant to photo-bleaching, and have excellent and appropriate optical properties. The main objective of this work is to use QDs for real-time imaging in live animals. Widespread use of QDs in biology is currently limited due to their questionable biocompatibility, and to the fact that some nanocrystals contain heavy metals, which are potentially hazardous, in their cores. In the present studies the mechanisms underlying the toxicity of cadmium telluride QDs was investigated in several stable cell lines. After long-term exposure to QDs, significant morphological and functional changes were observed at the cellular and subcellular levels. We showed that QD-induced toxicity includes the production of reactive oxygen species, peroxidation of membrane lipids, impairment of mitochondrial function, and changes in the genome and epigenome. Understanding how toxic QDs cause damage to the cells is a first step for i) the establishment of protocols to evaluate the safety of other nanomaterials, and ii) the development of new or improved nanocrystals that are non-toxic. We showed that modifications on QD surfaces with small drug molecules (e.g. N-acetylcysteine) or synthetic polymers can significantly decrease their toxicity, and in some cases, even render the QDs non-toxic. Utilizing a non-invasive route (i.e. intranasal) to deliver nano-probes and nano-therapeutics to the brain, we demonstrated the use of near-infrared fluorescence of non-toxic QDs to image cerebral microlesions in live animals. Repeated imaging in vivo allowed for the live monitoring of lesion size in animals; a reduction of lesion size is a measure of the effectiveness of nano-therapeutic interventions. Animals treated with micelle-incorporated nimodipine or minocycline had significantly smaller lesion volumes, and displayed better recovery of motor function. Quantitative evaluation and volume calculations were possible since the QD signal was isolated from autofluorescence and background after fluorescence lifetime gating. Taken together, the results from this work contribute to the development of QDs and fluorescence technology for biomedical imaging in two main ways: 1) by presenting in vitro measures as the first step in the evaluation of nanomaterial safety. 2) by demonstrating the advantages of using near-infrared QDs for non-invasive lifetime imaging in animals with unilateral cortical ischemic microlesions and for the determination of the spatio-temporal reduction of lesions upon nano-therapeutic interventions. These findings support the use of carefully designed and rigorously tested fluorescent QDs for lifetime optical imaging of the brain in experimental animals, and eventually extending to clinical studies.
L'imagerie par fluorescence reste à introduire dans les cabinets médicaux en raison du manque de fluorophores photo-stables, à haute intensité lumineuse, disponibles sur le marché. Les nanocristaux fluorescents ou boîtes quantiques (BQ), représentent une alternative intéressante par rapport aux teintures organiques car les BQ sont très petits, résistants au photoblanchiment et ont d'excellentes propriétés optiques. L'objectif principal de cette étude est d'utiliser les BQ pour une imagerie en temps réel sur les animaux vivants. L'usage étendu des BQ en biologie est limité en raison de leur biocompatibilité discutable et également en raison du fait que quelques nanocristaux sont composés en partie de métaux lourds. Dans cette étude, les mécanismes cellulaires impliquant la toxicité des BQ de cadmium telluride sont examinés. Après une exposition prolongée aux BQ, des modifications morphologiques et fonctionnelles significatives ont été observées à l'échelle cellulaire et infracellulaire. Nous démontrons que la toxicité induite par les BQ peut entrainer la production d'espèces réactives de l'oxygène, la peroxydation des lipides de la membrane biologique, l'altération du fonctionnement mitochondrial mais aussi des changements du génome et de l'épigénome. Comprendre comment les BQ toxiques endommagent les cellules est un premier pas dans l'établissement de protocoles d'évaluation de la sécurité des nanomatériaux et dans le développement de nouveau nanocristaux non-toxiques. Nous démontrons que la modification de la surface des BQ grâce à des médicaments (ex : N-acetylcysteine) ou des polymères synthétiques peut grandement diminuer leur toxicité, et dans quelques cas, peut aussi rendre les BQ non-toxiques. En utilisant de tel BQ non-toxiques, nous effectuons une démonstration de l'utilisation de la fluorescence infrarouge proche pour effectuer des clichés en temps réel de microlésions cérébrales sur des animaux vivants, à l'aide de méthodes non effractives (ex : voie intra-nasale) pour insérer des nano-sondes ou administrer des nano-thérapies au niveau du cerveau. Des imageries répétées permettent de surveiller la taille des lésions sur les animaux, et prouvent l'efficacité des nano-thérapies dans la prévention de l'expansion de la lésion. Les animaux traités par micelles chargées de nimodipine ou de minocycline ont des lésions moins volumineuses et une meilleure récupération de la fonction motrice. Une évaluation quantitative et un calcul de volume ont été possibles car le signal BQ était séparé de l'autofluorescence tissulaire grâce à de la synchronisation d'image fondé sur la durée de vie fluorescence. L'ensemble des résultats de ces études contribue au développement des BQ et des technologies par fluorescence en imagerie biomédicale, et ceci de deux façons : 1) en présentant des résultats in vitro qui constituent une première étape dans l'évaluation de la sécurité des nanomatériaux. 2) en démontrant des avantages de l'utilisation les BQ infrarouges proches pour l'imagerie non effractives sur les animaux vivants avec des lésions cérébrales et pour la détermination de la réduction des lésions après des nano-thérapies. Ces constatations appuient l'utilisation des BQ fluorescentes créés avec soin et ayant subi des essais précliniques rigoureux pour l'imagerie encéphalique in vivo et s'étendant finalement aux études cliniques.

Thesis (Ph.D.); Submitted to the University of California at Berkeley, Berkeley, CA (US); 1 Sep 2003.
Published through the Information Bridge: DOE Scientific and Technical Information. "LBNL--55024" Williams, Shara Carol. National Institutes of Health (US) 09/01/2003. Report is also available in paper and microfiche from NTIS.

Colloidal nanocrystals offer new and improved performance in applications as well as less environmental impact when compared to traditional device fabrication methods. The important properties that enable improved applications are a direct result of nanocrystal structure. While there have been many great advances in the production of colloidal nanocrystals over the past three decades, precise, atomic-level control of the size, composition, and structure of the inorganic core remains challenging. Rather than dictate these material aspects through traditional synthetic routes, this dissertation details the development and exploitation of a colloidal nanocrystal synthetic method inspired by polymerization reactions. Living polymerization reactions offer precise control of polymer size and structure and have tremendously advanced polymer science, allowing the intuitive production of polymers and block co-polymers of well-defined molecular weights. Similarly, living nanocrystal synthetic methods allow an enhanced level of structural control, granting the synthesis of binary, doped, and core/shell nanocrystals of well-defined size, composition, and structure. This improved control in turn grants enhanced nanocrystal property performance and deepens our understanding of structure/property relationships. This dissertation defines living nanocrystal growth and demonstrates the potential of the living methods in the colloidal production of oxide nanocrystals. After a brief introduction, living growth is defined and discussed in the context of synthetic prerequisites, attributes, and outcomes. Living growth is also compared to more traditional colloidal nanocrystal synthetic methods. The following chapters then demonstrate the precise control living approaches offer in three separate studies; the first highlights sub-nanometer control of nanocrystal size from 2-22+ nm in diameter. Next the improvement in nanocrystal composition is illustrated using several transition metal dopants into an oxide nanocrystal matrix at near thermodynamically allowed compositions. Additionally, precise radial dopant placement is demonstrated, which has striking implications for material properties. The radial position of tin in tin-doped indium oxide nanocrystals and the resulting differences on the localized surface plasmon resonance are discussed. Finally, future opportunities are reviewed. This dissertation includes previously published co-authored material.

Semiconductor nanocrystals are expected to play an important role in the development of new generation of microelectronic and photonic devices such as light emitting diodes and memory elements. Optimization of these devices requires detailed investigations. Various spectroscopic techniques have been developed for material and devices characterization. This study covers the applications of the following techniques for the analysis of nanocrystalline materials: Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, X-Ray Diffraction (XRD) and X-Ray Photoelectron (XPS). Transmission Electron Microscopy (TEM) and Secondary Ion Mass Spectrometry (SIMS) are also used as complementary methods. Crystallinity ratio, size, physical and chemical environment of the nanostructures were probed with these methods. Si and Ge nanocrystals were formed into the oxides Al2O3 and SiO2 by ion implantation, magnetron sputtering and laser ablation methods. FTIR and XPS are two methods used to extract information on the surface of the nanocrystals. Raman and XRD are non destructive and easy-to-operate methods used widely to estimate the crystallinity to amorphous ratio and the sizes of the nanocrystals. In this study, the structural variations of SiO2 matrix during the formation of Si nanocrystals were characterized by FTIR. The shift in position and changes in intensity of the Si-O-Si asymmetric stretching band of SiOx was monitored. An indirect metrology method based on FTIR was developed to show the nanocrystal formation. Ge nanocrystals formed in SiO2 matrix were investigated using FTIR, Raman and XRD methods. FTIR spectroscopy showed that Ge atoms segregate completely from the matrix at relatively low temperatures 900 oC. The stress between the Ge nanocrystals and the matrix can vary in samples produced by magnetron sputtering if the production conditions are slightly different. Si and Ge nanocrystals were formed into Al2O3 matrix by ion implantation of Si and Ge ions into sapphire matrix. Raman, XRD, XPS and TEM methods were employed to characterize the formed nanocrystals. XRD is used to estimate the nanocrystal sizes which are in agreement with TEM observations. The stress on nanocrystals was observed by Raman and XRD methods, and a quantitative calculation was employed to the Si nanocrystals using the Raman results. XPS and SIMS depth profiles of the sample implanted with Si, and annealed at 1000 oC were measured. Precipitation of Si atoms with the heat treatment to form the nanocrystals was observed using XPS. The volume fraction of the SiOx shell to the Si core in Si nanocrystals was found to be 7.9 % at projection range of implantation.

Cellulose nanocrystal surface modification is an expanding area in cellulose research and this thesis aims to add knowledge to this growing field. Two esterification techniques new to cellulose nanocrystal research were applied successfully to the formation of esters of (methylthio)acetic add, two pyridinum substituted benzoic acids and rnethacrylic acid. The efficacy of the two methods was compared with each other and with those used in the literature. Two click chemistry reactions were also applied to cellulose nanocrystals. Azidation of chlorinated cellulose nanocrystals allowed application of copper(I) catalysed azidealkyne cydoaddition to the grafting of two irnidazoliurn salts and ferrocene to cellulose nanocrystals. Attachment of a disulfide to cellulose nanocrystais lead to a one-pot disulfide reduction and thiol Michael addition to graft cellulose nanocrystals with pentabromobenzyl acrylate. These different surface modification strategies were used to prepare a variety of surface active nanopartides for further application. Cationic cellulose nanocrystals were produced with higher surface charge density than previously reported in the literature. The cationic nature of the nanocrystals was probed using an anionic dye adsorption methodology. The variation in anion affinity for imidazolium grafted cellulose nanocrystals was determined using a batch mixing methodology with ion chromatography. Cellulose (methylthio)acetate nanocrystals were tested as a potential supported sulfur ylid in the rhodiurn(II) acetate and sulfide co-catalysed conversion of aldehydes to epoxides. This proved unsuccessful with by-products suggesting fa ilure to form supported ylids. Finally, cellulose nanocrystals were modified with a multidentate amine ligand using a diisocyanate and the resulting nanocrystals used to bind palladiurn(II) acetate. These nanocrystals were tested in Sonogashlra reactions for recydability of the palladium catalyst. Significant leaching of the palladium catalyst occurred without the use of a copper co-catalyst and the exact nature of the palladium species present on the surface of the nanocrystals remains unknown

The achievement of luminescent nanocrystalline solid films, with good optical quality, will be crucial to the development of opto-electronic devices based on such materials. Although (CdSe)ZnS "core-shell" nanocrystals are typically found to have solution photoluminescence (PL) efficiencies in excess of 60%, the values associated with solid films are found to be an order of magnitude lower. Care of surface chemistry and control of nanocrystal/matrix interactions are of paramount importance. Furthermore, the PL efficiency exhibits a dependence on nanocrystal concentration consistent with a semi-quantitative model describing the effects of Förster energy transfer between nanocrystals and the associated trapping at surface sites. In addition to the ability to control optical properties by variation of the nanocrystal dimensions, it is also possible to alter the optical environment in which the nanocrystals are situated. By placing films of nanocrystals into high-Q, planar microcavities, it is possible to produce significant alteration of photoluminescence into very narrow resonant modes of the cavity. This is an important technical step towards the realisation of a nanocrystal laser. The combination of robust semiconductor emitters with the convenience of solution processing also offers considerable advantages over conventional molecular beam epitaxy (MBE) techniques. Finally, the PL emission from close-packed core-shell nanocrystalline thin films under intense picosecond UV excitation is studied. Strong, stable line-narrowing features are observed as the excitation intensity is increased, both at 77K and at room temperature; these are attributed to waveguiding and amplified spontaneous emission (ASE) in the films. Such behaviour would usually be considered as the signature of optical gain. Lasing from microcavities based on these films has yet to be observed, however, and a semi-empirical model of line-narrowing threshold intensities and cavity-photon lifetimes suggests that higher gain, lower losses or greater cavity finesse may be required for this.

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.

The primary focus of this thesis is to develop new synthesis routes for core-shell copper indium di-sulphide (CuInS₂)–zinc sulphide (ZnS), and core-only silver indium di-sulphide (AgInS₂) nanocrystals with enhanced photoluminescence quantum yield (PLQY) and study their application in luminescent concentrators. CuInS₂-ZnS nanocrystal photoluminescence (PL) properties were investigated using different synthesis conditions. Through a combination of PL, absorption (Abs), and transmission electron microscopy (TEM), the importance of relative stoichiometry between the two cations (copper and indium) are understood. To further study this effect, different injection temperatures and precursor ratios were employed. It was determined that with increasing indium (In) content, the PL blue shifted while increased PLQY was noted. Optimal synthesis conditions for monodispersed nanocrystals was found to be at an injection temperature of 30°C with a 1:4 Cu:In ratio, which achieved a PLQY of 40%. By using various halide precursor combinations of fluoride, chloride, bromide, and iodide, 16 combinations were evaluated in the control of the reactivity of the two cation precursors. Overall, this study demonstrated that the reactivity of Cu and In can be regulated to a certain extent. Additionally, the halide precursor counter ions potentially provided surface passivation, which enhanced PLQY by up to 60%. The synthesis method established was repeated with replacement of the easily oxidised Cu with more stable Ag to form AgInS₂ nanocrystals. It was shown that by-products of Ag and Ag₂S nanocrystals were observed, which reduced the overall PLQY. With in-situ PL, synthesis dynamics were examined. Ag₂S nanocrystals were formed from the Ag nanocrystals formed prior to sulfur injection. Through changes in reduction temperature and halide precursor combinations, the by-product quantity was reduced and a high PLQY of 89% was achieved. Using AgInS₂, the first ternary nanocrystal luminescent concentrator for visible light communication (VLC) was fabricated. Modifying the polymer and nanocrystal concentrations resulted in changes in transmittance and reflectance properties. It was determined that the best combination used was ethyl cellulose 8wt% with 9mg/ml AgInS₂ (AIS) nanocrystals. With the addition of a mirror and a low refractive polymer layer, the gain, when compared to a single photodiode (PD), was seven times higher.

In this work we present spectroscopic studies of the exciton dynamics in colloidal spherical cadmium sulfide CdS nanocrystals (NC) in the vicinity of a single indium gallium nitride InGaN quantum well (QW) in dependence of temperature. QWs of the alloy material InGaN exhibit a dependence of the exciton dimensionality on the thermal energy available. It was demonstrated that this dependence in uences the rate for uorescence resonant energy transfer from the QW to a layer of CdS-NC deposited on top of its capping layer. Investigations of different capping layer thicknesses demonstrated the dependence of the exciton dimensionality on the disorder potential of the QW. Furthermore, spectroscopic measurements of elongated asymmetric cadmium selenide/- cadmium sulfide CdSe/CdS nanorods (NR) under the application of external magnetic and electric fields are discussed. Asymmetric CdSe/CdS-NR represent a special case of elongated NR as the analytical treatment of spherical NC can be combined with numerical methods of calculating the electron and hole energies and wave functions. The results for the excitonic fine structure splitting in these nanomaterials is used to explain the dependence of the exciton dynamics under an external magnetic field. For the first time, a separate measurement of the Zeeman splitting and the magnetic field induced spin admixture in colloidal NR was performed. Electric field mediated carrier separation in asymmetric CdSe/CdS-NR is measured in time resolved luminescence quenching experiments. Retrieval of stored excitations is demonstrated employing a synchronised ultrafast voltage pulse detection scheme.

Les nanocristaux semi-conducteurs possèdent des tailles qui se situent entre celles des molécules et des matériaux cristallins. Leurs propriétés physiques sont donc dominées par des effets de confinement quantique et par des états électroniques discrets. Une étude approfondie de leur structure électronique et en particulier de la localisation des porteurs de charge s’avère nécessaire pour pouvoir à plus long terme faire de l’ingénierie de structure de bande des hétérostructures semi-conductrices. La microscopie à effet tunnel est l’outil idéal pour imager et sonder les propriétés électroniques de nanocristaux. Le système peut être comparé à une jonction tunnel à doublé barrière tunnel (chapitre 1). Pour caractériser les effets de Coulomb dans des objets quantiques par spectroscopie tunnel (technique détaillée au chapitre 2), mes travaux de recherche ont tout d’abord porté sur un système modèle : une liaison pendante silicium, dont l’état de charge a pu être modifié de manière contrôlée (chapitre 3). Des nanocristaux cœur-coquille (PbSe/CdSe) à symétrie sphérique ont ensuite été étudiés (chapitre 4). Contrairement aux nanocristaux sans coquille, les expériences révèlent que le transport est dominé par le même type de porteurs de charge à polarisation positive et négative de la jonction. Ces mesures donnent également accès à l’énergie de charge des nanocristaux. Un régime de transport similaire est obtenu pour des nanobâtonnets constitués d’un cœur sphérique CdSe enfermé dans un bâtonnet de CdS (chapitre 5), démontrant la reproductibilité des phénomènes observés par l’hétérostructures cœur-coquille
Semiconductor colloidal nanocrystals are quite attractive, because of their physical properties, such as discrete energy levels. However, devices prepared from semiconductor nanocrystals are still facing limitations due to a high environmental sensitivity of their organic shell. In order to increase their optical properties, core-shell nanocrystals have thus been synthesized. Scanning tunneling microscopy is the appropriate tool to image and probe the electronic properties of individual nanostructures and. This system can be compared to a double barrier tunnel junction, where the transport properties are governed by the transmission probability across both potential barriers (chapter 1). In order to investigate the Coulomb effect in those quantum objects by tunneling spectroscopy (this technique being described in chapter 2), the thesis has first focussed on a prototypical model: an isolated silicon dangling bond, where its charge state has been changed in a controlled manner (chapter 3). Then, PbSe/CdSe core-shell nanocrystals have been studied and a general method is described to correctly identify the electrical nature of the charge carriers in the tunneling spectra (chapter 4). In contrast to the core nanocrystals the transport through core-shell structures reveals, for a majority of nanocrystals, that the same type of charge carrier tunnel on both sides of the apparent gap. Charging peaks are also observed and allow the measurements of the charging energy in these systems. A similar transport regime is obtained for CdSe/CdS dot in rod nanocrystals (chapter 5), demonstrating the reproducibility of the characterized transport phenomena of nanoheterostructure

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Chemical and Biomolecular Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

We have investigated ways of modifying a common water soluble CdTe NCs to become non-photobleaching. Such NCs are capable of responding reversibly to an inter-switching of the oxygen and argon environments over multiple hours of photoexcitation. They are found to quench upon exposure to oxygen, but when the system is purged with argon, their photoluminescence (PL) revives to the original intensity. Such discovery could potentially be used as oxygen nanosensors. These PL robust CdTe NCs immobilized on glass substrates also exhibit significant changes in their PL when certain organic/bio molecules are placed in their vicinity (nanoscale). This novel technique also known as NC-organic molecule close proximity imaging (NC-cp imaging) has found to provide contrast ratio greater by a factor of 2-3 compared to conventional fluorescence imaging technique. PL of NCs is recoverable upon removal of these organic molecules, therefore validating these NCs as potential all-optical organic molecular nanosensors and, upon optimization, ultimately serving as point detectors for purposes of super-resolution microscopy (with proper instrumentation). No solvents are required for this sensing mechanism since all solutions were dried under argon flow. Furthermore, core graded shell CdSe/CdSeₓS(1-x)/CdS giant nanocrystal (g-NCs) were found to have very robust PL temperature response. At a size of 10.2 nm in diameter, these g-NCs undergo PL drop of only 30% at 355K (normalized to PL intensity at 85K). In comparison, the core step shells CdSe/CdS g-NCs at the same diameter exhibit 80% PL drop at 355K. Spectral shifting and broadening were acquired and found to be 5-10 times and 2-4 times smaller respectively than the standard CdSe core and CdSe/CdS core shell NCs. It is also discovered that these core graded shell g-NCs are largely nonblinking and have insignificant photoluminescence decay even after exciting the samples at very high irradiance (44 kW/cm²) for over an hour. These types of g-NCs have great potential to be used as the active medium for temperature insensitive laser devices in the visible range or temperature insensitive bioprobes for bioimaging applications.

Chemical surface modification of cellulose nanocrystals has had a fast development and increased interest from the scientific community as cellulose is the most abundantly available renewable polymer with many advantages such as nanoscale dimensions, high specific strength and modulus, high surface area, unique optical properties and the extraordinary modification potential to increase the application field. This thesis is aimed at expanding and improving upon the current knowledge in order to unlock new applications. Four esterification techniques were applied to the formation of cellulose nanocrystal esters of acrylic acid and methacrylic acid. The degree of surface substitution reached two to three surface hydroxyl groups (the maximum number) available for functionalization and this degree of substitution is very much dependent on the chosen esterification methodology. Two new fluorescently modified cellulose esters based on carbazole-9-yl-acetic acid and coumarin-3-carboxylic acid were synthesised using p-toluenesulfonyl chloride/pyridine and carbodiimide esterifications methods. Absorption and fluorescent properties were also measured and showed fluorescence proportional to the extent of surface functionalization. The maximum theoretically attainable degree of substitution could be reached while still maintaining the crystal structure of cellulose. Cationic cellulose nanocrystals were produced with a high positive surface charge when compared with the literature. The synthesis procedure was attempted in two steps and in a single step. The degree of modification for pyridinium acetate cellulose and methyl imidazolium acetate cellulose was found to depend significantly on the selected pathway. The cationic nature of the modifications was verified using zeta potential measurements and through adsorption of an anion dye. Synthesised cellulose acrylates and methacrylates were used in Thiol-Ene click reactions in which very mild and environmentally friendly reaction conditions proved to work from 10 min reaction times. Four different thiols were added, with and without hexylamine catalyst. In addition, an amidine functionalised cellulose nanocrystal was synthesised based on previously click-modified cellulose in a 2-hour reaction. Furthermore, the switchable behaviour of the synthesised nanoparticles was demonstrated by reverse bubbling with CO2 and Ar.

Les propriétés des nanozéolithes, à savoir leur grande surface, leur stabilité hydrothermale et leur nature non toxique, permettent leur utilisation dans des applications prospectives, notamment la biomédecine (capteurs, administration de médicaments et de gaz) et la microbiologie (agents antibactériens). De nombreuses recherches ont été consacrées à l’étude de nouvelles applications biomédicales utilisant des matériaux zéolithiques, toutefois leur plein potentiel n’a toujours pas été pleinement dévoilé.Il est bien connu que la résistance croissante aux traitements établis de tumeurs et d’infections bactériennes par radiothérapie et antibiotiques est un problème de première importance. Par conséquent, le développement de nouvelles stratégies thérapeutiques pour résoudre ces problèmes est très démandé.L'objectif de cette recherche de doctorat est de synthétiser et de modifier post-synthétiquement des zéolithes nanométriques pour des applications biomédicales. Cela implique l'échange d'ions de zéolithe avec divers cations pour trouver celui qui convient le mieux aux applications souhaitées : le traitement antimicrobien, la réoxygénation des tissus tumoraux et l’administration de gaz.Dans cette étude, nous rapportons: (i) l'effet de la zéolithe FAU de type nanométrique modifiée au cuivre sur les bactéries de type ESKAPE (chapitre 3), (ii) l’utilisation de nanozéolithes contenant du métal comme outil d'oxygénation et de visualisation tissulaire (chapitre 4) et enfin (iii) l'utilisation de nanozéolithes FAU comme vecteur de l'oxyde nitrique et du dioxyde de carbone pour prévenir des maladies potentiellement létales (chapitre 5)
The properties of nanozeolites, namely, large surface area, hydrothermal stability and non-toxic nature, enable their utilization in forward-looking applications, including biomedicine (sensors, drug and gas delivery) and microbiology (antibacterial agents). Hence, a lot of research has been devoted to study the new biomedical applications using zeolitic materials, their full potential has still not been fully unveiled.It is well recognised that growing resistance to already established treatments of tumors and bacterial infections using radiotherapy and antibiotics is a distressing matter. Therefore, the development of new therapeutic strategies towards above issues is of great demand.The goal of this PhD research is to synthesise and post-synthetically modify nanosized zeolites for biomedical applications. This involves the ion-exchange of zeolite with various cations to find the most suitable one for desired applications in regards to antimicrobial treatment, tumour tissue reoxygenation and gas delivery.In this study, we report: (i) the effect of copper modified nanosized FAU type zeolite on ESKAPE type bacteria (Chapter 3), (ii) the metal containing nanozeolite as a tool for tissue oxygenation and visualisation using MRI (Chapter 4), and lastly (iii) the use of FAU nanozeolite as nitric oxide and carbon dioxide gas vector to prevent life threatening conditions (Chapter 5)

Metal halide perovskites (particularly doped perovskites and lead free double perovskites) are starting to generate great interest in the scientific community due to their unique electronic and structural properties, such as high photoluminescent quantum yields (PLQY, up to 90%), chemical diversity in terms of elements employed and tunable optical properties. Consequently, their application in optoelectronic devices gained attention. During these three years, I intensively worked on the synthesis and characterization of inorganic perovskites nanocrystals, starting from lead halide perovskites (LHPs) in 3D and 0D structure and then moving to the double perovskites (DPs). Generally, the aim of these studies is to replace Pb with less toxic elements, producing materials more stable to atmosphere conditions and with good optical properties. Thus, synthesis and optimization are the key words of this part of the work. Pb has been replaced with a monovalent (Ag, Na) and a trivalent (In, Bi), or a bivalent (Cu, Mn) and a trivalent (Sb) metal cation, leading to DPs (e.g. Cs2AgInCl6) and layered perovskites (e.g. Cs4CuSb2Cl12), respectively. However, perovskites are not the only promising candidate for optoelectronic devices, in particular considering the increasing interest in studying NIR emitting materials. In this field, my work on silicates takes place. In fact, Cu - based silicates (e.g. CaCuSi4O10) possess a high emission in NIR region (900–1000 nm). Moreover, their high Stokes shift, which limits re-absorbance phenomenon, and the high stability to ambient condition and sun irradiation, suggest their use in solar absorbing devices. During my PhD I performed deep structural investigation using synchrotron radiation after an optimization of the material synthesis; then, I worked on their exfoliation leading to the formation of very homogeneous nanosheets.

Aim of this work was, to investigate the preparation of Si NC memories by sputter deposition. The milestones are as follows: - Review of relevant literature. - Development of processes for an ultrathin tunnel-oxide and high quality sputtered SiO2 for use as control-oxide. - Evaluation of methods for the preparation of an oxygen-deficient silicon oxide inter-layer (the precursor of the Si NC layer). - Characterization of deposited films. - Establishment of techniques capable of probing the phase separation of SiOx and the formation of Si NC. - Establishment of annealing conditions compatible with the requirements of current CMOS technology based on experimental results and simulations of Si NC formation. - Preparation Si NC memory capacitors using the developed processes. - Characterization of these devices by suitable techniques. Demonstration of their memory functionality.

Aim of this work was, to investigate the preparation of Si NC memories by sputter deposition. The milestones are as follows: - Review of relevant literature. - Development of processes for an ultrathin tunnel-oxide and high quality sputtered SiO2 for use as control-oxide. - Evaluation of methods for the preparation of an oxygen-deficient silicon oxide inter-layer (the precursor of the Si NC layer). - Characterization of deposited films. - Establishment of techniques capable of probing the phase separation of SiOx and the formation of Si NC. - Establishment of annealing conditions compatible with the requirements of current CMOS technology based on experimental results and simulations of Si NC formation. - Preparation Si NC memory capacitors using the developed processes. - Characterization of these devices by suitable techniques. Demonstration of their memory functionality.

The self-assembly of cellulose nanocrystals (CNCs) into a chiral nematic structure exhibiting photonic properties has garnered much interest in recent years. The development of free-standing chiral nematic films composed of mesoporous silica and organosilica using CNCs as a template has led to a number of studies on producing photonic films composed of inorganic compounds. These films can be tuned to reflect light within the visible spectrum, yielding an assortment of films that exhibit structural colour that is retained once formed, and can no longer be modified. The incorporation of a photonic structure into a flexible material, such as a hydrogel, would allow for colour changes to transpire after the film is formed. Here, the integration of a chiral nematic photonic structure into hydrogel films prepared from different monomers is reported. The swelling of the photonic hydrogels was explored through the use of UV-visible spectroscopy, and the strength of the gels was investigated. In addition to the formation of tunable photonic structures based on CNCs, also reported is the formation of hybrid photonic structures produced by combining two classes of photonic crystals. These films build onto the chiral nematic mesoporous silica films by introducing a secondary photonic structure, based on the close packed arrangement of nanospheres. These novel hybrid photonic structures were synthesized, and characterized using electron microscopy. The successful formation of composite photonic materials, such as CNC-hydrogels, and hybrid photonic films displays the potential for CNC to be used as a template to build photonic structures in a wide array of systems.
Science, Faculty of
Chemistry, Department of
Graduate

The understanding of surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. Therefore, it is important to understand the fundamental surface properties in order to use NCs efficiently in applications. In the work reported in this thesis ZnO NCs were studied. At MAX-lab in Lund, synchrotron radiation based Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) and X-ray Photoelectron Spectroscopy (XPS) were used. At Karlstad University characterization was done using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), Scanning Tunnelling Microscopy (STM), Auger Electron Spectroscopy (AES), and XPS. The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO2/Si surfaces. The conditions for distribution of ZnO NCs were determined to be beneficial when using ethanol as the solvent for ultrasonically treated dispersions. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO2/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs. Flat ZnO NCs with predominantly polar surfaces were grown using a rapid microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms.
Baksidestext The understanding of the surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. In the work in this thesis ZnO NCs were studied. The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO2/Si surfaces. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO2/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs. Flat ZnO NCs with predominantly polar surfaces were grown using a microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms.

This research emphasizes on the use of seeded growth in synthesis of noble metal nanocrystals with precise control over the size, shape, and composition. In the first part of this work, I have produced Au nanocrystals with single-crystal structure and truly spherical profiles and investigated their optical properties and self-assembly as induced by dilution with water. These Au nanospheres were generated in high yield and purity, together with controllable sizes continually increased from 5 to 150 nm. I also found these Au nanospheres self-assembled into dimers, larger aggregates, and wavy nanowires, respectively, as diluted with water. In the second part of this work, I demonstrate the kinetic control can be implemented to control the shape of mono- and bi-metallic nanocrystals in seeded growth. The as-prepared single-crystal nanospheres of Au were employed as seeds to synthesize of tetrahedral Au nanocrystals and Au@Pd core-shell nanocrystals with six distinct shapes. The success of the two demonstrations relies on manipulation of reaction kinetics to achieve different product shapes. The reaction kinetics was controlled by varying a set of reaction parameters, including the type and concentration of capping agent, the amount of reductant, and the injection rate of metal precursor solution. In the final part of this work, I will discuss an unusual change in crystallinity observed in seeded growth of Au nanocrystals on Au seeds. In particular, single-crystal Au seeds treated with a chemical species could develop twin defects during the seed-mediated growth process to yield multiply twinned products.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2001.
Includes bibliographical references.
Although several theoretical models for the behavior of magnetic crystals smaller than a single domain size were developed in the 1950's and 60's, they have hardly been verified experimentally because of the lack of appropriate material systems. This thesis is an attempt to develop such a system using metallic cobalt as a magnetic material and to verify its magnetic behavior in the context of a Stoner-Wohlfarth model of coherent rotation. The problem of preparing crystals of a desired shape and the effect of the crystal shape on its magnetic properties is also addressed. Cobalt nanocrystals are prepared by thermal decomposition of dicobalt octacarbonyl in solution and in the presence of suitable surfactants and coordinating ligands, which influence the shape of the resulting crystals as well as their internal structure. The presence of trialkylphosphines in the growth solution leads to the formation of spherical nanocrystals with mixed fcc-hcp structure, where as trioctylphosphine oxide leads to a newly discovered structure of [epsilon]-cobalt. The final size of the crystals is controlled by the precursor-to-ligand ratio, and low polydispersity is achieved by the separation of nucleation and growth stages. Size-selective precipitation is used to further reduce the size variation of the samples. As a result, cobalt nanocrystals in the size range of 4-12 nm in diameter can be routinely produced with size distributions as small as 6%. The study of magnetic properties reveals the superparamagnetic nature of cobalt nanocrystals of this size range at room temperature. At low temperatures, a good qualitative agreement with the theoretical (Stoner-Wohlfarth) model is found,
(cont.) although quantitative results are strongly influenced by the presence of an oxide shell around each nanocrystal. The presence of two surfactants (trialkylphosphines and sodium carboxylates) during the growth leads to the formation of a significant number of triangular and rod-shaped nanocrystals. Unlike disordered spherical particles, these nanocrystals have pure fcc structure without visible defects. The length of the rods is roughly controlled by the concentration of carboxylates in the growth solution and can be changed within a 40-400 nm range. Unlike spherical crystals of comparable volume, the rods are ferromagnetic even at room temperature due to an added effect of shape anisotropy. A growth mechanism for the formation of nanorods with cubic structure is also proposed.
by Dmitry P. Dinega.
Ph.D.

Oxidation is a key process for commercial applications, in the production of chemical intermediates, high tonnage commodity chemicals, high value fine chemicals, agrochemicals and pharmaceuticals. These oxidations often use stoichiometric oxygen donors such as chromate or permanganate, oxygen donors that give rise to pollutants of considerable environmental concern. Free solvent oxidation of 1-hexene with air using supported gold catalysts with a catalytic amount of tert-butyl hydroperoxide (TBHP) as initiator has been conducted in the liquid phase. Reaction conditions such as radical initiator concentration and reaction temperature were varied to obtain optimum conditions. The gold supported on graphite is an effective catalyst for such oxidations and that graphite was the best of the supports studied. Preparation of catalysts using modified sol-immobilisation including washing under reflux resulted in enhanced catalyst activity by a solvent treatment prior to the reaction. Gold, palladium and gold-palladium catalysts supported on TiO2 has been used for oxidation of trans-2-hexen-1-ol and 1-hexene-3-ol with air at 50 ˚C. The effect of the preparation method, catalyst mass, support, gold: palladium ratio and temperature have been investigated. The main aim was to determine if either the alcohol or alkene functional group can be oxidised selectively. However, based on the reaction products observed trans-2-hexen-1-ol forms trans-2-hexene, hexanal, trans-2-hexenal, trans-3-hexen-1-ol, 4-hexen-1-ol and trans-2-hexanoic acid. 1-hexen-3-ol forms 1-hexene, 3-hexanone, 1hexen-3-one and 3-hexenol), the main pathway in these reactions is isomerisation and, in addition, significant yields of the products are due to a VI disproportionation reaction. Controlling the selectivity in molecules with multiple function groups by manipulating the catalyst composition and reaction conditions can promote or hinder the various reaction pathways, thereby increasing the selectivity to the desired oxidation products. Oxidations of cyclic alkenes were carried out using supported gold nanoparticles under mild solvent-free conditions. The influences of support, preparation method and choice of metal have been investigated. The selectivity to the epoxide is dependent on the size of the cyclic alkene ring. In particular, the epoxide selectivity is very low for < C7, and the origins of this effect are discussed. The influence of the removing stabiliser form cycloalkene has been demonstrated that cycloalkene can be oxidized in the absence of radical initiators when cycloalkene is free of stabilisers.

xv, 153 p. : ill. (some col.)
Lead sulfide nanocrystals (PbS-NCs) are an important class of semiconductor nanomaterials that are active in the near-infrared and exhibit unique properties distinct from their bulk analogues, notably, size tunability of the band gap and solution processability. One factor influencing PbS-NC properties is the presence of an organic ligand shell, which forms the interface between the nanocrystal core and its environment. The specific focus of this dissertation is how ionic functionalization of the ligand shell alters the physical and chemical properties of the resulting PbS-NC/ligand complex. Short-chain ligands can improve photoconductivity in PbS-NC thin films, but there are few solution-based preparations available. Chapter II demonstrates how ionic groups can enable functionalization of PbS-NCs with two short- chain thiol ligands - sodium 3-mercaptopropanesulfonate (MT) and sodium 2,3-dimercaptopropanesulfonate (DT) - via a solution-phase exchange procedure. Despite a structural similarity, DT-functionalized PbS-NCs (PbS-DT) are more stable to oxidation than MT-functionalized PbS-NCs (PbS-MT). The relative stabilities are explained in terms of different binding modes to the nanocrystal surface (bidentate vs. monodentate) and oxidation pathways (intermolecular vs. intramolecular). Toxicology studies on nanomaterials have been limited by the availability of water-soluble samples with systematically controlled structures. As examples of such materials, PbS-DT and PbS-MT nanocrystals are studied in Chapter III for their toxicological impacts on embryonic zebrafish. PbS-DT solutions induce less toxicity than PbS-MT solutions, which is explained in terms of the relative stabilities of the nanocrystal solutions. Finally, Chapter IV investigates the hitherto unexplored effects of ionic functionalization on the optical/electrical properties of PbS-NC thin films, with an emphasis on understanding how counter ions affect the photoconductivity of PbS-DT thin films. Films containing small counter ions exhibit increased dark conductivity and responsivity with time under an applied bias, whereas films containing larger or multivalent counter ions show a suppression of this behavior. These results are discussed in terms of ion motion and ion-assisted carrier injection at the PbS-NC/electrode interface. This dissertation includes previously published and unpublished co-authored material.
Committee in charge: David R. Tyler, Chair; Mark C. Lonergan, Advisor; Catherine J. Page, Inside Member; Andrew Marcus, Inside Member; Hailin Wang, Outside Member

I materiali nanostrutturati ricoprono un ruolo di estrema prominenza nel panorama delle tecnologie avanzate grazie alle loro proprietà funzionali, le quali dipendono fortemente dalla dimensione dei cristalli. Ne consegue che l’ingegnerizzazione di tali materiali sia un argomento di primario interesse, al fine di poter ottenere nuovi dispositivi dalle proprietà superiori. Combinando diverse tipologie di materiali, quali ad esempio materiali metallici e/o materiali semiconduttori, è possibile concepire nuovi dispositivi per applicazioni biomediche, sensoristica ambientale e applicazioni energetiche, tra le varie. Le nanoparticelle di materiale semiconduttore sono ottenute in diversi modi; in questo lavoro verrà presentata la sintesi colloidale poiché è una tecnica economica, facile da configurare, ecologica e può portare a risultati molto interessanti in termini di affidabilità e ripetibilità. La cinetica è uno dei principali fattori a dover essere pienamente compreso per controllare il processo, avendo come obiettivo il raggiungimento di una specifica dimensione e distribuzione dei nuclei. Lo studio della cinetica di sintesi è il motivo per cui è stato investigata l’influenza del rapporto delle concentrazioni dei precursori: si è ottenuta evidenza che una maggiore concentrazione di acido oleico, molecola che si lega agli atomi di cadmio per stabilizzarli, provoca una velocità di crescita delle nanoparticelle più lenta. . Basandosi sui dati cinetici della sintesi di CdSe, è stata sviluppata una modifica della Teoria Classica della Nucleazione omogenea (CNT) nella quale si consideri il consumo di monomero durante la crescita di cristalli. Con questa modifica, nella classica curva di equilibrio il cui massimo rappresenta il raggio critico di nucleazione, appare un nuovo minimo assoluto, il quale rappresenterebbe la dimensione di equilibrio tra fase solida e fase liquida (dovuto al limite di solubilità). A conferma del modello di equilibrio di reazione proposto, risulta che gli andamenti sperimentali del raggio medio delle particelle a fine crescita corrispondono con quelli simulati tramite la modifica della CNT. Una strategia efficace per ottenere strutture complesse core-shell si basa sull’estensione della validità della tecnica colloidale Atomic Layer Deposition (c-ALD), proposta da Ithurria e Talapin ma studiata soltanto per sistemi a base di CdSe-CdS. Sono stati investigati il processo e la sua applicabilità a sistemi metallici (nello specifico core di Au, per il suo alto contrasto al TEM con la shell di CdS) e a base di semiconduttore, ma inserendo nuove configurazioni nella shell (molteplici combinazioni di CdS e ZnS). In base a quanto investigato per la c-ALD, un trasferimento di fase di nanocristalli da soluzione apolare a polare è stato sviluppato. Una volta ottenuta una soluzione stabile che presenti buona fotoluminescenza, i NCs sono stati depositati su substrati di silice e lasciati essiccare in camera a vuoto. È stato svolto uno studio cinetico delle trasformazioni che avvengono durante l’invecchiamento dei campioni in funzione della temperatura e del tempo, monitorando l’evoluzione del picco di fotoluminescenza della struttura. Tuttavia, il risultato più eclatante lo si è osservato eccitando la struttura con un laser a lunghezza d’onda inferiore rispetto l’energia richiesta dal band-gap del materiale barriera (ECdS=2.4 eV). Utilizzando il laser a 633 nm (E=1.96 eV), si è osservato un picco a 520 nm (E=2.38 eV). Sono state eseguite le opportune verifiche, scartando l’ipotesi che si tratti di un picco Raman (non era presente il relativo picco Stokes a 3400 cm-1) e nemmeno fotoluminescenza anti-Stokes, in quanto quest’ultima richiede una dipendenza non lineare con la potenza del laser, mentre il picco osservato ha dipendenza lineare.
Nanostructured materials are of upmost importance among advanced technologies thanks to their unique properties, which depends strongly on crystal size. Being able to tailor and engineer such materials, so that new devices with superior properties can be obtained, is a hot topic at the present. Functional nanocomposites are a brand new class of materials which possess features that traditional materials can't present; such features, for example opto-electronic properties, strongly depend on the nanoscale dimension of the system. Mixing different types of materials, i.e. creating a nanocomposite based on metallic and/or semiconductor nanocrystals, open the path to develop new devices for biomedics, energy application and environmental sensors, among others. Semiconductor nanoparticles (NPs) are obtained in several ways; we focused on the colloidal synthesis since it is a cheap, easy to set-up, environment-friendly technique that can lead to valuable results in terms of reliability and repeatability. The kinetics was one of the main factors to be fully understood to be able to control the process, aiming at reaching a certain NPs final dimension and distribution. A first study focused indicated how a higher concentration of oleic acid, the ligands for Cd atoms, causes a slower NPs growth rate, due to the increase of system viscosity. A modification of the classical homogeneous nucleation theory (CNT) has been developed to take in consideration the decrease of monomer concentration during crystal formation and growth. The new free energy curve, now, shows not only the typical maximum associated to nuclei critical size, but also an absolute minimum relative to the mean crystal size at equilibrium between solidus and liquidus (due to the solubility of the system). The trends of crystal size as function of OA concentration estimated using the modified CNT are in perfect agreement with experimental data. A novel strategy aimed to the extension of the validity of a recently proposed technique to create core-shell structure has been investigated: the colloidal atomic layer deposition. The goal was to apply such technique, applied only to CdSe-CdS core-shell structures, to metallic systems and investigate different shell compositions and crystal shapes. Based on the results of c-ALD, NCs phase transfer was performed changing the organic solvent (hexane) with a polar one (n-methylformamide) and stabilizing the nanoparticles with a surface layer of sodium sulfide. Once NCs are completely stripped out of organic ligands, but keep preserving stability and their emission properties, the solution has been drop casted on 0.2x0.2 mm2 microscope glass substrate and sealed in a vacuum chamber overnight for the evaporation of the solvent. A kinetic study has been performed on the films, investigating the effect of annealing time and temperature on the photoluminescence spectrum of samples. When excited with a wavelength (633 nm) larger than the gap of the barrier material (CdS, Egap=2.4 eV), annealed films presented a weak emission peak at 520 nm, associated to the band-edge emission of the shell. We proved that the peak at 520 nm is not a Raman peak, since there is no evidence of its Stokes counterpart located at 3400 cm-1, and it is no anti-Stokes PL. In this latter case, a multi-photon absorption would be requested, and the emission intensity shall present a non-linear dependence with the laser power. In our samples, on the contrary, a linear behavior is observed, indicating the absorption of one single photon per transition.

In conventional silicon solar cell, the collection probability of light generated carries shows a drop in the high energy range 280-400nm. One of the methods to reduce this loss, is to implement nanometre sized semiconductors on top of a solar cell where high energy photons are absorbed and low energy photons are re-emitted. This effect, called luminescence down-shifter (LDS), modifies the incident solar spectrum producing an enhancement of the energy conversion efficiency of a cell. We investigate this innovative effect using silicon nanoparticles dispersed in a silicon dioxide matrix as active material. In particular, I proposed to model these structures using a transfer matrix approach to simulate its optical properties in combination with a 2D device simulator to estimate the electrical performance. Based on the optimized layer sequences, high efficiency cells were produced within the european project LIMA characterized by silicon quantum dots as active layer. Experimental results demonstrate the validity of this approach by showing an enhancement of the short circuit current density with up to 4%. In addition, a new configuration was proposed to improve the solar cell performances. Here the silicon nanoparticles are placed on a cover glass and not directly on the silicon cells. The aim of this study was to separate the silicon nanocrystals (Si-NCs) layer from the cell. In this way, the solar device is not affected by the Si-NCs layer during the fabrication process, i.e. the surface passivation quality of the cell remains unaffected after the application of the LDS layer. Using this approach, the downshifting contribution can be quantified separately from the passivation effect, as compared with the previous method based on the Si-NCs deposition directly on the solar devices. By suitable choice of the dielectric structures, an improvement in short circuit current of up 1% due to the LDS effect is demonstrated and simulated.

In conventional silicon solar cell, the collection probability of light generated carries shows a drop in the high energy range 280-400nm. One of the methods to reduce this loss, is to implement nanometre sized semiconductors on top of a solar cell where high energy photons are absorbed and low energy photons are re-emitted. This effect, called luminescence down-shifter (LDS), modifies the incident solar spectrum producing an enhancement of the energy conversion efficiency of a cell. We investigate this innovative effect using silicon nanoparticles dispersed in a silicon dioxide matrix as active material. In particular, I proposed to model these structures using a transfer matrix approach to simulate its optical properties in combination with a 2D device simulator to estimate the electrical performance. Based on the optimized layer sequences, high efficiency cells were produced within the european project LIMA characterized by silicon quantum dots as active layer. Experimental results demonstrate the validity of this approach by showing an enhancement of the short circuit current density with up to 4%. In addition, a new configuration was proposed to improve the solar cell performances. Here the silicon nanoparticles are placed on a cover glass and not directly on the silicon cells. The aim of this study was to separate the silicon nanocrystals (Si-NCs) layer from the cell. In this way, the solar device is not affected by the Si-NCs layer during the fabrication process, i.e. the surface passivation quality of the cell remains unaffected after the application of the LDS layer. Using this approach, the downshifting contribution can be quantified separately from the passivation effect, as compared with the previous method based on the Si-NCs deposition directly on the solar devices. By suitable choice of the dielectric structures, an improvement in short circuit current of up 1% due to the LDS effect is demonstrated and simulated.

La nature quantique des noyaux produit des comportements inattendus et souvent paradoxaux. Du fait de sa légèreté, l'hydrogène est le candidat le plus susceptible de présenter de tels comportements. Nous avons étudié trois systèmes hydratés dont les mécanismes sont déterminés par les propriétés quantiques des protons (NQEs) : la Brucite (Mg(OH)2), l'hydrate de méthane (CH4-H2O) et l'hydroxyde de sodium (NaOH). Au sein des Brucites coexistent deux effets en compétition : un mécanisme de réorientation thermiquement activé, et un processus de dissociation déclenché par les NQEs. Ces deux effets s'opposent sous l’augmentation de la pression, entraînant l'existence d'un point de pression favorisant la diffusion des protons à mesure que se forme un plan d'hydrogène "quantique" quasi 2D. Sous pression, l’hydrate de méthane présentent une augmentation des interactions entre le réseau d’eau et les molécules de méthane qui y sont enfermées. Contrairement à la glace, la transition de symétrisation des liaisons hydrogène ne change pas par substitution isotopique du fait de la délocalisation du proton. Celle-ci déclenche également une transition vers une nouvelle phase, stable jusqu'à des pressions jamais atteintes par tout hydrate connu à ce jour. La soude présente une transition de phase en-dessous de la température ambiante et à pression ambiante uniquement dans sa version deutérée. Cet effet isotopique s'explique par la délocalisation quantique et par l'importance de l'énergie de point-zéro du proton par rapport au deutéron. Étonnement la substitution isotopique change la transition induite par la température dans NaOD en une transition déclenchée par la pression dans NaOH
The quantum nature of nuclei yields unexpected and often paradoxical behaviors. Due to the lightness of its nucleus, the hydrogen is a most likely candidate for such effects. During this thesis, we focus on complexe hydrated systems, namely, the brucite minerals (Mg(OH)2), the methane hydrate (CH4-H2O) and the sodium hydroxide (NaOH), which display complex mechanisms driven by the proton quantum properties. Brucite exhibits the coexistence of thermally activated hopping and quantum tunneling with opposite behaviors as pressure is increased. The unforeseen consequence is a pressure sweet spot for proton diffusion. Simultaneously, pressure gives rise to a «quantum» quasi two-dimensional hydrogen plane, non-trivially connected with proton diffusion. Upon compression, methane hydrate displays an important increase of the inter-molecular interactions between water and enclosed methane molecules. In contrast with ice, the hydrogen bond transition does not shift by H/D isotopic substitution. This is explained by an important delocalization of the proton which also triggers a transition toward a new MH-IV methane hydrate phase, stable up to 150 GPa which represents the highest pressure reached to date by any hydrate. Sodium hydroxide has a phase transition below room temperature at ambient pressure only in its deuterated version. This radical isotope effect can be explained by the quantum delocalization of the proton as compared with deuteron shifting the temperature-induced phase transition of NaOD towards a pressure-induced one in NaOH

This thesis investigates the photophysical properties of CdSe/ZnS/CdSe core/barrier/shell nanocrystals using a combination of steady state and time resolved spectroscopy. Interestingly, these materials exhibit photoluminescence (PL) from both the CdSe core and CdSe shell, which can be exploited for a variety of applications, such as white light emission and optical gain. Steady state measurements shed light on the coupling between the two CdSe phases. The CdSe shell is shown to have a significant influence upon the optical properties of the CdSe core. In fact, the CdSe shell acts as a light harvester, increasing the brightness of the CdSe core as compared to bare CdSe nanocrystals. Single nanocrystal spectra of CdSe/ZnS/CdSe revealed that both CdSe phases exhibited PL intermittency and spectral diffusion. No correlation was observed in the spectral diffusion of the two CdSe phases on the timescale of the measurement. However, the single nanocrystal PL linewidths suggest that spectral diffusion of the two CdSe phases differs on shorter timescales. Using ultrafast transient absorption spectroscopy, CdSe/ZnS/CdSe nanocrystals were shown to exhibit optical gain with enhancements over bare CdSe nanocrystals. The bandwidth of stimulated emission from CdSe/ZnS/CdSe nanocrystals was much broader than bare CdSe nanocrystals due to CdSe shell enabled states. CdSe/ZnS/CdSe nanocrystals were also found to have lower biexciton binding energies than CdSe nanocrystals, contributing to improved gain performance. In addition, higher energy bleaching features in the transient absorption spectra indicated that populations of excitons remain in higher energy states, enabling dual colour emission.
Cette thèse explore les propriétés photophysique du coeur/barrière/coquille des nanocrystaux de CdSe/ZnS/CdSe par une combinaison de spectroscopies à l'état d'équilibre et en temps résolu. Il est intéréssant de noter que ces matériaux présentent de la photoluminescence (PL) provenant du coeur CdSe et de la coquille CdSe qui pourrait être exploitée dans une variété d'applications comme l'émission de lumière blanche et gain optique. Des mesures à l'état d'équilibre ont illustré le couplage entre les deux phases de CdSe. On a démontré que la coquille de CdSe influence les propriétés optique du coeur de CdSe. En effect, la coquille de CdSe fonctionne comme collecteur de lumière, augmentant la luminosité du coeur de CdSe comparé aux nanocrystaux nus de CdSe. Les spectres de nanocrystaux simples de CdSe/ZnS/CdSe révèlent que les deux phases CdSe montrent de l'intermittence PL et de la diffusion spectrale. Aucune corrélation n'a été observée dans la diffusion spectrale des deux phases de CdSe à l'échelle de mesure. Par contre, les largeurs de raie de PL du simple nanocrystal suggère que la diffusion spectrale des deux phases de CdSe diffèrent à des échelles de temps plus courtes. Par spectroscopie ultrarapide d'absorption transitoire, on a démontré que les nanocrystaux de CdSe/ZnS/CdSe font preuve de gain optique amélioré sur les nanocrystaux de CdSe nus. La largeur de raie des émissions stimulées des nanocrystaux de CdSe/ZnS/CdSe était plus élargie que celle des nanocrystaux de CdSe à cause de la présence de la coquille de CdSe. On a aussi démontré que les nanocrystaux de CdSe/ZnS/CdSe ont des plus basses énergies de liaisons de biexciton que les nanocrystaux de CdSe, contribuant à une amélioration de performance de gain. De plus, des caractéristiques de blanchiment à haute énergie dans les spectres d'absorption transitoire indiquent que les populations d'excitons demeurent à des états d'énergie plus élevés, permettant une émission double en couleur.

Colloidally prepared nanocrystals of transparent conducting oxide (TCO) semiconductors have emerged in the past decade as an exciting new class of plasmonic materials. In recent years, there has been tremendous progress in developing synthetic methods for the growth of these nanocrystals, basic characterization of their properties, and their successful integration into optoelectronic and electrochemical devices. However, many fundamental questions remain about the physics of localized surface plasmon resonance (LSPR) in these materials, and how their optoelectronic properties derive from their underlying structural properties. In particular, the influence of the concentration and distribution of dopant ions and compensating defects on the optoelectronic properties of TCO nanocrystals has seen little investigation.

Indium tin oxide (ITO) is the most widely studied and commercially deployed TCO. Herein we investigate the role of the distribution of tin dopants on the optoelectronic properties of colloidally prepared ITO nanocrystals. Owing to a high free electron density, ITO nanocrystals display strong LSPR absorption in the near infrared. Depending on the particular organic ligands used, they are soluble in various solvents and can readily be integrated into densely packed nanocrystal films with high conductivities. Using a combination of spectroscopic techniques, modeling and simulation of the optical properties of the nanocrystals using the Drude model, and transport measurements, it is demonstrated herein that the radial distribution of tin dopants has a strong effect on the optoelectronic properties of ITO nanocrystals.

ITO nanocrystals were synthesized in both surface-segregated and uniformly distributed dopant profiles. Temperature dependent measurements of optical absorbance were first combined with Drude modeling to extract the internal electrical properties of the ITO nanocrystals, demonstrating that they are well-behaved degenerately doped semiconductors displaying finite conductivity at low temperature and room temperature conductivity reduced by one order of magnitude from that of high-quality thin film ITO.

Synchrotron based x-ray photoelectron spectroscopy (XPS) was then employed to perform detailed depth profiling of the elemental composition of ITO nanocrystals, confirming the degree of dopant surface-segregation. Based on free carrier concentrations extracted from Drude fitting of LSPR absorbance, an inverse correlation was found between surface segregation of tin and overall dopant activation. Furthermore, radial distribution of dopants was found to significantly affect the lineshape and quality factor of the LSPR absorbance. ITO nanocrystals with highly surface segregated dopants displayed symmetric LSPRs with high quality factors, while uniformly doped ITO nanocrystals displayed asymmetric LSPRs with reduced quality factors. These effects are attributed to damping of the plasmon by Coulombic scattering off ionized dopant impurities.

Finally, the distribution of dopants is also found to influence the conductivity of ITO nanocrystal films. Films made from nanocrystals with a high degree of surface segregation demonstrated one order of magnitude higher conductivity than those based on uniformly doped crystals. However, no evidence was found for differences in the surface electronic structure from one type of crystal to the other based on XPS and the exact mechanism for this difference is still not understood.

Several future studies to further illuminate the influence of dopant distribution on ITO nanocrystals are suggested. Using synchrotron radiation, detailed photoelectron spectroscopy on clean ITO nanocrystal surfaces, single-nanoparticle optical measurements, and hard x-ray structural studies will all be instructive in elucidating the interaction between oscillating free electrons and defect scattering centers when a plasmon is excited. In addition, measurements of temperature and surface treatment-dependent conductivity with carefully controlled atmosphere and surface chemistry will be needed in order to better understand the transport properties of ITO nanocrystal films. Each of these studies will enable better fundamental knowledge of the plasmonic properties of nanostructures and improve the development of nanocrystal based plasmonic devices.

Thesis (Ph. D.)--Dept. of Physics and Dept. of Mathematics. University of Missouri--Kansas City, 2007.
"A dissertation in physics and mathematics." Advisor: Michael B. Kruger. Typescript. Vita. Title from "catalog record" of the print edition Description based on contents viewed July 16, 2008. Includes bibliographical references (leaves 137-143). Online version of the print edition.

This thesis focuses on different aspects of NCs colloidal synthesis, the exploration of the relevant surface chemistries that afford NC assembly and the NC implementation into porous nanomaterials. The work is divided into two blocks. The first block is devoted to developing and optimizing the synthesis of NCs followed by the examination of their suitability for potential applications in catalysis and photocatalysis. The second block is dedicated to establish procedures to fabricate single-component or multicomponent porous nanomaterials from NC building blocks. To embrace the use of the developed strategies in different application fields, several kind of materials were under research. Namely, metals (e.g. Au), metal oxides (e.g. CeO2, TiO2, Fe2O3), metal chalcogenides (e.g. In2S3, ZnS, PbS, CuGaS2 and Cu2ZnSnSe4), and their composites. CeO2 NCs synthesis was deeply investigated with the aim to achieve a proper control on the NCs morphology, facets exposed, crystal phase, composition, etc., required for application. Overall, CeO2 NCs with spherical, octapod-like branched, cubic hyperbranched, and kite-like morphology with sizes in the range 7 to 45 nm were produced by adjusting experimental conditions of the synthetic protocol. Branched and hyperbranched NCs showed higher surface areas, porosities and oxygen capacity storage values compared to quasi-spherical NCs. The NCs morphology-controlled synthesis has been extended to quaternary Cu2ZnSnSe4 (CZTSe). CZTSe NCs with narrow size distribution and controlled composition were produced. It was shown how off-stoichiometric CZTSe compositions were characterized by higher charge carrier concentrations and thus electrical conductivities. The strategy to functionalize the metal oxide NC surface composition by applying different ligands is proposed. This enables to develop a novel approach to assemble metal oxide NCs into porous gel and aerogel structures. Propylene oxide has been found to trigger the gelation process of glutamine functionalized NCs. The detailed investigation of the gelation mechanism is demonstrated for the case of ceria. The method is applied for NCs with different morphologies. Eventually, the versatility of the concept is proved by using of the proposed approach for the TiO2 and Fe2O3 nanocrystals. The assembly method has been extended to metal chalcogenides - In2S3 NCs - starting from the NCs synthesis, with further surface chemistry manipulation and eventually follows by the NC assembly into gels and aerogels. The optimization of NC surface chemistry was achieved by testing different ligand exchange approaches via applying short-chain organic and inorganic ligands. The assembly method based on ligand desorption from the NC surface and chalcogenide-chalcogenide bond formation has been established for In2S3. The comparison of the different ligands impact on the NC performance in colloidal form, when assembled into gels and when supported onto substrate is investigated towards photoelectrocatalysis. The oxidative ligand desorption assembly approach has been extended for multicomponent NCs for the case of CuGaS2 and CuGaS2-ZnS. Optimization of spin-coating process of the formed NCs inks followed by applying of sol-gel chemistry led to formation of highly porous layers from TGA-CuGaS2 and TGA-ZnS. Applied results of CuGaS2/ZnS nanocrystal-based bilayers and CuGaS2–ZnS nanocrystal-based composite layers have been shown by testing their photoelectrochemical energy conversion capabilities. The approach to adjust NC surface chemistry has been proposed and tested for performing multicomponent NC assemblies. Applying of different ligands for NC surface functionalization endows their surface with different charges which usually provides colloidal NCs stabilization. It has been found that mixing of oppositely charged NCs with certain concentration enabled their assembly/gelation via electrostatic interaction. The proposed approach has been applied and optimized to produce multicomponent NC gels and aerogels. The detailed investigation of the gelation mechanism is shown for combination of metal-metal oxide and metal oxide-metal chalcogenide NCs (Au-CeO2, CeO2-PbS). Applied results of the Au-CeO2 aerogels were demonstrated for CO-oxidation.
Esta tesis se centra en la síntesis coloidal de nanocristales (NCs), en la exploración de su química de superficie y en su ensabanado en nanomateriales porosos funcionales. Para demostrar la versatilidad de aplicación de dichas estructuras, en este estudio se han considerado NCs de distintos tipos de materiales: metales (Au), óxidos metálicos (CeO2, TiO2, Fe2O3), calcogenuros metálicos (In2S3, ZnS, PbS, CuGaS2,Cu2ZnSnSe4) y sus materiales compuestos. El trabajo se dividió en dos bloques. En el primero se desarrolló y optimizó la síntesis de NCs de óxidos y calcogenuros metálicos y se evaluó su potencial para aplicaciones de catálisis y fotocatálisis. Se investigó en profundidad la síntesis de NCs de CeO2, poniendo énfasis en controlar su morfología. Se consiguió producir NCs de CeO2 de forma controlada (esférica, octapodo ramificado, cúbico ramificado y romboidal) y con tamaño controlado (7-45 nm). Asimismo, se obtuvieron NCs de Cu2ZnSnSe4 con una fina distribución de tamaños y composición controlada. En el segundo bloque se establecieron y estudiaron procedimientos para fabricar nanomateriales porosos mono- o multicomponentes a partir del ensamblado de NCs. Se desarrolló una estrategia basada en el ajuste de la química de superficie de NCs de óxidos metálicos (CeO2, Fe2O3,TiO2) y de calcogenuros metálicos (In2S3, CuGaS2-ZnS) que permitió su ensamblaje controlado en estructuras porosas de tipo gel y aerogel. En el caso de los óxidos metálicos, se determinó que el ensamblado se inicia con la adición de un epóxido a NCs funcionalizados con glutamina, causando la gelación. La desorción oxidativa de ligandos basada en la formación de enlaces calcogenuro-calcogenuro se propuso como mecanismo de gelación en calcogenuros mono- (In2S3) y multicomponente (CuGaS2-ZnS). Se investigó el impacto del empleo de distintos ligandos en la eficiencia foto-electrocatalítica de NCs en forma coloidal, ensamblados en geles y soportados en sustratos. Se desarrolló y estudió el ajuste de la química de superficie de NCs para la obtención de ensamblajes multicomponente mediante interacción electrostática de coloides en suspensión. El mecanismo de gelación fue investigado al detalle para materiales compuestos de NCs de oxido metálico (CeO2) con NCs de óxido de calcogenuro (PbS-CeO2) y metálicos (Au-CeO2). Los aerogeles de Au-CeO2 demostraron potencial para la oxidación de CO.

Primerament, la síntesis de quinze tipus de nanocristalls inorgànics i la tendència general dels nanocristalls de fluorurs metàl·lics ha estat satisfactòriament desentranyada. Utilitzant el mètode de la coprecipitació, es reporta la fàcil, ràpida i reproduïble síntesis de nanocristalls de LnF3 i el detallat estudi mecanístic de les diferents condicions sintètiques. Mitjançant el complert estudi de la química de superfície, un nou tipus de self-assembly iònic en sistemes col·loidals ha estat proposat. Utilitzant mètodes experimentals i simulacions de dinàmica molecular, es postula aquest mecanisme de self-assembly aplicable a diversos sistemes i no només en el sistema estudiat. També s’han obtingut nanocristalls patchy utilitzant un mètode fàcil, ràpid i reproduïble. El comportament d’aquests nanocristalls patchy ha estat investigat en detall utilitzant mètodes experimentals i simulacions de dinàmica molecular. Els nostres resultats revelen la espontània i selectiva coordinació de cations i anions en les diferents cares exposades, com també interaccions selectives amb el solvent. Avançant en la temàtica de nanocristalls patchy, hem demostrat que les diferents cares dels nanocristalls obtinguts poden ser modificades selectivament. Els cations i anions poden ser modificats mitjançant l’addició de nous lligands que continguin un grup amina o carboxílic. També, utilitzant una molècula zwitterionica podem aconseguir la homogeneïtzació de la superfície eliminant al mateix temps cations i anions. Addicionalment s’han estudiat diferents processos de creixement per millorar les partícules obtingudes, permetent la obtenció de nanocristalls més grans i definits al mateix temps que modifiquem l’estabilitzant orgànic. La tècnica de EGA-MS ha estat també provada per a simplificar el complex camí de la completa caracterització en sistemes col·loidals. Hem demostrat que utilitzant una única tècnica experimental, la completa caracterització de sistemes col·loidals es possible comparat amb els nostres estudis previs en les mateixes partícules. Aquesta tesis està basada principalment en el estudi mecanístic de la síntesis i el comportament de la química de superfície de nanocristalls de LnF3. Conseqüentment, aquest coneixement permetrà el control i la manipulació del pont que hi ha entre la síntesis i les aplicacions, actualment anomenat química de superfície. Finalment, algunes aplicacions son presentades com a diferents rutes a seguir després d’aquest treball, essent aquestes excel·lents candidats en ciència de materials i medicina.
Starting from the synthesis of fifteen different types of inorganic nanocrystals, the general trends of metal fluoride nanocrystals have been successfully unravelled. Using the co-precipitation method, we reported the easy, fast and reproducible synthesis of LnF3 nanocrystals and the detailed mechanistic studies of different synthetic conditions. Through the complete study of the surface chemistry, a new kind of ionic self-assembly in colloidal systems has been proposed. Using experimental techniques and molecular dynamics simulations, we postulated this self-assembly mechanism not only specific for the studied case but also applicable to other kind of systems. In addition, thermodynamically stable patchy nanocrystals have been also obtained using an easy, fast and reproducible method. The behaviour of these patchy nanocrystals has been investigated in detail using this dual approximation, from experimental techniques to all-atomistic molecular dynamics simulations. Our results revealed the spontaneous and selective attachment of cations and anions in their different exposed faces, as well as, selective solvent interactions. Going one step further in patchy nanocrystals, we demonstrated that the different facets of the obtained nanocrystals can be modified selectively. Cations and anions can be removed from nanocrystal surface via the addition of a new molecule containing an amino group or a carboxylate respectively. Likewise, using a zwitterionic molecule, the homogenisation of the surface was possible releasing at same time cations and anions. Additionally, some growing process were carried out to enhance the obtained particles, allowing bigger hexagonal-faceted nanocrystals while trying to modify the organic stabilisers. In addition, EGA-MS technique has been tested to simplify the complex pathway to full-characterise colloidal systems. We demonstrated that using a simple experimental technique, the full characterisation of a colloidal system is possible, comparing the results with our previous characterisations. This thesis is mainly based on the mechanistic understanding of the synthesis and the final behaviour of the surface of LnF3 nanocrystals. In consequence, this knowledge will allow the control and manipulation of the bridge between synthesis and applications, currently called surface chemistry. Finally, some initial applications will be presented as different pathways emerged from the manipulation of the unravelled systems, being promising candidates for material science and medical fields.

This thesis focuses on the synthesis of different types of NCs, their application to energy conversion and storage technologies, particularly LIBs, KIBs and DEFCs, and their structural transformation during electrochemical processes within these energy storage and conversion applications. The morphology and composition of target transition metal oxides, bimetallic NCs and phosphorous incorporated intermetallic NRs are characterized in detail to follow the alterations during application. An understanding of the correlation between structural, chemical and electrochemical properties will allow a more rational design of functional nanomaterials. The 1st chapter gives a general introduction to the rapid development and importance of renewable energy technologies in modern human society. Among which electrochemical energy storage and conversion technologies are particularly appealing in terms of cost, safety and environmental friendliness. The basic principles of Li-, Na- and K- ion battery technologies are discussed, including the battery structures, electrode materials and working mechanisms. Additionally, I describe the working principle of DEFCs and the electrocatalytic EOR. Strategies for synthesizing high performance NCs for electrochemical energy storage and conversion applications are also explained. Finally, in this chapter I discuss the phenomenon of NCs structural and chemical evolution during electrochemical operations and how their characterization in each system is needed for a thorough understanding of nanomaterials properties and applications. Chapter 1 also includes the objectives of the thesis. Chapter 2 describes a simple seed-mediated growth method at low temperature to grow heterstructered Mn3O4 on hollow Fe3O4 seeds. A moderate temperature (500 °C) annealing process is conducted to promote the solid-state reaction for hollow MnxFe3-xO4 NPs while conserving the original morphology. When serving as anode electrode materials, the polycrystalline shell, the internal void space and the high surface area of MnxFe3-xO4 NPs can effectively buffer the volume change of the NCs during lithiation and delithiation process to improve the stability and cycle life. The electrochemical activity of MnxFe3-xO4 NPs toward lithium reaction is evaluated and the relationship between the structure and electrochemical properties is explored. The excellent performance of hollow MnxFe3-xO4 NPs is associated with their crystal structure and composition, and with the presence of carbonized ligands, which further promote electrical conductivity and rapidly accommodate and release lithium ions while retaining a stable structure even after continuous charge/discharge cycles. This work was published in Nano Energy in 2019. Chapter 3 talks about the performance of bimetallic NPs as anodes in LIBs and KIBs. Monodisperse CoSn and NiSn NPs are synthesized through co-reduction and supported on commercial carbon materials. The obtained nanocomposites are tested as anode materials in half-cell LIBs, KIBs and full-cell LIBs. CoSn@C electrodes display excellent charge-discharge capacities with LIB half-cell and LIB full-cells. The capacities for KIB are stabilized at around 200 mAh g-1 with high coulombic efficiency over 400 cycles for CoSn@C and 100 mAh g-1 for NiSn@C over 300 cycles. The oxidation of NPs, the formation of SEI layer, the vast volume change during lithiation and delithiation processed caused the capacities decrease. This work was published in ACS Applied Materials & Interfaces in 2020. In chapter 4, a simple approach to produce intermetallic Pd3Pb nanocubes with well-defined cubic geometry and average size ranged from 6 nm to 10 nm is detailed. Pd3Pb/C catalysts present improved EOR electrocatalytic activities and stabilities. The EOR activity of Pd3Pb nanocubes is investigated through CV and CA techniques, which is size-dependent. All the catalysts exhibit a pronounced current decay during the first 500 s of continuous EOR operation, which is associated to the accumulation of strongly adsorbed reaction intermediates and the related blockage of reaction sites. The catalysts can be reactivated by simply cycling to effectively remove the poisoning species adsorbed on the surface and recover the electrocatalytic activity. A reorganization of Pd and Pb elements happens on Pd3Pb nanocubes during EOR, involving an outward/inward diffusion of Pd/Pb to equilibrate the stoichiometry of the NCs surface, which is driven by the different affinity of Pb and Pd towards oxygen and possibly ethanol, and the electrochemical oxidation/reduction of Pd. This work was published in Chemistry of Materials in 2020. Chapter 5 demonstrates the synthesis of colloidal Pd2Sn:P NRs through phosphorization of Pd2Sn NPs with a highly active reagent- hexamethylphosphorous triamide (HMPT) in a one-pot two-steps reaction. The Pd2Sn:P/C catalyst exhibits significantly enhanced activity toward EOR in alkaline media compared with Pd2Sn/C, PdP2/C and commercial Pd/C catalysts. The performance improvement is rationalized with the aid of DFT calculations considering the different phosphorous chemical environments. Depending on its oxidation state, surface phosphorus introduces sites with low energy OH- adsorption and/or strongly influences the electronic structure of palladium and tin to facilitate the oxidation of the acetyl to acetic acid, which is considered the EOR rate limiting step. The Pd2Sn:P NRs is characterized with Sn- and P-rich surface, which correlates well with the higher percentages of oxidized tin and phosphorous, and the higher tendency to oxidation of Sn compared with Pd. DFT calculations prove that the presence of P can induce a higher chemical adsorption of OH- to facilitate the formation of CH3COOH, resulting in EOR activity increase. This work was accepted in Nano Energy in 2020.
Esta tesis se centra en la síntesis de diferentes tipos de nanocristales, su aplicación a las tecnologías de conversión y almacenamiento de energía, particularmente LIBs, KIBs y DEFCs, y su transformación estructural durante los procesos electroquímicos dentro de estas aplicaciones de almacenamiento y conversión de energía. La morfología y composición de nanocristales de óxidos de metales de transición, bimetálicos e intermetálicos que incroporan fósforo se caracterizan en detalle para seguir las alteraciones durante la aplicación. La comprensión de la correlación entre las propiedades estructurales, químicas y electroquímicas permitirá un diseño más racional de nanomateriales funcionales. El primer capítulo ofrece una introducción general al rápido desarrollo y la importancia de las tecnologías de energía renovable en la sociedad moderna. Entre ellas, las tecnologías de conversión y almacenamiento de energía electroquímica son particularmente atractivas en términos de costo, seguridad y respeto al medio ambiente. Se discuten los principios básicos de las tecnologías de baterías de iones de litio, sodio y potasio, incluidas las estructuras de las baterías, los materiales de los electrodos y los mecanismos de trabajo. Además, describo el principio de funcionamiento de las DEFCs y el EOR electrocatalítico. También se explican las estrategias para sintetizar nanocristales de alto rendimiento para aplicaciones de almacenamiento y conversión de energía electroquímica. Finalmente, en este capítulo discuto el fenómeno de la evolución estructural y química de los nanocristales durante las operaciones electroquímicas y cómo se necesita su caracterización en cada sistema para una comprensión profunda de las propiedades y aplicaciones de los nanomateriales. El capítulo 1 también incluye los objetivos de la tesis. El Capítulo 2 describe un método de crecimiento simple mediado por semillas a baja temperatura para crecer Mn3O4 en nanoparticulas huecas de Fe3O4. Se lleva a cabo un proceso de sinterizado a temperatura moderada (500 °C) para promover la reacción en estado sólido de las NPs y obtener partículas huecas de MnxFe3-xO4. Al ser usados como materiales de electrodo anódico, la cubierta policristalina, el espacio vacío interno y la gran área de superficie de las NPs de MnxFe3-xO4 pueden amortiguar de manera efectiva el cambio de volumen de los nanocristales durante el proceso de litiación y delitizacion para mejorar la estabilidad y la vida útil del ciclo. Se evalúa la actividad electroquímica de las NPs de MnxFe3- xO4 hacia la reacción de litio y se explora la relación entre la estructura y las propiedades electroquímicas. El excelente rendimiento de las NPs huecas de MnxFe3-xO4 está asociado con su estructura y composición cristalinas, y con la presencia de ligandos carbonizados, que promueven aún más la conductividad eléctrica y acomodan y liberan rápidamente iones de litio mientras retienen una estructura estable incluso después de ciclos continuos de carga/descarga . Este trabajo fue publicado en Nano Energy en 2019. El Capítulo 3 vesra sobre el rendimiento de los NPs bimetálicos como ánodos en LIBs y KIBs. NPs monodispersas de CoSn y NiSn se sintetizan mediante co-reducción y se soportan en materiales comerciales de carbono. Los nanocompuestos obtenidos se prueban como materiales anódicos en LIBs de media celda y KIBs y LIBs de celda completa. Los electrodos CoSn@C muestran excelentes capacidades de carga y descarga en media celda y celdas completas LIB. Las capacidades para KIB se estabilizan alrededor de 200 mAh g-1 con alta eficiencia culombiana durante 400 ciclos para CoSn@C y 100 mAh g-1 para NiSn@C durante 300 ciclos. La oxidación de las NPs, la formación de la capa SEI, el vasto cambio de volumen durante la litiación y la delitiación causaron la disminución de las capacidades. Este trabajo fue publicado en ACS Applied Materials & Interfaces en 2020. En el capítulo 4, se detalla un enfoque simple para producir nanocubos intermetálicas de Pd3Pb con geometría cúbica bien definida y un tamaño promedio de 6 nm a 10 nm. Los catalizadores de Pd3Pb/C presentan actividades y estabilidades electrocatalíticas EOR mejoradas. La actividad EOR de las NPs de Pd3Pb se investiga en función de su tamaño a través de técnicas CV y CA. Todos los catalizadores exhiben una disminución de corriente pronunciada durante los primeros 500 s de operación EOR continua, que está asociada con la acumulación de intermedios de reacción fuertemente adsorbidos y el bloqueo relacionado de los sitios de reacción. Los catalizadores pueden reactivarse simplemente ciclando para eliminar eficazmente las especies adsorbidas en la superficie y recuperar la actividad electrocatalítica. Una reorganización de los elementos Pd y Pb ocurre en las NPs de Pd3Pb durante EOR, lo que implica una difusión hacia afuera/hacia adentro de Pd/Pb para equilibrar la estequiometría de la superficie de los NCs, que es impulsada por la diferente afinidad de Pb y Pd hacia el oxígeno y posiblemente el etanol, y la oxidación/reducción electroquímica de Pd. Este trabajo fue publicado en Chemistry of Materials en 2020. El Capítulo 5 demuestra la síntesis de NRs coloidales de Pd2Sn que incorporan P a través de la fosforización de las NPs de Pd2Sn con un reactivo altamente activo. El catalizador Pd2Sn:P/C exhibe una actividad significativamente mejorada hacia EOR en medios alcalinos en comparación con Pd2Sn/C, PdP2/C y catalizadores comerciales de Pd/C. La mejora del rendimiento se racionaliza con la ayuda de los cálculos de DFT teniendo en cuenta los diferentes entornos químicos de fósforo. Dependiendo de su estado de oxidación, el fósforo superficial introduce sitios con adsorción de OH de baja energía y/o influye fuertemente en la estructura electrónica del paladio y el estaño para facilitar la oxidación del acetilo al ácido acético, que se considera el paso limitante de la tasa de EOR. El Pd2Sn:P se caracteriza por una superficie rica en Sn y P, que se correlaciona bien con los porcentajes más altos de estaño oxidado y fósforo, y la mayor tendencia a la oxidación de Sn en comparación con Pd. Los cálculos de DFT demuestran que la presencia de P puede inducir una mayor adsorción química de OH- para facilitar la formación de CH3COOH, lo que resulta en un aumento de la actividad EOR. Este trabajo fue aceptado en Nano Energy en 2020.

This work investigates the electronic properties of very small gold and semiconductor particles using Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Ballistic Electron Emission Spectroscopy (BEES). Complementary theoretical works were also performed. The first theoretical work was to calculate the quantized states in the CdS/HgS/CdS quantum-well-quantum-dot nanocrystals. An eight-band envelope function method was applied to this system. This method treats exactly the coupling between the conduction bands, the light-hole bands, the heavy-hole bands, and the spin-orbit split bands. The contributions of all other bands were taken into account using second order perturbation theory. Gold nanocrystals with diameters of 1.5 nm have discrete energy levels with energy spacings of about 0.2 eV. These values are comparable to the single electron charging energy, which was about 0.5 eV in our experimental configuration. Since bulk gold doesnt have an energy gap, we expect the electron levels both below and above the Fermi level should be involved in the tunneling. Measured spectroscopy data have rich features. In order to understand and relate these features to the electronic properties of the nanocrystals, we developed a tunneling model. This model includes the effect of excited states that have electron-hole pairs. The relaxation between discrete electron energy levels can also be included in this model. We also considered how the nanocrystals affect the BEES current. In this work an ultra-high vacuum and low-temperature STM was re-designed and rebuilt. The BEEM/BEES capabilities were incorporated into the STM. We used this STM to image gold nanocrystals and semiconductor nanocrystals. STS and BEES spectra of gold nanocrystals were collected and compared with calculations.