Die strukturelle Charakterisierung im Nano- und Mikrometerbereich ist unerlässlich, da die Materialstruktur eng mit den makroskopischen Eigenschaften verbunden ist. Die vierdimensionale Rastertransmissionselektronenmikroskopie (4DSTEM) ist für diese Charakterisierung gut geeignet. Bei 4DSTEM wird ein konvergenter Elektronenstrahl über eine elektronentransparente Probe gerastert, während ein pixelierter Detektor an jeder Rasterposition ein Transmissionselektronenbeugungsbild aufnimmt. Die Analyse der in diesen Bildern kodierten Information erlaubt die Kartierung der Kristallorientierung, Kristallinität und weiteren Probeneigenschaften. 4DSTEM wird typischerweise in Rastertransmissionselektronenmikroskopen (STEMs) mit Elektronenergien von 60 keV bis 300 keV eingesetzt. Rasterelektronenmikroskope (SEMs) werden hingegen meist für die Oberflächenanalyse verwendet und nutzen Elektronenenergien bis 30 keV. 4DSTEM kann auch in SEMs integriert werden, indem ein pixelierter Detektor unter der elektronentransparenten Probe platziert wird. Während 4DSTEM in STEM eine sub-Ångström-Auflösung und die Durchstrahlung dickerer Proben ermöglicht, stellt 4DSTEM-in-SEM eine preiswertere und verfügbarere Alternative mit größerem Rastersichtfeld dar. Wir haben 4DSTEM-in-SEM in einem regulären SEM mithilfe einer szintillatorbasierten fasergekoppelten Kamera und einem Timepix3 hybriden Pixeldetektor (HPD) realisiert. Deren Kombination mit motorisierten Probentischen ermöglichte uns die Probe relativ zum Elektronenstrahl des SEMs zu bewegen und zu kippen, was die Probenkontrolle in STEMs widerspiegelt. Zur Demonstration des Potenzials von 4DSTEM-in-SEM, haben wir ein komplettes TEM-Netzchen sowie eine C60/MoS₂-van-der-Waals-Heterostruktur kartiert. Außerdem ermöglichte uns der datengesteuerte Modus des Timepix3 HPD eine Datenaufnahme mit mehr als 500k Bildern pro Sekunde, was der Verweilzeit von ringförmig integrierenden STEM-Detektoren ähnelt während das gesamte Beugungsbild aufgenommen wird. / Nano- and micro-scale structural characterization is essential for understanding, predicting, and optimizing the properties of materials since the material's structure is closely linked to its macroscopic properties. Four dimensional scanning transmission electron microscopy (4DSTEM) is well suited for this characterization. In 4DSTEM, a convergent electron beam is rastered over an electron-transparent sample, while a pixelated detector records a transmission diffraction pattern at each raster position. The encoded structural information in these patterns can be analyzed to map sample properties like crystal orientation, grain size, crystallinity, and more. 4DSTEM is typically employed in scanning transmission electron microscopes (STEMs) operating at electron energies of 60 keV to 300 keV. Scanning electron microscopes (SEMs) are widely used for surface morphology analysis operating at electron energies up to 30 keV. 4DSTEM can also be implemented in SEMs by placing a pixelated detector beneath the electron-transparent sample. While 4DSTEM in STEM achieves sub-Ångström spatial resolution and transmits through thicker samples, 4DSTEM-in-SEM offers a cost-effective and accessible alternative with a larger scan field of view while still transmitting through samples tens of nanometers thick. We implemented 4DSTEM-in-SEM using a standard SEM equipped with a custom scintillator-based fiber-coupled camera and a Timepix3 hybrid pixel detector (HPD). The combination of these detectors with motorized sample stages allowed us to move and tilt the sample relative to the SEM's electron beam mirroring the sample control in STEM. To demonstrate the potential of 4DSTEM-in-SEM, we mapped an entire TEM grid and analyzed a C60/MoS₂ van der Waals heterostructure. Additionally, the Timepix3 HPD's data-driven mode enabled acquisition speeds exceeding 500k frames per second, achieving dwell times comparable to annular integrating STEM detectors while recording the full diffraction patterns.

This dissertation is devoted to the development of novel devices for optoelectronic and photovoltaic applications using the promise of inkjet printing with two-dimensional (2D) materials. A systematic approach toward the characterization of the liquid exfoliated 2D inks comprising of graphene, molybdenum disulfide (MoS2), tungsten diselenide (WSe2), and 2D perovskites is discussed at depth. In the first study, the biocompatibility of 2D materials -- graphene and MoS2 -- that were drop cast onto flexible PET and polyimide substrates using mouse embryonic fibroblast (STO) and human esophageal fibroblast (HEF) cell lines, was explored. The polyimide samples for both STO and HEF showed high biocompatibility with a cell survival rate of up to ~ 98% and a confluence rate of 70-98%. An inkjet printed, biocompatible, heterostructure photodetector was constructed using inks of photo-active MoS2 and electrically conducting graphene, which facilitated charge collection of the photocarriers. The importance of such devices stems from their potential utility in age-related-macular degeneration (AMD), which is a condition where the photosensitive retinal tissue degrades with aging, eventually compromising vision. The biocompatible inkjet printed 2D heterojunction devices were photoresponsive to broadband incoming radiation in the visible regime, and the photocurrent scaled proportionally with the incident light intensity, exhibiting a photoresponsivity R ~ 0.30 A/W. Strain-dependent measurements were also conducted with bending, that showed Iph ~ 1.16 µA with strain levels for curvature up to ~ 0.262 cm-1, indicating the feasibility of such devices for large format arrays printed on flexible substrates. Alongside the optoelectronic measurements, temperature-dependent (~ 80 K to 573 K) frequency shifts of the Raman-active E12g and A1g modes of multilayer MoS2 exhibited a red-shift with increasing temperature, where the temperature coefficients for the E12g and A1g modes were determined to be ~ - 0.016 cm-1/K and ~ - 0.014 cm-1/K, respectively. The phonon lifetime τ was determined to be in the picosecond range for the E12g and A1g modes, respectively, for the liquid exfoliated multilayer MoS2. Secondly, an all inkjet printed WSe2-graphene hetero-structure photodetector on flexible polyimide substrates is also studied, where the device performance was found to be superior compared to the MoS2-graphene photodetector. The printed photodetector was photo responsive to broadband incoming radiation in the visible regime, where the photo responsivity R ~ 0.7 A/W and conductivity σ ~ 2.3 × 10-1 S/m were achieved at room temperature. Thirdly, the synthesis of solution-processed 2D layered organo-halide (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 2, 3, and 4) perovskites is presented here, where inkjet printing was used to fabricate heterostructure flexible photodetector devices on polyimide substrates. The ON/OFF ratio was determined to be high, ~ 2.3 × 103 while the photoresponse time on the rising and falling edges was measured to be rise ~ 24 ms and fall ~ 65 ms, respectively. The strain-dependent measurements, conducted here for the first time for inkjet printed perovskite photodetectors, revealed the Ip decreased by only ~ 27% with bending (radius of curvature of ~ 0.262 cm-1). This work demonstrates the tremendous potential of the inkjet printed, composition tunable, organo-halide 2D perovskite heterostructures for high-performance photodetectors, where the techniques are readily translatable toward flexible solar cell platforms as well. Fourthly, metal contacts and carrier transport in 2D (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 4) perovskites is a critical topic, where the use of silver (Ag) and graphene (Gr) inks as metallic contacts to 2D perovskites was investigated. The all inkjet printed Gr-perovskite and Ag-perovskite photodetectors were found to be photo-responsive to broadband incoming radiation where measurements were conducted from λ ~ 400 nm to 2300 nm. The photoresponsivity R and detectivity D were compared between the Gr-perovskite and Ag-perovskite photodetectors, which revealed the higher performance for the Ag-perovskite photodetector. The superior performance of the Ag-perovskite photodetector was also justified with the Schottky barrier analysis using the thermionic emission model through temperature-dependent transport measurements. Finally, this dissertation ends with the description of the first steps for using solution-processed, inkjet printed perovskites for solar cells. The preliminary investigations include the discussion of the chemical formulations for the carrier separation layers, dispersion route, and the variation of solar cell figures of merit with processing.

<div>Silicon based semiconductor technology is facing more and more challenges to continue the Moore's law due to its fundamental scaling limitations. To continue the pace of progress of device performance for both logic and memory devices, researchers are exploring new low-dimensional materials, e.g. nanowire, nanotube, graphene and hexagonal boron nitride. Transition metal dichalcogenides (TMDs) are attracted considerable attention due their atomically thin nature and proper bandgap at the initial study. Recently, more and more interesting properties are found in these materials, which will bring out more potential usefulness for electronic applications. Competing with the silicon device performance is not the only goal in the potential path finding of beyond silicon. Low-dimensional materials may have other outstanding performances as an alternative materials in many application realms. </div><div><br></div><div>This thesis explores the potential of TMD based devices in memory and logic applications. For the memory application, TMD based vertical devices are fully studied. Two-terminal vertical transition metal dichalcogenide (TMD) based memory selectors were firstly built and characterized, exhibiting better overall performance compared with some traditional selectors. Polymorphism is one of unique properties in TMD materials. 2D phase engineering in TMDs attracted great attention. While electric switching between semiconductor phase to metallic phase is the most desirable. In this thesis, electric field induced structural transition in MoTe₂ and Mo_1-xW_xTe₂ is firstly presented. Reproducible bipolar resistive random access (RRAM) behavior is observed in MoTe₂ and Mo_1-xW_xTe₂ based vertical devices. Direct confirmation of a phase transition from a 2H semiconductor to a distorted 2H_d metallic phase was obtained after applying an electric field. Set voltage is changed with flake thickness, and switching speed is less than 5 ns. Different from conventional RRAM devices based on ionic migration, the MoTe₂-based RRAMs offer intrinsically better reliability and control. In comparison to phase change memory (PCM)-based devices that operate based on a change between an amorphous and a crystalline structure, our MoTe₂-based RRAM devices allow faster switching due to a transition between two crystalline states. Moreover, utilization of atomically thin 2D materials allows for aggressive scaling and high-performance flexible electronics applications. Both of the studies shine lights on the new application in the memory field with two-dimensional materials.<br></div><div><br></div><div>For the logic application, the ultra thin body nature of TMDs allows for more aggressive scaling compared with bulk material - silicon. Two aspects of scaling properties in TMD based devices are discussed, channel length scaling and channel width scaling. A tunability of short channel effects in MoS₂ field effect transistor (FET) is reported. The electrical performance of MoS₂ flakes is governed by an unexpected dependence on the effective body thickness of the device which in turn depends on the amount of intercalated water molecules that exist in the layered structure. In particular, we observe that the doping stage of a MoS₂ FET strongly depends on the environment (air/vacuum). For the channel width scaling, the impact of edge states in three types of TMDs, metallic T_d-phase WTe₂ as well as semiconducting 2H-phase MoTe₂ and MoS₂ were explored, by patterning thin flakes into ribbons with varying channel widths. No obvious charge depletion at the edges is observed for any of these three materials, which is different from what has been observed in graphene nanoribbon devices. </div>

Discovery of grapheme has paved way for experimental realization of many physical phenomena such as massless Dirac fermions, quantum hall effect and zero-field conductivity. Search for other two dimensional (2D) materials led to the discovery of boron nitride, transition metal dichalcogenides(TMDs),transition metal oxides(MO2)and silicene. All of these materials exhibit different electronic and transport properties and are very promising for nanodevices such as nano-electromechanical-systems(NEMS), field effect transistors(FETs),sensors, hydrogen storage, nano photonics and many more. For practical utility of these materials in electronic and photonic applications, varying the band gap is very essential. Tuning of band gap has been achieved by doping, functionalization, lateral confinement, formation of hybrid structures and application of electric field. However, most of these techniques have limitations in practical applications. While, there is a lack of effective method of doping or functionalization in a controlled fashion, growth of specific sized nanostructures (e.g., nanoribbons and quantum dots),freestanding or embedded is yet to be achieved experimentally. The requirement of high electric field as well as the need for an extra electrode is another disadvantage in electric field induced tuning of band gap in low dimensional materials. Development of simpler yet effective methods is thus necessary to achieve this goal experimentally for potential application of these materials in various nano-devices. In this thesis, novel methods for tuning band gap of few 2D materials, based on strain and stacking, have been proposed theoretically using first principles based density functional theory(DFT) calculations. Electronic properties of few layered nanomaterials are studied subjected to mechanical and chemical strain of various kinds along with the effect of stacking pattern. These methods offer promising ways for controlled tuning of band gap in low dimensional materials. Detailed methodology of these proposed methods and their effect on electronic, structural or vibrational properties have also been studied. The thesis has been organized as follows: Chapter1 provides a general introduction to the low dimensional materials: their importance and potential application. An overview of the systems studied here is also given along with the traditional methods followed in the literature to tune their electronic properties. The motivation of the current research work has also been highlighted in this chapter. Chapter 2 describes the theoretical methodology adopted in this work. It gives brief understanding of first principles based Density Functional Theory(DFT) and various exchange and correlation energy functionals used here to obtain electronic, structural, vibrational and magnetic properties of the concerned materials. Chapter 3 deals with finding the origin of a novel experimental phenomenon, where electromechanical oscillations were observed on an array of buckled multiwalled carbon nanotubes (MWCNTs)subjected to axial compression. The effect of structural changes in CNTs in terms of buckling on electronic properties was studied. Contribution from intra-as well as inter-wall interactions was investigated separately by using single-and double-walled CNTs. Chapter 4 presents a method to manipulate electronic and transport properties of graphene bilayer by sliding one of the layers. Sliding caused breaking of symmetry in the graphene bilayer, which resulted in change in dispersion in the low energy bands. A transition from linear dispersion in AA stacking to parabolic dispersion in AB stacking is discussed in details. This shows a possibility to use these slid bilayers to tailor graphene based devices. Chapter 5 develops a method to tune band gap of bilayers of semiconducting transition metal dichalcogenides(TMDs) by the application of normal compressive strain. A reversible semiconductor to metal(S-M) transition was reported in this chapter for bilayers of TMDs. Chapter 6 shows the evolution of S-M transition from few layers to the bulk MoS2 under various in-plane and out of plane strains. S-M transition as a function of layer number has been studied for different strain types. A comparison between the in-plan and normal strain on modifying electronic properties is also presented. Chapter 7 discusses the electronic phase transition of bulk MoS2 under hydrostatic pressure. A hydrostatic pressure includes a combined effect of both in-plane and normal strain on the structure. The origin of metallic transition under pressure has been studied here in terms of electronic structure, density of states and charge analysis. Chapter 8 studies the chemical strain present in boron nitride nanoribbons and its effect on structural, electronic and magnetic properties of these ribbons. Properties of two achiral (armchair and zig-zag) edges have been analyzed in terms of edge energy and edge stress to predict stability of the edges. Chapter9 summarizes and concludes the work presented in this thesis.

This work is focused on the study of luminescence of atomic thin layers of transition metal chalkogenides (eg. MoS2). In the experimental part, the work deals with the preparation of atomic thin layers of semiconducting chalcogenides and the subsequent manufacturing of plasmonic interference structures around these layers. The illumination of the interference structure will create a standing plasmonic wave that will excite the photoluminescence of the semiconductor. Photoluminescence was studied both by far-field spectroscopy and near-field optical microscopy.

<div>Surface level molecules act as an interface that mediates between the surface and the environment. In this way, interfacial molecules are responsible for conferring characteristics of relevance to many modern material science problems, such as electrical conductivity and wettability. For many applications, such as organic photovoltaics and nanoelectronics, macroscopic placement of chemical patterns at the sub-10 nm must be achieved to advance next generation device applications.</div><div><br></div><div>In the work presented here, we show that sub-10 nm orthogonal features can be prepared by translating the building principles of the lipid bilayer into striped phase lipids on 2D materials (e.g. highly ordered pyrolytic graphite (HOPG), MoS2). Macroscopic patterning of these nanoscopic elements is achieved via Langmuir Schafer deposition of polymerizable diyne amphiphiles. On the Langmuir trough, amphiphiles at the air water interface are ordered into features that can be observed on the macroscale using Brewster angle microscopy. Upon contact of the 2D material with the air-water interface the macroscopic pattern on the trough is transferred to the 2D material creating a macroscopic pattern consisting of sub-10 nm orthogonal chemistries. We also show here how hierarchical ordering can be accomplished via noncovalent microcontact printing of amphiphiles onto 2D materials. Microcontact printing allows a greater measure of control over the placement and clustering of interfacial molecules.</div><div><br></div><div>The alkyl chain/surface enthalpy has a great deal of influence over the ordering of amphiphiles at the sub-nm scale. Here, we examine this influence by depositing diyne amphiphiles onto MoS2 which has a weaker alkyl adsorption enthalpy compared to HOPG. We found that dual-chain amphiphiles deposited on MoS2 adopt a geometry that maximized the molecule-molecule interaction compared to the geometry adopted on HOPG.</div><div><br></div><div>Finally, we show how the hierarchical pattern of diyne amphiphiles can be transferred off of the 2D material onto an amorphous material. This is done by reacting the amorphous material with the conjugated backbone of the diyne moiety through a hydrosilylation reaction to exfoliate the film from the 2D crystalline material. The resulting polymer ‘skin’ has many applications were controlling interfacial properties of an amorphous material is important.</div>

La microscopie optique sur substrats antireflets est un outil de caractérisation simple et puissant qui a notamment permis l'isolation du graphène en 2004. Depuis, le domaine d'étude des matériaux bidimensionnels (2D) s'est rapidement développé, tant au niveau fondamental qu'appliqué. Ces matériaux ultraminces présentent des inhomogénéités (bords, joints de grains, multicouches, etc.) qui impactent fortement leurs propriétés physiques et chimiques. Ainsi leur caractérisation à l'échelle locale est primordiale. Cette thèse s'intéresse à une technique récente de microscopie optique à fort contraste, nommée BALM, basée sur l'utilisation originale de couches antireflets très minces (2-5 nm) et fortement absorbantes (métalliques). Elle a notamment pour but d'évaluer les mérites de cette technique pour l'étude des matériaux 2D et de leur réactivité chimique. Ainsi, les différents leviers permettant d'améliorer les conditions d'observation des matériaux 2D ont tout d'abord été étudiés et optimisés pour deux matériaux modèles : l'oxyde de graphène et les monocouches de MoS₂. L'étude de la dynamique de dépôt de couches moléculaires a notamment permis de montrer à la fois l'extrême sensibilité de BALM pour ce type de mesures et l'apport significatif des multicouches antireflets pour l'augmentation du contraste lors de l'observation des matériaux 2D. L'un des atouts principaux de BALM venant de sa combinaison à d'autres techniques, nous nous sommes particulièrement intéressés au couplage de mesures optiques et électrochimiques pour lesquelles le revêtement antireflet sert d'électrode de travail. Nous avons ainsi pu étudier optiquement la dynamique de réduction électrochimique de l'oxyde de graphène (GO), l'électro-greffage de couches minces organiques par réduction de sels de diazonium sur le GO et sa forme réduite (r-GO), ainsi que l'intercalation d'ions métalliques entre feuillets de GO. En combinant versatilité et fort-contraste, BALM est ainsi établi comme un outil prometteur pour l'étude des matériaux 2D et en particulier pour la caractérisation locale et in situ de leur réactivité chimique et électrochimique. / Optical microscopy based on anti-reflective coatings is a simple yet powerful characterization tool which notably allowed the first observation of graphene in 2004. Since then, the field of two-dimensional (2D) materials has developed rapidly both at the fundamental and applied levels. These ultrathin materials present inhomogeneities (edges, grain boundaries, multilayers, etc.) which strongly impact their physical and chemical properties. Thus their local characterization is essential. This thesis focuses on a recent enhanced-contrast optical microscopy technique, named BALM, based on ultrathin (2-5 nm) and strongly light-absorbing (metallic) anti-reflective layers. The goal is notably to evaluate the benefits of this technique for the study of 2D materials and their chemical reactivity. The various levers to improve 2D materials observation were investigated and optimized for two model materials: graphene oxide and MoS₂ monolayers. The investigation of molecular layer deposition dynamic notably showed the extreme sensitivity of BALM for such measurements and the significant contribution of multilayers anti-reflective coatings to enhance contrast during the observation of 2D materials. One of the main assets of BALM comes from its combination to other techniques. We particularly considered the coupling between optical measurements and electrochemistry for which the anti-reflective layer serves as working electrode. We investigated optically the dynamic of electrochemical reduction of Graphene Oxide (GO), the electrografting of organic layers by diazonium salts reduction on GO and its reduced form (rGO), as well as the intercalation of metallic ions within GO sheets. By combining versatility and high-contrast, BALM is established as a promising tool for the study of 2D materials, especially for the local and in situ characterization of their chemical and electrochemical reactivity.

In this work, the crystal growth as well as structural and magnetic investigations of several metal trichalcogenides compounds with a general formula M2X2Ch6 are presented. M stands for a main group metal or transition metal, X is an element of the IV or V main group and Ch is a chalcogen. In particular, these compounds are the phosphorus sulfides Fe2P2S6, Ni2P2S6 as well as intermediate compounds of the substitution regime (Fe1-xNix)2P2S6, the quarternary phosphorus sulfides CuCrP2S6 and AgCrP2S6 and the germanium tellurides Cr2Ge2Te6 and In2Ge2Te6. As members of the metal trichalcogenides, all these compounds have a van der Waals layered honeycomb structure in common. This layered structure in combination with their magnetic properties makes these compounds interesting candidate materials for the production of magnetic monolayers by exfoliation from bulk crystals. Crystals of the phosphorus sulfides were grown by the chemical vapor transport technique and, for the growth of the germanium tellurides, the self-flux growth technique was used. Crystals of all phases were extensively characterized regarding their morphology, chemical composition and homogeneity as well as regarding their crystal structure. The structural analysis, especially for Ni2P2S6, yields insight into details of the stacking order and disorder of the corresponding quasi-two-dimensional layers in the bulk. Regarding the magnetic properties, both Fe2P2S6 and Ni2P2S6 order antiferromagnetically but exhibit different magnetic anisotropies (i.e. Ising-like anisotropy for Fe2P2S6 and XYZ anisotropy for Ni2P2S6). In this context, it is surprising to find that compounds in the solid solution regime of (Fe1-xNix)2P2S6 up to x = 0.9 exhibit an anisotropic magnetic behavior that is comparable to Fe2P2S6 and, thus, indicative of Ising-like anisotropy. For CuCrP2S6 and AgCrP2S6, the ordering of the two different transition elements on the honeycomb sites yields more complex magnetic structures. The magnetic Cr3+ atoms in CuCrP2S6 order in a triangular arrangement and form an antiferromagnetic ground state with notable ferromagnetic interactions. AgCrP2S6 exhibits pronounced features of low dimensional magnetism resulting from the (quasi-)one-dimensional stripe-like arrangement of magnetic Cr3+ atoms and no onset of long-range magnetic order is unambiguously observed. Cr2Ge2Te6 exhibits ferromagnetic order and an anisotropic feature in the temperature dependence of the magnetization. Based on the magnetic phase diagrams for two orientations between the magnetic field and the crystallographic directions, the temperature dependence of the magnetocrystalline anisotropy constant as well as the critical exponents of the magnetic phase transition are extracted. Concluding from this, the magnetic interactions in Cr2Ge2Te6 are dominantly of two-dimensional nature and the anisotropy is uniaxial with the before mentioned anisotropic feature resulting from the interplay between magnetocrystalline anisotropy, magnetic field, and temperature. In2Ge2Te6 is diamagnetic as to be expected for a closed-shell system. Additional to the investigations on single crystals, the quasi-binary phase diagram of (Cu1-xAgx)CrP2S6 was investigated for regimes of solid solution behavior based on polycrystalline samples. Accordingly, isostructural substitution is most likely possible in the composition range of (Cu0.25Ag0.75)CrP2S6 to AgCrP2S6, potentially allowing to tune the magnetic interactions of the Cr sublattice indirectly by substitution on the Cu/Ag sublattice.:1. Introduction 1.1. M2X2Ch6 Class of Materials 1.2. Magnetism in Solid State Materials 1.2.1. Diamagnetism 1.2.2. Paramagnetism 1.2.3. Cooperative Magnetism 1.2.4. Magnetic Anisotropy 1.2.5. Magnetism in D < 3 1.2.6. Critical Exponents 2. Methods 2.1. Synthesis and Crystal Growth 2.1.1. Solid State Synthesis 2.1.2. Crystal Growth via the Liquid Phase 2.1.3. Crystal Growth via the Vapor Phase 2.2. X-ray Diffraction 2.2.1. Single Crystal X-ray Diffraction 2.2.2. Powder X-ray Diffraction 2.3. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy 2.3.1. Scanning Electron Microscopy 2.3.2. Energy Dispersive X-ray Spectroscopy 2.4. Magnetometry 2.5. Nuclear Magnetic Resonance Spectroscopy 2.6. Specific Heat Capacity 3. M2P2S6 3.1. Ni2P2S6 3.1.1. Crystal Growth 3.1.2. Characterization 3.1.3. Magnetic Properties 3.1.4. 31P-NMR Spectroscopy 3.1.5. Stacking (Dis-)Order in Ni2P2S6 3.2. (Fe1-xNix)2P2S6 3.2.1. Synthesis and Crystal Growth 3.2.2. Characterization 3.2.3. Evolution of Magnetic Properties 3.3. Summary and Outlook 4. M1+CrP2S6 4.1. CuCrP2S6 4.1.1. Crystal Growth 4.1.2. Characterization 4.1.3. Magnetic Properties 4.2. AgCrP2S6 4.2.1. Crystal Growth 4.2.2. Characterization 4.2.3. Magnetic Properties 4.3. Polycrystalline (Cu1-xAgx)CrP2S6 4.3.1. Synthesis 4.3.2. Phase Analysis 4.4. Summary and Outlook 5. M2(Ge,Si)2Te6 5.1. Cr2Ge2Te6 5.1.1. Crystal Growth 5.1.2. Characterization 5.1.3. Magnetic Properties 5.1.4. Analysis of the Critical Behavior 5.2. In2Ge2Te6 5.2.1. Crystal Growth 5.2.2. Characterization 5.2.3. Magnetic Properties 5.2.4. Specific Heat 5.3. Summary and Outlook 6. Conclusion Bibliography List of Publications Acknowledgements Eidesstattliche Erklärung A. Appendix A.1. Scanning Electron Microscopic Images A.1.1. (Fe1-xNix)2P2S6 A.2. scXRD A.2.1. (Fe1-xNix)2P2S6 / In dieser Arbeit werden die Kristallzüchtung sowie strukturelle und magnetische Untersuchungen an mehreren Metalltrichalkogenid-Verbindungen mit der allgemeinen Summenformel M2X2Ch6 vorgestellt. M steht für ein Hauptgruppen- oder Übergangsmetall, X ist ein Element der IV- oder V-Hauptgruppe und Ch ein Chalkogen. Insbesondere handelt es sich bei diesen Verbindungen um die Phosphorsulfide Fe2P2S6, Ni2P2S6 sowie um Verbindungen der Substitutionsreihe (Fe1-xNix)2P2S6, die quaternären Phosphorsulfide CuCrP2S6 und AgCrP2S6 sowie die Germaniumtelluride Cr2Ge2Te6 und In2Ge2Te6. Als Mitglieder der Metalltrichalkogenide haben alle diese Verbindungen eine van-der-Waals-Schichtstruktur mit Honigwabenmotiv gemein. Diese Schichtstruktur in Kombination mit ihren magnetischen Eigenschaften macht diese Verbindungen zu interessanten Kandidaten für die Herstellung von magnetischen Monolagen durch Exfoliation aus Volumenkristallen. Kristalle der Phosphorsulfide wurden mit der chemischen Dampfphasentransporttechnik gezüchtet und für die Züchtung der Germaniumtelluride wurde die Selbstflusstechnik verwendet. Die Kristalle aller Phasen wurden sowohl hinsichtlich ihrer Morphologie, chemischen Zusammensetzung und Homogenität als auch hinsichtlich ihrer Kristallstruktur umfassend charakterisiert. Die Strukturanalyse, insbesondere für Ni2P2S6, gibt Aufschluss über Details der Stapelordnung und -unordnung der entsprechenden quasizweidimensionalen Schichten im Volumen. Bezüglich der magnetischen Eigenschaften ordnen sowohl Fe2P2S6 als auch Ni2P2S6 antiferromagnetisch, zeigen aber unterschiedliche magnetische Anisotropien (d.h. Ising-artige Anisotropie für Fe2P2S6 und XYZ-Anisotropie für Ni2P2S6). In diesem Zusammenhang ist es überraschend, dass Verbindungen im Mischkristallregime von (Fe1-xNix)2P2S6 bis x = 0.9 ein anisotropes magnetisches Verhalten zeigen, das mit dem von Fe2P2S6 vergleichbar ist und daher auf Ising-artige Anisotropie hindeutet. Bei CuCrP2S6 und AgCrP2S6 führt die Anordnung der beiden unterschiedlichen Übergangselemente auf den Gitterplätzen der Wabenstruktur zu komplexeren magnetischen Strukturen. Die magnetischen Cr3+ Atome in CuCrP2S6 ordnen sich in einer Dreiecksanordnung an und bilden einen antiferromagnetischen Grundzustand mit ausgeprägten ferromagnetischen Wechselwirkungen. AgCrP2S6 weist deutliche Merkmale von niederdimensionalem Magnetismus auf, welche aus der (quasi-)eindimensionalen, streifenartigen Anordnung der magnetischen Cr3+ Atome resultieren, und das Einsetzen von langreichweitiger magnetischer Ordnung kann nicht eindeutig beobachtet werden. Cr2Ge2Te6 weist ferromagnetische Ordnung und einen anisotropen Verlauf der Temperaturabhängigkeit der Magnetisierung auf. Anhand von magnetischen Phasendiagrammen für zwei Orientierungen zwischen Magnetfeld und kristallographischen Richtungen wurden die Temperaturabhängigkeit der magnetokristallinen Anisotropiekonstante sowie die kritischen Exponenten des magnetischen Phasenübergangs extrahiert. Hieraus ergibt sich, dass die magnetischen Wechselwirkungen in Cr2Ge2Te6 überwiegend zweidimensionaler Natur sind und die Anisotropie uniaxial ist, wobei der zuvor erwähnte anisotrope Verlauf aus dem Zusammenspiel von magnetokristalliner Anisotropie, Magnetfeld und Temperatur resultiert. In2Ge2Te6 ist diamagnetisch, wie es für ein System mit geschlossener Schale zu erwarten ist. Zusätzlich zu den Untersuchungen an Einkristallen wurde das quasibinäre Phasendiagramm von (Cu1-xAgx)CrP2S6 anhand von polykristallinen Proben auf Bereiche mit Mischkristallverhalten hin untersucht. Folglich ist eine isostrukturelle Substitution höchstwahrscheinlich im Zusammensetzungsbereich von (Cu0.25Ag0.75)CrP2S6 bis AgCrP2S6 möglich, was es erlauben könnte, die magnetischen Wechselwirkungen des Cr-Untergitters indirekt durch Substitution auf dem Cu/Ag-Untergitter zu beeinflussen.:1. Introduction 1.1. M2X2Ch6 Class of Materials 1.2. Magnetism in Solid State Materials 1.2.1. Diamagnetism 1.2.2. Paramagnetism 1.2.3. Cooperative Magnetism 1.2.4. Magnetic Anisotropy 1.2.5. Magnetism in D < 3 1.2.6. Critical Exponents 2. Methods 2.1. Synthesis and Crystal Growth 2.1.1. Solid State Synthesis 2.1.2. Crystal Growth via the Liquid Phase 2.1.3. Crystal Growth via the Vapor Phase 2.2. X-ray Diffraction 2.2.1. Single Crystal X-ray Diffraction 2.2.2. Powder X-ray Diffraction 2.3. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy 2.3.1. Scanning Electron Microscopy 2.3.2. Energy Dispersive X-ray Spectroscopy 2.4. Magnetometry 2.5. Nuclear Magnetic Resonance Spectroscopy 2.6. Specific Heat Capacity 3. M2P2S6 3.1. Ni2P2S6 3.1.1. Crystal Growth 3.1.2. Characterization 3.1.3. Magnetic Properties 3.1.4. 31P-NMR Spectroscopy 3.1.5. Stacking (Dis-)Order in Ni2P2S6 3.2. (Fe1-xNix)2P2S6 3.2.1. Synthesis and Crystal Growth 3.2.2. Characterization 3.2.3. Evolution of Magnetic Properties 3.3. Summary and Outlook 4. M1+CrP2S6 4.1. CuCrP2S6 4.1.1. Crystal Growth 4.1.2. Characterization 4.1.3. Magnetic Properties 4.2. AgCrP2S6 4.2.1. Crystal Growth 4.2.2. Characterization 4.2.3. Magnetic Properties 4.3. Polycrystalline (Cu1-xAgx)CrP2S6 4.3.1. Synthesis 4.3.2. Phase Analysis 4.4. Summary and Outlook 5. M2(Ge,Si)2Te6 5.1. Cr2Ge2Te6 5.1.1. Crystal Growth 5.1.2. Characterization 5.1.3. Magnetic Properties 5.1.4. Analysis of the Critical Behavior 5.2. In2Ge2Te6 5.2.1. Crystal Growth 5.2.2. Characterization 5.2.3. Magnetic Properties 5.2.4. Specific Heat 5.3. Summary and Outlook 6. Conclusion Bibliography List of Publications Acknowledgements Eidesstattliche Erklärung A. Appendix A.1. Scanning Electron Microscopic Images A.1.1. (Fe1-xNix)2P2S6 A.2. scXRD A.2.1. (Fe1-xNix)2P2S6

<p dir="ltr">This thesis deals with optical cavity-coupled two-dimensional (2D) materials. First, we describe a new theoretical approach to model the properties of cavity-coupled plasmons in 2D conductors. Next, we propose an optical cavity architecture for enhanced light-matter interaction with potential for performance and functionality beyond that of traditional approaches and describe an initial investigation of one example of such a system. Finally, we provide a thorough description of the fabrication techniques used to produce the previously mentioned optical cavities.</p><p dir="ltr">The advent of 2D materials has opened exciting possibilities for controlling light-matter interactions at the nanoscale. The first major contribution of this work is the investigation of coupling between patterned 2D Van der Waals materials and Fabry-Perot cavities, focusing on how system parameters like pattern shape and material properties influence these interactions. Using a quasistatic eigenmode expansion approach, we develop a theoretical framework to predict and manipulate optical behavior in these systems. Our work opens new pathways for engineering light-matter interactions within patterned 2D material platforms, paving the way for the engineering of novel optical phenomena.</p><p dir="ltr">The second major contribution of this work is the development of a versatile platform for light-matter coupling experiments in Van der Waals materials. It is well-known that light-matter interaction can be used to realize unprecedented functionality in the coupled materials. However, few---if any---approaches to date utilize this phenomenon to its fullest extent. We have provided a platform that can be used to realize light-matter coupling efficiencies beyond what is possible in conventional systems, can be easily integrated with 2D materials, and provides new opportunities to engineer the photonic environment of the coupled material. In particular, we focus on silicon dielectric bowtie cavities (DBCs) coupled to few-layer flakes of $\rm WSe_2$. This approach leverages topology-optimized cavity architectures to achieve simultaneous spatial and spectral confinement, yielding Purcell factors exceeding 2500, mode volumes as small as $\sim10^{-3}(\lambda/2n)^3$, and quality factors up to $\sim200$---performance metrics limited only by material losses. The lithographically defined DBCs enable deterministic emission hotspot placement and tunability across a broad wavelength range with minimal performance impact. Photoluminescence imaging and spectroscopy reveal comparable $\rm WSe_2$ exciton emission enhancement to plasmonic structures. This platform surpasses the limitations of conventional cavity architectures by enabling unprecedented coupling efficiencies and unique functionality while maintaining sufficient mechanical robustness for 2D material transfer.</p><p dir="ltr">The final chapter outlines the fabrication process for the cavities described in the previous chapter. The fabrication involves advanced nanolithography techniques to define patterns with high resolution, addressing challenges such as proximity effects and process blur. Techniques such as proximity effect correction (PEC) are used to enhance pattern accuracy, while careful optimization of exposure and development parameters ensures minimal distortion. The process utilizes high-anisotropy reactive ion etching to transfer the patterns onto the substrate, where precise optimization of the etching parameters has been performed to achieve high resolution and selectivity. The final optimized process yields structures with a minimum feature size of approximately 20 nm and minimum radius of curvature of approximately 10 nm, allowing for the repeatable fabrication of complex inverse-designed cavities.</p>

In this work, we explored one material from the broad family of 2D semiconductors, namely WSe2 to serve as an enabler for advanced, low-power, high-performance nanoelectronics and optoelectronic devices. A 2D WSe2 based field-effect-transistor (FET) was designed and fabricated using electron-beam lithography, that revealed an ultra-high mobility of ~ 625 cm2/V-s, with tunable charge transport behavior in the WSe2 channel, making it a promising candidate for high speed Si-based complimentary-metal-oxide-semiconductor (CMOS) technology. Furthermore, optoelectronic properties in 2D WSe2 based photodetectors and 2D WSe2/2D MoS2 based p-n junction diodes were also analyzed, where the photoresponsivity R and external quantum efficiency were exceptional. The monolayer WSe2 based photodetector, fabricated with Al metal contacts, showed a high R ~502 AW-1 under white light illumination. The EQE was also found to vary from 2.74×101 % - 4.02×103 % within the 400 nm -1100 nm spectral range of the tunable laser source. The interfacial metal-2D WSe2 junction characteristics, which promotes the use of such devices for end-use optoelectronics and quantum scale systems, were also studied and the interfacial stated density Dit in Al/2D WSe2 junction was computed to be the lowest reported to date ~ 3.45×1012 cm-2 eV-1. We also examined the large exciton binding energy present in WSe2 through temperature-dependent Raman and photoluminescence spectroscopy, where localized exciton states perpetuated at 78 K that are gaining increasing attention for single photon emitters for quantum information processing. The exciton and phonon dynamics in 2D WSe2 were further analyzed to unveil other multi-body states besides localized excitons, such as trions whose population densities also evolved with temperature. The phonon lifetime, which is another interesting aspect of phonon dynamics, is calculated in 2D layered WSe2 using Raman spectroscopy for the first time and the influence of external stimuli such as temperature and laser power on the phonon behavior was also studied. Furthermore, we investigated the thermal properties in 2D WSe2 in a suspended architecture platform, and the thermal conductivity in suspended WSe2 was found to be ~ 1940 W/mK which was enhanced by ~ 4X when compared with substrate supported regions. We also studied the use of halide-assisted low-pressure chemical vapor deposition (CVD) with NaCl to help to reduce the growth temperature to ∼750 °C, which is lower than the typical temperatures needed with conventional CVD for realizing 1L WSe2. The synthesis of monolayer WSe2 with high crystalline and optical quality using a halide assisted CVD method was successfully demonstrated where the role of substrate was deemed to play an important role to control the optical quality of the as-grown 2D WSe2. For example, the crystalline, optical and optoelectronics quality in CVD-grown monolayer WSe2 found to improve when sapphire was used as the substrate. Our work provides fundamental insights into the electronic, optoelectronic and quantum properties of WSe2 to pave the way for high-performance electronic, optoelectronic, and quantum-optoelectronic devices using scalable synthesis routes.