Croissance et transfert de graphène pour la fabrication d'électrodes transparentes

Ce mémoire de maîtrise porte sur la caractérisation et l’optimisation d’électrodes transparentes et conductrices de graphène. Nous avons utilisé un procédé de croissance CVD à basse pression, avec du méthane comme source de carbone et du cuivre comme support et catalyseur de croissance, pour synthétiser le graphène. Les couches de graphène ainsi déposées sont complètes et uniformes, tel qu’observé par microscopie électronique à balayage et par spectroscopie Raman. Une méthode utilisant un polymère (PMMA) comme support mécanique a été optimisée pour le transfert du graphène vers différents substrats, tels du verre ou du silicium recouvert d’oxyde de silicium. La caractérisation par microscopie optique et électronique à balayage et par spectroscopie Raman des échantillons transférés démontre que le procédé de transfert n’endommage pas le graphène et laisse la surface exempte de résidus apparents de contamination. Les mesures de cartographie Raman montrent une très faible intensité du pic associé aux défauts, preuve que la croissance produit du graphène de haute qualité. Ce procédé de transfert a été appliqué pour réaliser des électrodes transparentes comportant entre une à quatre couches de graphène empilées. Ces électrodes ont été caractérisées par spectroscopie Raman, par des mesures en transmission optique et par des mesures de résistance électrique par la méthode de van der Pauw. Les résultats de spectroscopie Raman montrent que le niveau de dopage des échantillons diminue lorsqu’on augmente le nombre de couches de graphène. Les mesures optiques montrent une transparence variant entre 97,5 et 90%pour une longueur d’onde entre 400 et 1000 nm en fonction du nombre de couches empilées. La résistance de feuille des électrodes varie entre 560 et 360, une performance équivalente, voire supérieure à celle des électrodes de graphène présentées dans la littérature. Une méthode électrochimique est utilisée pour doper les échantillons à des niveaux très élevés,diminuant la résistance de feuille jusqu’à 260 pour une électrode de graphène monocouche. Ce dopage permet de varier le travail de sortie du graphène entre 3,9 et 5,6 eV. L’ensemble de ces résultats démontre que le graphène est un matériau idéal pour être utilisé comme électrode transparente, étant donné ses excellentes propriétés optiques et électriques ainsi que la possibilité de modifier simplement son travail de sortie, permettant de minimiser la résistance de contact avec divers matériaux semiconducteurs.---------- Abstract This master’s thesis focuses on the characterization and optimization of transparent and conductive electrodes made from graphene layers. We used a low pressure CVD method, with methane as the carbon source and copper as a catalyst and growth support, to synthesize graphene. Scanning electron microscopy and Raman spectroscopy were used to demonstrate that the graphene layers are complete and uniform. A method using a polymer (PMMA) as a mechanical support has been optimized for the transfer of graphene films onto various substrates, such as glass or silicon dioxide on silicon. Characterization of transferred samples by optical and scanning electron microscopy and Raman spectroscopy reveals that transferred films are undamaged and that the surface is free of visible residues. The intensity of the defect-related peak in Raman mapping measurements is low, indicating that the as-grown and transferred graphene layers are of high quality. This transfer process has been applied to fabricate transparent electrodes with one to four layers of graphene. The electrodes were characterized by Raman spectroscopy, optical transmission analyses and electrical resistance measurements using the van der Pauw method. Raman results indicate that the average doping level of the samples decreases with an increasing number of graphene layers. The optical measurements yield transparency values ranging from 97.5 to 90 % for wavelengths between 400 and 1000 nm, depending on the number of stacked layers. The sheet resistance of the electrodes varies between 560 and 360, which is an equivalent or superior performance to that of graphene electrodes reported in the literature. An electrochemical method was used to dope the samples in an ionic liquid to vary the Fermi level and to decrease the sheet resistance down to 260for a monolayer graphene electrode. This doping allows to vary the work function of graphene between 3.9 and 5.6 eV. Taken together, these results demonstrate that graphene appears as an ideal material to be used as transparent electrode given its excellent optical and electrical properties as well as the ability to easily modify its work function and thus minimize the contact resistance with various semiconductor materials

Trajectoires et visées de l'hydrogéomorphologie au Québec

L'hydrogéomorphologie étudie la dynamique des rivières en se concentrant sur lesinteractions liant la structure des écoulements, la mobilisation et le transport dessédiments et les morphologies qui caractérisent les cours d'eau et leur bassin‐versant. Elleoffre un cadre d'analyse et des outils pour une meilleure intégration des connaissancessur la dynamique des rivières pour la gestion des cours d'eau au sens large, et plusspécifiquement, pour leur restauration, leur aménagement et pour l'évaluation et laprévention des risques liés aux aléas fluviaux. Au Québec, l'hydrogéomorphologie émergecomme contribution significative dans les approches de gestion et d'évaluation du risqueet se trouve au cœur d'un changement de paradigme dans la gestion des cours d'eau parlequel la restauration des processus vise à augmenter la résilience des systèmes et dessociétés et à améliorer la qualité des environnements fluviaux. Cette contribution exposela trajectoire de l'hydrogéomorphologie au Québec à partir des publications scientifiquesde géographes du Québec et discute des visées de la discipline en recherche et enintégration des connaissances pour la gestion des cours d'eau. Hydrogeomorphology studies river dynamics, focusing on the interactions between flowstructure, sediment transport, and the morphologies that characterize rivers and theirwatersheds. It provides an analytical framework and tools for better integratingknowledge of river dynamics into river management in the broadest sense, and morespecifically, into river restoration as well as into the assessment and prevention of risksassociated with fluvial hazards. In Quebec, hydrogeomorphology is emerging as asignificant contribution to risk assessment and management approaches, and is at theheart of a paradigm shift in river management whereby process restoration aims toincrease the resilience of fluvial systems and societies, and improve the quality of fluvialenvironments. This contribution outlines the trajectory of hydrogeomorphology inQuebec, based on scientific publications by Quebec geographers, and discusses thediscipline's aims in research and knowledge integration for river management

Can the Morphological Quality Index (MQI) be used to determine the ecological status of lowland rivers?

Stream assessment indices have become increasingly important in quantifying the overall status of river networks to define specific targets for restoration initiatives. Such an assessment is particularly needed in degraded environments, such as agricultural streams. Some of these evaluation tools, for instance the Qualitative Habitat Evaluation Index (QHEI), are resource intensive because they are field based. Indices that are less dependent on detailed field observations, such as the Morphological Quality Index (MQI), can provide a greater spatial coverage at a lower cost. The objectives of this study are to (1) verify whether a river's morphological quality, quantified using the MQI, can predict the fish habitat quality of a stream determined with the QHEI for eastern Canadian lowland streams, (2) compare the morphological quality, estimated solely from remotely sensed data (RMQI), to the standard MQI, (3) test whether a modified MQI (MMQI) is more appropriate than the original index for the eastern Canadian landscape when comparing with the QHEI fish habitat assessment, and (4) considering the near absence of dams in lowland eastern Canadian streams, compare an MQI where dam-related indicators are optional (MQI-OD) with the QHEI. The hydrogeomorphological and ecological conditions of 118 stream reaches, including 97 in agricultural areas, were assessed across Quebec and southern Ontario using the MQI and QHEI. Each stream was initially evaluated using remotely sensed data: 1-m LiDAR (Quebec) and 5-m DEMs (Ontario), historical aerial photography (1964–2010), and orthophotos. Field assessments were conducted to validate fish habitat and morphological data for both indices. A strong correlation was observed at the reach scale between the MQI and QHEI (r = 0.81) and MQI and RMQI (r = 0.95). Both modified MQI (MMQI and MQI-OD) showed stronger correlation than the standard MQI with the QHEI (r = 0.87), likely because the standard MQI slightly overestimates the current quality status at the habitat scale in the case of small agricultural catchments where main pressures are not always represented by artificial structures. However, because the standard MQI performs generally well in the study area, we suggest using it to ensure consistency and facilitate comparison with other regions. The results from this study demonstrate that the MQI can be used to provide an assessment of ecological (fish habitat) quality. In addition, our results indicate that a remote hydrogeomorphological assessment can be conducted to estimate the overall status of stream networks

Graphene CVD: Interplay Between Growth and Etching on Morphology and Stacking by Hydrogen and Oxidizing Impurities

The growth of high quality graphene layers by chemical vapor deposition (CVD) has been found to strongly depend on growth conditions with results varying greatly from one laboratory to another for nominally identical conditions. We report the results of a systematic investigation of the role of hydrogen and oxidizing impurities present in the gas feedstock during the growth and cooling stages in low-pressure CVD. First, we show that for a partial pressure of oxidizing impurities below 1 ppb, hydrogen is not required for graphene growth from methane. Second, we demonstrate that purified hydrogen does not etch graphene films at typical growth temperatures. Third, a flow of purified hydrogen during cooling counterbalances graphene etching by oxygen, thus protecting the films. Films grown under high purity conditions (low level of oxidizing impurities) exhibit a higher bilayer and multilayer coverage; Surprisingly some of these bi- and multilayer graphene islands are twisted with respect to the first graphene layer as revealed by hyperspectral Raman imaging. Overall, this growth behavior suggests a competitive action between film growth from the carbon precursors and etching by the oxidative species. Our results provide new fundamental insights on the graphene CVD growth, highlighting the important yet indirect role of hydrogen and its major influence on controlling the action of oxidizing impurities on nucleation and etching during the growth process