Дисертації з теми "Nanoparticle Superlattices"

Current global energy usage is largely dependent on non-renewable resources such as fossil fuels. Research is expanding into alternative, renewable energy sources such as solar energy. Of specific focus is research into the use of metal chalcoginide semiconductor nanoparticles as a cost-efficient platform for future use in solar applications. These semiconductor nanoparticles have size-dependent electronic band gaps within the solar spectrum and can be deposited into thin films from colloidal solutions. To date, most electronic studies have focused on thin films with disordered morphologies, where the dominant inter-nanoparticle charge transport mechanism is hopping. Highly spatially ordered metal chalcoginide nanoparticle films may have the ability to form extended Bloch states, thereby resulting in more efficient charge transport. This work focuses on fabricating both highly spatially ordered and highly disordered PbSe nanoparticle thin films to compare their electronic properties and elucidate charge transport mechanisms.

Comme de nombreux colloïdes, des nanoparticules métalliques recouvertes de ligands en suspension s’organisent au-delà d’une fraction volumique seuil et forment ce que l’on appelle un « supracristal ». Ce sont ainsi des systèmes modèles, déjà largement étudiés à partir de suspensions dans des solvants volatils, qui permettent de mieux comprendre l’auto-assemblage de sphères déformables. Les interactions qui conduisent à l’auto-assemblage sont couramment décrites par une compétition entre une attraction de van der Waals entre les cœurs métalliques et une répulsion entre les ligands qui va dépendre de l’affinité entre les ligands et le solvant. Un effet du solvant a déjà été observé sur l’auto-organisation de nano-objets. En mesurant par diffusion de rayons X aux petits angles le facteur de structure de suspensions de nanoparticules d’or greffées, j’ai pu sonder de façon systématique les interactions entre des nanoparticules en suspension avec plusieurs tailles de cœur, des ligands alcane-thiols de longueur différente et dans différents solvants à la fois volatils et non volatils. J’ai ainsi pu mettre en évidence une interaction attractive inattendue dans des alcanes linéaires flexibles et dont l’intensité augmente avec la longueur de l’alcane. Pour corréler les interactions entre particules à leur diagramme de phase, j’ai suivi le processus de cristallisation dans des suspensions en solvant volatil ou partiellement volatil ainsi qu’en émulsion, techniques qui permettent d’augmenter lentement la concentration en nanoparticules. Les interactions attractives induites par le solvant contribuent ainsi à la formation de supracristaux à de très faibles fractions volumiques. A de fortes concentrations, la structure des supracristaux ne dépend pas du solvant utilisé mais, à forte densité de greffage, du rapport R entre la longueur des ligands et le diamètre du cœur d’or. Pour un rapport R voisin de 0.7, la structure finale observée est cubique centrée, la structure à concentration intermédiaire étant cubique à faces centrées. Pour un rapport R deux fois plus petit, une structure originale a été mise en évidence. Il s’agit d’une structure hexagonale de grand paramètre de maille, analysée comme une phase de Frank et Kasper de type MgZn2 ou C14. C’est la première fois qu’une telle phase à empilement local tétraédrique est observée dans un système de sphères monodisperses molles. L’existence de cette phase ainsi que le rôle du rapport R a pu être interprétée en estimant quantitativement la compétition entre l’attraction de van der Waals forte et l’entropie des ligands
As many colloids, metallic nanoparticles grafted with hydrophobic ligands self-assemble above a volume fraction threshold and thus build superlattices. These model systems, which are widely studied when suspended in volatile oils, enable a better understanding of soft spheres self-assembly.Interactions which lead to self-assembly are commonly described by the combination of van der Waals attraction with interaction between the ligand shells. The shell behavior is controlled by the ligand affinity with the solvent. An effect of the solvent on the self-assembly of nanoparticles has already been observed. Using a small angle X-ray scattering, I measured, through the structure factor, the interactions between gold nanoparticles grafted with alkanethiols in different oils, at various concentrations, for different lengths of ligands and core diameters. I noticed an attractive interaction when using flexible linear alkanes as solvent. It has also been shown that the attraction intensity increases with the solvent length.In order to correlate the interactions between particles to their phase diagram, I studied the crystallization process by concentrating nanoparticles using evaporation in capillaries or Ostwald ripening in emulsions. I showed that attractive interactions induced by the solvent lead to superlattices formation at very low volume fractions.At high concentrations, the superlattice structure depends on the ratio of the ligand length over the gold core diameter. For a ratio around 0.7, the final structure observed is body centered cubic, whereas at lower concentration, it is face centered cubic. When this ratio is halved, an unexpected structure is observed. It is a hexagonal structure with a large lattice parameter. It has been analyzed as a Frank and Kasper’s phase named MgZn2 or C14. It is the first time that this topologically close-packed structure is observed for monodisperse soft spheres. The existence of this phase and the role of the ratio R have been interpreted by considering quantitatively the competition between ligands entropy and the strong van der Waals attraction

Electronic memory and computing devices currently rules our digital lives, creating and consuming more than 10^21 bytes of data per year. This amount is expected to grow exponentially, questing for a so-called “hardware revolution”. Bit size-reduction governed the development of solid-state-memory and semiconductor technologies in the last decades. Yet, the innovative concept of an “universal memory” emerged for transcending the size-node. Indeed, this novel system combines high-speed computation and high-density storage skills. Memory devices based on Phase-Change Materials (PCMs) may serve to the scope, defying to scale reduction and both having a competitive erase/read speed with respect to the primary memory systems -as Static/Dynamic Random Access Memory (SRAM, DRAM)- and the large capacity of non-volatile secondary/tertiary storage systems -as Solid State Disk (SSD), Hard Disk Drive (HDD), Digital Video Disc (DVD). The appeal of Phase-Change Materials arises from their ability to rapidly and reversibly switch between the amorphous and crystalline states, via optical or electrical pulses. Interestingly, the two structural phases own significantly different physical properties, in particular in terms of reflectivity and conductivity. Many binary or ternary compounds display phase-change features, still Ge-Sb-Te (GST) alloys are the prominent members of this class of materials and are largely employed in industry. However, the main drawback is the PCMs relatively high operation power consumption, in the form of energy required for the phase transformation and of energy dissipation through -primarily- thermal diffusion. This thesis follows a GST dimensional strategy for power optimization -while maintaining advanced performances- for future integration in novel memory devices. The followed approach includes the case of 2-D highly-textured superlattice structures -of alternately deposited GeTe bilayers and Sb2Te3 quintuple layers- and of 0-D Ge2Sb2Te5 nanoparticles. Chapter 1 reviews the state-of-the-art of Phase-Change Materials, that stimulated the research questions addressed in the present work. Extended X-Ray Absorption Spectroscopy (EXAFS) -briefly presented in the first part of Chapter 2- is a powerful experimental tool for investigating a material’s atomic structure. As described in Chapter 3, one of the two allotrope crystalline phases of (GeTe)-(Sb2Te3) superlattices is revealed in details via EXAFS measurements performed at the Ge and Sb K-edges. The emerged structural picture is commented in light of the proposed models in literature, advising a power-saving yet over-simplified switching process occurring in the superlattice structure. Chapter 4 tackles the problem of power consumption by experimentally demonstrating the energy boost on the optical phase-change process occurring in 0-D Ge2Sb2Te5 nanoparticles. Here, a stable but reversible transition from the crystalline to the amorphous state of nanoparticles is induced with a single low-fluence femtosecond laser pulse. Thermodynamic, optical and structural considerations corroborate the experimental evidence. The laser source together with the setup used for the optical measurements are described in the second part of Chapter 2. The optical arrangement -conceived for time-resolved measurements- led to follow also the relaxation pathways of photo-excited nanoparticles below the threshold fluence for permanent amorphization. The results of this study are unveiled in Chapter 5. The ultrafast dynamics are compared to theoretical simulation and modelled with a phenomenological rate equation. Remarks on the resulting time-scales and the underlying interaction mechanisms -questioning the nature of the resonant bonding- close the Chapter.

Layered crystals with magnetic elements as Co and Fe have been studied. In TlCo2Se2, where Co atoms in one sheet are separated by Tl and Se from the next Co sheet, magnetic interaction within and between the sheets have been studied. Samples doped with 4% 57Fe replaced Co, show a magnetic spiral character with hyperfine fields in a flower shape in the ab-plane. The magnetic moment of 0.46 μB per Co atom derived from the average field is in good agreement with the result from neutron diffraction. In TlCu1.73Fe0.27Se2 the easy axis of magnetisation is the c-axis. The magnetic moment calculated from the Mössbauer data and SQUID magnetrometry is 0.97 μB per Fe atom with TC = 55(5) K. Multilayers of different elements have been studied. The effect of vanadium atoms on iron atoms at the interface of FeNi/V multilayers has been determined and the intermixing at the interface has been calculated to be 2-3 monolayers. For FeNi/Co 1/1 monolayer the magnetic hyperfine field (Bhf) is 45° out-of-plane, while for superlattices containing 2 to 5 monolayers it is in the plane. An study on Fe/Co superlattice were done by experimental, theoretical and simulational methods. The Bhf is highest for the Fe at the second layer next to the interface and gets the bulk value in the centre of thicker Fe layers. Studied magnetic nanoparticles coated with a lipid bilayer (magnetoliposomes) are found to have the magnetite structure but being non-stoichiometric as a result of the manufacturing process. The composition was approximately 32% γ-Fe2O3 and 68% Fe3O4. The oxidation evolution and its effect on magnetic properties of Fe clusters were also studied by means of different techniques. The extraction and insertion mechanism of lithium in the cathode material Li2FeSiO4 has been monitored by in situ x-ray diffraction and Mössbauer spectroscopy during the first two cycles. The relative amount of Fe+3/ Fe+2 at each end state was in good agreement with the results obtained from electrochemical measurements. A possible explanation to the observed lowering of the potential plateau from 3.10 to 2.80 V occurring during the first cycle, involves a structural rearrangement process in which some of the Li ions and the Fe ions are interchanged. The behaviour of small amounts of Fe in brass is investigated using Mössbauer spectroscopy. It was shown that a heat treatment can increase the amount of the precipitates of γ-Fe and ~650° C is the optimal treatment for having the highest amount of this phase.

Doctor of Philosophy
Department of Chemistry
Kenneth J. Klabunde
This is an account of the synthesis of several drastically different forms of silver nanoparticles: Bare metal nanoparticles, dry nanoparticulate powders, aqueous soluble particles, and organic ligand coated monodisperse silver nanoparticles were all produced. The synthetic method was adapted from previous studies on gold nanoparticles and investigated to understand the optimal conditions for silver nanoparticle synthesis. Also the procedure for refinement of the nanoparticles was studied and applied to the formation of alloy nanoparticles. This extraordinary procedure produces beautifully colored colloids of spherical metal nanoparticles of the highest quality which under suitable conditions self-assemble into extensive three dimensional superlattice structures. The silver nanoparticle products were later tested against several biological pathogens to find dramatic increases in antimicrobial potency in comparison to commercially available silver preparations.

Size and Shape Controlled Synthesis and Superparamagnetic Properties of Spinel Ferrites Nanocrystals Qing Song 216 pages Directed by Dr. Z. John Zhang The correlationship between magnetic properties and magnetic couplings is established through the investigations of various cubic spinel ferrite nanocrystals. The results of this thesis contribute to the knowledge of size and shape controlled synthesis of various spinel ferrites and core shell architectured nanocrystals as well as the nanomagnetism in spinel ferrites by systematically investigating the effects of spin orbital coupling, magnetocrystalline anisotropy, exchange coupling, shape and surface anisotropy upon superparamagnetic properties of spinel ferrite nanocrystals. A general synthetic method is developed for size and shape control of metal oxide nanocrystals. The size and shape dependent superparamagnetic properties are discussed. The relationship between spin orbital coupling and magnetocrystalline anisotropy is studied comparatively on variable sizes of spherical CoFe2O4 and Fe3O4 nanocrystals. It also addresses the effect of exchange coupling between magnetic hard phase and soft phase upon magnetic properties in core shell structured spinel ferrite nanocrystals. The role of anisotropic shapes of nanocrystals upon self assembled orientation ordered superstructures are investigated. The effect of thermal stability of molecular precursors upon size controlled synthesis of MnFe2O4 nanocrystals and the size dependent superparamagnetic properties are described.

The aim of this work is to create a process which allows the tailored growth of Ge nanocrystals for use in photovoltic applications. The multilayer systems used here provide a reliable method to control the Ge nanocrystal size after phase separation. In this thesis, the deposition of GeOx/SiO2 and Ge:SiOx~ 2/SiO2 multilayers via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation within the GeOx and Ge:SiOx~ 2 sublayers during subsequent annealing is investigated. Mostly the focus of this work is on the determination of the proper deposition conditions for tuning the composition of the systems investigated. For the GeOx/SiO2 multilayers this involves changing the GeOx composition between elemental Ge (x = 0) and GeO2 (x = 2), whereas for the Ge:SiOx~ 2/SiO2 multilayers this involves changing the stoichiometry of the Ge:SiOx~ 2 sublayers in the vicinity of stochiometric silica (x = 2). The deposition conditions are controlled by the variation of the deposition rate, the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows the sequential deposition of GeOx/SiO2 or Ge:SiOx ~2/SiO2 without changing the oxygen partial pressure during deposition. For stoichiometry determination Rutherford back-scattering spectrometry has been applied extensively. The phase separation in the spatially confined GeOx and Ge:SiOx ~2 sublayers was investigated by X-ray absorption spectroscopy at the Ge K-edge. The Ge sub-oxides content of the as-deposited multilayers diminishes with increasing annealing temperature, showing complete phase separation at approximately 450° C for both systems (using inert N2 at ambient pressure). With the use of chemical reducing H2 in the annealing atmosphere, the temperature regime where the GeOx phase separation occurs is lowered by approximately 100 °C. At temperatures above 400° C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H2. The Ge nanocrystal formation after subsequent annealing was investigated with X-ray scattering, Raman spectroscopy and electron microscopy. By these methods the existence of 2 - 5 nm Ge nanocrystals at annealing temperatures of 550 (GeOx) - 700° C (Ge:SiOx ~2) has been confirmed which is within the multilayer stability range. The technique used allows the production of extended multilayer stacks (50 periods ~ 300 nm) with very smooth interfaces (roughness ~ 0.5 nm). Thus it was possible to produce Ge nanocrystal layers with ultra-thin SiO2 separation layers (thickness ~ 1 nm) which offers interesting possibilities for charge transport via direct tunneling.

Binary nanoparticle superlattices or BNSL’s have been a major area of research in nanotechnology for the last decade. The conventional lithographic techniques are limited to fabricate features only within a plane while these self-assembled structures are formed within a given volume. The potential applications of self-assembly leading to crystalline structures include nanoelectronics, photonics and improving the fundamental understanding of the bottom-up approach as well. The present work deals with the crystallization of nanoparticles in a colloidal suspension leading to three dimensional crystals(BNSL’s). The goal is to generate phase diagrams for binary nanoparticle suspensions with varying charge ratios between the two particles. Monte Carlo molecular simulations in various equilibrium ensembles are the basis for the generation of equations of state and calculation of free energies of the two coexisting phases. Coexistence here implies the solid-fluid thermodynamic equilibrium and consequently the equivalence of free energies. The calculation framework is standard for pure substances, but advanced techniques need to be used for mixtures. The focal point of this work is the effect of charge asymmetry on the formation and stability of BNSL’s from the binary suspension. There are six phase diagrams for charge ratios of 0, 􀀀0:2, 􀀀0:4, 􀀀0:6, 􀀀0:8 and 􀀀1:0. The variables are the reduced pressure P_ and the mole fraction XA. The proposed applications of BNSL’s are generally dependent on substitutional order. From the results, symmetric mixtures are favorable for the formation of BNSL’s while charge asymmetry gives rise to solid solutions.

No
Make it connected! 2D close-packed layers of inorganic nanoparticles are interconnected by organic fibrils of oleic acid as clearly visualized by electron holography. These fibrils can be mineralised by PbS to transform an organic-inorganic framework to a completely interconnected inorganic semiconducting 2D array.

碩士
國立中山大學
化學系研究所
96
Complex nano-architectures of different materials have very interesting geometry. Combining different metallic nanoparticles should allow the manufacture of novel nanocomposite materials with a plethora of exploitable electronic, optical, and magnetic properties. 　Thiolate-capped Au nanoparticles prepared by Brust-Schiffrin two phase method and carboxylate-capped Ag nanoparticles prepared by our one-step synthetic method are reported. 　We also developed and prepared Ag colloidal solution which can be used to form a high valuable conductive thin film by spin coating on Si wafer. Specific resistivity of 6.097 μΩ•cm for the silver metallic film (0.7 μm) on the Si wafer can be simply produced by thermal annealing of Ag MPCs film under an atmosphere of 10 % H2-90 % N2 at 300 ℃ for 1 h. Furthermore, it can be applied to make a micro-circuit by ink-jet printing technique. 　The characterizations of TEM, PXRD, UV-Visible, NMR, FT-IR, ESCA, TGA, TA-MS, EI-MS and SEM of Au and Ag nanoparticles are described. 　We hope the thiolate-capped Au nanoparticles and carboxylate-capped Ag nanoparticles could spontaneously self-connect to form the nanoscale alloy superlattice structure by the molecular recognizable bifuctional linkage.

No
The latest advances in mesocrystal formation and non-classical crystallization of pre-synthesised nanoparticles have been reviewed with the focus on providing a fuller description of a number of complex systems and their properties and applications through examination of the crystallisation mechanisms at work. Two main crystallization principles have been identified; classical crystallization and particle based aggregation modes of non-classical pathways. To understand the non-classical pathways classical crystallization and its basics are introduced before non-classical pathways, such as oriented attachment and mesocrystal formation, are examined. In particular, the various destabilization mechanisms as applied to the pre-synthesized building blocks in order to form mesocrystalline materials as well as the interparticular influences providing the driving forces are analyzed and compared to the mechanisms at work within classical crystallization. Furthermore, the new properties of the mesocrystalline materials that derive from the collective properties of the nanoparticular building units, and their applications potential are presented. It is shown that this new class of materials has the potential to impact in a number of important areas such as sensor applications, energy conversion, photonic crystals as well as for energy storage, optoelectronics and heterogeneous catalysis or photocatalysis.

Nanopartikel auf Basis von NaYF4 erfreuen sich großer Beliebtheit durch ihre vielseitigen Einsatzmöglichkeiten. Durch die Dotierung mit Ytterbium und Erbium im Wirtsgitter ist es beispielsweise möglich, niedrigenergetisches Infrarotlicht in höher-ergetisches, sichtbares Licht umzuwandeln. Zudem lässt sich NaYF4 auch im Nanometermaßstab präparieren, sodass ein Einsatz in Zellen oder lebenden Organismen möglich ist, wo die zur Anregung verwendete infrarote Strahlung ohne Probleme das Gewebe durchdringen kann. Zu Beginn dieser Arbeit zeigten aufwärtskonvertierende Nanomaterialien wie NaYF4 :Yb,Er jedoch auch nach Umhüllen mit einer inaktiven Schale aus undotiertem NaYF4 nur sehr geringe Lumineszenz-Quantenausbeuten und kurze Energieniveau-Lebenszeiten. Im Rahmen dieser Arbeit wurde die Synthesemethode zur Herstellung von aufwärtskonvertierenden NaYF4 -Nanopartikeln durch den Einsatz neuer Eduktmaterialien modifiziert und die Auswirkung der Modifikationen auf die Partikeleigenschaften näher untersucht. So konnte gezeigt werden, dass durch den Einsatz einer alternativen Fluoridquelle (NaHF2) Partikel mit sehr engen Partikelgrößenverteilungen hergestellt werden können. Jedoch zeigte sich auch, dass die mit NaHF2 präparierten Partikel sich nicht mit einer Schale aus undotiertem NaYF4 umhüllen ließen. Im zweiten Teil dieser Arbeit wurde daher der Fokus auf die Verbesserung der optischen Eigenschaften gelegt. Durch die Verwendung von getrockneten Lösungsmitteln und wasserfreien Seltenerdacetaten, sowie NH4F als Fluoridquelle gelang es erstmals, aufwärtskonvertierende Kern/Schale-Nanopartikel (<50 nm) mit einer sehr hohen Lumineszenz-Quantenausbeute, ähnlich dem des makrokristallinen Referenzmaterials, herzustellen. Auch bei sehr kleinen Kern/Schale-Partikeln (≤15 nm) konnten Quantenausbeuten erzielt werden, die nur um einen Faktor 3-4 niedriger sind als beim Referenzmaterial. Dabei zeigte sich durch die Messung der Energieniveau-Lebenszeiten, dass die größten Verlustprozesse durch die Yb3+ Emission bei 940 nm auftraten und diese durch aufbringen einer Schale unterbunden werden konnten.