Caractérisation structurale des hétérostructures à base de GaSb et de GaP épitaxiées sur silicium (001)

Monolithique integration of III-V compound semiconductors on silicon makes possible the large scale integration of compound semiconductors for optical and electronic devices. However, the growth of III-V semiconductors on silicon generate several defects (threading dislocations, twins and antiphase boundaries). In this PhD thesis, we studied structural properties of GaSb-based and GaP-based hetero-structures grown on silicon using X-Ray diffraction and Transmission Electon Microscopy. Threading dislocations are the major defects in the growth of GaSb on Si because of the high lattice mismatch between the two materials(12.2%). Other defects like twins are presents on the growth of GaSb on Si. Twins are related to the crystalline quality of surface substrate (contaminants and roughness). We developed a cleaning process of surfaces which shows a high efficiency on twins density reduction. We reduced the high defects density using super-lattices . The super-lattices act not only as a dislocations filter but also help antiphase domains closure. The efficiency of super-lattices depends on its nature (thickness and strain) and its position on the structure. With our dislocations geometrical recombination model, we bring out the global interaction between dislocations and we define essential interaction parameters between dislocations. For the growth of GaP on Si, We have shown that the initial coverage of gallium on the substrate in the early stages of growth has a major effect on the presence of micro-twins, but also on the size and density of the antiphase domains. Due to the small lattice mismatch between GaP and Si (0.37%), antiphase boundaries and domains are the major defects on the GaP-based heterostructures. Antiphase domains can be blocked near the interface using specific growth conditions (substrate miscut, growth temperature, strained thin films). We showed with the two heterostructures (GaP-based and GaSb-based) that the suppression of antiphase boundaries decreases semiconductors roughness. We studied the influence of Nitride incorporation on the STEM-HAADF contrast of GaPN films. This inversed contrast (GaPN layers are more brilliant than GaP ) depend on two parameters: the deformation state of GaPN lattice compared to GaP one and the punctual defects related to the Nitride incorporation. Those defects can be Interstitial or anti-site Ga atoms.L'intégration monolithique des semi-conducteurs III-V sur silicium est une voix prometteuse pour la fabrication de composants électroniques et photoniques. Cependant, cette croissance s'accompagne de la génération d'une forte densité de défauts cristallins (dislocations résiduelles, macles et parois d'inversion). Au cours de ce travail de thèse, nous avons étudié les propriétés structurales, par diffraction des rayons X et microscopie électronique en transmission, d'hétérostructures à base de GaSb et de GaP épitaxiées sur silicium (001). Vu le fort désaccord paramétrique entre le GaSb et le Si (12.2%), la croissance s'accompagne de la génération d'une très forte densité de dislocations résiduelles. D'autres défauts sont présents dans la croissance de GaSb sur Si comme les macles. Ces défauts dépendent de la qualité cristalline du surface du substrat . La préparation de surface du substrat permet de diminuer la rugosité et de supprimer les contaminants présents. Dans ce contexte, nous avons mis au point un protocole de préparation adapté permettant de réduire la densité de macles générées à l'interface GaSb sur Si. Nous avons aussi réussi à réduire la densité de défauts en utilisant un super-réseau capable de filtrer les dislocations mais également d'aider à la fermeture des domaines d'inversion. L'efficacité du super-réseau dépend beaucoup de sa nature (nombre de périodes, épaisseur et contrainte des couches constituant le super-réseau) ainsi que sa position dans la structure. Nous avons aussi développer un modèle géométrique de recombinaison des dislocations. Ce modèle, nous a permis de mettre en évidence les interactions globales entre dislocations et de donner des paramètres d'interaction entre dislocations. Pour la croissance de GaP sur Si, le très faible désaccord paramétrique (0.37%) permet d'éviter le problème de relaxation plastique des structures à base de GaP. Les défauts essentiellement présents sont les domaines et les parois d'inversion. Nous avons montré que le taux de couverture initial en gallium sur le substrat en tout début de croissance a un effet prépondérant sur la présence de micro-macles mais également sur la taille et la densité des domaines d'inversion. D'autres paramètres de croissance sont également étudiés, comme la température, la vicinalité du substrat et l'utilisation de fines couches contraintes pour limiter le développement des domaines d'inversion. Nous avons observé que pour ces deux types d'hétérostructure (GaP/Si mais aussi GaSb/Si), la suppression de domaines d'inversion permet de réduire la rugosité. En troisième partie, nous avons étudié l'effet d'incorporation de l'azote sur le contraste des images STEM-HAADF des couches de GaPN. Contrairement à ce qui est attendu, les couches GaPN apparaissent toujours plus claires que le substrat de GaP, quelque soit la concentration en azote. Nous avons montré que le rapport des intensités HAADF des couches GaPN et GaP dépend de deux paramètres : la déformation effective de la maille de GaPN (par rapport à celle de GaP) et la présence des défauts ponctuels liés à l'incorporation d’azote. Une hypothèse avancée serait la présence de gallium en site substitutionnel du phosphore, voire en site interstitiel

A study of the formation of fuzzy tungsten in a HiPIMS plasma system

Abstract Nanostructured “fuzzy” tungsten has been grown for the first time in a high-power impulse magnetron sputtering HiPIMS system. The fuzzy layers were formed over range of surface temperatures Ts, from 1025 to 1150 K, for helium ion fluences of 5.02 × 1024 m-2, and mean ion bombardment energy of 55 eV. The time-evolution of the helium ion flux (ΓHe) and incident energy (EHe) were determined during the HiPIMS pulse (of width of 150 µs) using a planar Langmuir probe. The micrographic findings revealed that, the thickness of HiPIMS-grown nano-tendrill layers increased by 83 % (from 274 to 501 nm) for only a 125 K rise in Ts. This result is explained by the fact that higher surface temperatures led to larger helium bubbles which ultimately produce a thicker nanostructured layer. The growth rate of fuzzy tungsten layers in HiPIMS conditions is approximately 50 % lower than those observed for DC magnetron operation.</jats:p

Efficient hole abstraction for highly selective oxidative coupling of methane by Au-sputtered TiO2 photocatalysts

Photocatalytic oxidative coupling of methane (OCM) produces C2 molecules that can be used as building blocks for synthesis of fuels and chemicals. However, the yield rate and the selectivity of C2 products are still moderate due to the stable nature of methane molecules. Here we develop a Au nanocluster-loaded TiO2 photocatalyst by a sputtering approach, achieving a high methane conversion rate of 1.1 mmol h−1, C2 selectivity of ~90% and apparent quantum efficiency of 10.3 ± 0.6%. The high C2/C2+ yield rate is on the same order of magnitude as the benchmark thermal catalysts in OCM processes operated at high temperature (&gt;680 °C). Au nanoparticles are shown to prolong TiO2 photoelectron lifetimes by a factor of 66 for O2 reduction, together with Au acting as a hole acceptor and catalytic centre to promote methane adsorption, C–H activation and C–C coupling. This work underscores the importance of multifunctional co-catalysts and mechanistic understanding to improve photocatalytic OCM

Compressive Scanning Transmission Electron Microscopy

Scanning Transmission Electron Microscopy (STEM) offers high-resolution images that are used to quantify the nanoscale atomic structure and composition of materials and biological specimens. In many cases, however, the resolution is limited by the electron beam damage, since in traditional STEM, a focused electron beam scans every location of the sample in a raster fashion. In this paper, we propose a scanning method based on the theory of Compressive Sensing (CS) and subsampling the electron probe locations using a line hop sampling scheme that significantly reduces the electron beam damage. We experimentally validate the feasibility of the proposed method by acquiring real CS-STEM data, and recovering images using a Bayesian dictionary learning approach. We support the proposed method by applying a series of masks to fully-sampled STEM data to simulate the expectation of real CS-STEM. Finally, we perform the real data experimental series using a constrained-dose budget to limit the impact of electron dose upon the results, by ensuring that the total electron count remains constant for each image

Photocatalytic overall water splitting under visible light enabled by a particulate conjugated polymer loaded with iridium

Polymer photocatalysts have received growing attention in recent years for photocatalytic hydrogen production from water. Most studies report hydrogen production with sacrificial electron donors, which is unsuitable for large-scale hydrogen energy production. Here we show that the palladium/iridium oxide-loaded homopolymer of dibenzo[b,d]thiophene sulfone (P10) facilitates overall water splitting to produce stoichiometric amounts of H2 and O2 for an extended period (>60 hours) after the system stabilized. These results demonstrate that conjugated polymers can act as single component photocatalytic systems for overall water splitting when loaded with suitable co-catalysts, albeit currently with low activities. Transient spectroscopy shows that the IrO2 co-catalyst plays an important role in the generation of the charge separated state required for water splitting, with evidence for fast hole transfer to the co-catalyst

Insight on Thermal Stability of Magnetite Magnetosomes: Implications for the Fossil Record and Biotechnology

Magnetosomes are intracellular magnetic nanocrystals composed of magnetite (Fe3O4) or greigite (Fe3S4), enveloped by a lipid bilayer membrane, produced by magnetotactic bacteria. Because of the stability of these structures in certain environments after cell death and lysis, magnetosome magnetite crystals contribute to the magnetization of sediments as well as providing a fossil record of ancient microbial ecosystems. The persistence or changes of the chemical and magnetic features of magnetosomes under certain conditions in different environments are important factors in biotechnology and paleomagnetism. Here we evaluated the thermal stability of magnetosomes in a temperature range between 150 and 500 °C subjected to oxidizing conditions by using in situ scanning transmission electron microscopy. Results showed that magnetosomes are stable and structurally and chemically unaffected at temperatures up to 300 °C. Interestingly, the membrane of magnetosomes was still observable after heating the samples to 300 °C. When heated between 300 °C and 500 °C cavity formation in the crystals was observed most probably associated to the partial transformation of magnetite into maghemite due to the Kirkendall effect at the nanoscale. This study provides some insight into the stability of magnetosomes in specific environments over geological periods and offers novel tools to investigate biogenic nanomaterials

Accelerated Synthesis and Discovery of Covalent Organic Framework Photocatalysts for Hydrogen Peroxide Production

[Image: see text] A high-throughput sonochemical synthesis and testing strategy was developed to discover covalent organic frameworks (COFs) for photocatalysis. In total, 76 conjugated polymers were synthesized, including 60 crystalline COFs of which 18 were previously unreported. These COFs were then screened for photocatalytic hydrogen peroxide (H(2)O(2)) production using water and oxygen. One of these COFs, sonoCOF-F2, was found to be an excellent photocatalyst for photocatalytic H(2)O(2) production even in the absence of sacrificial donors. However, after long-term photocatalytic tests (96 h), the imine sonoCOF-F2 transformed into an amide-linked COF with reduced crystallinity and loss of electronic conjugation, decreasing the photocatalytic activity. When benzyl alcohol was introduced to form a two-phase catalytic system, the photostability of sonoCOF-F2 was greatly enhanced, leading to stable H(2)O(2) production for at least 1 week

Enhanced production and control of liquid alkanes in the hydrogenolysis of polypropylene over shaped Ru/CeO2 catalysts

The hydrogenolysis of polypropylene waste to liquid hydrocarbons offers a promising pathway for the chemical recycling of waste polymers. This work describes the importance of reaction conditions and support morphology to produce high liquid yields with enhanced control of chain length over highly active shaped and non-shaped Ru/CeO2 catalysts. The shaped 2 wt% Ru/CeO2 exhibit high liquid alkane yields (58–81%) when compared to the non-shaped 2 wt% Ru/CeO2 (liquid yield: 34–58%) under optimized reaction conditions (220 °C, 16 h, 30 bar H2). In particular, the 2 wt% Ru/CeO2 nanocube catalyst exhibits the highest activity yielding lighter hydrocarbons. This was rationalized to be a combination of small Ru cluster formation and enhanced metal-support interactions. The influence of larger Ru particles (≥1.5 nm) was confirmed mechanistically using a computational density functional theory study on the hydrogenolysis of pentane (C5) to determine the favorable formation of methane in the non-shaped Ru/CeO2 catalyst

Photocatalytic overall water splitting under visible light enabled by a particulate conjugated polymer loaded with palladium and iridium

Abstract: Polymer photocatalysts have received growing attention in recent years for photocatalytic hydrogen production from water. Most studies report hydrogen production with sacrificial electron donors, which is unsuitable for large‐scale hydrogen energy production. Here we show that the palladium/iridium oxide‐loaded homopolymer of dibenzo[b, d]thiophene sulfone (P10) facilitates overall water splitting to produce stoichiometric amounts of H2 and O2 for an extended period (>60 hours) after the system stabilized. These results demonstrate that conjugated polymers can act as single component photocatalytic systems for overall water splitting when loaded with suitable co‐catalysts, albeit currently with low activities. Transient spectroscopy shows that the IrO2 co‐catalyst plays an important role in the generation of the charge separated state required for water splitting, with evidence for fast hole transfer to the co‐catalyst

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides

The origins of the superior catalytic activity of poorly crystallized Ir-based oxide material for the OER in acid is still under debate. Here, authors synthesize porous IrMo oxides to deconvolute the effect of Ir oxidation state from short-range ordering and show the latter to be a key factor