The first-principles calculation method has been used to obtain electronic and structural properties of few-layer-graphenes (FLG) with and without an external electric potential Vext. For Vext=0, the AB stacked two-layer FLG has a band overlapping of 9meV. However, an energy gap (Eg) emerges when Vext is greater than about 0.04Volts. Beyond this threshold, Eg increases monotonically with the increase of Vext. The Eg vs. Vext result suggests a semi-metal-semiconductor transition in the AB stacked two-layer FLG, which can be utilized as a nanoscale electronic switch. Three- and four-layer AB stacked FLG¡¦s don¡¦t have a similar dependence of Eg on Vext

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This thesis is devoted to the calculation of thermal contact resistance in various molecular systems based on carbon nanotubes (CNTs) and few layer graphene (FLG). This work has been performed through equilibrium molecular dynamics (EMD) simulations. We adopted the temperature difference fluctuations method in our EMD calculations. This method only needs the input of the temperatures of the subsystems whereas the heat flux, which is involved in all the other approaches, remains more difficult to compute in terms of simulation time and algorithm. Firstly, three cases were studied to validate this method, namely: (i) Si/Ge superlattices; (ii) diameter modulated SiC nanowires; and (iii) few-layer graphenes. The validity of the temperature difference fluctuations method is proved by equilibrium and non-equilibrium MD simulations. Then, by using this method, we show that an azide-functionalized polymer (HLK5) has a lower contact resistance with CNT than the one between CNT and PEMA, because HLK5 could form covalent bonds (C-N bonds) with CNT through its tail group azide, while only weak Van der Waals interactions exist in the case of CNT-PEMA contact. The data from our EMD simulations match with the results from experiments in a reasonable range. We then report the thermal contact resistance between FLG and a SiO2 substrate, which could be tuned with the layer number. Taking advantage of the resistive interface, we show that a SiO2 /FLG superlattices have a thermal conductivity as low as 0.30 W/mK, exhibiting a promising prospect in nano-scale thermal insulation. In the last part, we investigated the layer number dependence of the cross-plane thermal resistances of suspended and supported FLGs. We show that the existence of a silicon dioxide substrate can significantly decrease the cross-plane resistances of FLGs with low layer numbers, and the effective thermal conductivities were increased accordingly. The Frenkel-Kontorova model was introduced to explain the substrate-induced band gaps in FLG dispersion relations and the corresponding thermal energy transfer. The enhanced thermal conduction in the cross-plane direction is ascribed to the phonon radiation that occurs at the FLG-substrate interface, which re-distributes the FLG in-plane propagating energy to the cross-plane direction and to the substrate.

Cette thèse se concentre sur le calcul des résistances thermique de contact dans plusieurs systèmes moléculaires à base de nanotubes de carbone (NTCs) et de quelques couches de graphène (QCG). Ce travail a été réalisé en utilisant la méthode de simulation par dynamique moléculaire à l’équilibre (DME). Nous avons utilisé la méthode basée sur les fluctuations de la différence de température dans nos simulations de DME. Cette méthode ne nécessite que l’entrée des températures des sous-systèmes quand le flux thermique, qui intervient dans toutes les autres approches, reste plus difficile à calculer en terme de durée de simulation et d’algorithme. Premièrement, trois cas ont été étudiés pour valider cette méthode : (i) des super-réseaux Si/Ge ; (ii) des nanofils de SiC de plusieurs diamètres ; et (iii) QCGs. La validité de la méthode par fluctuation de la différence de température est démontrée par des simulations de dynamique moléculaire à l’équilibre et hors-équilibre. Ensuite, avec cette méthode, nous montrons qu’un polymère fonctionnalisé azoture (HLK5) a une plus faible résistance de contact avec un NTC que la résistance entre un NTC et un PEMA, car HLK5 forme des liaisons covalentes (C-N bonds) avec un NTC par le groupement azoture de sa queue, quand seul de faibles interactions de van der Waals existent dans le cas d’un contact NTC-PEMA. Les données de nos simulations de DME concordent raisonnablement avec les résultats expérimentaux. Nous rapportons ensuite la résistance thermique de contact entre QCG et un substrat de SiO2, qui peut être contrôlée par le nombre de couches de graphène. Avec l’avantage d’une interface résistante, nous montrons que des super-réseaux SiO2/QCG ont une conductivité thermique descendant sous 0.30 W/mK, ce qui est une perspective prometteuse pour l’isolation thermique à l’échelle du nanomètre. Dans la dernière partie, nous recherchons la dépendance de la résistance thermique inter-plan avec le nombre de couches de graphène pour des QCG suspendus ou supportés. Nous montrons que la présence d’un substrat de dioxyde de silicium peut significativement réduire les résistances inter-plan de QCG possédant peu de couches de graphène, et la conductivité thermique effective est augmentée en accord. Le modèle de Frenkel-Kontorova a été introduit pour expliquer les bandes interdites induites par le substrat dans la relation de dispersion de QCG et le transfert d’énergie thermique correspondant. L’augmentation de la conduction thermique inter-plan est attribuée au rayonnement de phonons à l’interface QCG-substrat, qui redistribue l’énergie se propageant dans le plan du QCG en énergie dans la direction inter-plan et dans le substrat. / This thesis is devoted to the calculation of thermal contact resistance in various molecular systems based on carbon nanotubes (CNTs) and few layer graphene (FLG). This work has been performed through equilibrium molecular dynamics (EMD) simulations. We adopted the temperature difference fluctuations method in our EMD calculations. This method only needs the input of the temperatures of the subsystems whereas the heat flux, which is involved in all the other approaches, remains more difficult to compute in terms of simulation time and algorithm. Firstly, three cases were studied to validate this method, namely: (i) Si/Ge superlattices; (ii) diameter modulated SiC nanowires; and (iii) few-layer graphenes. The validity of the temperature difference fluctuations method is proved by equilibrium and non-equilibrium MD simulations. Then, by using this method, we show that an azide-functionalized polymer (HLK5) has a lower contact resistance with CNT than the one between CNT and PEMA, because HLK5 could form covalent bonds (C-N bonds) with CNT through its tail group azide, while only weak Van der Waals interactions exist in the case of CNT-PEMA contact. The data from our EMD simulations match with the results from experiments in a reasonable range. We then report the thermal contact resistance between FLG and a SiO2 substrate, which could be tuned with the layer number. Taking advantage of the resistive interface, we show that a SiO2 /FLG superlattices have a thermal conductivity as low as 0.30 W/mK, exhibiting a promising prospect in nano-scale thermal insulation. In the last part, we investigated the layer number dependence of the cross-plane thermal resistances of suspended and supported FLGs. We show that the existence of a silicon dioxide substrate can significantly decrease the cross-plane resistances of FLGs with low layer numbers, and the effective thermal conductivities were increased accordingly. The Frenkel-Kontorova model was introduced to explain the substrate-induced band gaps in FLG dispersion relations and the corresponding thermal energy transfer. The enhanced thermal conduction in the cross-plane direction is ascribed to the phonon radiation that occurs at the FLG-substrate interface, which re-distributes the FLG in-plane propagating energy to the cross-plane direction and to the substrate.

Au delà du graphène, de nouveaux semiconducteurs 2D tels que MoS2, GaS, GaSe et InSe deviennent pertinents pour les applications et dispositifs émergents. Dans cette thèse, nous fabriquons des échantillons de quelques feuillets atomiques de ces matériaux pour des dispositifs de photo-détecteurs et les caractérisons par microscopie optique, AFM et TEM. L'interaction de la lumière avec le substrat et le dispositif ultra-mince étant critique pour son fonctionnement, nous calculons et mesurons le contraste et l'intensité de la lumière diffusée par le dispositif. Nous caractérisons également la réponse Raman et la photoluminescence en fonction du nombre de couches pour étudier les propriétés vibrationnelles et électroniques. Plusieurs dispositifs ont été fabriqués et analysés. Nous examinons d'abord les dispositifs homogènes basés sur MoS2, GaSe ou InSe, et trouvons une excellente photosensibilité pour notre photo-détecteur MoS2. Nous examinons ensuite plusieurs hétéro-structures pour combiner les propriétés de chaque matériau et atteindre de meilleures performances. Le premier exemple est un photo-détecteur graphène/InSe dont la photosensibilité augmente de quatre ordres de grandeur par rapport à un dispositif basés sur InSe seul. Nous montrons également que la couche supérieure de graphène prévient la dégradation de couches atomiques ultra-minces dans l'air. Des hétéro-structures plus complexes graphène/InSe/graphène et graphène/InSe/Au montrent un effet photovoltaïque. Enfin, nous combinons InSe avec MoS2 et obtenons un dispositifs avec photo-réponse rapide, un comportement de type photo-diode, une distribution de photo-courant uniforme et un fort effet photovoltaïque. / Novel two dimensional (2D) semiconductors beyond graphene such as MoS2, GaS, GaSe and InSe are increasingly relevant for emergent applications and devices. In this thesis, we fabricate these 2D samples for photo-detector applications and characterize them with optical microscopy, atomic force microscopy, Raman and photoluminescence (PL) spectroscopy and transmission electron microscopy. Since the interaction of light with the substrate and the ultra-thin photodetector device is critical for its functioning we calculate and measure optical contrast and intensity of light scattered from the device. We also characterize the Raman and PL response as a function of number of layers to study both vibrational properties and the band gap transition. For the device application, we first examine homogenous devices based on few-layer MoS2, GaSe and InSe respectively and find an excellent photoresponsivity in our few-layer MoS2 photo-detector. We then examine several geometries for heterostructure devices, which have the advantage of combining favorable properties of each material to reach better performances. The first example is a graphene/InSe photo-detector where the photoresponsivity increases by four orders of magnitude with respect to a few-layer InSe device while the top graphene layer is also shown to prevent degradation of ultra-thin atomic layers in air. Still more complex graphene/InSe/graphene and graphene/InSe/Au heterostructures show a photovoltaic effect. Finally for the first time, we combine InSe with MoS2 and obtain a high performance device with fast photo-response, photodiode like behavior, uniform photocurrent distribution and high photovoltaic effect.

The continued growth of the optoelectronics industry and the emergence of wearable and flexible electronics will continue to place an ever increasing pressure on replacing ITO, the most widely used transparent conducting electrode (TCE). Among the various candidates, graphene shows the highest optical transmittance in addition to promising electrical transport properties. The currently available large-scale synthesis routes of graphene result in polycrystalline samples rife with grain boundaries and other defects which limit its transport properties. Chemical doping of graphene is a viable route towards increasing its conductivity and tuning its work function. However, dopants are typically present at the surface of the graphene sheet, making them highly susceptible to degradation in environmental conditions. Few-layers graphene (FLG) is a more resilient form of graphene exhibiting higher conductivity and performance stability under stretching and bending as contrasted to single-layer graphene. In addition FLG presents the advantage of being amenable bulk doping by intercalation. Herein, we explore non-covalent doping routes of CVD FLG, such as surface doping, intercalation and combination thereof, through in-depth and systematic characterization of the electrical transport properties and energy levels shifts. The intercalation of FLG with Br2 and FeCl3 is demonstrated, showing the highest improvements of the figure of merit of TCEs of any doping scheme, which results from up to a five-fold increase in conductivity while maintaining the transmittance within 3% of that for the pristine value. Importantly the intercalation yields TCEs that are air-stable, due to encapsulation of the intercalant in the bulk of FLG. Surface doping with novel solution-processed metal-organic molecular species (n- and p-type) is demonstrated with an unprecedented range of work function modulation, resulting from electron transfer and the formation of molecular surface dipoles. However, the conductivity increases compared modestly to intercalation as the electron transfer is limited to the uppermost graphene layers. Finally, a novel and universal multi-modal doping strategy is developed, thanks to the unique platform offered by FLG, where surface and intercalation doping are combined to mutually achieve high conductivity with an extended tunability of the work function. This work presents doped-FLG as a prospective and versatile candidate among emerging TCEs, given the need for efficient and stable doping routes capable of controllably tuning its properties to meet the criteria of a broad range of applications.

As the family of two−dimensional(2D) materials has grown, two−dimensional heterostructure devices have emerged as great alternatives to replace conventional electronic materials and enable new functionality such as flexible and bendable electronics. The fabrication and performance of these devices depend critically on the understanding and ability to manipulate the mechanical interplay between the stacked materials. In this dissertation, we investigate adhesive interactions and determine the shear modulus of heterostructure devices made from Molybdenum Disulfide (MoS2). MoS2 has been attracting attention recently due to its semiconductor nature (having a direct band gap of 1.9 eV) along with its exceptional mechanical strength and flexibility. As the first step of our research, we suspended MoS2 flakes grown through chemical vapor deposition (CVD) over substrates made of metal (gold, titanium, chromium), semiconductor (germanium, silicon), insulator (silicon oxide), and semi-metal (graphite). Then, by creating pressure differences across the membrane, we forced MoS2 to bulge upward until we observe separation from the surface of the substrates. We demonstrated that MoS2 on graphite has the highest work of separation within the tested surface materials. Furthermore, we measured considerable adhesion hysteresis between the work of separation and the work of adhesion. We proposed that surface roughness and chemical interactions play a role in surface adhesion and separation of 2D materials. These experiments are critical to guiding the future design of electrical and mechanical devices based on 2D materials. Next, we measured the effective shear modulus of MoS2/few−layer graphene (FLG) heterostructures by employing a blister test. Again, by introducing a pressure differential across the suspended MoS2 membrane over the FLG substrate, the MoS2/FLG heterostructure peeled off from the silicon oxide surface once the critical pressure is exceeded. Incorporating a modified free energy model and Hencky’s axisymmetric membrane solution, we determine the average effective shear modulus of the heterostructure. This is the first experimental measurement of the shear modulus of heterostructure devices using a blister test and this platform can be extended to determine the shear modulus of other 2D heterostructures as well. / 2024-05-24T00:00:00Z

Dans cette thèse, des films à base de graphène ont été étudiés comme alternatives viables dans la fabrication d'électrodes transparentes (TCE). Elle met l'accent sur des couches fines de graphène (FLG), sur l'oxyde de graphène réduit (RGO) et leurs hybrides avec des nanotubes de carbone (NTCs) pour être utilisé comme TCE dans les cellule solaires organiques (OSC). Les FLGs et RGO ont été préparés par des méthodes d'exfoliation mécanique ou en phase liquide assistée par micro-ondes. Ces nanomatériaux dilués dans un solvant liquide ont été déposé en couche mince par aérographe. Des caractérisations de transport de charge ont été obtenues grâce à la méthode des 4 pointes. Ces échantillons ont été caractérisés: leur transparence(UV-Visible), leur morphologie et leur topographique (MEB, MET, AFM) ainsi que le travail de sortie (UPS). Pour obtenir des informations sur la qualité structurelle des échantillons, nous avons utilisés les méthodes de spectroscopie XPS, Raman et la photoluminescence. / Graphene mate rials have been researched as viable alternatives of transparent conductors electrodes (TCEs) in this thesis. Current study focuses on few layer graphene (FLG), reduced graphene oxide (rGO) and their hybrids with carbon nanotubes (CNTs) for TCE applications inorganic solar cells (OSCs). FLGs and rGOs have been prepared by mechanical and microwave-assisted exfoliation methods. This films of these materials have been produced by hot-spray method. Results of charge transport characterizations by four-point probes, transparency (UV-Vis), measurements, along with morphological (SEM, TEM) and topgraphic (AFM) studies of films have been presented. UPS studies were performed to determine for a work-function. XPS,Raman and Photoluminescence studies have been employed to obtain the information about the structural quality of the samples.

Le présent travail de thèse décrit le développement de systèmes actifs de polymérisation d’oléfines basés sur des métaux de fin de transition (nickel et fer) supportés sur des nanomatériaux. Le chapitre I décrit l’état de l’art des systèmes catalytiques supportés ou non pour la polymérisation d’oléfines. Dans le chapitre II, nous décrivons la polymérisation de l’éthylène en utilisant des catalyseurs de nickel contenant un groupement –NH2 pour leur immobilisation covalente sur nanotubes de carbone ; montrant l’influence positive de l’immobilisation : les catalyseurs ainsi supportés sont en effet à la fois plus actifs et conduisant à des polymères de plus haut poids moléculaire. Dans le chapitre III, des complexes de fer contenant un groupement pyrène sont décrits et immobilisés sur nanotubes de carbone par interaction non covalente π-π. Dans ce cas, à la fois les systèmes homogènes et leurs analogues supportés catalysent la réaction de polymérisation de l’éthylène avec des activités particulièrement élevées. Il a également pu être mis en évidence l’importante influence du support carboné sur les performances du système catalytique ainsi que sur la structure des polymères obtenus. Différents types de complexes de nickel contenant un ligand imino-pyridine et différents groupes polyaromatiques ont été synthétisés et leur utilisation en polymérisation de l’éthylène est décrite dans le chapitre IV. L’influence de l’addition de faibles quantités de matériaux nanocarbonés (nanotubes de carbone ou graphène) au milieu réactionnel a ainsi été étudiée. Le graphène s’est dans ce cas révélé particulièrement bénéfique sur les performances du catalyseur. Enfin, le chapitre V décrit la polymérisation de l’isoprène à l’aide de catalyseurs de fer contenant des groupements polyaromatiques permettant leur immobilisation à la surface de nanoparticules de fer. Ces systèmes ont ensuite pu être confinés dans des nanotubes de carbone. Les systèmes catalytiques décrits sont particulièrement actifs produisant des polyisoprènes à température de transition vitreuse élevée et avec une haute sélectivité trans-1,4-polyisoprène. / This present thesis deals with the development of active olefin polymerization catalysts based on late transition metal (nickel and iron) imino-pyridine complexes supported on nanomaterial. Chapter I gives a comprehensive literature review of unsupported and supported ethylene polymerization catalyst. In Chapter II we report the ethylene polymerization studies using nickel complexes containing an –NH2 group for covalent immobilization on multi-walled carbon nanotubes (MWCNTs) of the corresponding precatalysts. Comparison of the homogeneous catalysts with their supported counterparts evidenced higher catalytic activity and higher molecular weights for the polymers produced. In Chapter III, iron complexes containing a pyrene group have been synthesized and immobilized on MWCNTs through non-covalent π-π interactions between pyrene group and surface of MWCNTs. Activated by MMAO, both the iron complexes and immobilized catalysts show high activities for ethylene polymerization. It was possible to evidence that MWCNTs have a great influence on the catalytic activity and on the structure of the resulting polyethylenes. Imino-pyridine nickel complexes containing various kinds of aromatic groups have been synthesized in Chapter IV and polymerization conditions in the presence and in the absence of nanocarbon materials, such as MWCNTs or few layer graphene (FLG), are discussed. For those nickel catalysts bearing 1-aryliminoethylpyridine ligands, the presence of MWCNTs in the catalytic mixture allows the formation of waxes of lower molecular weight and polydispersity, whereas the presence of FLG proved to be beneficial for the catalytic activity. In Chapter V, isoprene polymerization catalyzed by iron complexes containing polyaromatic groups and non-covalently supported on nanoparticles and confined into the inner cavity of MWCNTs (Cat@NPs and Cat@NPs@MWCNTs) are investigated. Iron complexes show excellent activity for the isoprene polymerization and produced high glass temperature polyisoprene with a high trans-1,4-polyisoprene selectivity. Polymer nanocomposites are produced by supported catalysts and, transmission electron microscopy (TEM) evidenced efficient coating of the resulting polyisoprene around the oxygen sensitive iron nanoparticles.