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This dissertation describes preparation and characterization of Ni₂P catalysts and their application in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). The work carried out includes synthesis of Ni₂P on different siliceous supports, SiO2 and MCM-41. It also includes characterization of these catalytic materials using X-ray diffraction (XRD), temperature-programmed reduction (TPR), Fourier transform infrared (FTIR) spectroscopy and X-ray absorbance fine structure (XAFS) spectroscopy. In situ FTIR was employed to study the acidity of Ni₂P/SiO₂ and probe the catalytic sites involved in the HDN of pyridine. Transient and steady-state kinetics of a surface intermediate that is formed upon pyridine adsorption and reaction was studied to elucidate the mechanism of HDN over Ni₂P/SiO₂. Additionally, in situ FTIR and X-ray absorption near edge structure (XANES) spectroscopy was utilized to probe the bonding, mechanism and kinetics of thiophene HDS over a novel MCM-41-supported Ni₂P catalyst. The use of these techniques allowed for better understanding of the surface intermediates, mechanisms and the nature of the active sites involved in HDN and HDS. / Ph. D.

This dissertation describes preparation and characterization of Ni₂P catalysts and their application in deep hydrodesulfurization (HDS) of a model sulfur compound, 4,6-dimethyldibenzothiophene (4,6-DMDBT), one of the most refractory S-compounds. This topic is of great importance in addressing recently enacted environmental regulations limiting the sulfur content in fuels. The work carried out includes synthesis of Ni₂P on different siliceous supports, SiO₂, MCM-41, and ultra-stable Y zeolite (USY). It also includes determining the characteristics of the supported Ni₂P catalysts with a wide range of techniques: X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and X-ray absorption fine structure (XAFS) spectroscopy. The use of these techniques allowed better understanding of the nature of the active sites as well as the effect of supports. Activity tests were conducted in the HDS of 4,6-DMDBT and the HDN of quinoline. The performance of the catalysts will be compared to that of a conventional sulfide hydrotreating catalyst, Ni-Mo-S/Al₂O₃. Investigation of the reaction mechanism in the hydrodenitrogenation (HDN) of 2-methylpiperidine together with in situ FT-IR measurements were conducted to understand how catalyst properties affect activity and selectivity. / Ph. D.

The high degree of control over the morphology and optoelectronic properties of semiconductor nanowires (NWs) makes them attractive for applications such as thermoelectrics, quantum emitters, and photodetectors. However, NW growth is still not fully understood as many parameters play a role in the determination of NW morphology and crystal structure, which in turn governs resulting optoelectronic properties. We report tellurium-doped GaP NWs with positive tapering and radii measuring as low as 5 nm grown by the self-assisted vapor–liquid–solid mechanism using selective-area molecular beam epitaxy. The occurrence of ultrathin nanoantenna showed a dependence on pattern pitch (separation between NWs) with a predominance at 600 nm pitch, and exhibited radius oscillations that correlate with polytypic zincblende (ZB)/wurtzite (WZ) segments. A growth model explains the positive tapering of the NW leading to an ultrathin tip from the suppression of surface diffusion of Ga adatoms on the NW sidewalls by Te dopant flux. The model also provides a relationship between the radius modulations and the oscillations of the droplet contact angle, predicting the quasi-periodic radius oscillations and corresponding crystal phase transitions. Photoluminescence and cathodoluminescence at 10 K reveal distinct spectra corresponding to either the ZB or WZ phase. Emission above and below ~2.15 eV are associated with ZB and WZ, respectively. The characteristic WZ spectrum arises from a bound exciton and its phonon replicas, consistent with published results. The origin of emission in the ZB regime is less conclusive, but may originate from the splitting of a bound exciton by the field of an axial defect. The results presented in this thesis establish a link between NW growth, morphology, and optoelectronic properties to inform future work involving ultrathin NWs. / Thesis / Master of Applied Science (MASc) / A nanowire (NW) is a tiny rod with a length on the order of one millionth of a meter and diameter on the order of one billionth of a meter. We made gallium phosphide (GaP) NWs by stacking gallium and phosphorus atoms in a column. The NWs were separated by a constant distance. In some cases, we also added beryllium and tellurium atoms to our NWs. The addition of tellurium caused our NWs to grow into extremely sharp points, which we measured with a microscope that uses electrons instead of light. The microscope images also revealed that the arrangement of the atoms in the NW changes along its length. By detecting the light emission from the NWs, it was possible to distinguish between two unique arrangements. Overall, the small dimensions of our GaP NWs make them interesting for applications that require the emission or detection of single particles of light.

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / The semiconductor boron phosphide (BP) has many outstanding features making it attractive for developing various electronic devices, including neutron detectors. In order to improve the efficiency of these devices, BP must have high crystal quality along with the best possible electrical properties. This research is focused on growing high quality crystalline BP films on a variety of superior substrates like AIN, 4H-SiC, 3C-SiC and ZrB₂ by chemical vapor deposition. In particular, the influence of various parameters such as temperature, reactant flow rates, and substrate type and its crystalline orientation on the properties of BP films were studied in detail. Twin-free BP films were produced by depositing on off-axis 4H-SiC(0001) substrate tilted 4° toward [1-100] and crystal symmetry matched zincblende 3C-SiC. BP crystalline quality improved at higher deposition temperature (1200°C) when deposited on AlN, 4H-SiC, whereas increased strain in 3C-SiC and increased boron segregation in ZrB₂ at higher temperatures limited the best deposition temperature to below 1200°C. In addition, higher flow ratios of PH₃ to B₂H₆ resulted in smoother films and improved quality of BP on all substrates. The FWHM of the Raman peak (6.1 cm⁻¹), XRD BP(111) peak FWHM (0.18°) and peak ratios of BP(111)/(200) = 5157 and BP(111)/(220) = 7226 measured on AlN/sapphire were the best values reported in the literature for BP epitaxial films. The undoped films on AlN/sapphire were n-type with a highest electron mobility of 37.8 cm²/V·s and a lowest carrier concentration of 3.15x1018 cm⁻ᶟ. Raman imaging had lower values of FWHM (4.8 cm⁻¹) and a standard deviation (0.56 cm⁻¹) for BP films on AlN/sapphire compared to 4H-SiC, 3C-SiC substrates. X-ray diffraction and Raman spectroscopy revealed residual tensile strain in BP on 4H-SiC, 3C-SiC, ZrB₂/4H-SiC, bulk AlN substrates while compressive strain was evident on AlN/sapphire and bulk ZrB₂ substrates. Among the substrates studied, AlN/sapphire proved to be the best choice for BP epitaxy, even though it did not eliminate rotational twinning in BP. The substrates investigated in this work were found to be viable for BP epitaxy and show promising potential for further enhancement of BP properties.

by Wong Kin Sang. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 61-62). / Text in English; abstract also in Chinese. / by Wong Kin Sang. / Table of contents --- p.I / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Interest in ZnxCd1-x Se/InP --- p.1 / Chapter 1.2 --- Conditions of thermal annealing --- p.2 / Chapter 1.3 --- Advantages of using photoluminescence (PL) --- p.3 / Chapter 1.4 --- Our work --- p.4 / Chapter Chapter 2 --- Experimental setup and procedures / Chapter 2.1 --- PL measurements --- p.6 / Chapter 2.1.1 --- Setup --- p.6 / Chapter 2.1.2 --- Types of PL measurements --- p.6 / Chapter 2.2 --- Annealing experiments --- p.8 / Chapter 2.2.1 --- Setup --- p.8 / Chapter 2.2.2 --- Types of annealing --- p.10 / Chapter 2.2.3 --- Procedures --- p.11 / Chapter Chapter 3 --- Results and discussions / Chapter 3.1 --- Room temperature PL studies of ZnxCd1-xSe/InP --- p.12 / Chapter 3.1.1 --- As-grown ZnxCd1-x Se/InP --- p.12 / Chapter 3.1.1.1 --- Peak energy vs concentration --- p.12 / Chapter 3.1.2 --- Annealing studies --- p.15 / Chapter 3.1.2.1 --- Isothermal annealing --- p.15 / Chapter 3.1.2.2 --- Isochronal annealing --- p.20 / Chapter 3.2 --- PL studies of ZnxCd1-xSe/InP at 10 K temperature --- p.22 / Chapter 3.2.1 --- As-grown ZnxCd1-xSe/InP --- p.22 / Chapter 3.2.1.1 --- Excitation power density dependence --- p.22 / Chapter 3.2.1.2 --- Peak energy vs Zn concentration --- p.26 / Chapter 3.2.2 --- Annealing studies --- p.29 / Chapter 3.2.2.1 --- Isothermal annealing --- p.29 / Chapter 3.2.2.2 --- Isochronal annealing --- p.33 / Chapter 3.3 --- Temperature dependent PL studies of ZnxCd1-xSe/InP --- p.37 / Chapter 3.3.1 --- As-grown ZnxCd1-xSe/InP --- p.37 / Chapter 3.3.1.1 --- Peak energy vs temperature --- p.37 / Chapter 3.3.1.2 --- Peak width vs temperature --- p.46 / Chapter 3.3.2 --- Annealing studies --- p.50 / Chapter 3.3.1.1 --- Peak energy vs temperature --- p.50 / Chapter 3.3.1.2 --- Peak width vs temperature --- p.55 / Chapter Chapter 4 --- Conclusions --- p.59 / References --- p.61

"August 1999." / Thesis (Ph.D.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

by So King Lung, Benny. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 102-109). / ABSTRACT --- p.v / ACKNOWLEDGEMENTS --- p.vii / LIST OF FIGURES --- p.viii / LIST OF TABLES --- p.xiii / Chapter Chapter 1 --- Background of the study --- p.1 / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- Surface passivation techniques --- p.3 / Chapter 1.2.1 --- Sulfide solution passivation --- p.3 / Chapter 1.2.2 --- Gas-phase sulfide passivation --- p.4 / Chapter 1.3 --- Surface structure of sulfide-passivated surface --- p.5 / Chapter 1.4 --- Objectives of the present study --- p.7 / Chapter Chapter 2 --- Instrumentation --- p.9 / Chapter 2.1 --- Introduction --- p.9 / Chapter 2.2 --- X-ray photoelectron spectroscopy (XPS) --- p.9 / Chapter 2.2.1 --- The development of XPS --- p.9 / Chapter 2.2.2 --- Basic principle of XPS --- p.9 / Chapter 2.2.3 --- Quantitative analysis of XPS --- p.14 / Chapter 2.2.3.1 --- Atomic concentration of a homogenous material --- p.14 / Chapter 2.2.3.2 --- Layer structure --- p.15 / Chapter 2.2.3.3 --- Simulation of XPS atomic concentration ratios from proposed surface structural models --- p.17 / Chapter 2.2.4 --- XPS experiment --- p.19 / Chapter 2.3 --- Low energy electron diffraction (LEED) --- p.21 / Chapter 2.3.1 --- The development of LEED --- p.21 / Chapter 2.3.2 --- Basic principle of LEED --- p.23 / Chapter 2.3.3 --- LEED experiment --- p.28 / Chapter 2.3.3.1 --- The ultra high vacuum chamber (UHV) --- p.28 / Chapter 2.3.3.2 --- The electron gun --- p.28 / Chapter 2.3.3.3 --- The sample --- p.30 / Chapter 2.3.3.4 --- The detector system --- p.30 / Chapter Chapter 3 --- Surface treatments --- p.31 / Chapter 3.1 --- Semiconductor wafers --- p.31 / Chapter 3.2 --- Cleaning procedure --- p.31 / Chapter 3.3 --- Polysulfide passivation --- p.33 / Chapter Chapter 4 --- Gas-phase polysulfide passivation of the InP(100) surface --- p.37 / Chapter 4.1 --- Introduction --- p.37 / Chapter 4.2 --- Sulfide-assisted reordering at the InP(100) surface --- p.38 / Chapter 4.2.1 --- Gas-phase polysulfide-treated InP( 100) surface --- p.38 / Chapter 4.2.2 --- Further annealing of the gas-phase polysulfide-treated surface --- p.47 / Chapter 4.2.3 --- Comparison with the UV/O3-HF treatment --- p.48 / Chapter 4.2.4 --- Sulfide at the interface of SiNx/InP --- p.49 / Chapter 4.3 --- Conclusions --- p.53 / Chapter Chapter 5 --- Gas-phase polysulfide passivation of the GaAs(lOO) surface --- p.55 / Chapter 5.1 --- Introduction --- p.55 / Chapter 5.2 --- Gas-phase poly sulfide-passivated GaAs( 100) surface --- p.56 / Chapter 5.2.1 --- Surface structure of the as-treated surface --- p.56 / Chapter 5.2.2 --- Surface structure after further annealing --- p.64 / Chapter 5.2.3 --- Mechanism of the gas-phase polysulfide passivation --- p.67 / Chapter 5.3 --- Conclusions --- p.68 / Chapter Chapter 6 --- Gas-phase polysulfide passivation of the GaAs(100) surface --- p.69 / Chapter 6.1 --- Introduction --- p.69 / Chapter 6.2 --- Reordering at the gas-phase polysulfide-passivated GaAs(100) surface --- p.70 / Chapter 6.2.1 --- Adsorption of polysulfide on the GaAs(100) surface --- p.70 / Chapter 6.2.2 --- Ordered sulfide at the GaAs(l 10) surface --- p.73 / Chapter 6.2.3 --- Further analysis of the LEED pattern --- p.80 / Chapter 6.3 --- Conclusions --- p.83 / Chapter Chapter 7 --- Sulfide Solution passivation of the GaAs(100) surface --- p.84 / Chapter 7.1 --- Introduction --- p.84 / Chapter 7.2 --- Sulfide solution passivation on the GaAs(l 10) surface --- p.85 / Chapter 7.2.1 --- Etching of sulfide solution on the GaAs(l 10) surface --- p.85 / Chapter 7.2.2 --- Annealing of sulfide solution-passivated GaAs( 110) surface --- p.88 / Chapter 7.2.3 --- Further analysis of the LEED pattern --- p.92 / Chapter 7.2.4 --- Shift of XPS peak position during annealing --- p.95 / Chapter 7.3 --- Conclusions --- p.97 / Chapter Chapter 8 --- Conclusions and further work --- p.99 / Chapter 8.1 --- Conclusions --- p.99 / Chapter 8.2 --- Further work --- p.100 / References --- p.102

Doctor of Philosophy / Department of Chemistry / Emily McLaurin / Quantum dots are nanocrystalline semiconductors in which the size is so small that optoelectronic properties are size dependent. QDs have a lot of applications in displays, solar cells, lasers, light emitting diodes, etc. The optoelectronic properties of QDs depend on their size, composition, the shape of the particles and also the surface chemistry of the QDs. Phosphine based precursors have been mostly used in the synthesis of QDs. Due to the lack of tunable reactivity, this class of precursors, QDs with different shape are obtained by under different reaction conditions. With that, branched QDs are less likely to be obtained in one step reaction using phosphine based precursors. To synthesis QDs with a branched structure, in a single step synthesis, mixtures of precursors with different reactivity were used. Using dichalcogenides mixture, CdSe₁-xSx hyperbranched supra-quantum dots (HSQDs) where synthesized in a one-step microwave-assisted synthesis and shape evolution mechanism of formation of NCs studied. It is shown that the NCs formed in three steps of nucleation, aggregation, and growth. By controlling the reaction conditions, simple branched tetrapod NCs are prepared, but the obtained NCs have no emission due to unpassivated surface and defects which work as trap. To obtain luminescent NCs obtained through doping. Hyperbranched Mn²+:ZnSe₁-xSx NCs also prepared using a mixture of Ph₂Se₂ and Me₂S₂. The shape evolution mechanism of the formation of NCs was studied and it is shown that the NCs are formed via oriented attachment of initially formed nanoparticles. The NCs used for thiol sensing, and it observed that they have a better sensitivity and detection limit than spherical QDs. Although hyperbranched NCs have higher sensitivities over nonbranched NCs but, the spherical NCs have better detection limit and can dispersed in aqueous medium by ZnS shell growth followed by silica shell formation. To study the effect of ZnS shell thickness on sensing property of NCs, a set of spherical Mn:ZnSe@ZnS with different ZnS shell thickness were prepared and used for thiol sensing. It observed that in organic medium, thinner ZnS layer gives the highest sensitivity and QDs with thick ZnS shell layer have less sensitivity. For measurement in aqueous medium, QDs transferred to PBS buffer after formation of silica shell over QDs. It observed that NCs with a thin ZnS shell layer lose their emission and sensing completely. Thick ZnS shell protects NCs in the silica shell formation step but they show very low sensitivity to thiol compounds as well. ZnS shell with medium thickness gives the best sensitivity in an aqueous medium. The emission of Mn:ZnSe@ZnS QDs originated from d-d electron transition of Mn(II) ions and is independent to the size of QDs. To extend our study to QDs with band edge emission, preparation of luminescent InP QDs by post-synthetic modification is studied. InP NCs were synthesized using heat up method and successive injection of precursors. Narrow size distribution NCs obtained after size selection precipitation. Emissive NCs obtained after etching using InCl3 and fluoride containing salts. The study showed that more InCl3 case more etching and presence of fluoride-containing salt is necessary for band edge emission of the NCs.

Dans le cadre de la transition énergique, le déploiement de sources d’énergies ne produisant pas de gaz à effet de serre devient primordial. Bénéficiant de la surabondante énergie fournie par le Soleil, le photovoltaïque est un des éléments-clés du bouquet énergétique du futur. Le marché du photovoltaïque est actuellement dominé par les technologies à base de silicium et les meilleurs rendements de conversion dépassent les 26% avec la technologie de cellules à hétérojonction de silicium amorphe hydrogéné (a-Si:H) sur silicium monocristallin (c-Si).Le silicium amorphe hydrogéné, déposé par PECVD, permet d’obtenir une excellente passivation de la surface du substrat de silicium cristallin, et ainsi d’obtenir des tensions de circuit ouvert au-delà de 730 mV. Cependant l’a-Si:H montre une absorption parasite des photons ultraviolets, et sa faible conductivité limite la longueur de diffusion des porteurs de charge générés en son sein, limitant la performance électrique et aussi leur contribution au courant de la cellule.Pour augmenter le rendement de cette technologie, nous proposons de fabriquer et de caractériser une nouvelle structure de cellules photovoltaïques à base d'hétérojonction de phosphure de gallium (GaP) sur c-Si, déposé par dépôt en phase vapeur aux organométalliques (MOCVD). Matériau III-V, cristallin, et à énergie de bande interdite élevée (2.26 eV contre 1.6-1.9 eV pour l’a-Si:H et 1.12 eV pour le c-Si), le GaP permettrait une croissance par épitaxie sur le c-Si, une meilleure transparence face à l’a-Si:H, ainsi qu’une passivation par effet de champ repoussant les trous, porteurs de charge positive, loin de l’interface GaP/Si. Les améliorations des caractéristiques courant-tension de telles cellules avec seulement 10 nm de GaP ont précédemment montré, par simulation, une amélioration des rendements de 2% en absolu.Dans le cadre de cette thèse, nous avons étudié expérimentalement l’effet du dépôt de GaP sur le c-Si. Nous avons mis en évidence une dégradation de la durée de vie des porteurs dans le c-Si lors d’une étape de préparation de surface pour améliorer l’épitaxie du GaP, qui favoriserait la diffusion de contaminants issus de la chambre de dépôts III-V dans le substrat. Cette étape pourrait être retirée, mais elle est nécessaire pour limiter l’émergence de domaines d’antiphase, défauts cristallins liés à la nature polaire des liaisons Ga-P qui limitent aussi la durée de vie des porteurs. De plus, la durée de vie à l’interface GaP/Si est demeure inférieure à 150 µs, malgré l’hypothétique passivation par effet de champ et sans défauts cristallins.Se basant sur ces découvertes, nous avons cherché à comprendre et améliorer la passivation de l’interface GaP/Si. Des techniques d’analyses avancées ont montré la présence de traces de carbone et d’arsenic dans le GaP, accompagné de fluor à l’interface, ainsi qu’une oxydation du GaP post-épitaxie. Différentes couches de mouillage ont été testées, permettant de corréler la rugosité, la défectuosité du GaP à la durée de vie des porteurs.D’autre part, l’intégration d’étapes de décontamination du substrat (gettering) a permis avec succès de restaurer la durée de vie volumique des charges tout en maintenant le recuit de reconstruction de surface dans le procédé de fabrication. Ces étapes ont été optimisées pour minimiser leur impact sur la couche de GaP. Un cellule avec GaP déposé sans pré-recuit atteint 11.2% tandis qu’en reléguant le GaP à une couche fenêtre, une cellule GaP/(n+)c-Si/(p)c-Si a montré un rendement amélioré à 13.8% avec le recuit et les étapes de gettering.Ce travail s'appuie sur l'expertise du CEA-INES en cellules solaires à hétérojonctions et du CNRS-LTM en épitaxie et caractérisation des matériaux III/V. / In the frame of energy transition, the development of energy sources that do not generate greenhouse gases is paramount. Benefiting from the overabundant energy provided by the Sun, photovoltaics is a key element of the future energy mix. Photovoltaics market is currently led by the silicon-based technologies, and best conversion efficiencies exceed 26% with the heterojunction solar cells technology with hydrogenated amorphous silicon (a-Si:H) on monocrystalline silicon (c-Si).Hydrogenated amorphous silicon, deposited by PECVD, enables high surface passivation of crystalline silicon, and to reach over 730 mV of open-circuit voltage. However, the parasitic absorption in the Ultra Violet region limits photon collection, and its low conductivity limits the diffusion length of charge carriers it generates, limiting the electrical performance and their contributions to the cell current.To enhance the efficiency of this technology, we propose to fabricate and characterise a new structure of photovoltaic solar cells based on heterojunction of gallium phosphide on crystalline silicon, made by metalorganic chemical vapour deposition (MOCVD). This crystalline III-V material, with high bandgap energy (2.26 eV vs 1.6-1.9 for a-Si:H and 1.12 eV for c-Si), allows its pseudomorphic epitaxy on silicon, with higher transparency vs a-Si:H along with field effect passivation that repels the holes, positive charge carriers, away from the GaP/Si interface. The improvement of current-voltage characteristics, with only 10-nm-thick GaP, have previously shown by simulation an absolute improvement of the efficiency by 2%.In the frame of this thesis, we have experimentally studied the effect of GaP deposition on c-Si. We have outlined a carrier lifetime degradation in c-Si during a surface preparation annealing that favours the diffusion of contaminants from the III-V MOCVD chamber into the substrate. This step could be removed, but it is required to limit the formation of antiphase domains, which are crystalline defects linked to the polarity of Ga-P bonds that also limit the carrier lifetime. Moreover, GaP/Si interface lifetime remains below 150 µs, despite the hypothetic field effect passivation and without crystalline defects.From these conclusions, we sought to understand and improve the GaP/Si interface passivation. Advanced analysis techniques have shown carbon and arsenic traces in the GaP, with fluorine at the interface, as well as post-epitaxy GaP oxidation. Different wetting layers were tested, correlating the roughness and defectivity of Gap to the carrier lifetime.Furthermore, integration of substrate decontamination steps (gettering) enables successful bulk carrier lifetime recovery while maintaining the surface reconstruction annealing in the process flow. These steps were optimised to minimise their impact the GaP layer. A solar cell with GaP deposited on unannealed silicon reached 11.2% while, making GaP a window layer in a GaP/(n+)c-Si/(p)c-Si stack produced a solar cell with 13.8% with annealing and gettering steps.This work relies on the expertise of CEA-INES on heterojunction solar cells and CNRS-LTM on the epitaxy of III-V materials and their characterisation.