The ability to isolate single atomic layers of van der Waals materials has led to renewed interest in the electronic and optical properties of these materials as they can be fundamentally different at the monolayer limit. Moreover, these 2D crystals can be assembled together layer by layer, with controllable sequence and orientation, to form artificial materials that exhibit new features that are not found in monolayers nor bulk. Twisted bilayer graphene is one such prototype system formed by two monolayer graphene layers placed on top of each other with a twist angle between their lattices, whose electronic band structure depends on the twist angle. This thesis presents the efforts to explore the electronic and optical properties of twisted bilayer graphene by Raman spectroscopy and scanning tunneling microscopy measurements. We first synthesize twisted bilayer graphene with various twist angles via chemical vapor deposition. Using a combination of scanning tunneling microscopy and Raman spectroscopy, the twist angles are determined. The strength of the Raman G peak is sensitive to the electronic band structure of twisted bilayer graphene and therefore we use this peak to monitor changes upon doping. Our results demonstrate the ability to modify the electronic and optical properties of twisted bilayer graphene with doping. We also fabricate twisted bilayer graphene by controllable stacking of two graphene monolayers with a dry transfer technique. For twist angles smaller than one degree, many body interactions play an important role. It requires eight electrons per moire unit cell to fill up each band instead of four electrons in the case of a larger twist angle. For twist angles smaller than 0.4 degree, a network of domain walls separating AB and BA stacking regions forms, which are predicted to host topologically protected helical states. Using scanning tunneling microscopy and spectroscopy, these states are confirmed to appear on the domain walls when inversion symmetry is broken with an external electric field. We observe a double-line profile of these states on the domain walls, only occurring when the AB and BA regions are gaped. These states give rise to channels that could transport charge in a dissipationless manner making twisted bilayer graphene a promising platform to realize controllable topological networks for future applications.

La caractérisation des complexes de van der Waals est déterminante pour la compréhension et la modélisation des atmosphères planétaires et du milieu interstellaire. Cependant, ces agrégats moléculaires sont assez peu étudiés, particulièrement dans l’infrarouge proche. Cette thèse a pour but d’enrichir les connaissances sur les complexes d’eau et d’ammoniac dans cette région spectrale. Pour cela, nous avons utilisé un montage expérimental appellé FANTASIO+, couplant une expansion su- personique à un spectromètre à temps de déclin (CRDS). Plusieurs complexes de van der Waals ont pu être observés et analysés, à savoir 14/15NH3−Ar/Kr, (H2O)2, H2O−Ar/Kr et HDO−N2O.Ces complexes peuvent être décrits par deux régimes selon la hauteur de la barrière du potentiel intermoléculaire. Les complexes caractérisés par un potentiel inter- moléculaire élevé peuvent être traités par le régime semi-rigide, à savoir comme une molécule à part entière avec des constantes rotationelles bien définies [(H2O)2 et HDO−N2O]. Dans le régime du rotateur libre, au contraire, le monomère du complexe est soumis à une faible barrière de potentiel, et donc sa rotation est pra- tiquement libre (tous les complexes 14/15NH3− et H2O−gaz rare).Le déplacement et la séparation des niveaux d’énergie robivrationels du monomère dépend de l’effet de l’anisotropie du potentiel intermoléculaire par rapport au lien intermoléculaire. Les transitions entre ces sous-états sont appelées des sous-bandes.Les spectres observés dans ce travail de thèse correspondent à l’excitation vibra- tionnelle de 14/15NH3, H2O ou HDO. L’analyse rotationelle des sous-bandes a pu être réalisée pour toutes les espèces (y compris les 4 isotopes principaux du krypton dans H2O−Kr), excepté pour 15NH3−Kr car la sous-bande observée était à peine visible. Nous avons pu caractériser la structure rotationnelle de 1 (15NH3−Ar) à 14 (H2O−Ar) sous-états vibrationnellement excités et de l’état fondamental pour toutes les espèces sauf 14NH3−Kr, 15NH3−Ar/Kr et (H2O)2. Notre travail sur ce dernier, basé sur des calculs théoriques et sur une étude similaire en matrice de néon, est la première analyse rotationelle de ce complexe dans cette région spectrale. Les analyses des spectres de H2O−Ar et HDO−N2O s’appuient sur des calculs ab initio. Pour HDO−N2O, nous avons pu déterminer que le lien OD se trouve plus proche de N2O que le lien OH. Les spectres de 14/15NH3−Ar/Kr présentent des sous-bandes appartenant à des complexes de taille plus importante. Un séjour à l’Université d’Alberta a été organisé afin d’identifier la nature de ces complexes.Quand l’analyse rotationelle a pu être réalisée, nous avons déterminé les temps de vie de prédissociation des sous-états excités (sauf pour HDO−N2O car le signal-sur- bruit était trop faible). Ils sont de 20 ps pour (H2O)2 à 4 ns pour H2O−Ar. Nous avons observé que les temps de vie déterminés étaient inversement proportionnels à l’énergie de dissociation des complexes. Le déplacement observé entre l’origine des sous-bandes par rapport à la bande correspondante du monomère suit égale- ment la même tendance que l’énergie de dissociation du complexe. Nous avons pu déterminer les températures rotationnelles pour toutes les espèces dans le jet super- sonique. Celles-ci sont différentes uniquement dans le cas de 14NH3−Ar.Des spectres des monomères 14NH3 et 15NH3 ont également été enregistrés, notamment en utilisant un spectromètre à transformée de Fourier, enrichissant ainsi les données spectroscopiques de ces molécules dans l’infrarouge proche. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Abstract This thesis focuses on the molecular-level chemistry of the solvation of aluminum salts. Fundamental aspects such as, structural characteristics of the aluminum molecules, hydrolysis, acidity, solvation structure, effect of counter ions, and chemical stability are discussed herein. Static computations augmented with the conductor-like screening model (COSMO) were used to investigate hundreds of planar and cyclic configurations of dimeric, trimeric, tetrameric, and pentameric aluminum complexes. Car–Parrinello molecular dynamics (CPMD) calculations were used to expand investigations to aqueous environments. This thesis consists of four articles and one additional article. The first paper focuses on the structural analysis of the hydrolysis products of AlCl3 · 6H2O. Dimeric, trimeric, and tetrameric aluminum (chloro)hydroxides were investigated in both gas and liquid phase. The liquid environment was modeled by using COSMO. The second and the additional paper concentrate on the chemistry of aluminum sulfate complexes. The second article focuses on identifying hydrolysis products of AlCl3 · 6H2O in the presence of sulfate (H2SO4). The additional paper focuses on the structural characteristics of the hydrolysis products of Al2 (SO4)3 · 18H2O. Structural information was deduced from the ESI MS results with the aid of computational methods. Detected cationic structures closely resembled the aluminum chlorohydrate analogues introduced in the first paper. The third and fourth articles are devoted to the hydrolysis, stability, and dynamics of dimeric and pentameric aluminum (chloro)hydroxides in aquatic environments. During the CPMD simulations, several spontaneous associative hydration reactions were detected in the primary hydration shell of the complexes. Dimeric aluminum chlorohydrates were detected to be stable in liquid conditions, whereas the pentameric aluminum complexes experienced significant topological changes during the simulations. Constrained simulations were used to reveal the role of chloride ions in the hydrolysis processes of dimeric complexes. The effect of the empirical van der Waals corrections to the dynamics of the simulations was also tested for the pentameric system. The results of this thesis showed unequivocally that computational chemistry provides effective tools for structural analysis of inorganic complexes such as, aluminum chlorohydrates and sulfates in both gas and liquid phase. In addition, calculations provided answers to the anomalies detected in the experiments. Hence, theoretical methods are highly recommended to be used alongside with conventional experimental methods in the interpretation of the aluminum species in aqueous solutions and to widen the overall chemical perspective of the hydrolysis of aluminum salts.

Au cours de cette thèse nous avons utilisé la théorie de la fonctionnelle de la densité et les calculs Monte Carlo quantiques pour analyser l'impact des effets de van der Waals sur la structure, l'énergie, et les propriétés des polymorphes de SiO2 et B2O3. Nous avons mis en évidence un phénomène de compensation d'erreur, lié à l'utilisation de fonctionnelle d'échange et corrélation incluant les effets de van der Waals, dans les polymorphes basse densité de SiO2 entre une sur-évaluation des longueurs Si-O et une sous-estimation des angles Si-O-Si. Nous avons effectué des calculs Monte-Carlo quantiques afin de prédire la structure et l'énergie d'un nouveau polymorphe de B2O3 avec une grande précision, ce qui nous a permis d'évaluer les performances de différentes fonctionnelles d'échange et corrélation sur B2O3. Nous avons ensuite utilisé la théorie de la fonctionnelle de la densité pour prédire la structure et l'énergie de 25 nouveaux polymorphes de B2O3 , ainsi que leurs propriétés mécaniques et électroniques. Cette étude permet de proposer une explication à l'anomalie de cristallisation de B2O3, et réconcilie le comportement de B2O3 avec celui des autres oxydes formateurs de réseaux. Elle souligne la possibilité de créer des borates cristallins aux propriétés mécaniques remarquables, et confirme qu'il existe lien entre polymorphisme de basse énergie et facilité de vitrification. / During this PhD I use density functional theory and quantum Monte Carlo to evaluate the importance of van der Waals effects on the structures, the energies, and the properties of SiO2 and B2O3 polymorphs. I show that exchange-correlation functionals including dispersion effects lead to an error cancellation between an overestimation of the Si-O distances and an underestimation of the Si-O-Si angles in low densities SiO2 polymorphs. By using quantum Monte Carlo calculations, I have predicted with high accuracy the relative energy of a new B2O3 polymorph, which allowed me to evaluate the performances of different exchange-correlation functionals on this material. I then use the best functional possible to compute the mechanical and electronic properties of 25 predicted B2O3 polymorphs. Some of the predicted polymorphs exhibit intriguing mechanical properties, such as negative linear compressibility, auxeticity and anisotropy. These calculations allow me to make a hypothesis explaining the crystallization anomaly in B2O3. They underline a seemingly universal link between low energy polymorphism and ease of vitrification.

La plupart des fonctions énergie potentielle utilisées pour simuler les systèmes biologiques complexes ne traitent qu'implicitement la polarisation électronique et ce, de façon très incomplète. Bien qu'efficaces pour un large éventail d'applications, ces champs de force atteignent rapidement leurs limites dès lors que les effets de polarisation électronique sont importants. Tel est le cas par exemple au site actif des métalloprotéines où l'ion métallique polarise fortement son environnement. Dans cette thèse, j'ai développé une approche basée sur la mécanique quantique pour obtenir des paramètres d'un champ de force polarisable ayant pour composantes des charges, des polarisabilités distribuées d'ordre zéro et un (isotrope) et un potentiel de van der Waals décrit par une fonction de Buckingham. L'énergie d'induction peut être atténuée par une fonction de Tang et Toennies pour décrire l'énergie d'échange-induction. Cette approche a été effectuée avec succès pour l'interaction d'ions avec l'eau et le benzène mais aussi dans le cas d'un dimère d'eau. Une première étude des résultats en dynamique moléculaire montre que les paramètres obtenus en phase gazeuse peuvent se transférer pour les simulations en phase condensée / Most of the energy potential functional used in biological systems only treat electronic polarization implicitely and this, in an incomplete way. Even very effective for many applications, those force fields reach there limits when the polarization effects are important. This is the case, for example, at the active site of metalloproteins where the metallic ion heavily polarizes its environment. We will present a strategy to take into account the polarization through polarizability distributed on atoms obtained by quantum chemical calculations. The interaction energies have been compared with a reference SAPT (\textit{Symmetry Adapted Perturbation Theory}) calculation which permits to expand the interaction in electrostatics, induction and van der Waals contributions. I will present the interaction of ions with water, benzene and also water dimer interactions. Preliminary results in molecular dynamics seems to confirm that gas phase parameters can be transfered to condensed phase

Carbon nanotubes have unique electronic, optical, mechanical, and transport properties which make them an important element of nanoscience and nanotechnology. However, successful application and integration of carbon nanotubes into new nanodevices requires fundamental understanding of their property changes under the influence of many external factors. This dissertation presents qualitative and quantitative theoretical understanding of property changes, while carbon nanotubes are exposed to the deformations, defects, external electric fields, and adsorption. Adsorption mechanisms due to Van der Waals dispersion forces are analyzed first for the interactions of graphitic materials and biological molecules with carbon nanotubes. In particular, the calculations are performed for the carbon nanotubes and graphene nanoribbons, DNA bases, and their radicals on the surface of carbon nanotubes in terms of binding energies, structural changes, and electronic properties alterations. The results have shown the importance of many-body effects and discrete nature of system, which are commonly neglected in many calculations for Van der Waals forces in the nanotube interactions with other materials at nanoscale. Then, the effect of the simultaneous application of two external factors, such as radial deformation and different defects (a Stone Wales, nitrogen impurity, and mono-vacancy) on properties of carbon nanotubes is studied. The results reveal significant changes in mechanical, electrical, and magnetic characteristics of nanotubes. The complicated interplay between radial deformation and different kinds of defects leads to the appearance of magnetism in carbon nanotubes which does not exist in perfect ones. Moreover, the combined effect of radial deformation and external electric fields on their electronic properties is shown for the first time. As a result, metal-semiconductor or semiconductor-metal transitions occur and are strongly correlated with the strength and direction of external electric field and the degree of radial deformations.

abstract: Liquid-phase exfoliation (LPE) is a straightforward and scalable method of producing two-dimensional nanomaterials. The LPE process has typical been applied to layered van der Waals (vdW) solids, such as graphite and transition metal dichalcogenides, which have layers held together by weak van der Waals interactions. However, recent research has shown that solids with stronger bonds and non-layered structures can be converted to solution-stabilized nanosheets via LPE, some of which have shown to have interesting optical, magnetic, and photocatalytic properties. In this work, two classes of non-vdW solids – hexagonal metal diborides and boron carbide – are investigated for their morphological features, their chemical and crystallographic compositions, and their solvent preference for exfoliation. Spectroscopic and microscopic techniques are used to verify the composition and crystal structure of metal diboride nanosheets. Their application as mechanical fillers is demonstrated by incorporation into polymer nanocomposite films of polyvinyl alcohol and by successful integration into liquid photocurable 3D printing resins. Application of Hansen solubility theory to two metal diboride compositions enables extrapolation of their affinities for certain solvents and is also used to find solvent blends suitable for the nanosheets. Boron carbide nanosheets are examined for their size and thickness and their exfoliation planes are computationally analyzed and experimentally investigated using high-resolution transmission electron microscopy. The resulting analyses indicate that the exfoliation of boron carbide leads to multiple observed exfoliation planes upon LPE processing. Overall, these studies provide insight into the production and applications of LPE-produced nanosheets derived from non-vdW solids and suggest their potential application as mechanical fillers in polymer nanocomposites. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

The evolution of electronics has become the staple thrust of modern scientific innovation: a need for advancing materials engineered for our equally rapidly advancing needs and computing requirements has fueled recent wealth of new materials. Here, I use the ideals of exotic materials design to answer this need, specifically for 2D materials. Two-dimensional (2D) van der Waals materials with in-plane anisotropy are of great interest for directional transport of charge and energy. I perform solid state synthesis to produce several such materials: an intrinsic antiferromagnet, superatomic semiconductors, and a polytype system with a component that displays the possibilities of Weyl nodes.The former, chromium sulfur bromide (CrSBr), is first synthesized, then fully studied structurally, compositionally, electronically, and magnetically. Second harmonic generation (SHG), more advanced than older techniques such as magneto-optical Kerr spectroscopy or Raman spectroscopy, allows us to fully understand the magnetic symmetry in this system as an interlayer antiferromagnetic and intralayer ferromagnetic in-plane anisotropic material. I also introduce published work in which we integrate CrSBr into different devices to show the utility of this fundamental research into a more practical application setting. It is used to stimulate more magnetic response from graphene — promising ultra-thin magnetic memory or sensory devices in future projects. Applying strain and external magnetic fields provides another tuning knob through which to access different functional modalities. In the latter third of this dissertation, we report a layered van der Waals semiconductor with in-plane anisotropy built upon the superatomic units of Mo₆S₃Br₆ (MSB), a robust construction with a direct gap of 1.64 eV. Next, MSB and Re₆Se₈Cl₂, another analogous superatomic vdW material, are potential candidates for optoelectronic applications; we qualify this by studying their Auger dynamics as a measure of quantum efficiency. Finally, layered van der Waals (vdW) materials belonging to the MM’Te₄ structure class have recently received intense attention due to their ability to host exotic electronic transport phenomena, such as in-plane transport anisotropy, Weyl nodes, and superconductivity. In summary, we have discovered two ternary exfoliatable vdW TMD polytypes with the composition TaFeTe₄, one of which (ꞵ) shows the prerequisite symmetry elements to be a type-II Weyl semimetal. This dissertation is a treatise to solid state synthesis, exploration into the more exotic spectrum of 2D materials, and robust and eclectic methods used to paint a full picture of different magnetic and electronic systems within.

This licentiate thesis addresses two important aspects of the van der Waals and the Casimir-Polder ground-state and excited-state (resonance) interactions between two atoms or molecules. The first is the finite-size effect and the second is the non-perturbative effect. Going beyond the usual assumption of atoms and molecules as point particles and adopting a description of finite size, the divergence inherent in such interaction energies in the limit of zero separation distance between the two interacting atoms or molecules is removed. The attainment of finite interaction energy at such close separation distance facilitates the estimation of van der Waals force contribution to the binding energy of the molecules, and towards surfaces. This is particularly important for noble atoms. We investigate in detail for a pair of helium (He) atoms and krypton (Kr) atoms, and for a pair of methane (CH4) molecules considering its environmental importance. The application of finite size further leads to finite self energies of the atoms. The expression of the interaction energy, as is discussed in detail in this thesis, typically contains a logarithmic factor of the form ln(1-x). Formerly, in evaluating the interaction energies, this factor is customarily series-expanded and truncated in the leading order with certain assumptions. This thesis explores the effect of using the full expression, which we refer to as the non-perturbative (or, the non-expanded) theory, analytically wherever possible as well as numerically. The combined application of the finite-size theory and the non-perturbative theory results in as much as 100% correction in the self energy of atoms in vacuum. This may give rise to significant physical consequences, for example, in the permeabilities of atoms across dielectric membranes. The non-perturbative theory, in addition, exhibits interesting behaviour in the retarded resonance interaction. / <p>QC 20160509</p>

Since the first successful isolation and subsequent characterization of graphene, the interest in two dimensional (2-D) materials has expanded exponentially. Despite the dozens of graphene-like van der Waals materials that have been found and their interesting properties, a significant obstacle in realizing their promise is their instability especially for monolayer and thin layers at elevated temperature. To overcome the obstacle of passivating the 2-D materials and study their properties at elevated temperature, we take advantage of the potential improvements afforded by assembling heterostructures by stacking the atomic thick 2-D materials together hexagonal boron nitride (ℎ-BN) which possess high chemical stability and thermal stability. In this dissertation, several experiments are described in detail in which we utilized h-BN encapsulation to passivate atomically-thin transition metal dichalcogenide and studied their properties at elevated temperature. In the first project we demonstrated that chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS₂ can be stabilized at elevated temperatures by encapsulation with only top ℎ-BN layers in the presence of ambient air, N₂ or forming gas. The best passivation occurs for ℎ-BN covered samples with flowing N₂. In the second project, we demonstrated that encapsulating monolayer MoSe₂ and WS₂ with top and bottom ℎ-BN can improve their thermal stability at high temperature and increase their photoluminescence (PL). The increased PL likely occurs because impurities are laterally expelled from the TMD stack during heating. In the third project, we demonstrated the passivation of different modes of ℎ-BN encapsulation on thin layer FeSe sample by using temperature dependent Raman scattering. The complete encapsulation showed the best protection of thin layer FeSe. Finally, we utilized the temperature dependence of the Raman mode of thin-layer FeSe with complete encapsulation and applied a noncontact method to measure the thermal conductivity of the thin-layer FeSe.