Tesi: "Matériaux de Van der Waals"

1

Henck, Hugo. "Hétérostructures de van der Waals à base de Nitrure". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS319/document.

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Le sujet de cette thèse est à l’interface entre l’étude de composés à base de nitrure et des structures émergeantes formées par les matériaux bidimensionnels (2D) d’épaisseur atomique. Ce travail se consacre sur l’hybridation des propriétés électriques et optiques des semi-conducteurs à larges bandes interdites que sont les nitrures et des performances mécaniques, électriques et optiques des matériaux lamellaires, récemment isolé à l’échelle d’un plan atomique, qui sont aujourd’hui considérées avec attention aux regards de futures applications et d’études plus fondamentales. En particulier, une étude des propriétés électroniques, optiques et structurelles d’hétérostructures composées de plusieurs matériaux lamellaires et d’interfaces entre matériaux 2D et 3D a été réalisé par des moyens de microscopie et de spectroscopie tel que la spectroscopie Raman, de photoémission et d’absorption.Ce manuscrit traite dans un premier temps des propriétés structurelles et électroniques du nitrure de bore hexagonal (h-BN), matériau isolant aux propriétés optiques exotiques et essentiel dans la future intégration de ce type de matériaux 2D permettant de mettre en valeur leurs propriétés intrinsèques.En utilisant le graphène comme substrat les problèmes de mesures par photoémission rencontrés pour des matériaux isolant ont pu être surmonté dans le cas du h-BN et une étude des défauts structurels a pu être réalisée. Par conséquent, les premières mesures directes de la structure de bande électronique de plusieurs plans de h-BN sont présentées dans ce manuscrit.Dans un second temps, une approche d’intégration de ces matériaux 2D différente a été étudiée en formant une hétérostructure 2D/3D. L’interface de cette hétérojonction, composée d’un plan de disulfure de molybdène (MoS2) de dopage intrinsèque N associé à 300 nm de nitrure de gallium (GaN) intentionnellement dopé P à l’aide de magnésium, a été caractérisée. Un transfert de charge du GaN vers le MoS2 a pu être identifié suggérant un contrôle des propriétés électroniques de ce type de structure par le choix de matériaux.Ces travaux ont permis de révéler les diagrammes de bandes électroniques complet des structures étudiées a pu être obtenu permettant une meilleur compréhension de ces systèmes émergeants
This thesis is at the interface between the study of nitride based compounds and the emerging structures formed by atomically thin bi-dimensional (2D) materials. This work consists in the study of the hybridization of the properties of large band gap materials from the nitride family and the mechanical, electronic and optical performances of layered materials, recently isolated at the monolayer level, highly considered due to their possible applications in electronics devices and fundamental research. In particular, a study of electronics and structural properties of stacked layered materials and 2D/3D interfaces have been realised with microscopic and spectroscopic means such as Raman, photoemission and absorption spectroscopy.This work is firstly focused on the structural and electronic properties of hexagonal boron nitride (h-BN), insulating layered material with exotic optical properties, essential in in the purpose of integrating these 2D materials with disclosed performances. Using graphene as an ideal substrate in order to enable the measure of insulating h-BN during photoemission experiments, a study of structural defects has been realized. Consequently, the first direct observation of multilayer h-BN band structure is presented in this manuscript. On the other hand, a different approach consisting on integrating bi-dimensional materials directly on functional bulk materials has been studied. This 2D/3D heterostructure composed of naturally N-doped molybdenum disulphide and intentionally P-doped gallium nitride using magnesium has been characterised. A charge transfer from GaN to MoS2 has been observed suggesting a fine-tuning of the electronic properties of such structure by the choice of materials.In this work present the full band alignment diagrams of the studied structure allowing a better understanding of these emerging systems

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2

Abdukayumov, Khasan. "Conversion spin-charge dans les matériaux 2D et les hétérostructures de van der Waals". Electronic Thesis or Diss., Université Grenoble Alpes, 2024. http://www.theses.fr/2024GRALY016.

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Après la première exfoliation du graphène en 2004, de nombreux autres matériaux 2D ont été étudiés pour diverses applications, notamment la spintronique, un domaine qui exploite le degré de liberté du spin des électrons par opposition à la charge en électronique. La pierre angulaire de la spintronique fondamentale est le phénomène d'interconversion entre un courant de spin et un courant de charge, plus connu sous le nom de conversion spin-charge (SCC). Les matériaux 2D sont caractérisés par une faible interaction de van der Waals (vdW) entre les couches, ce qui permet de relâcher la contrainte d’accord de paramètre de maille pour l'épitaxie et de produire des hétérostructures vdW complexes. Cela peut également offrir de nouvelles plates-formes de croissance difficilement accessibles aux matériaux 3D conventionnels et, en raison des faibles liaisons vdW, les films produits peuvent être transférés sur un autre substrat. En outre, les matériaux 2D présentent une structure de bande dépendant de l'épaisseur et diverses hétérostructures peuvent être formées, ce qui ouvre la voie à un grand nombre de nouvelles applications en spintronique. Cependant, la plupart des recherches actuelles sont basées sur des flocons exfoliés dont la taille ne dépasse pas quelques dizaines de µm, ce qui limite les possibilités d'intégration. Dans cette thèse, je présente la croissance sur de grandes surfaces de matériaux 2D de haute qualité et d'hétérostructures vdW par épitaxie par jets moléculaires (EJM) et j'étudie les effets SCC par émission THz spintronique sondée par spectroscopie THz dans le domaine temporel. Tout d'abord, des hétérostructures CoFeB/PtSe2 avec une épaisseur variable de PtSe2 ont été étudiées et une transition de l'effet Rashba-Edelstein inverse dans quelques monocouches (ML) à l'effet Hall de spin inverse dans des films plus épais a été observée. C'est la première fois qu'un matériau présente une telle transition. Le second système était une bicouche PtSe2/MoSe2 dans laquelle nous avons observé une bande électronique hybridée présentant une texture de spin opposée à celle de PtSe2. Nous avons ainsi pu démontrer la possibilité d'inverser le signe de l’effet Rashba-Edelstein inverse en insérant une seule couche de MoSe2, ce qui ouvre une nouvelle voie pour moduler l'intensité et le signe de la SCC dans les hétérostructures vdW avec un contrôle à l’échelle de la monocouche. Enfin, la SCC dans quelques couches de PtSe2/MoSe2 a été étudiée en fonction d'un champ électrique externe variable ou rémanent par effet de proximité avec un matériau ferroélectrique 3D. En effet, cette hétérostructure vdW est semi-conductrice avec une efficacité de SCC possiblement plus grande dans les bandes électroniques loin du niveau de Fermi accessibles par l'application d'un champ électrique. Ces résultats nous incitent à explorer davantage le monde des matériaux 2D par divers moyens, tels que les champs électriques, et à rapprocher les matériaux 2D des applications dans le domaine de la spintronique
After the first-time successful exfoliation of graphene in 2004 many more 2D materials have been studied for various applications, including spintronics, a field that exploits the spin degree of freedom of electrons as opposed to the charge in electronics. The cornerstone of fundamental spintronics is the spin current-charge current interconversion phenomena, shortly known as spin-charge conversion (SCC). 2D materials are characterized by weak van der Waals (vdW) interaction between the layers, thus, relaxing the lattice-matching requirement for the epitaxy, enabling to grow complex vdW heterostructures. This can also offer new growth platforms not easily accessible by conventional 3D materials, and, due to the weak nature of the vdW forces, grown films can be transferred onto another substrate. Moreover, 2D materials show thickness dependent band structure and various heterostructures can be formed, opening up a vast number of possibly new physics for spintronic applications that can be explored. However, most of the current research is based on exfoliated flakes that are at most tens of µm in size, limiting their possible implementation for applications. In this thesis, I present large-area growth of high quality 2D materials and vdW heterostructures by molecular beam epitaxy (MBE) and study SCC effects by spintronic THz emission probed by THz time domain spectroscopy. First, CoFeB/PtSe2 heterostructures with varying the thickness of PtSe2 were studied and a transition from the inverse Rashba-Edelstein effect in a few monolayers (ML) to the inverse spin Hall effect in thicker films was observed. This is the first time a material showed such a transition. The second system was PtSe2/MoSe2 bilayer where we observed a hybridized electronic band showing an opposite spin texture to that of PtSe2. By this, we could demonstrate the possibility to reverse the sign of the inverse Rashba-Edelstein effect by inserting a single MoSe2 layer opening up a new route to modulate SCC intensity and sign in vdW heterostructures with monolayer control. Finally, SCC in few layer PtSe2/MoSe2 was investigated as a function of an external electric field either variable or remanent by proximity with a 3D ferroelectric material. Indeed, this vdW heterostructure is semiconducting with possibly larger SCC efficiency in electronic bands far from the Fermi level accessible through the application of an electric field. Those findings push us to explore the world of 2D materials even more by various means, such as electric fields, and bring 2D materials closer to spintronic device applications

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Nayak, Goutham. "Amélioration des propriétés physiques de matériaux de basse-dimensionnalité par couplage dans des hétérostructures Van der Waals". Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY084/document.

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Les propriétés intrinsèques extraordinaires de ces matériaux de faible dimension dépendent fortement de l'environnement auquel ils sont soumis. Par conséquent, ils doivent être préparés, traités et caractérisés sans défauts. Dans cette thèse, je discute de la manière de contrôler l'environnement des nanomatériaux de faible dimension tels que le graphène, le MoS$_{2}$ et les nanotubes de carbone afin de préserver leurs propriétés physiques intrinsèques. De nouvelles solutions pour l'amélioration des propriétés sont discutées en profondeur. Dans la première partie, nous fabriquons des dispositifs d'hétérostructure à base de graphène de Van der Waals (VdW) de dernière génération, en contact avec les bords, encapsulés dans du nitrure de bore hexagonal (hBN), afin d'obtenir un transport balistique. Nous utilisons une technique basée sur des mesures de bruit 1 / f pour sonder le transport de masse et de bord lors de régimes Quantum Hall entiers et fractionnaires. Dans la deuxième partie, le même concept de fabrication des hétérostructures VdW a été étendu pour encapsuler la couche monocouche MoS $_{2}$ dans le hBN afin d'en modifier les propriétés optiques. À cet égard, nous présentons une étude approfondie sur l'origine et la caractérisation des défauts intrinsèques et extrinsèques et leur incidence sur les propriétés optiques. En outre, nous décrivons une technique pour sonder le couplage entre couches ainsi que la génération de lumière avec une résolution spatiale inférieure à la limite de diffraction de la lumière. Enfin, nous discutons d'un processus systémique naturel visant à améliorer les propriétés mécaniques de la soie polymérique naturelle à l'aide d'une nanotubes de carbone à paroi unique fabriqués par HipCO comme aliment pour le ver à soie
The extraordinary intrinsic properties of low dimensional materials depend highly on the environment they are subjected to. Hence they need to be prepared, processed and characterized without defects. In this thesis, I discuss about how to control the environment of low dimensional nanomaterials such as graphene, MoS2 and carbon nanotubes to preserve their intrinsic physical properties. Novel solutions for property enhancements are discussed in depth. In the first part, we fabricate state-of-the-art, edge-contacted, graphene Van der Waals(VdW) heterostructuredevices encapsulated in hexagonal-boron nitride(hBN), to obtain ballistic transport. We use a technique based on 1/f-noise measurements to probe bulk and edge transport during integer and fractional Quantum Hall regimes. In the second part, the same fabrication concept of VdW heterostructures has been extended to encapsulate monolayer MoS2 in hBN to improve optical properties. In this regard we present an extensive study about the origin and characterization of intrinsic and extrinsic defects and their affect on optical properties. Further, we describe a technique to probe the interlayer coupling along with the generation of light with spatialresolution below the diffraction limit of light. Finally, we discuss a natural systemic process to enhance the mechanical properties of natural polymer silk using HipCO-made single walled carbon nanotubes as a food for silkworm

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4

Islam, Md Samiul. "Coherent ultrafast spectroscopy of excitons in Van der Waals materials". Electronic Thesis or Diss., Strasbourg, 2024. http://www.theses.fr/2024STRAE011.

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Dans cette thèse, basée sur un développement original de microscopie de mélange à quatre ondesultrarapide, la première mesure directe de la dynamique de cohérence excitonique dans le disulfurede rhénium a été obtenue. Ces résultats ont démontré une robustesse unique de la cohérenceexcitonique par rapport à d'autres matériaux dichalcogénures de métaux de transition(TMD). Le potentiel de contrôle des propriétés intrinsèques des excitons dans les matériaux van der Waals (vdW) a été exploré dans des assemblages bidimensionnels innovants. En particulier, l'impact du graphène sur l'environnement excitonique d'une hétérostructure et sur les propriétés dynamiques de ces excitons a été étudié. Enfin, une avancée significative vers la compréhension et l'ingénierie des propriétés optiques des défauts émetteurs de photons uniques dans le nitrure de bore hexagonal (hBN) a été réalisée
In this thesis, based on an original development of ultrafast four wave mixing microscopy, the firstdirect measurement of excitonic coherence dynamics in rhenium disulfide was obtained. Theseresults demonstrated a unique robustness of excitonic coherence compared to other Transition metal dichalcogenide (TMD) materials. The potential for controlling the intrinsic properties of excitons in van der Waals (vdW) materials was explored in innovative two-dimensional assemblies. In particular, the impact of graphene in the excitonic environment of a heterostructure on the dynamic properties of these excitons has been investigated. Finally, a significant step towards understanding and engineering the optical properties of single photon emitting defects in hBN has been achieved

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5

Lorchat, Étienne. "Optical spectroscopy of heterostructures based on atomically-thin semiconductors". Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAE035.

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Au cours de cette thèse, nous avons fabriqué et étudié par spectroscopie optique, des hétérostructures de van der Waals, composées de monofeuillets semi-conducteurs (dichalcogénures de métaux de transition, DMT) couplés à une monocouche de graphène ou à un résonateur plasmonique. Nous avons observé des modifications importantes de la dynamique des états excités optiquement dans le DMT (excitons) lorsque celui-ci est en contact avec le graphène. Le graphène neutralise la couche de DMT et permet un transfert non-radiatif d’excitons en moins de quelques picosecondes. Ce transfert d’énergie peut s’accompagner d’un photodopage extrinsèque considérablement moins efficace. La réduction de la durée de vie des excitons du DMT en présence de graphène a été exploitée pour montrer que leur pseudo-spin de vallée maintenait un degré de polarisation et de cohérence important jusqu’à température ambiante. Enfin, en couplant fortement les excitons d’un DMT aux modes d’un résonateur plasmonique à phase géométrique, nous avons mis en évidence, à température ambiante, le verrouillage du pseudo-spin de vallée sur la direction de propagation des polaritons chiraux (chiralitons) issus du couplage
During this thesis, we have fabricated and studied by optical spectroscopy, van der Waals heterostructures composed of semiconductor monolayers (transition metal dichalcogenides, TMD) coupled to a graphene monolayer or to a plasmonic resonator. We have observed significant changes in the dynamics of the TMD optically excited states (excitons) when it is in direct contact with graphene. Graphene neutralizes the TMD monolayer and enables non-radiative transfer of excitons within less than a few picoseconds. This energy transfer process may be accompanied by a considerably less efficient, extrinsic photodoping. The reduced lifetime of TMD excitons in the presence of graphene has been exploited to show that their valley pseudo-spin maintains a high degree of polarization and coherence up to room temperature. Finally, by strongly coupling TMD excitons to the modes of a geometric phase plasmonic resonator, we have demonstrated, at room temperature, that the momentum of the resulting chiral polaritons (chiralitons) is locked to their valley pseudo-spin

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Di, Felice Daniela. "Electronic structure and transport in the graphene/MoS₂ heterostructure for the conception of a field effect transistor". Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS267/document.

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L'isolement du graphène, une monocouche de graphite composée d'un plan d’atomes de carbone, a démontré qu'il est possible de séparer un seul plan d'épaisseur atomique, que l'on appelle matériau bidimensionnel (2D), à partir des solides de Van de Waals (vdW). Grâce à leur stabilité, différents matériaux 2D peuvent être empilés pour former les hétérostructures de vdW. L'interaction vdW à l'interface étant suffisamment faible, les propriétés spécifiques de chaque matériau demeurent globalement inchangées dans l’empilement. En utilisant une démarche théorique et computationnelle basée sur la théorie de la fonctionnelle de la densité (DFT) et le formalisme de Keldysh-Green, nous avons étudié l'hétérostructure graphène/MoS₂ . Le principal intérêt des propriétés spécifiques du graphène et du MoS₂ pour la conception d'un transistor à effet de champ réside dans la mobilité du graphène, à la base d'un transistor haute performance et dans le gap électronique du MoS₂, à la base de la commutation du dispositif. Tout d'abord, nous avons étudié les effets de la rotation entre les deux couches sur les propriétés électroniques à l'interface, en démontrant que les propriétés électroniques globales ne sont pas affectées par l'orientation. En revanche, les images STM (microscope à effet tunnel) sont différentes pour chaque orientation, en raison d'un changement de densité de charge locale. Dans un deuxième temps, nous avons utilisé l’interface graphène/MoS₂ en tant que modèle très simple de Transistor à Effet de Champ. Nous avons analysé le rôle des hétérostructures de vdW sur la performance du transistor, en ajoutant des couches alternées de graphène et MoS₂ sur l'interface graphène/MoS₂. Il a ainsi été démontré que la forme de la DOS au bord du gap est le paramètre le plus important pour la vitesse de commutation du transistor, alors que si l’on ajoute des couches, il n’y aura pas d’amélioration du comportement du transistor, en raison de l'indépendance des interfaces dans les hétérostructures de vdW. Cependant, cela démontre que, dans le cadre de la DFT, on peut étudier les propriétés de transport des hétérostructures de vdW plus complexes en séparant chaque interface et en réduisant le temps de calcul. Les matériaux 2D sont également étudiés ici en tant que pointe pour STM et AFM (microscope à force atomique) : une pointe de graphène testée sur MoS₂ avec défauts a été comparée aux résultats correspondants pour une pointe en cuivre. La résolution atomique a été obtenue et grâce à l'interaction de vdW entre la pointe et l’échantillon, il est possible d’éviter les effets de contact responsables du transfert d'atomes entre la pointe et l'échantillon. En outre, l'analyse des défauts est très utile du fait de la présence de nouveaux pics dans le gap du MoS₂ : ils peuvent ainsi être utilisés pour récupérer un pic de courant et donner des perspectives pour améliorer la performance des transistors
The isolation of graphene, a single stable layer of graphite, composed by a plane of carbon atoms, demonstrated the possibility to separate a single layer of atomic thickness, called bidimensional (2D) material, from the van der Waals (vdW) solids. Thanks to their stability, 2D materials can be used to form vdW heterostructures, a vertical stack of different 2D crystals maintained together by the vdW forces. In principle, due to the weakness of the vdW interaction, each layer keeps its own global electronic properties. Using a theoretical and computational approach based on the Density Functional Theory (DFT) and Keldish-Green formalism, we have studied graphene/MoS₂ heterostructure. In this work, we are interested in the specific electronic properties of graphene and MoS₂ for the conception of field effect transistor: the high mobility of graphene as a basis for high performance transistor and the gap of MoS₂ able to switch the device. First, the graphene/MoS₂ interface is electronically characterized by analyzing the effects of different orientations between the layers on the electronic properties. We demonstrated that the global electronic properties as bandstructure and Density of State (DOS) are not affected by the orientation, whereas, by mean of Scanning Tunneling Microscope (STM) images, we found that different orientations leads to different local DOS. In the second part, graphene/MoS₂ is used as a very simple and efficient model for Field Effect Transistor. The role of the vdW heterostructure in the transistor operation is analyzed by stacking additional and alternate graphene and MoS₂ layers on the simple graphene/MoS₂ interface. We demonstrated that the shape of the DOS at the gap band edge is the fundamental parameter in the switch velocity of the transistor, whereas the additional layers do not improve the transistor behavior, because of the independence of the interfaces in the vdW heterostructures. However, this demonstrates the possibility to study, in the framework of DFT, the transport properties of more complex vdW heterostructures, separating the single interfaces and reducing drastically the calculation time. The 2D materials are also studied in the role of a tip for STM and Atomic Force Microscopy (AFM). A graphene-like tip, tested on defected MoS₂, is compared with a standard copper tip, and it is found to provide atomic resolution in STM images. In addition, due to vdW interaction with the sample, this tip avoids the contact effect responsible for the transfer of atoms between the tip and the sample. Furthermore, the analysis of defects can be very useful since they induce new peaks in the gap of MoS₂: hence, they can be used to get a peak of current representing an interesting perspective to improve the transistor operation

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Vergnaud, Céline. "Optimisation de la croissance de MoSe2 - WSe2 par épitaxie de Van der Waals pour la valleytronique". Thesis, Université Grenoble Alpes, 2020. http://www.theses.fr/2020GRALY038.

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Cette thèse a pour objet l’optimisation de la croissance par épitaxie par jets moléculaires dans le régime de van der Waals de couches semi-conductrices bidimensionnelles (2D) de diséléniures de métaux de transition (MoSe2, WSe2) pour les études magnéto-optiques et électriques. Cette optimisation passe par l’amélioration de la qualité cristallographique des couches sur de grandes surfaces en ajustant les paramètres de croissances (température et flux). En particulier, la maîtrise de l’état de surface du substrat est déterminante sur les mécanismes de croissance de ces couches. L’élaboration de ces matériaux de basse dimensionnalité a nécessité l’utilisation de techniques de caractérisation avancées (Diffraction de rayons X en incidence rasante, Microscopie électronique en transmission en mode haute résolution, ect). Dans cette thèse, nous nous sommes concentrés sur deux substrats particuliers : l’oxyde de silicium et le mica. Ils présentent tous les deux la particularité d’être isolants et inertes d’un point de vue électronique, ce qui est indispensable pour sonder les propriétés optiques et électriques intrinsèques des couches 2D. Finalement, nous avons développé les dopages électrique (dopage p) pour la microélectronique et magnétique (dopage Mn) pour la valleytronique
The purpose of this thesis is to optimize growth by molecular beam epitaxy in the van der Waals regime of two-dimensional (2D) semiconductor layers of transition metal diselenides (MoSe2, WSe2) for magneto-optical and electric studies. This optimization involves improving the crystallographic quality of the layers over large areas by adjusting the growth parameters (temperature and flux). In particular, the control of the surface state of the substrate is decisive on the growth mechanisms of these layers. The development of these low-dimensional materials required the use of advanced characterization techniques (Grazing incidence X-ray diffraction, High Resolved Transmission Electronic Microscopy, ect). In this thesis, we focused on two specific substrates : silicon oxide and mica. They both have the particularity of being insulating and inert from an electronic point of view, which is essential to probe the optical and electrical intrinsic properties of 2D layers. Finally, we developed electrical doping (p doping) for microelectronics and magnetic (Mn doping) for valleytronics

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Touhari, Françoise. "Etude de l'interaction de Van der Waals en microscopie à force atomique. Simulation numérique d'images de nanostructures et effet de la nature chimique des matériaux". Montpellier 2, 1998. http://www.theses.fr/1998MON20060.

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Cette these presente une methode numerique permettant de modeliser l'interaction de van der waals entre la pointe d'un microscope a force atomique (afm) et une surface comportant des nanostructures. Le modele developpe permet non seulement de prendre en compte la geometrie reelle de la pointe mais aussi de simuler des images d'objets nanometriques. Nous nous sommes interesses a deux modes de fonctionnement du microscope : le mode non-contact dit a hauteur constante et celui dit a gradient de force constant. Dans ce modele, la pointe - supposee pyramidale - ainsi que les objets sont representes par des empilements de cubes de dimensions nanometriques. En nous basant sur l'hypothese d'additivite par paires appliquee aux forces de van der waals, nous utilisons le potentiel de lennard-jones pour calculer les interactions pointe-surface et pointe-objet. Notre etude est en accord avec des resultats experimentaux publies dans la litterature portant sur l'effet de la geometrie de la pointe ou encore celui de la distance pointe-surface. L'originalite de ce travail reside dans la mise en evidence par une simulation numerique du contraste chimique visible lorsque l'on realise l'image afm de deux zones presentant des natures chimiques differentes. Ce phenomene est lie a la nature meme des forces de van der waals, forces qui dependent des constantes dielectriques des materiaux en interaction. Nous montrons ainsi que l'interpretation des images non-contact est rendue difficile par la combinaison des effets geometriques et chimiques.

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Wang, Hangtian. "Interfacial Engineering of the Magnetism in 2D Magnets, Topological Insulators, and Their Heterostructures". Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0206.

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Alors que le nœud critique des circuits intégrés (CI) entre dans la phase 1 nm, les matériaux tridimensionnels traditionnels ne peuvent pas conserver leurs propriétés physiques d'origine et ne peuvent donc pas répondre aux besoins des processus de fabrication des circuits intégrés. Parallèlement, la diminution de la largeur des lignes entraîne également une augmentation inévitable de la consommation d'énergie statique. Par conséquent, la recherche de nouveaux matériaux et de nouvelles technologies pour briser le « mur de taille » et le « mur de puissance » est devenue une direction cruciale dans l'industrie des circuits intégrés. En tant que nouveau membre de la famille des matériaux bidimensionnels (2D), les aimants 2D peuvent maintenir leur ordre magnétique à longue portée à l'échelle atomique avec leurs propriétés physiques facilement contrôlées par des stimuli externes, ce qui constitue une plate-forme idéale pour la haute densité et les dispositifs spintroniques de faible puissance. Cependant, en raison de l'effet dimensionnel, le magnétisme 2D ne peut pas exister à haute température. Bien que plusieurs méthodes puissent améliorer la température de Curie (Tc) des aimants 2D (comme le dopage, l'intercalation ionique ou le pompage laser), elles sont loin d'être faciles à contrôler et à haut rendement. Plus important encore, la méthode de préparation largement utilisée par exfoliation mécanique abandonne le mérite de l'effet interfacial 2D, qui s'est avéré être une approche importante pour une manipulation magnétique 2D efficace. Par conséquent, l'étude de l'effet interfacial dans les aimants 2D épitaxiaux est considérée comme un domaine clé pour obtenir un ordre ferromagnétique 2D stable, à grande échelle, à haute Tc, facile à contrôler. L'isolant topologique (TI) est un autre matériau 2D avec un fort couplage spin-orbital. Les états de surface protégés par la topologie ont fourni à TI de nombreux effets fascinants liés au spin, tels que le verrouillage de l'impulsion de spin, l'effet d'échange de spin, etc., ce qui fait de ce matériau un candidat potentiel pour fabriquer des dispositifs spintroniques efficaces. De plus, le TI peut être intégré à des aimants 2D pour former une hétérostructure 2D, dans laquelle non seulement le magnétisme peut être amélioré via l'effet d'interface, mais également les propriétés liées au spin de l'hétérostructure peuvent être manipulées grâce aux avantages de ces aimants
With the critical node of integrated circuits (IC) entering the 1 nm stage, traditional three-dimensional materials cannot maintain their original physical properties, and thus cannot meet the needs of IC manufacturing processes. Meanwhile, the shrinking line width also introduces an inevitable increase in static power consumption. Therefore, researching new materials and new technologies to break through the "Size Wall" and "Power Wall" has become a crucial direction in the IC industry. As a new member of the two-dimensional (2D) material family, the 2D magnets can maintain its long-range magnetic order at the atomic scale with its physical properties easily controlled by external stimuli, which provides an ideal platform for the high-density and low-power spintronic devices. However, due to the dimensional effect, 2D magnetism cannot exist at high temperatures. Although several methods can enhance the Curie temperature (Tc) of 2D magnets (such as doping, ion intercalation, or laser pumping), they are far from easy-controllability and high-efficiency. More importantly, the widely-used preparation method via mechanical exfoliation abandons the merit of 2D interfacial effect, which was proved to be an important approach to efficient 2D magnetic manipulation. Therefore, studying the interfacial effect in epitaxial 2D magnets is regarded as a key field to achieving large-scale, high-Tc, easy-controlling, and stable 2D ferromagnetic order. Topological insulator (TI) is another 2D material with strong spin-orbital coupling. The topology-protected surface states provided TI with numerous fascinates spin-related effects, such as spin-momentum locking, spin exchange effect, etc., which makes this material a potential candidate to fabricate effective spintronic devices. In addition, the TI can be integrated with 2D magnets to form a 2D heterostructure, in which not only the magnetism can be enhanced via the interfacial effect, but also the spin-related properties of the heterostructure can be manipulated due to the advantages of these two materials

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10

Ben, Jabra Zouhour. "Study of new heterostructures : silicene on graphene". Electronic Thesis or Diss., Aix-Marseille, 2021. http://www.theses.fr/2021AIXM0583.

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Le but de ce travail est la croissance du silicène sur Gr. J'ai décrit le substrat en fonction des conditions d’élaboration par CVD. Lorsque la proportion de H2 est faible il est possible d’obtenir du Gr homogène sur couche tampon (BL) sur SiC. Le STM et LEED montrent la superposition de la maille du Gr et de la reconstruction de la BL représentatif du Gr épitaxié. Lorsque la proportion de H2 est élevée la couche de Gr obtenue est totalement hydrogénée. Ceci est un résultat nouveau car aucun procédé d’intercalation d’hydrogène n’avait permis jusqu’à présent d’hydrogéner totalement les échantillons de (6x6)Gr épitaxié sur BL. Pour des proportions intermédiaires de H2/Ar, la coexistence de (6x6)Gr et H-Gr est observée. En fonction de la proportion de H2 dans le mélange gazeux, soit la surface du SiC reste passivée pendant toute la croissance du Gr et on obtient du H-Gr, soit le H2 désorbe partiellement, ou totalement et on obtient soit la coexistence des deux structures soit du (6x6)Gr pleine plaque. J’ai étudié la croissance par MBE de Si-ene sur (6x6)Gr. J’ai démontré qu'il est possible de former des flaques de Si-ene pour des épaisseurs de dépôt <0.5MC. Nous observons la présence de zones planes d’une épaisseur de 0.2-0.3nm correspondant à une monocouche de Si-ene, entourées d’îlots dendritiques 3D de Si. Les spectres Raman mettent en évidence un pic allant jusqu’à 563cm-1 ce qui est la valeur la plus proche du Si-ene FS jamais obtenue. Ces démontrent la formation de Si-ene quasi-FS. Ce travail contribue à une meilleure compréhension du mécanisme de croissance CVD du Gr et à l’avancement des recherches dans le domaine de la croissance épitaxiale des matériaux 2D
The topic of this thesis deals with the study of the growth and properties of silicene (Si-ene) on graphene (Gr) on 6H-SiC(0001) with the final goal of forming free-standing (FS) Si-ene on an insulating or semiconductor substrate. I have described the substrate as a function of the CVD processing conditions. When the proportion of H2 is low it is possible to obtain homogeneous Gr on buffer layer (BL) on SiC. The STM and LEED show the superposition of the Gr mesh and the BL reconstruction representative of the epitaxial Gr. When the proportion of H2 is high, the resulting Gr layer is fully hydrogenated. This is a new result as no hydrogen intercalation process has been able to fully hydrogenate (6x6)Gr samples epitaxial on BL until now. For intermediate proportions of H2/Ar, the coexistence of (6x6)Gr and H-Gr is observed. Depending on the proportion of H2 in the gas mixture, either the SiC surface remains passivated during the entire Gr growth and H-Gr is obtained, or the H2 partially or totally desorbs and either both structures coexist or full plate (6x6)Gr is obtained. I have studied the MBE growth of Si-ene on (6x6)Gr. I have shown that it is possible to form Si-ene puddles for deposit thicknesses <0.5MC. We observe the presence of flat areas of 0.2-0.3nm thickness corresponding to a Si-ene monolayer, surrounded by 3D dendritic islands of Si. The Raman spectra show a peak up to 563cm-1 which is the closest value to Si-ene FS ever obtained. This demonstrates the formation of quasi-FS Si-ene. This work contributes to a better understanding of the CVD growth mechanism of Gr and to the advancement of research in the field of epitaxial growth of 2D materials

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11

Marcon, Paul. "Calcul ab-initio des propriétés physiques d'hétérostructures associant des matériaux ferromagnétiques à anisotropie magnétique perpendiculaire et des dichalcogénures de métaux de transition". Electronic Thesis or Diss., Toulouse 3, 2023. http://www.theses.fr/2023TOU30273.

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La possibilité de synthétiser des hétérostructures formées de matériaux 2D offre des perspectives majeures pour l'amélioration des composants spintroniques actuels ou la réalisation de nouveaux dispositifs. Le contrôle et la bonne compréhension des propriétés physiques de ces systèmes constituent de fait un enjeu technologique majeur. Au cours de cette thèse, nous avons étudié, à l'aide de calculs ab initio basés sur la théorie de la fonctionnelle de la densité (DFT), des hétérostructures formées de monocouches de dichalcogénures de métaux de transition (TMDCs) et de cristaux ferromagnétiques présentant une anisotropie magnétique perpendiculaire. Trois objectifs principaux ont été définis : (i) comprendre comment utiliser la proximité magnétique pour lever la dégénérescence des vallées et quantifier l'effet Zeeman des vallées ; (ii) évaluer la possibilité d'injecter un gaz d'électrons polarisé en spin dans des vallées spécifiques du feuillet de TMDC ; (iii) examiner l'impact de la proximité sur le couplage spin-orbite dans le feuillet de TMDC et sur les phénomènes Rashba et Dresselhaus dans ces systèmes. Nous avons d'abord étudié des multicouches possédant une électrode constituée d'un métal et d'une barrière isolante non 2D. Dans le système Fe/MgO/MoS2, nous avons calculé qu'un transfert d'électrons spontané s'opère de la couche de Fe vers le monofeuillet de MoS2, donnant lieu à la formation d'un gaz d'électrons non polarisé en spin. Nous avons établi un modèle expliquant la compétition entre les effets spin-orbite de type Rashba et Dresselhaus et les effets de proximité magnétique sur les bandes de valence de MoS2 : Ce modèle nous a permis de montrer que les effets de proximité sont prédominants pour une faible épaisseur de MgO (<0.42 nm), et tendent à disparaître au profit des effets spin-orbite pour à plus forte épaisseur (> 1.06 nm). Nous avons prédit qu'il est possible d'obtenir des effets spin-orbites plus forts en remplaçant l'électrode de Fe par une électrode non-magnétique de V. Afin d'augmenter les effets de proximité magnétique, nous avons finalement décider d'étudier des hétérostructures [Co1Ni2]n/h-BN/WSe2, dans lesquelles [Co1Ni2]n est un super réseau à anisotropie magnétique perpendiculaire et h-BN un isolant bidimensionnel. Pour ce système, nous prédisons qu'il serait possible d'avoir une polarisation en spin des vallées aux points K et K'. Finalement, nous avons étudié les propriétés particulières de l'hétérostructure de van der Waals Graphène/CrI3/WSe2,dans laquelle l'électrode magnétique est également remplacée par des matériaux 2D
The ability to synthesize heterostructures made up of 2D materials provides significant opportunities for improving current spintronic components or developing new devices. Thus, the control and deep understanding of the physical properties of these systems become a critical technological challenge. During this thesis, we examined heterostructures composed of transition metal dichalcogenide (TMDC) monolayers and ferromagnetic crystals exhibiting perpendicular magnetic anisotropy, using ab initio calculations based on density functional theory (DFT). We focus on three main goals: (i) understanding how to use magnetic proximity to lift valley degeneracy and quantify the valley Zeeman effect; (ii) assessing the possibility of injecting spin-polarized electron gas into specific valleys of the TMDC sheet; (iii) investigating the impact of proximity on spin-orbit coupling in the TMDC sheet and on the Rashba and Dresselhaus phenomena in these systems. We first studied multilayers with an electrode made up of a metal and a non-2D insulating barrier. In the Fe/MgO/MoS2 system, we computed that a spontaneous electron transfer occurs from the Fe layer to the MoS2 monolayer, leading to the formation of a non-spin-polarized electron gas. We established a model explaining the competition between Rashba and Dresselhaus-type spin-orbit effects and magnetic proximity effect on the MoS2 valence bands: This model allowed us to show that proximity effect predominate for thin MgO (<0.42 nm) and tend to disappear in favor of spin-orbit effects for thicker layers (> 1.06 nm). We predicted that stronger spin-orbit effects can be achieved by replacing the Fe electrode with a non-magnetic V electrode. To boost the magnetic proximity effects, we finally decided to study [Co1Ni2]n/h-BN/WSe2 heterostructures, in which [Co1Ni2]n is a superlattice with perpendicular magnetic anisotropy, and h-BN is a two-dimensional insulator. For this system, we predict that it could be possible to have a spin polarization of the valleys at the K and K' points. Ultimately, we explored the unique properties of the van der Waals heterostructure Graphene/CrI3/WSe2, where the magnetic electrode is also replaced by 2D materials

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Sant, Roberto. "Exploration par rayonnement synchrotron X de la croissance et de la structure de dichalcogénures 2D". Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY075.

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Les dichalcogenures de métaux de transition bidimensionnels (2D) suscitent un grand intérêt pour des applications variées, principalement en optoelectronique. Toutefois, la faible compréhension des mécanismes liés à leur épitaxie, de leur microstructure et de la nature de leur interaction avec le substrat représentent encore des problèmes ouverts. Nous avons exploré un certain nombre de croissances épitaxiales des dichalcogenures 2D préparés par épitaxie à jets moléculaires sur des substrats différents. Nous en avons examiné la structure atomique et essayé d’en modifier certains in situ. Plusieurs systèmes et processus ont été étudiés: (i) des tellurures de métaux de transition, ZrTe2, MoTe2 et TiTe2, épitaxiés sur un substrat de InAs(111), (ii) l’intercalation d’espèces atomique alcalines entre une monocouche de MoS2 et son substrat d’Au(111), (iii) la croissance et le traitement thermique sous atmosphère de H2S d’une monocouche de PtSe2 sur Pt(111). Notre travail s’appuie sur des approches à la fois phénoménologiques et quantitatives de diffraction de rayons X de surface, souvent complétées par ses analyses effectuées à l’aide d’autres techniques (STM, TEM, XPS et ARPES). Les principaux résultats sont que: (i) une phase orthorhombique et une onde de densité de charge sont stabilisées à température ambiante dans les couches de MoTe2 et TiTe2 par un effet de déformation induite par l’épitaxie; (ii) l’intercalation de césium (Cs) au-dessous du MoS2 induit un découplage structurel mais aussi électronique de la monocouche de son substrat; (iii) la sulfurisation de PtSe2 à chaud en conditions contrôlées permet de substituer des atomes de Se par des atomes de S dans la couche supérieure du dichalcogenure, formant ainsi un alliage ordonné de SPtSe, structure de type Janus
Two-dimensional transition metal dichalcogenides (TMDCs) are promising materials for a variety of applications, especially in optoelectronics. However, the lack of understanding of their epitaxy - i.e. growth mechanism, microscopic structure, nature of the 2D layer-substrate interaction, etc. - is still a crucial issue to address. In this PhD thesis we explored a series of epitaxial growths of monolayer and thin film TMDCs grown by molecular beam epitaxy (MBE) on a variety of substrates. We studied their atomic structures and we attempted the modifications of some of them with various in situ methods. Several systems and processes have been investigated: (i) transition metal ditellurides, ZrTe2 , MoTe2 and TiTe2 on InAs(111) substrate, (ii) the intercalation of alkali metal species between single layer MoS2 and its Au(111) substrate, (iii) the growth and the thermal treatments in H2S atmosphere of monolayer PtSe2 on Pt(111). Our work relies on both phenomenological and quantitative methods based on surface X-ray diffraction, often complemented by parallel analysis performed with other probes, e.g. STM, TEM, XPS, ARPES. Most notably, we found that: (i) a metastable orthorhombic phase and a charge density wave phase can be stabilized at room temperature in MoTe2 and TiTe2 owing to the epitaxial strain in the materials; (ii) the intercalation of Cs atoms under MoS2 induces structural and electronic decoupling of the 2D MoS2 layer from its Au(111) substrate; (iii) the sulfurization of PtSe2 promotes the Se-by-S substitution in one (or both) of its two chalcogen layers, leading either to the full conversion of the selenide into a sulfide or even to an ordered Janus alloy

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Rocha, Alberto Sperotto dos Santos. "Um modelo de sacola difusa para a matéria nuclear". reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2004. http://hdl.handle.net/10183/6706.

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Neste trabalho desenvolvemos um modelo efetivo para a descrição da matéria nuclear, que incorpora os resultados obtidos, para a descrição de um núcleon, pelo modelo de sacola difusa. O sistema nuclear será descrito via uma função de energia interna, que compreende um termo livre e outro que leva em conta a interação entre os núcleons. A parte livre, por se tratar de um sistema de férmions, corresponderá à energia de um gásde Fermi livre. Além disso, para evitar a superposição de dois ou mais núcleons, introduzimos um volume de exclusão a la Van der Waals. Na parte integrante, a troca de píons entre os núcleons será levada em conta via um potêncial efetivo. A função energia interna dependerá da densidade da matéria nuclear e também de um parâmetro que determinará o volume esperado de cada núcleon na matéria nuclear. O valor deste parâmetro será um pouco diferente do valor encontrado para um núcleons isolado, devido à interação entre eles. Obtém-se então resultados para a energia de ligação por núcleon para a matéria nuclear simétrica e para a matéria de nêutrons, bem como para a equação de estado da matéria de nêutrons.

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Chevereau, Élodie. "Synthèse et caractérisation physico-chimique d'un matériau membranaire à structure contrôlée : étude expérimentale de la sélectivité". Lorient, 2011. http://www.theses.fr/2011LORIS249.

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L’objectif de ce travail était de synthétiser des membranes inorganiques type mordénite en vue d’une application en nanofiltration ou en ultrafiltration à bas-seuil de coupure. Cette étude repose sur l’interprétation des mécanismes de transport régissant la rétention d’ions monovalents et divalents au travers d’une membrane minérale à structure contrôlée. La démarche qui a été mise en place nous a conduit, dans un premier temps, à s’assurer de la nature des produits synthétisés et à identifier leurs propriétés de surface, morphologique, et électrique. Nous avons choisi de déterminer les paramètres spécifiques des mordénites synthétisées (tels que le rayon de pore, la surface spécifique, et la charge de surface) à partir des poudres utilisées pour préparer les membranes associées. Parallèlement à ces caractérisations physico-chimiques, des essais de filtration ont été réalisés sur des pilotes de laboratoire afin d’étudier la sélectivité d’une membrane mordénite vis-à-vis des sels monovalents et divalents. Pour cela, un protocole expérimental a été mis en place. Nous avons réussi à observer la rétention d’ions monovalents de petites tailles après modification des propriétés de surface de la mordénite. Le passage facilité ainsi que la rétention spécifique d’ions mono- et divalents ont également été observés dans certaines conditions opératoires. Ainsi, les résultats obtenus lors des essais de filtration en solutions pures et en mélanges nous ont conduit à formuler plusieurs hypothèses concernant les interactions membrane-solutés : la membrane se comporte comme une surface dont la polarité peut être modifiée suite à un traitement alcalin, la polarisabilité des ions hydratés diminue avec leurs enthalpies d’hydratation, les interactions entre la surface traitée et ces ions dépendent des forces de Van der Waals
The aim of this work was to synthesize mordenite membrane for an application in nanofiltration or in low cut-off ultrafiltration and to understand the transport mechanisms governing the retention of monovalent and divalent ions through a controlled structure mineral membrane. The approach what has been implemented has led us, at first, to ascertain the nature of synthesized products and identify their surface, morphological and electrical properties. We chose to determine the specific parameters of the synthesized mordenite (such as pore radius, surface area and surface charge) from the powders used to prepare the related membrane. In addition to these physical and chemical characterizations, filtration tests were carried out on pilot unit in order to study the mordenite membrane selectivity toward monovalent and divalent salt-water solution. For this, an experimental protocol was defined. We managed to observe the retention of small monovalent ions after changing the surface properties of mordenite. The preferential transport and the specific retention of mono- and divalent ions were also observed under certain operating conditions. Thus, the results obtained during the filtration tests in single and mixed solutions led us to make several hypotheses about solute-membrane interactions: the membrane behaves as a surface whose the polarity can be changed after an alkaline treatment, the polarizability of hydrated ions decreases with their enthalpies of hydration, the interaction between the treated surface and theses ions depend on the Van der Waals forces

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Mahmoudi, Aymen. "Propriétés électroniques des dichalcogénures bi-dimensionnels de métaux de transition". Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP106.

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Le sujet de cette thèse s'inscrit dans la thématique des matériaux bidimensionnels (2D) d'épaisseur atomique. L'étude des propriétés optiques et électroniques des hétérostructures hybrides à base de dichalcogénures de métaux de transition (TMD) MX₂ (M = Mo, W ; X = S, Se, Te) est aujourd'hui considérée avec attention en raison de futures applications et d'études plus fondamentales. Plus que leurs propriétés physiques intrinsèques, en configuration multicouche, ces matériaux offrent des phénomènes physiques prometteurs tels que la modulation de la valeur de la bande interdite, la ferroélectricité pour des configurations cristallines spécifiques, etc. En particulier, ce travail se consacre aux hétérostructures hybrides à base de diséléniure de tungstène (WSe₂) sur des substrats de graphène et de phosphate de gallium (GaP). En mettant en œuvre des techniques de microscopie et de spectroscopie telles que la spectroscopie Raman et la spectroscopie de photoémission résolue en angle (ARPES), une étude des propriétés électroniques, optiques et structurelles d'hétérostructures composées de plusieurs matériaux 2D a été réalisée afin de fournir une meilleure compréhension de ces systèmes émergents. Par conséquent, les premières mesures directes de la structure de bande électronique de la phase rhomboédrique de la structure bicouche de WSe₂ déposée sur un substrat 2D de graphène sont présentées dans ce manuscrit. La croissance directe de ce matériau 2D sur un substrat 3D de GaP a été étudiée pour plusieurs épaisseurs. Ces travaux ont permis d'identifier l'effet de la nature de la phase cristalline ainsi que la méthode de croissance sur les structures de bandes électroniques, ce qui permet une meilleure compréhension de ces systèmes émergents
The subject of this thesis is two-dimensional (2D) materials of atomic thickness. The study of the optical and electronic properties of hybrid heterostructures based on MX₂ transition metal dichalcogenides (TMDs) (M = Mo, W; X = S, Se, Te) is now being carefully considered with a view to future applications and more fundamental studies. Beyond their intrinsic physical properties, in multilayer configurations, these materials offer promising physical phenomena such as modulation of bandgap values, ferroelectricity for specific crystal configurations, and so on. In particular, this work focuses on hybrid heterostructures based on tungsten diselenide (WSe₂) on graphene and gallium phosphate (GaP) substrates. Using microscopy and spectroscopy techniques such as Raman spectroscopy and angle-resolved photoemission spectroscopy (ARPES), we investigated the electronic, optical, and structural properties of heterostructures composed of several 2D materials to better understand these emerging systems. Accordingly, the first direct measurements of the electronic band structure of the rhombohedral phase of the WSe₂ bilayer structure deposited on a 2D graphene substrate are presented in this manuscript. The direct growth of this 2D material on a 3D GaP substrate has been studied for several thicknesses. This work has enabled us to identify the effect of the nature of the crystalline phase and the growth method on the electronic band structures, providing a better understanding of these emerging systems

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16

Bezzi, Luca. "Materiali 2D van der Waals". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.

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Dalla scoperta del grafene, molte ricerche sono state condotte sui cosiddetti “materiali 2D”. Questo elaborato si focalizza sulle proprietà strutturali, elettroniche, ottiche ed eccitoniche di due materiali bidimensionali, ossia il grafene il disolfuro di molibdeno (MoS2-1H), quest’ultimo un importante semiconduttore. Le proprietà di questi materiali sono diverse rispetto alla loro controparte massiva (bulk) grafite e MoS2-2H, e un loro confronto è stato preso in considerazione. Come metodo di indagine sono state scelte simulazioni quanto- meccaniche ab initio dei sistemi in esame, un approccio che, negli ultimi decenni, sta avendo un impatto sempre più importante sulla fisica, sulla chimica dello stato solido e sulla scienza dei materiali, promuovendo non solo una comprensione più profonda, ma anche la possibilità di contribuire in modo significativo alla progettazione di materiali per nuove tecnologie. Questo importante passo avanti è stato possibile grazie a: (i) una descrizione migliorata ed efficiente degli effetti elettronici a molti corpi (many-body) nella teoria del funzionale della densità (DFT), nonché lo sviluppo di metodi post-DFT per lo studio di proprietà specifiche; (ii) un’accurata implementazione di questi metodi in software altamente efficienti, stabili e versatili, capaci di sfruttare il potenziale delle architetture informatiche moderne. Tra i possibili software ab initio basati su DFT, abbiamo scelto il pacchetto di simulazione di Vienna ab initio VASP, considerato un gold standard per questo tipo di indagini. I risultati ottenuti per le varie proprietà di bulk e di superficie (bidimensionale) dei materiali scelti sono in ottimo accordo con dati ottenuti in precedenza, sia a livello teorico, sia sperimentale. Questo elaborato getta quindi le basi per futuri studi nel campo dei materiali 2D per comprendere, analizzare, ingegnerizzare nuovi materiali con proprietà desiderabili e per sviluppare nuove applicazioni degli stessi.

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Boddison-Chouinard, Justin. "Fabricating van der Waals Heterostructures". Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/38511.

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The isolation of single layer graphene in 2004 by Geim and Novoselov introduced a method that researchers could extend to other van der Waals materials. Interesting and new properties arise when we reduce a crystal to two dimensions where they are often different from their bulk counterpart. Due to the van der Waals bonding between layers, these single sheets of crystal can be combined and stacked with diferent sheets to create novel materials. With the goal to study the interesting physics associated to these stacks, the focus of this work is on the fabrication and characterization of van der Waals heterostructures. In this work, we first present a brief history of 2D materials, the fabrication of heterostructures, and the various tools used to characterize these materials. We then give a description of the custom-built instrument that was used to assemble various 2D heterostructures followed by the findings associated with the optimization of the cleanliness of the stack's interface and surface. Finally, we discuss the results related to the twisting of adjacent layers of stacked MoS2 and its relation to the interlayer coupling between said layers.

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18

Tiller, Andrew R. "Spectra of Van der Waals complexes". Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.333415.

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19

Mauro, Diego. "Electronic properties of Van der Waals heterostructures". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10565/.

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L’interazione spin-orbita (SOI) nel grafene è attualmente oggetto di intensa ricerca grazie alla recente scoperta di una nuova classe di materiali chiamati isolanti topologici. Questi materiali, la cui esistenza è strettamente legata alla presenza di una forte SOI, sono caratterizzati dall’interessante proprietà di avere un bulk isolante ed allo stesso tempo superfici conduttrici. La scoperta teorica degli isolanti topologici la si deve ad un lavoro nato con l’intento di studiare l’influenza dell’interazione spin-orbita sulle proprietà del grafene. Poichè questa interazione nel grafene è però intrinsecamente troppo piccola, non è mai stato possibile effettuare verifiche sperimentali. Per questa ragione, vari lavori di ricerca hanno recentemente proposto tecniche volte ad aumentare questa interazione. Sebbene alcuni di questi studi abbiano mostrato un effettivo aumento dell’interazione spin-orbita rispetto al piccolo valore intrinseco, sfortunatamente hanno anche evidenziato una consistente riduzione della qualità del grafene. L’obbiettivo che ci si pone in questa tesi è di determinare se sia possibile aumentare l’interazione spin-orbita nel grafene preservandone allo stesso tempo le qualità. La soluzione proposta in questo lavoro si basa sull’utilizzo di due materiali semiconduttori, diselenio di tungsteno WSe2 e solfuro di molibdeno MoS2, utilizzati da substrato su cui sopra verrà posizionato il grafene formando così un’eterostruttura -nota anche di “van der Waal” (vdW)-. Il motivo di questa scelta è dovuto al fatto che questi materiali, appartenenti alla famiglia dei metalli di transizione dicalcogenuri (TMDS), mostrano una struttura reticolare simile a quella del grafene, rendendoli ideali per formare eterostrutture e ancora più importante, presentano una SOI estremamente grande. Sostanzialmente l’idea è quindi di sfruttare questa grande interazione spin-orbita del substrato per indurla nel grafene aumentandone così il suo piccolo valore intrinseco.

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20

Klein, Andreas. "Energietransferprozesse in matrixisolierten van-der-Waals-Komplexen". [S.l. : s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=962344761.

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21

Odeyemi, Tinuade A. "Numerical Modelling of van der Waals Fluids". Thèse, Université d'Ottawa / University of Ottawa, 2012. http://hdl.handle.net/10393/22661.

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Many problems in fluid mechanics and material sciences deal with liquid-vapour flows. In these flows, the ideal gas assumption is not accurate and the van der Waals equation of state is usually used. This equation of state is non-convex and causes the solution domain to have two hyperbolic regions separated by an elliptic region. Therefore, the governing equations of these flows have a mixed elliptic-hyperbolic nature. Numerical oscillations usually appear with standard finite-difference space discretization schemes, and they persist when the order of accuracy of the semi-discrete scheme is increased. In this study, we propose to use a Chebyshev pseudospectral method for solving the governing equations. A comparison of the results of this method with very high-order (up to tenth-order accurate) finite difference schemes is presented, which shows that the proposed method leads to a lower level of numerical oscillations than other high-order finite difference schemes, and also does not exhibit fast-traveling packages of short waves which are usually observed in high-order finite difference methods. The proposed method can thus successfully capture various complex regimes of waves and phase transitions in both elliptic and hyperbolic regimes

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22

Marsden, Alexander J. "Van der Waals epitaxy in graphene heterostructures". Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/77193/.

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Graphene — a two-dimensional sheet of carbon atoms — has surged into recent interest with its host of remarkable properties and its ultimate thinness. However, graphene combined with other materials is starting to attract more attention. These heterostructures can be important for production routes, incorporating graphene into existing technologies, or for modifying its intrinsic properties. This thesis aims to examine the role of van der Waals epitaxy within these heterostructures. First, the graphene-copper interaction during chemical vapour deposition of graphene is investigated. Graphene is found to grow with a mismatch epitaxy of 8 relative to the [001] direction of the Cu(100) surface, despite a mismatch in symmetry and lattice parameter between two. Further, the electronic structure of both graphene and copper is unchanged by the interaction. This highlights the weak interaction between the two, owing to its van der Waals nature. Functionalised graphene is another important heterostructure, and is intensively studied for both graphene production routes and for altering graphene’s properties. Here, it is the change to the homogeneous graphene surface that makes it interesting for van der Waals epitaxy. The effect of functionalisation of graphene with atomic oxygen and nitrogen is presented next. In both cases, only small amounts of functionalisation ( 5 at%) is sufficient to significantly deteriorate the -band structure of the graphene through localisation. For small amounts of nitrogen functionalisation, and greater amounts of oxygen functionalisation, extended topological defects are formed in the graphene lattice. Unlike epoxide oxygen groups, these disruptions to the pristine graphene are found to be irreversible by annealing. Next, the interaction between graphene and the organic semiconducting molecule vanadyl-phthalocyanine (VOPc) is presented. As a result of the van der Waals nature of the graphene surface, VOPc molecules can form crystals microns in size when deposited onto a substrate with an elevated temperature of 155 C; at ambient temperatures, the crystals are only tens of nanometres across. In contrast, the functionalised graphene oxide surface prevents large crystal growth, even at elevated temperatures, because surface functionalities inhibit molecule diffusion. This highlights the importance of graphene as a substrate for molecular crystal growth, even when the growth is not epitaxial. Finally, the supramolecular assembly of trimesic acid (TMA) and terephthalic acid (TPA) is presented. Despite their chemical similarity they display different behaviour as they transition from monolayers to three-dimensional structures: for TMA, the epitaxial chicken wire structure seen at a monolayer templates up through the layers as molecules stack, until a thickness of 20 nm, when random in-plane orientations appear; on the other hand, TPA forms a brickwork structure at the monolayer, which quickly transitions to fibre-like crystals with a bulk structure for the thin films. However, the TPA orientation is still determined by the epitaxy with the graphene substrate, although this is significantly weaker than for TMA.

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23

Connelly, James Patrick. "Microwave studies of Van der Waals complexes". Thesis, University of Oxford, 1993. http://ora.ox.ac.uk/objects/uuid:3865eb1d-d288-44c9-8d42-84f7ff2c0608.

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This thesis describes the commissioning and development of a pulsed supersonic nozzle, Fourier-transform microwave spectrometer and its application to the study of several weakly bound van der Waals complexes. A pulsed supersonic expansion, Fourier-transform microwave spectrometer based on the Flygare design with a number of modifications has been constructed with an operating range of 6-18 GHz. A homodyne detection circuit mixing signals to modulus values between dc and 1 MHz is used, requiring two measurements to determine absolute transition frequencies. Transition frequencies are measured from the power spectrum by determining the first derivative zero crossing point in a least squares fitting procedure. Semiautomation of many of the spectrometer operations has been achieved allowing unattended data collection over scans of up to 300 MHz. The microwave spectrum of Ar₂-OCS and Ar₂-OC³⁴S has been observed and analysed using conventional Watson S reduction hamiltonian parameters. Effective structural parameters are derived and used in a harmonic force field analysis, based on the centrifugal distortion constants, to compare the trimer interations with a model based on the sum of dimer interactions. A series of complexes containing the nitrogen molecule undergoing tunnelling motions have been studied. Hyperfine matrix elements for the first order nuclear quadrupole interaction are derived for the coupled identical nuclei case appropriate to the rapid tunnelling motions observed. The microwave spectrum of N₂-OCS is described. Tunnelling and nuclear spin statistical effects for two symmetry states are observed arising from the interchange of nitrogen nuclei. Rotational and quadrupole constants are derived; an accidental near degeneracy of two rotational levels allows the off-diagonal quadrupole coupling constant to be determined from second order effects. A tunnelling hamiltonian fitting the quadrupole coupling constants to an angular potential has been used to calculate the tunnelling frequency and barrier to N₂ rotation. The microwave spectrum of N₂-O₃ and a preliminary spectrum of N₂-SO₃ have been observed. Rotation-inversion motions of the O₃ and SO₂ moieties must be considered in addition to the N₂ tunnelling to fit the spectrum. Tunnelling frequencies for the O₃/SO₂ and geared motions with the N₂ are derived as well as structural parameters. Modifications for production of refractory molecules and complexes by laser ablation have been made. A modified nozzle employing rods of material is used with the ablation process taking place in the nozzle throat. Modifications to obtain an expansion along the axis of the microwave cavity employ a hemispherical Fabry-Perot cavity configuration. The system has been tested on a number of diatomic molecules including PbS and CuCl.

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24

Wright, Nicholas J. "Bound states of Van der Waals trimers". Thesis, Durham University, 1998. http://etheses.dur.ac.uk/5048/.

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A method for calculating the energy levels and wave functions of floppy tri- atomic systems such as rare-gas trimers has been developed. It is based upon a potential-optimized discrete variable representation and takes into account the wide-amplitude vibrations that occur in such systems. The quadrature error that occurs in DVR calculations is analysed and a method of correction implemented. The diagonalisation procedure is based upon a combination of successive diagonalisation and truncation and a Lanczos diagonaliser. Using this method the wave functions of the Ar(_3) Van der Waals trimer have been calculated. The wave functions for the low-lying states show very regular behaviour. Above the barrier to linearity, most of the wave functions are irregular but some have simple nodal patterns that suggest localization along periodic orbits. In addition to the "horseshoe" states previously described for H(^+)(_3), localized features corresponding to symmetric and antisym metric stretching vibrations around a linear configuration have been identified. The different localized modes can be combined to form more complex states in a manner analogous to normal modes. A preliminary study of the rotational states of Ar(_3) has also been performed. The rotational constants for the low lying states of Ar(_3) reflect the increasing average size of Ar(_3) with increasing vibrational excitation. The rotational constants are obtained from two methods, expectation values and energy level differences. The results for the levels above the barrier to isomerisation reveal that the simple models used for obtaining the rotational constants are no longer valid and indicate that a more sophisticated treatment is necessary.

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25

Bryan, Robert. "Theoretical studies of Van der Waals clusters". Thesis, Durham University, 1997. http://etheses.dur.ac.uk/4712/.

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The vibrational energy levels of various rare gas trimers, Ar(_3), Ne(_3), He(_3), Ar(_2)Ne and Ne(_2)Ar, have been calculated using a coupled channel approach. We have compared results obtained with previous calculations. The existence of Efi-mov states in He(_3) has been investigated, and no evidence of their existence has been found. The affect of the Eckart conditions on embedding axis into a rotating-vibrating system has been investigated for several rare gas systems. A wide range of rare gas trimers have been studied, Ar(_3), He(_2)Ar, Ar(_2)He, Ar(_2)Ne and Ne(_2)Ar. For each trimer the full range of molecular motion is investigated. The low energy minima for the Ar(_n)N(_2) and Ne(_n)N(_2) systems have been found using simulated annealing search, and a gradient based minimisation technique, of a pairwise potential energy surface. Clusters with n ≥ 12 have been studied, and first solvation shells for both systems have been proposed. For each value of n, for n = 1 - 12, the first few low energy minima of the potential energy surface have been found. From these studies, we have gained a detailed understanding of the interplay of forces that determine the low energy structures for these systems. The affect of three-body interactions on the low energy minima both rare gas-N(_2) systems has been studied. In both system, rare gas-rare gas and rare gas- threebody interactions have been taken into account. This study has shown that the three-body forces have a small affect on the low energy structures of each system.

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26

Tulegenov, Akyl S. "SIMPER method for van der Waals complexes". Thesis, University of Nottingham, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.431329.

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27

McDowell, Sean Alistair Courtney. "Theoretical studies of Van der Waals molecules". Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259733.

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28

Le, Sueur Catherine Ruth. "Induction effects in Van der Waals complexes". Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.385523.

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29

Willberg, Dean Michael Zewail Ahmed H. "Picosecond spectroscopy of van der Waals clusters /". Diss., Pasadena, Calif. : California Institute of Technology, 1994. http://resolver.caltech.edu/CaltechETD:etd-04042008-110156.

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30

Columberg, Gieri. "Mikrowellen-Spektroskopie T-förmiger Van der Waals Komplexe /". [S.l.] : [s.n.], 1996. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=11636.

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31

Coy, Diaz Horacio. "Preparation and Characterization of Van der Waals Heterostructures". Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6212.

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In this dissertation different van der Waals heterostructures such as graphene-MoS2 and MoTe2-MoS2 were prepared and characterized. In the first heterostructure, polycrystalline graphene was synthesized by chemical vapor deposition and transferred on top of MoS2 single crystal. In the second heterostructure, MoTe2 monolayers were deposited on MoS2 by molecular beam epitaxy. Characterization of graphene-MoS2 heterostructures was conducted by spin and angle resolve spectroscopy which showed that the electronic structure of the bulk MoS2 and graphene in this van der Waals heterostructures is modified. For MoS2 underneath the graphene, a band structure renormalization and spin polarization are observed. The band structure of MoS2 is modified because the graphene induces screening which shifts the Г-point ~150 meV to lower binding compared to the sample without graphene. The spin polarization is explained by the dipole arising from band bending which breaks the symmetry at the MoS2 surface. For graphene, the band structure at lower binding energy shows that the Dirac cone remains intact with no significant doping. Instead, away from the Fermi level the formation of several gaps in the pi-band due to hybridization with states from the MoS2 is observed. For the heterostructures made depositing monolayer of MoTe2 on MoS2, the morphology, structure and electronic structure were studied. Two dimensional growth is observed under tellurium rich growth conditions and a substrate temperature of 200 °C but formation of a complete monolayer was not achieved. The obtained MoTe2 monolayer shows a high density of the mirror-twins grain boundaries arranged in a pseudo periodic wagon wheel pattern with a periodicity of ~2.6 nm. These grain boundary are formed due to Te-deficiency during the growth. The defect states from these domain boundary pin the Fermi level in MoTe2 and thus determine the band alignment in the MoTe2-MoS2 heterostructures.

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32

Lawrence, Stuart John. "High-resolution spectroscopy of van der Waals molecules". Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318824.

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33

Kettley, J. C. "Van der Waals complexes of large aromatic molecules". Thesis, University of Nottingham, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371128.

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34

Althorpe, Stuart C. "Bound state calculations for van der Waals dimers". Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319937.

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35

Ma, Qiong Ph D. Massachusetts Institute of Technology. "Optoelectronics of graphene-based Van der Waals heterostructures". Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104523.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Research on van der Waals (vdW) materials (homo- or hetero-) is a rapidly emerging field in condensed matter physics. They are layered structures with strong chemical bonding within layers and relatively weak van der Waals force to combine layers together. This unique layer-bylayer nature makes it easy to exfoliate layers out and at the same time to re-assemble in arbitrary sequences with different combinations. The versatility, flexibility, and relatively low cost of production make the scientific community enthusiastic about their future. In this thesis, I investigate the fundamental physical processes of light-matter interactions in these layered structures, including graphene, boron nitride, transition metal dichalcogenides and heterostructures formed from these materials. My research involves state-of-the-art nanoscale fabrication and microscale photocurrent spectroscopy and imaging. In Chapter 1, 1 will briefly discuss basic physical properties of the vdW materials involved in this thesis and introduce the main nanofabrication and measurement techniques. Chapter 2-4 are about hot electron dynamics and electron-phonon coupling in intrinsic graphene systems, among which Chapter 2 is focusing on the generation mechanism of the photocurrent at the p-n interface, which is demonstrated to have a photothermoelectric origin. This indicates a weak electron-phonon coupling strength in graphene. Chapter 3 is a direct experimental follow-up of the work in Chapter 2 and reveals the dominant electron-phonon coupling mechanism at different temperature and doping regimes. In Chapter 4, I present the observation of anomalous geometric photocurrent patterns in various devices at the charge neutral point. The spatial pattern can be understood as a local photo-generated current near edges being collected by remote electrodes. The anomalous behavior as functions of change density and temperature indicates an interesting regime of energy and charge dynamics. In Chapter 5 and 6, 1 will show the photoresponse of graphene-BN heterostuctures. In graphene-BN stack directly on SiO₂, we observed strong photo-induced doping phenomenon, which can be understood as charge transfer from graphene across BN and eventually trapped at the interface between BN and SiO₂. By inserting another layer of graphene between BN and SiO₂ , we can measure an electrical current after photoexcitation due to such charge transfer. We further studied the competition between this vertical charge transfer and in-plane carrier-carrier scattering in different regimes. In Chapter 7, I will briefly summarize collaborated work with Prof. Dimitri Basov's group on near-field imaging of surface polariton in two-dimensional materials. This technique provides a complementary tool to examine the intriguing light-matter interaction (for large momentum excitations) in low-dimensional materials. Chapter 8 is the outlook, from my own point of view, what more can be done following this thesis.
by Qiong Ma.
Ph. D.

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36

Waage, Magnus Heskestad. "Radiative corrections to van der Waals interaction in fluids". Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for fysikk, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18872.

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The van der Waals interaction energy is derived for a homogenous fluid of polarizable particles. Low-temperature corrections to the van der Waals interaction energy are computed numerically for three cases: Dilute media with radiative interactions, arbitrarily dense media without radiative interactions, and arbitrarily dense media with radiative interactions. The validity of the model used to calculate the corrections at arbitrary densities is argued for, by comparison with the results for low densities.

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37

DelRio, Frank William. "Van der Waals and capillary adhesion in microelectromechanical systems". Diss., Connect to online resource, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3239374.

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38

Peet, Andrew Charles. "Vibrational predissociation of Van der Waals complexes containing ethylene". Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329168.

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39

Howson, Joanna M. M. "Obtaining potential energy surfaces of Van der Waals molecules". Thesis, Durham University, 1999. http://etheses.dur.ac.uk/4488/.

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Two different methods were used to obtain a potential energy surface for the Arco molecule. One involved choosing a functional form for the repulsion and dispersion energies whose parameters were determined by a fit to experimental data. A physically justified potential that agreed with experiment could not be obtained. The other method was based on calculating ab initio interaction energies at different configurations of the molecule and interpolating between them. The resulting surface was scaled in the energy and the co-ordinates. Improved agreement was achieved for most observed bound states. Errors in the surface may have been due to an inadequate density of ab initio energies. So, how the molecular configurations chosen for interaction energy calculations affected the potential obtained was investigated. Both the co-ordinate system and the interpolation scheme also significantly affected the quality of surface obtained. The best compromise between accuracy and number of configurations, was points distributed on a regular grid in elliptic co-ordinates with Gauss-Legendre quadrature points in the angular co-ordinate. This information was employed to obtain a potential energy surface for the weakly bound HeOCS molecule which was in close agreement with experiment. A co-ordinate and energy scaling, guided by experiment, was applied to the ab initio surface. Only three scaling parameters were required due to the high quality of the initial surface.

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40

Sanz-Garcia, Aranzazu. "Modelling the dispersion energy for Van der Waals complexes". Thesis, University of Nottingham, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.252013.

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41

Musgrave, Adam. "Electronic spectroscopy of Van der Waals clusters and complexes". Thesis, University of Nottingham, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.445684.

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42

Khestanova, Ekaterina. "Van der Waals heterostructures : fabrication, mechanical and electronic properties". Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/van-der-waals-heterostructures-fabrication-mechanical-and-electronic-properties(047ce24b-7a58-4192-845d-54c7506f179f).html.

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The fast progress in the exploration of 2D materials such as graphene became possible due to development of fabrication techniques that allowed these materials to be protected from e.g. undesirable doping and gave rise to new functionalities realized within van der Waals heterostructures. Attracted by van der Waals interaction the constituent layers of such heterostructures preserve their exceptional electronic quality and for example in graphene allow for high electron mobility to be achieved. However, the studies of atomically thin layers such as NbSe2 that exhibit metallic behavior have been impeded by their reactivity and hence oxidation during exposure to ambient or oxidizing agents such as solvents. In this thesis, the existing heterostructure assembly technique was improved by the introduction of exfoliation and re-stacking by a fully motorized system placed in an inert atmosphere. This approach allowed us to overcome the problem of environmental degradation and create Hall bars and planar tunnel junctions from atomically thin superconducting NbSe2. Furthermore, this versatile approach allowed us to study the thickness dependence of the normal and superconducting state transport properties of NbSe2, uncovering the reduction of the superconducting energy gap and transition temperature in the thinnest samples. On the other hand, 2D materials being just 1-3 atoms thick represent an ultimate example of a membrane - thin but laterally extended object. Consisting of such atomically thin membranes the van der Waals heterostructures can be used for purposes other than the studies of electronic transport. In this work, ubiquitous bubbles occurring during van der Waals heterostructure assembly are employed as a tool to explore 2D materials' mechanical properties and mutual adhesion. This allowed us to measure Young's modulus of graphene and other 2D materials under 1-2% strain and deduce the internal pressure that can reach up to 1 GPa in sub-nanometer size bubbles.

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43

Schofield, Robert Christopher. "Raman studies of 2-dimensional van der Waals materials". Thesis, University of Sheffield, 2018. http://etheses.whiterose.ac.uk/21313/.

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Presented herein are results of optical studies with emphasis on the Raman response, providing significant contribution to the knowledge of the field. In Mox W(1−x) S2 , confirmation of the behaviour of the excitonic properties is made. Raman measurements performed in this system allow investigation with unprecedented resolution, highlighting deviations in the high frequency A1g optical phonon mode from theoretical predictions, and previous experimental studies. In the low frequency, data confirms the trends in the shear and breathing interlayer modes in alloys between WS2 and MoS2 are well described by the modification in the square density. Resonant excitation for [Mo] < 0.4, highlights new evidence for the understanding of the hitherto unexplained ‘Peak X’ resonant feature. Diverse indium-selenium compounds isolated by novel means are studied. The ULF Raman modes of PDMS exfoliated InSe are documented for the first time, demonstrating the ε-phase with ABA stacking, with flake of thickness N manifesting (N − 1) shear modes owing to resonant excitation of few layer samples. InSe flakes encapsulated in hexagonal boron nitride manifest different stacking orders to those of PDMS exfoliated InSe, and were found to have significant contamination, with crystalline degradation of the monolayer flake, and peaks corresponding N2 & O2 rotational modes present. In2Se3 films grown epitaxially on GaSe display substrate-selective polymorphism, where α-, β-, & γ- phases are identified, in addition to regions of InSe. Laser photo-annealing is shown to drive a phase change from the groundstate β → α phase, which is against the thermodynamic gradient. MoSe2/WS2 twisted hetero-bi-layer structures are studied, where shear modes showing a linear softening from AA′ stacking towards the AB at 60° indicating reduced interlayer coupling, as expected from the difference in interlayer spacing of AA′ and AB ordering. High frequency modes in the heterobilayer also demonstrate some sensitivity in the relative angle, and are analysed in detail.

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44

David, Lamuel Abraham. "Van der Waals sheets for rechargeable metal-ion batteries". Diss., Kansas State University, 2015. http://hdl.handle.net/2097/32796.

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Doctor of Philosophy
Department of Mechanical and Nuclear Engineering
Gurpreet Singh
The inevitable depletion of fossil fuels and related environmental issues has led to exploration of alternative energy sources and storage technologies. Among various energy storage technologies, rechargeable metal-ion batteries (MIB) are at the forefront. One dominant factor affecting the performance of MIB is the choice of electrode material. This thesis reports synthesis of paper like electrodes composed for three representative layered materials (van der Waals sheets) namely reduced graphene oxide (rGO), molybdenum disulfide (MoS₂) and hexagonal boron nitride (BN) and their use as a flexible negative electrode for Li and Na-ion batteries. Additionally, layered or sandwiched structures of vdW sheets with precursor-derived ceramics (PDCs) were explored as high C-rate electrode materials. Electrochemical performance of rGO paper electrodes depended upon its reduction temperature, with maximum Li charge capacity of 325 mAh.g⁻¹ observed for specimen annealed at 900°C. However, a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. More importantly, annealing of GO in NH₃ at 500 °C showed negligible cyclability for Na-ions while there was improvement in electrode's Li-ion cycling performance. This is due to increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH₃. Further enhancement in rGO electrodes was achieved by interfacing exfoliated MoS₂ with rGO in 8:2 wt. ratios. Such papers showed good Na cycling ability with charge capacity of approx. 225.mAh.g⁻¹ and coulombic efficiency reaching 99%. Composite paper electrode of rGO and silicon oxycarbide SiOC (a type of PDC) was tested as high power-high energy anode material. Owing to this unique structure, the SiOC/rGO composite electrode exhibited stable Li-ion charge capacity of 543.mAh.g⁻¹ at 2400 mA.g⁻¹ with nearly 100% average cycling efficiency. Further, mechanical characterization of composite papers revealed difference in fracture mechanism between rGO and 60SiOC composite freestanding paper. This work demonstrates the first high power density silicon based PDC/rGO composite with high cyclic stability. Composite paper electrodes of exfoliated MoS₂ sheets and silicon carbonitride (another type of PDC material) were prepared by chemical interfacing of MoS₂ with polysilazane followed by pyrolysis . Microscopic and spectroscopic techniques confirmed ceramization of polymer to ceramic phase on surfaces on MoS₂. The electrode showed classical three-phase behavior characteristics of a conversion reaction. Excellent C-rate performance and Li capacity of 530 mAh.g⁻¹ which is approximately 3 times higher than bulk MoS₂ was observed. Composite papers of BN sheets with SiCN (SiCN/BN) showed improved electrical conductivity, high-temperature oxidation resistance (at 1000 °C), and high electrochemical activity (~517 mAh g⁻¹ at 100 mA g⁻¹) toward Li-ions generally not observed in SiCN or B-doped SiCN. Chemical characterization of the composite suggests increased free-carbon content in the SiCN phase, which may have exceeded the percolation limit, leading to the improved conductivity and Li-reversible capacity. The novel approach to synthesis of van der Waals sheets and its PDC composites along with battery cyclic performance testing offers a starting point to further explore the cyclic performance of other van der Waals sheets functionalized with various other PDC chemistries.

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Gée, Christelle. "Reactions chimiques isolees sur agregats de van der waals". Paris 11, 1997. http://www.theses.fr/1997PA112092.

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Cette these porte sur l'etude des collisions reactives ou non reactives en presence d'un milieu materiel: un agregat forme de plusieurs centaines d'atomes ou molecules. L'agregat par sa taille finie permet d'isoler un couple de reactifs qui peuvent migrer sur l'agregat, entrer en collision et eventuellement declencher une reaction chimique. Notre dispositif experimental permet de generer un faisceau d'agregats homogenes (d'argon, de methane et d'azote) et d'y deposer les reactifs en faible quantite, par une technique collisionnelle. Nous etudions ces reactions chimiques isolees a la fois par des diagnostics optiques et par spectrometrie de masse. Les travaux realises sur les systemes reactionnels (ba/n#2o), (ba/co#2), (ba/sf#6) et (ba/ch#4) font appel a une technique d'analyse introduite dans cette these: la methode c. I. C. R. Acronyme de cluster isolated chemical reaction. Sa puissance reside dans la capacite a isoler une reaction sur un agregat pour en determiner la stoechiometrie. Nous avons suivi 2 orientations dans cette these. La premiere orientation porte sur l'effet de solvatation en observant l'influence de l'agregat sur une reaction chimique. L'agregat est parfois spectateur (reaction ba+n#2o), mais il peut aussi perturber considerablement, voire inhiber la reaction comme pour les paires 1 ba+1 sf#6 et 1 ba+1 co#2 ou l'agregat bloque la reaction en voie d'entree. Le deuxieme theme aborde est la reactivite de petits agregats de baryum. Nous avons etudie l'evolution de leur reactivite en fonction de leur taille et du caractere oxydo/reducteur du reactif moleculaire (cl#2, n#2o, o#2, co#2, sf#6, ch#4). A cette occasion, l'etude des systemes ba#2+co#2 et ba#2+n#2o a permis d'assigner un spectre de chimiluminescence jusque la non assigne: celui de ba#2o. La technique c. I. C. R. A montre toute sa force sur cet exemple en permettant l'identification du produit de reaction grace a la comprehension de la stoechimetrie de la reaction

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46

Yu, Geliang. "Transport properties of graphene based van der Waals heterostructures". Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/transport-properties-of-graphene-based-van-der-waals-heterostructures(5cbb782f-4d49-42da-a05e-15b26606e263).html.

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In the past few years, led by graphene, a large variety of two dimensional (2D) materials have been discovered to exhibit astonishing properties. By assembling 2D materials with different designs, we are able to construct novel artificial van der Waals (vdW) heterostructures to explore new fundamental physics and potential applications for future technology. This thesis describes several novel vdW heterostructures and their fundamental properties. At the beginning, the basic properties of some 2D materials and assembled vdW heterostructures are introduced, together with the fabrication procedure and transport measurement setups. Then the graphene based capacitors on hBN (hexagonal Boron Nitride) substrate are studied, where quantum capacitance measurements are applied to determine the density of states and many body effects. Meanwhile, quantum capacitance measurement is also used to search for alternative substrates to hBN which allow graphene to exhibit micrometer-scale ballistic transport. We found that graphene placed on top of MoS2 and TaS2 show comparable mobilities up to 60,000cm2/Vs. After that, the graphene/hBN superlattices are studied. With a Hall bar structure based on the superlattices, we find that new Dirac minibands appear away from the main Dirac cone with pronounced peaks in the resistivity and are accompanied by reversal of the Hall effects. With the capacitive structure based on the superlattices, quantum capacitance measurement is used to directly probe the density states in the graphene/hBN superlattices, and we observe a clear replica spectrum, the Hofstadter-butterfly fan diagram, together with the suppression of quantum Hall Ferromagnetism. In the final part, we report on the existence of the valley current in the graphene/hBN superlattice structure. The topological current originating from graphene’s two valleys flows in opposite directions due to the broken inversion symmetry in the graphene/hBN superlattice, meaning an open band gap in graphene.

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Gani, Yohanes Satrio. "Electronic Properties of Two-Dimensional Van Der Waals Systems". W&M ScholarWorks, 2019. https://scholarworks.wm.edu/etd/1563899012.

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In this dissertation we study the electronic structure of van der Waals systems. A van der Waals systems is a heterostructure in which the different constituents are held together by van der Waals forces. We study two different types of van der Waals systems: van der Waals systems formed by graphene and a monolayer of NbSe2, van der Waals systems obtained by placing graphene nanoribbons on a two-dimensional crystal. For the first type of systems we build a continuous low-energy effective model that takes into account the presence of a twist angle between graphene and NbSe2, and of spin-orbit coupling and superconducting pairing in NbSe2. We then obtain how the superconducting pairing induced by proximity into the graphene layer depends on the twist angle. For the second type of systems we obtain using ab-initio methods the electronic structure of graphene nanoribbons placed on hexagonal boron nitride, and of graphene nanoribbons placed on monolayers of transition metal dichalcogenide. For both cases we show how the electronic structure depends on the stacking configuration.

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48

Tomarken, Spencer Louis. "Thermodynamic and tunneling measurements of van der Waals heterostructures". Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/123567.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 201-212).
In certain electronic systems, strong Coulomb interactions between electrons can favor novel electronic phases that are difficult to anticipate theoretically. Accessing fundamental quantities such as the density of states in these platforms is crucial to their analysis. In this thesis, I explore the application of two measurement techniques towards this goal: capacitance measurements that probe the thermodynamic ground state of an electronic system and planar tunneling measurements that access its quasiparticle excitation spectrum. Both techniques were applied to van der Waals materials, a class of crystals composed of layered atomic sheets with weak interplane bonding which permits the isolation of single and few-layer sheets that can be manually assembled into heterostructures. Capacitance measurements were performed on a material system commonly known as magic-angle twisted bilayer graphene (MATBG).
When two monolayers of graphene, a single sheet of graphite, are stacked on top of one another with a relative twist between their crystal axes, the resultant band structure is substantially modified from the cases of both monolayer graphene and Bernal-stacked (non-twisted) bilayer graphene. At certain magic angles, the low energy bands become extremely flat, quenching the electronic kinetic energy and allowing strong electron-electron interactions to become relevant. Exotic insulating and superconducting phases have been observed using conventional transport measurements. By accessing the thermodynamic density of states of MATBG, we estimate its low energy bandwidth, Fermi velocity, and interaction-driven energy gaps. Time-domain planar tunneling was performed on a heterostructure that consisted of monolayer graphene and hexagonal boron nitride (serving as the dielectric and tunnel barrier) sandwiched between a graphite tunneling probe and metal gate.
Tunneling currents were induced by applying a sudden voltage pulse across the full parallel plate structure. The lack of in-plane charge motion allowed access to the tunneling density of states even when the heterostructure was electrically insulating in the quantum Hall regime. These measurements represent the first application of time-domain planar tunneling to the van der Waals class of materials, an important step in extending the technique to new material platforms.
by Spencer Louis Tomarken.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics

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Scheele, Iris. "Hochauflösende Infrarot-Spektroskopie an schwach gebundenen Van-der-Waals-Systemen". [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=963558668.

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50

Quayle, Christopher John Kendrick. "Alignment effects in the photodissociation of van der Waals molecules". Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357431.

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Tesi sul tema "Matériaux de Van der Waals"

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