e van der Waals (vdW) interactions, or dispersion forces, are crucial in many chem- ical, physical and biological processes and received much attention from developers of density functional theory (DFT) methods. e most popular non-empirical DFT method for treating vdW interactions is the vdW density functional by Dion et al. (vdW-DF). Despite its success, vdW-DF is not accurate enough for many chemical applications. Here, we investigate two possible ways how to improve its accuracy. First, we reoptimize the only weakly speci ed parameter of vdW-DF for several semi-local functionals. On the S benchmark database set, we nd that revPBE is the best performer, decreasing the error from . % to . %. Second, a system-speci c but very accurate (∼ . kcal/mol) DFT correction scheme is proposed for precise calcula- tions of adsorbent−adsorbate interactions by combining vdW-DF and the empirical DFT/CC correction scheme. e new approach is applied to small molecules (CH , CO , H , H O, N ) interacting with a quartz surface and a lamella of UTL zeolite. e very high accuracy of the new scheme and its relatively easy use and numerical stability compared to the earlier DFT/CC scheme o er a straightforward solution for obtaining reliable predictions of adsorption energies.

In this study, the interaction energies between aromatic organic molecules (AOMs) and cellulose are explored using density functional theory (DFT) through the software SIESTA and the exchange-correlational functional VDW-DRSLL. Three AOMs will be modelled: benzene, benzamide and benzoic acid. Firstly, the interaction energies of the dimers of the AOMs are determined. Then, the obtained interaction energies of the cellulose-AOM complexes are compared to the former in order to decide which interaction is stronger. It is found that the studied AOMs are more likely to interact with cellulose than with another identical monomer; benzamide has the highest propensity to interact, followed by benzoic acid and benzene. Furthermore, for all interaction energy calculations a counterpoise correction term will be introduced as an addition to the SIESTA optimisation and it will be shown that without this correction the acquired energy minima will deviate significantly from accepted values from previous studies.

With the emerging interest in nanoscale materials, the fascinating field of surface science is rapidly growing and presenting challenges to the design of both experimental and theoretical studies. The primary aim of this dissertation is to shed some light on the physical and chemical properties of selected nanoscale materials at the interface. Furthermore, we will discuss the effective application of cutting edge theoretical and experimental techniques that are invaluable tools for understanding the systems at hand. To this effect, we use density functional theory (DFT) with the inclusion of van der Waals (vdW) interactions to study the effect of long-range interactions on the adsorption characteristics of various organic molecules (i.e. benzene, olympicene radical, and sexithiophene) on transition metal surfaces. Secondly, the detailed analysis of x-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements will be presented. These investigations will be dedicated to the study of (i) the effect of pre-treatment on the coarsening behavior of Pt nanoparticles (NPs) supported on ?-Al2O3 and (ii) deconvoluting the intrinsic (size effects) and extrinsic (ligand effects) physical and electronic properties of Au NPs encapsulated by polystyrene 2-vinylpiridine ligands.

Ce travail de thèse s’inscrit dans le cadre d'études théoriques ayant pour but l’établissement de la structure des chalcogénures binaires et ternaires sous différentes conditions thermodynamiques. Des techniques de modélisation numérique ab-initio ont été employées. En particulier, nous avons utilisé la dynamique moléculaire par premiers principes selon l’approche de Car et Parrinello ainsi que sa version dite "deuxième génération". La première partie est consacrée à l’étude des chalcogénures binaires, notamment les verres GeSe2 et GeSe4 sous pression ainsi qu'à l’étude des effets des forces de van der Waals et des fonctionnelles d’échange et corrélation DFT sur la structure de l’amorphe GeTe4. Dans la deuxième partie, l’intérêt a été porté à l’étude d'un matériau à changement de phase récemment proposé par les expérimentateurs comme un bon candidat pour le stockage de données, le ternaire Ga4Sb6Te3. / This thesis reflects efforts toward an accurate understanding of the atomic scale structure of chalcogenide glasses. These compounds have an impact on electronics, optoelectronics and memory devices. I resorted to the most advanced first-principles molecular dynamics simulations such as the standard Car-Parrinello method as well as its second generation version. In the first part of this thesis we provide a detailed study of the topological changes undergone under pressure by glassy GeSe2 and by glassy GeSe4. Structural transition and bonding features are described and compared to the results of neutron and X-ray diffraction experiments. Furthermore, in the case of glassy GeTe4 we demonstrated that the inclusion of van der Waals forces leads to substantial improvements in the description of the structure. In the second part of this thesis, we established the atomic-scale organization of a promising candidate for phase change memory applications, glassy Ge4Sb6Te3.

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Tailoring and precisely controlling the propagation of confined light precisely at the nanometer-length scale is an ubiquitous requirement for the realization of integrated optics and, hence, a major goal of the current research in nano-optics. Phonon polaritons (PhPs) - quasi-particles formed by the interaction of light with excitations of the crystal lattice - propagating in thin layers of van-der-Waals (vdW) materials are key to solve this challenge, as they intrinsically offer high confinement of light coupled with low losses, as well as an inherent directional energy transport profiting from the lattice anisotropy of the polariton-hosting material. Furthermore, their strong interaction with surrounding media due to their high surface-to-volume ratio allows for tuning of their wavelengths, shape of wavefront and type of propagation, e.g. via changing the substrate permittivity or stacking multiple layers of polariton-hosting vdW materials under defined twist angles (twistoptics). However, due to the limited access to light sources and detectors operating at frequencies in the so-called THz gap between 0.1 and 10 THz, the research so far had focused to the mid-infrared (MIR) frequency range, omitting the reststrahlen bands of numerous materials spectrally located at or in the THz range. The present work focuses precisely on such PhPs spectrally located at THz frequencies and the tailoring of their propagation behavior. For that purpose, the vdW materials α-molybdenum trioxide (α-MoO3) and α-germanium monosulfide (α-GeS), featuring THz reststrahlen bands between 7.85 to 11.8 THz and 6.06 to 9.70 THz, respectively, have been employed as polariton-hosting materials. Specifically, within the framework of this thesis, the manipulation of the PhP propagation of THz PhPs is achieved via two approaches: First, the deposition of the two polariton-hosting materials onto different substrate materials, including three isotropic materials (Si, SiO2, and Ge) featuring different dielectric responses, and the highly anisotropic β-gallium oxide (bGO) under different twist angles. By doing so the tuning of the PhP wavelengths and the directionality of their propagation as well as the observation of unidirectional ray-like PhPs is achieved. Second, the stacking of two flakes of the polariton-hosting materials to homo- and heterostacks. This extension of the concept of twistoptics from the MIR frequency range to the THz regime enables the tuning of the THz PhP propagation between in-plane hyperbolic and in-plane elliptic, including the first observation of canalized THz PhPs. All studies presented in this work are conducted by combining theoretical simulations, including analytical transfer-matrix calculations and numerical full-wave approaches, with experimental measurements, performed with scattering-type scanning near-field optical microscopy (s-SNOM) utilizing the free-electron laser FELBE (Dresden, Germany) as the tunable THz light source. This unique combination allows for the measurement of the optical properties of materials at frequencies between 1.2 and 60 THz, covering a large part of the THz gap, with a lateral resolution down to 20 nm, thus enabling the real-space visualization of the target PhP propagations at the 2D surfaces. Overall, these studies explore different methods to tune the propagation of THz PhPs, extending the frequency range of tailoring polaritons and thus contributing to the development in the booming field of THz nano-optics.:1 Introduction 1 2 Fundamentals 5 2.1 Phonon Polaritons 5 2.1.1 Phonons and their Interaction with Light 6 2.1.2 Types of Phonon Polaritons 7 2.1.3 Theoretical Models Describing Phonon Polaritons 13 2.2 Van-der-Waals (vdW) Materials 18 2.2.1 Properties of vdW Materials 19 2.2.2 Twistoptics 20 2.3 Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) 22 2.3.1 Historic Development of SNOM 23 2.3.2 Scanning Force Microscopy (SFM) 24 2.3.3 Decoupling Near- and Far-Field in s-SNOM 27 2.3.4 Examples of s-SNOM applications 31 3 Setup and Materials 33 3.1 Experimental Setup 33 3.1.1 s-SNOM instrument neaSNOM 34 3.1.2 Free-Electron Laser FELBE 39 3.1.3 Light Detection 42 3.1.4 Utilized Setup Parameters 44 3.2 Data Acquisition 44 3.2.1 Data Processing 45 3.2.2 Extraction of Polariton Parameters 50 3.2.3 2D Fast Fourier Transformation with 'Zero Padding' 52 3.3 Materials 53 3.3.1 α-Molybdenum Trioxide (α-MoO3) 53 3.3.2 α-Germanium Monosulfide (α-GeS) 57 3.3.3 β-Gallium Oxide (β-Ga2O3) 61 3.3.4 Isotropic Substrates 64 3.4 Sample Preparation 65 3.4.1 Exfoliating Layers of vdW Materials 65 3.4.2 Pre-Characterization 67 3.4.3 Sample Structuring for the PhP Propagation-Shape Imaging 70 4 Results I: Tuning Phonon Polaritons in vdW Single Layers via Substrates 75 4.1 General Theoretical Considerations 76 4.1.1 Tuning via an Isotropic Substrate 76 4.1.2 Tuning via an Anisotropic Substrate 79 4.2 Tuning Hyperbolic Phonon Polaritons in Single-Layers of α-Molybdenum Trioxide 82 4.2.1 Impact of the Substrate Permittivity vs. Layer Thickness 82 4.2.2 Enhanced Tuning via Substrate Anisotropy: Utilizing β-Gallium Oxide as Substrate 86 4.3 Tuning Phonon Polaritons in Single-Layers of α-Germanium Monosulfide 94 4.3.1 The Complex Dispersion of α-Germanium Monosulfide 95 4.3.2 Tuning the Dispersion Splitting via the Substrate Permittivity 99 4.3.3 Impact of the Substrate Permittivity on the In-Plane Propagation Shape 104 4.3.4 Analyzing the Origin of the Dispersion Splitting 109 4.4 Chapter Conclusion 115 5 Results II: Tuning Phonon Polaritons in Twisted Bi-Layer Stacks 117 5.1 Twisted Bilayer α-Molybdenum Trioxide at THz frequencies 117 5.1.1 Theoretical Considerations 118 5.1.2 Experimental Observations 120 5.1.3 Quantitative Characterization of the Measured THz Phonon Polaritons 126 5.1.4 Canalization of Hyperbolic Phonon Polaritons 129 5.2 Heterostacks made up from α-Germanium Monosulfide on α-Molybdenum Trioxide 130 5.2.1 Preliminary Experimental Observations 131 5.2.2 Theoretical Predictions 134 5.3 Chapter Conclusion 138 6 Final Conclusion 141 Appendices 145 A1 Comprehensive Overview of the s-SNOM Performance for Different Experimental Settings 147 A1.1 Extracted Signal-to-Noise Ratios 148 A1.2 Images of the Performed Retracts 156 A1.3 Images of the Performed Scans 162 A2 Additional Measurements of Twisted α-MoO3 Homostacks 163 Bibliography 165 Own Publications 177