A logical progression from the maturing field of colloidal semiconductor quantum dots to the emerging subclass of impurity-doped colloidal semiconductor nanoparticles is underway. To this end, the present work describes the formation and analysis of a new form of Tin-doped Indium Oxide (ITO). The form is that of a colloidal dispersion comprised of pure-phase, 4-6 nanometer ITO particles possessing an essentially single crystalline character. This system forms a non-agglomerated, optically clear solution in a variety of non-polar solvents and can remain in this state, at room temperature, for months and potentially, years. ITO is the most widely used member of the exotic materials family known as Transparent Conductive Oxides (TCOs) and is the primary enabling material behind a wide variety of opto-electronic device technologies. Material synthesis was achieved by initiating a series of interrelated nucleophilic substitution reactions that provided sufficient intensity to promote doping efficiencies greater than 90% for a wide range of tin concentrations. The optical clarity of this colloidal system allowed the intrinsic properties of single crystalline ITO particles to be evaluated prior to their use in thin-films or composite structures. Monitoring the temporal progression of n-type degeneracy by its effects on the optical properties of colloidal dispersions shed light on the fundamental issues of particle formation, band filling (Burstein-Moss) dynamics, and the very origin of n-type degeneracy in ITO. Central to these studies was the issue of excess electron character. The two limiting cases of entirely free and entirely confined electron motion were evaluated by application of bulk-like band dispersion analysis and the effective mass approximation, respectively. This provided a means to estimate the number of excess conduction band electrons present within an individual particle boundary. The ability to control and optimize the level of n-type degeneracy within the colloidal ITO nanoparticle form by compositional variation was also demonstrated. A key to the widespread adoption of a new material by industry is an ability to produce multi-gram and perhaps, kilogram quantities with no significant sacrifice in quality. Accordingly, a modified synthesis process was developed to allow for the mass production of high-quality colloidal ITO nanocrystals.

Colloidal semiconductor nanocrystals exist at the interface of inorganic chemistry, solid-state physics, and materials applications. The highly tunable and size-dependent properties position them as prime candidates for advancing a range of technologies, including improving efficiency in solid-state lighting devices and high color-purity displays. To be successful in these endeavors, quantum dots require excellent optical properties, such as bright emission. Optimization of a zinc sulfide coating is widely regarded as a key requirement to achieving these necessary performances. Even so, zinc sulfide nanocrystal chemistry remains underdeveloped. This dissertation addresses these shortcomings and provides comprehensive synthetic and analytical tools to harness the potential of zinc sulfide containing nanocrystals. Chapter 1 introduces semiconductor nanocrystals, also referred to as quantum dots, and begins with a description of the size-dependent optical properties. Factors that lead to poorer emission properties, such as undercoordinated surface atoms are discussed. Methods to alleviate these issues, including controlling the surface coordination environment, and design and growth of heterostructures are introduced. Lastly, synthetic approaches and nanocrystal formation mechanisms are described. Chapter 2 covers the synthesis and size-dependent optical properties of zinc sulfide nanocrystals. We find that commonly used solvents in nanocrystal reactions lead to the formation of polymeric byproducts that are challenging to purify away, and thus design the zinc sulfide synthesis such that these can be avoided. Leveraging a library of rate tunable thioureas the final nanocrystal size can be carefully controlled. The reactions follow a thermally activated growth process, with larger zinc sulfide nanocrystals accessible at higher temperatures. Most relevantly for later chapters, the surface coordination environment is highly important; bulkier zinc carboxylate ligands that cannot achieve high surface coverages result in higher growth rates. These results represent the most tunable size controls reported for zinc sulfide nanocrystals. Chapter 3 uses high resolution electron microscopy techniques to study the shape (morphology) of zinc sulfide nanocrystals, synthesized using the methods developed in the second chapter. Irregular, anisotropic growth is commonly seen in zinc sulfide shell growth and is attributed to core/shell interfacial strain. We find that this growth also occurs in the binary zinc sulfide system. Synthetic conditions favoring fast growth result in unselective, isotropic growth of spherical zinc sulfide. Conversely, slower conditions can lead to irregular, anisotropic shapes. The shape is also highly dependent on the coordination environment during growth. Small, sterically unencumbered ligands stabilize specific crystal facets, leading to selective, anisotropic growth. These findings are translated to shelling procedures in Chapter 6, and further emphasize the need to understand and characterize zinc sulfide surfaces. Chapter 4 establishes an empirical relationship between the band gap energy of a zinc sulfide nanocrystal and its diameter. The literature reports a wide spread of diameters for a given energy, meaning zinc sulfide sizes could not previously be easily calculated from their optical properties. Leveraging the size- and shape-control discussed in Chapters 2 and 3, we assess the utility of a range of nanocrystal characterization techniques for accurately sizing quantum confined zinc sulfide. Using electron microscopy and X-ray scattering methods we present an updated energy-size (“sizing curve”) relationship for zinc sulfide. These results represent the most comprehensive zinc sulfide nanocrystal sizing study and enable the rapid size characterization of zinc sulfide from its absorbance spectrum. This provided crucial insight into the reaction progressions described in Chapter 2. Chapter 5 covers our endeavors to characterize and quantify the zinc sulfide nanocrystal surface chemistry, which we believe is imperative to improving shelling procedures and optical properties in zinc sulfide heterostructures. With no published extinction coefficient, the surface coverages of zinc sulfide cannot be obtained. Using the size- and shape-controlled syntheses, in conjunction with optical absorption spectroscopy and elemental analysis, we calculate extinction coefficients for a range of zinc sulfide nanocrystal sizes. The size-dependence is well described by a power law, and this represents the first reported extinction coefficient for zinc sulfide. Using this, we report the first surface coverages of zinc sulfide nanocrystals and assess the binding affinity of zinc carboxylates to the surface by monitoring their displacement by L-type ligands. Chapter 6 widens the zinc sulfide synthetic methods developed in earlier chapters to deposit zinc sulfide shells onto blue-emitting II-VI and red-emitting III-V nanocrystals. The reaction shows versatility, shelling nanocrystals over a wide range of temperatures. We demonstrate morphology control over the zinc shell by altering the deposition kinetics and coordination environment. Usually, thick, homogenous shells are desired by the nanocrystal field. However, by correlating the shell morphology to its optical properties, we see that the anisotropic shells generally achieve higher photoluminescence quantum yields (PLQYs). We also report progress towards cadmium-free quantum dot downconverters for use in solid-state lighting applications. Among other things, the photoluminescence intensity evolution throughout the shelling procedure is highly dependent on the initial surface termination of the nanocrystal core. Application of surface treatments allows brighter zinc sulfide shelled III-V heterostructures to be accessed.

M. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology / The understanding of the fundamental properties and potential use of semiconductor materials in nanotechnology has stimulated the interest of many researchers. Coordination compounds containing ligands with chalcogenide atoms as donors have received considerable attention. Among these chalcogenide ligands, thiourea and urea have been extensively used previously to form single source precursors for the synthesis of group II–VI semiconductor nanoparticles. The synthesis and study of semiconductor nanocrystals has become a subject area of considerable research interest because they have potentially useful applications such as biomedical imaging, sensing, light-emitting diodes (LEDs), photovoltaics and displays. In this work, special attention has been given to the synthesis and characterization of cadmium and nickel chalcogenides nanoparticles because of their interesting and unique optical properties, using a single source precursor method. The mixed chalcogenide sources are also explored for their complexes or simple reactions to produce semiconductor nanoparticles. The advantages of single molecular precursors over other existing methods for the synthesis of metal chalcogenide nanoparticles has proven to be a more efficient route for the synthesis of high-quality nanocrystals. Cadmium and nickel complexes of urea and thiourea were successfully synthesized by refluxing metal salts of cadmium and nickel with thiourea and urea at 30-40 °C for an hour and the complex mixture was cooled at room temperature. The synthesized complexes were washed with methanol and acetone to remove impurities and dried in air. All other complexes were synthesized using the same refluxing synthesis method mentioned above. The characterization of all complexes was done using Fourier-Transform infrared spectroscopy, thermogravimetric analysis and CARLO ERBA elemental analysis. Nickel thiourea and cadmium thiourea complexes were found to be coordinating with the center metal through a sulfur atom, and the urea complexes through an oxygen atom. This observation of the coordination of metals with ligands makes these complexes suitable for use as the single source precursor for the synthesis of metal chalcogenides nanoparticles. The six resulted complexes from above synthesis are of cadmium sulphide, cadmium oxide, and nickel sulphide, nickel oxide. The TGA showed that all the complexes were stable at room temperature, as they decomposed around 200 °C, which makes these complexes suitable to be used for the synthesis of nanoparticles. The synthesized complexes reported in this study were used as a single source molecular precursor in the preparation of cadmium oxide, cadmium sulfide, nickel oxide, and nickel sulfide nanoparticles. The precursors were thermalized under nitrogen gas while refluxing at 160 °C for 1 hour, using HDA as the capping agent and TOP was used as the solvent. Crystalline semiconducting nanoparticles were obtained as the end product for all complexes, the resulting nanoparticles were washed with acetone to remove any impurities. The UV-Vis spectra of all the nanoparticles were blue-shifted, with their PL spectra red-shifted from the maximum absorption peak due to change in size of particles from bulk to nano-size. XRD pattern of CdS nanoparticles from Cd-thiourea suggested that a hexagonal phase of CdS was formed, and TEM analysis showed large particle sizes that were polydispersed with the dominance of cubic and rod shaped particles. The XRD patterns of CdS nanoparticles from cadmium thiourea and urea mixture showed the mixture of hexagonal and cubic phase nanoparticles with the predominance of cubic phase and its TEM images shows small particles size ranging from less than 50 nm, and the particles were polydispersed with the predominance of spherical nanoparticles. The XRD pattern of CdO nanoparticles showed the cubic phase nanoparticles with the existence of broad peaks indicating small particle size distribution. The TEM images of CdO nanoparticles confirmed the XRD data showing small particle size distribution with a size average of 6.8 nm. The NiS nanoparticles synthesized from Ni-thiourea complex showed narrow peaks with hkl indices indicating hexagonal phase. Particles has no clear morphology due to the agglomeration of the nanoparticles that can be caused by the instability of the nanoparticles because of their high surface area. A cluster of particles can be observed from the TEM images, making it difficult to determine the particle size and shape of NiS particles from Ni-thiourea complex. NiS particles from nickel thiourea and urea mixture also showed XRD patterns of the hexagonal plane and TEM showed small size conjugated nanocrystals. The TEM of NiO indicated a spherical morphology, existence of other morphology rather than spherical can also be observed from the image. The average particle size was 8 nm, and XRD pattern showed the cubic phase of NiO nanocrystals.

Les nanocristaux semi-conducteurs colloïdaux possèdent des propriétés photo-physiques qui en font des objets de choix pour des applications variées, comme le marquage biologique, le photovoltaïque ou encore l’optique quantique. Leur interaction avec une structure photonique peut modifier leurs propriétés d’émission (durée de vie, intensité…). Le microscope optique de champ proche est un outil privilégié pour venir sonder ces modifications à l’échelle nanométrique.Cette thèse porte sur la réalisation d’une sonde active de champ proche réalisée à partir d’un nanocristal cœur/coquille CdSe/CdS greffé à l’apex d’une fibre optique amincie. Cette sonde est utilisée pour cartographier, dans les 3 dimensions de l’espace et à l’échelle nanométrique, les variations de durée de vie de l’émetteur. Elle permet de rendre compte des variations des modes photoniques sur la surface.Une partie de cette thèse porte sur la réalisation de la sonde active elle-même. Grâce à cette sonde les études sont alors développées sur un réseau de trous dans un film mince d’or. Des simulations FDTD ont été réalisées dans le but de déterminer les paramètres pertinents du réseau et d’analyser leur réponse en champ proche.Les résultats expérimentaux des durées de vie en divers points de différents réseaux, obtenus avec la sonde active, sont confrontés aux résultats numériques. / Colloidal semiconductor nanocrystals have photo-physical properties that make them objects of choice for various applications, such as biological marking, photovoltaics or quantum optics. Their interaction with a photonic structure can modify their emission properties (lifetime, intensity, etc.). The near-field optical microscope is a privileged tool to probe these changes at the nanoscale.This thesis deals with the realization of an active near-field probe made from a CdSe / CdS core / shell nanocrystal grafted to the apex of a thinned optical fiber. This probe is used to map, in the 3 dimensions of the space and on the nanometric scale, the variations in the lifetime of the emitter. It makes it possible to account for variations in photonic modes on the surface.A part of this thesis concerns the realization of the active probe itself. Thanks to this probe the studies are then developed on a hole grating made in a thin film of gold. FDTD simulations were performed to determine relevant grating parameters and to analyze their near field response.The experimental results of the lifetimes at various points of different gratings, obtained with the active probe, are compared with the numerical results.

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フランス国ボルドー大学との共同学位プログラムによる学位 / 京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23585号 / エネ博第431号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 石原 慶一, 教授 萩原 理加 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

Doping in bulk semiconductors (e.g., n- or p- type doping in silicon) allows for precise control of their properties and forms the basis for the development of electronic and photovoltaic devices. Recently, there have been reports on the successful synthesis of doped semiconductor nanocrystals (or quantum dots) for potential applications in solar cells and spintronics. For example, nanocrystals of ZnSe (with zinc-blende lattice structure) and CdSe and ZnO (with wurtzite lattice structure) have been doped successfully with transition-metal (TM) elements (Mn, Co, or Ni). Despite the recent progress, however, the underlying mechanisms of doping in colloidal nanocrystals are not well understood. This thesis reports a comprehensive theoretical analysis toward a fundamental kinetic and thermodynamic understanding of doping in ZnO, CdSe, and ZnSe quantum dots based on first-principles density-functional theory (DFT) calculations. The theoretical predictions of this thesis are consistent with experimental measurements and provide fundamental interpretations for the experimental observations. The mechanisms of doping of colloidal ZnO nanocrystals with the TM elements Mn, Co, and Ni is investigated. The dopant atoms are found to have high binding energies for adsorption onto the Zn-vacancy site of the (0001) basal surface and the O-vacancy site of the (0001) basal surface of ZnO nanocrystals; therefore, these surface vacancies provide viable sites for substitutional doping, which is consistent with experimental measurements. However, the doping efficiencies are affected by the strong tendencies of the TM dopants to segregate at the nanocrystal surface facets, as indicated by the corresponding computed dopant surface segregation energy profiles. Furthermore, using the Mn doping of CdSe as a case study, the effect of nanocrystal size on doping efficiency is explored. It is shown that Mn adsorption onto small clusters of CdSe is characterized by high binding energies, which, in conjunction with the Mn surface segregation characteristics on CdSe nanocrystals, explains experimental reports of high doping efficiency for small-size CdSe clusters. In addition, this thesis presents a systematic analysis of TM doping in ZnSe nanocrystals. The analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets, as well as dopant-induced nanocrystal morphological transitions, and leads to a fundamental understanding of the underlying mechanisms of dopant incorporation into growing nanocrystals. Both surface kinetics (dopant adsorption onto the nanocrystal surface facets) and thermodynamics (dopant surface segregation) are found to have a significant effect on the doping efficiencies in ZnSe nanocrystals. The analysis also elucidates the important role in determining the doping efficiency of ZnSe nanocrystals played by the chemical potentials of the growth precursor species, which determine the surface structure and morphology of the nanocrystals.

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