<p>Since the successful exfoliation of graphene, two-dimensional (2D) materials have attracted a lot of scientific interest due to their electronic, chemical, and mechanical properties. Due their reduced dimensionality, these 2D materials exhibit superior mechanical and optoelectronic properties when compared to their bulk counterparts. Within the family of 2D materials, the ultrathin transition metal dichalcogenides (TMDs) such as Tungsten diselenide and Molybdenum disulphide have gained significant attention due to their chemical versatility and tunability. Furthermore, it is possible to leverage the distinct characteristic properties of these 2D materials, which are held together by van der Waals forces, by stacking different 2D layers on top of each other resulting in van der Waals (vdW) heterostructures. Due to the absence of feasible methods to effectively deform the crystal structures of these 2D materials and vdW heterostructures, their mechanical properties have not been thoroughly understood. The atomistic simulations can effectively capture the material behavior at the nanoscale level and help us not only not only understand the mechanical properties of these materials but also aid in the development of tailored processes to tune the material properties for the design of novel metamaterials. Using atomistic simulations, we develop the process - property relationships which can guide the direction of experimentation efforts, thereby making the process of discovering and designing new metamaterials efficient. </p><p>In this work, we have used laser shock nanostraining technique which is a scalable approach to modulate the optomechanical properties of 2D materials and vdW materials for practical semiconductor industry applications. The deformation mechanisms of 2D materials such as graphene, boron nitride (BN) and TMDs such as WSe₂ and MoS₂ are examined by employing a laser shocking process. We report studies on crystal structure deformation of multilayered WSe₂ and monolayer graphene at ultra-high strain rate using laser shock . The laser shocking process generates high pressure at GPa level, causing asymmetric 3D straining in graphene and a novel kinked-like locking structure in multilayered WSe₂. The deformation processes and related mechanical behaviors in laser shocked 2D materials are examined using atomistic simulations. Moiré heterostructures can be obtained by introducing a twist angle between these 2D layers, which can result into vdW materials with different properties, thereby adding an additional degree of freedom in the process-property design approach. We were able to successfully create a tunable stain profile in 2D materials and vdW heterostructures to modulate the local properties such as friction, and bandgap by controlling the level of laser shock, twist angle between the 2D layers and by applying appropriate laser shock pressure . We thus extend this knowledge to further explore the pathways of strain modulation using a combination of laser shocking process, moiré engineering, and strain engineering in 2D materials consisting of graphene, BN, and MoS₂ and to develop the process - property relationships in vdW materials. </p><p>In summary, this research presents a systematic understanding of the effect of laser shocking process on the van der Waals materials and demonstrates the modulation of mechanical and opto-electronic property using laser nanostraining approach. This understanding provides us with opportunities for deterministic design of 2D materials with controllable properties for semiconductor and nanoelectronics applications.</p>

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

In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. In this dissertation, we achieve narrow bands by twisting two atomically thin layers of the semiconducting van der Waals material WSe₂. The resulting moiré potential from the twist angle modulates the electronic bands, yielding minibands of tens of meV on the valence band. We perform transport measurements at cryogenic temperatures and observe signatures of collective phases over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. Near the boundary between ordered and disordered quantum phases, several experiments have demonstrated metallic behaviour that defies the Landau Fermi paradigm. We find that the metal-insulator transition as a function of both density and displacement field is continuous. At the metal–insulator boundary, the resistivity displays strange metal behaviour at low temperatures, with dissipation comparable to that at the Planckian limit. Further into the metallic phase, Fermi liquid behaviour is recovered at low temperature, and this evolves into a quantum critical fan at intermediate temperatures, before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. We further show via magnetotransport measurements that new correlated electronic phases can exist independent of moiré commensurability, and are instead driven by weak interactions in twisted WSe₂. The first of these phases is an antiferromagnetic metal that is driven by proximity to the van Hove singularity (vHS), which trails a range of incommensurate dopings. The temperature, magnetic field and density dependence of the Hall effect carry signatures of the reconstructed Fermi surface due to itinerant magnetic ordering. The second is an excitonic metal-insulator phase that exists at high external magnetic field in the vicinity of half-filling of the moiré superlattice. For a 4.2° sample, magnetic field dependence of the longitudinal resistance shows metallic behavior at fields above 5 T, but transitions to an insulating state above ∼ 24 T. A detailed analysis of of the Landau fans and the high field 𝝆_𝜘𝛾 near the gap rules out the possibility of a trivial insulator. We propose an Ising excitonic insulator as the most likely scenario. Moreover, in the electron-imbalanced excitonic metal, a set of correlated Landau levels emerge. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.

Spin-orbit coupling (SOC) is a relativistic effect whose origin lies in the Dirac’s equation – a relativistic analogue of Schrödinger’s equation. SOC corrects the electronic states of a quantum mechanical system up to ~1 eV in case of semiconductors and ~ 2 – 3.6 eV in case of actinides and heavy elements by considering not only the coordinates but also the spin of the electrons in the system. Most of the applications of the present day technology are based on manipulating the electronic structure of a system with very high accuracy and precision. This demands availability of correct electronic structure of a material or molecule within a feasible computational time. Some direct consequences of SOC in materials can be noticed in analyzing the charge-transport properties of a semiconductor, evaluating the candidature of transition metal dichalcogenides (TMDCs) for spintronic, twistronic and valleytronic applications, and in the origin of topological properties of a material. Not only in materials but also in molecules the SOC effects can be observed. Fine-structure of atomic spectra was explained on the account of SOC. Several additional peaks and wavelength shift in UV-vis spectroscopy of Gold Superatoms can only be explained by correctly considering the energy level splittings caused by SOC. SOC allows intersystem and reverse intersystem crossing by mixing the spin states, ultimately opening various chemical reaction pathways which were spin forbidden before. Current advancements in computational power enrich us to work shoulder to shoulder with experiments where one can simulate the synthesized structures containing thousands of atoms using semi-empirical methods as in DFTB, GFN-XTB. These methods so far considered SOC effects but only as case studies in testing the implementation of SOC Hamiltonian rather than a systemic extension of SOC parameters to most part of the periodic table and studying SOC effects for different categories of materials and molecules. This motivated us to implement the SOC either in the form of highly accurate parameters throughout the periodic table or as addition in hamiltonian in such methods. Twisted van der Waals 2D materials as in twisted TMDC bilayers shows exciting electronic and optoelectronic properties and depending on the twist angle and chemical composition they can have thousands of atoms in their superlattices. A correct electronic analysis of such structures with SOC corrected DFT is computationally very expensive but is feasible at semi-empirical level. Here, we have applied our implementation on TMDC homo and heterobilayer twisted superlattices and studied the effect of SOC on the excitonic properties of the system. Therefore, this work opens the way for realizing various exotic applications of present day materials as well as molecules.:Table of Contents Abstract 4 1 Introduction 8 1.1 Quantum Chemistry: 8 1.2 HF based Semi-Empirical Methods 9 1.3 DFT based Semi-Empirical Methods 11 1.3.1 Density Functional based Tight-Binding Method (DFTB) 11 1.3.2 Geometry, Frequency, Non-Covalent, extended Tight Binding (GFN-xTB) 12 1.4 Spin-Orbit Coupling (SOC) 14 1.4.1 SOC in Materials 18 1.4.2 SOC in Molecular Structures 22 1.5 Theoretical Models for Accounting SOC 24 1.6 Motivation, Objective and Outline of thesis 26 2 Methodology 29 2.1 Quantum Chemistry 30 2.1.1 Schrödinger equation 30 2.2 Density Functional Theory 33 2.2.1 Generalized Gradient Approximations 39 2.3 Spin-orbit Coupling (SOC) 41 2.3.1 Classical Picture of SOC in LS model 42 2.3.2 Quantum Picture of SOC in LS model: 43 2.3.3 Calculation of SOC Paramentes 45 2.4 Density Functional Based Semi-empirical Quantum Mechanical Methods 48 2.4.1 Self-Consistent Charge Density Functional Based Tight Binding Method (SCC-DFTB) 48 2.4.2 Extended Tight-Binding (GFN1-xTB) 51 2.4.3 Addition of Spin-Orbit Coupling Hamiltonian in DFTB and GFN-xTB 54 3 Benchmarking Spin-Orbit Coupling Parameters for DFTB 56 3.1 Introduction 58 3.2 Computational Details of the DFT benchmark calculations 60 3.3 Benchmarking Spin-Orbit Coupling Parameters 60 3.3.1 III-V Bulk Semiconductor 61 3.3.2 Transition Metal Dichalcogenide 2D Crystals 65 3.3.3 Topological Insulators 68 3.4 Conclusions 70 4 Spin-Orbit Coupling Corrections for the GFN-xTB method 71 4.1.1 Introduction 73 4.2 Computational Details of The Benchmark Calculations 75 4.3 Results & Discussion 76 4.3.1 Geometries 76 4.3.2 Effect of SOC on Charge Transport Properties of Chromophores in MOFs 77 4.3.3 Superatoms 82 4.3.4 Effect of SOC on Binding of O2 on Ferrous Deoxyheme 85 4.4 Conclusions 86 5 Spin Orbit Coupling Effects on The Excitonic Properties of Twisted Moiré Transition Metal Dichalcogenides 88 5.1 Introduction 90 5.2 Computational Details 92 5.3 Results & Discussions 93 5.4 Excitons in Twisted Moiré Homobilayers 93 5.5 Excitons in Twisted Moiré Heterobilayers 102 5.6 Conclusions 109 6 Summary 112 A. Acronym 116 B. Appendices 120 SOC Parameters 120 7 References 147 C. Acknowledgement 173

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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24116号 / エネ博第449号 / 新制||エネ||84(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 大垣 英明, 教授 松田 一成, 教授 宮内 雄平 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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Ever since the discovery of van der Waals materials, the condensed matter community has developed a wide spectrum of techniques to probe various phases in these fascinating materials. Among these phases, correlated phenomena are of great importance to physicists, and recent progress on moiré heterostructures offers a highly flexible and tunable platform to study them. It has been established in previous works that twisted WSe₂, a type of semiconductor in the van der Waals family, has great potential in hosting a large number of correlated phases and phase transitions. However, it is believed that commensurability plays a critical role in the stability of correlations. In this thesis, we demonstrate correlated physics in twisted WSe₂ beyond commensurate fillings, as well as their magnetic field dependence, via electric transport measurements. At modest magnetic fields, a Stoner-like instability in the system near van Hove singularities causes a reconstruction of the Fermi surface. On the other hand, at extremely high magnetic fields, the system exhibits reentrant insulating behaviors that are possibly due to the presence of strong excitonic interactions. Furthermore, correlated topological states are observed away from half-filling in the imbalanced excitonic metallic regime. This wide range of tunability once again proves moiré heterostructures as a promising platform to simulate quantum correlation effects on a lattice.

In this thesis, two-dimensional (2D) van der Waals materials have been explored, focusing on the encapsulation of graphene with hexagonal boron nitride (hBN), as well as detailed study of 2D magnets Cr2Ge2Te6 and CuCrP2S6. Graphene has been studied to establish the connection between its quantum Hall phase with the non-hermitian Hatano-Nelson model. Additionally, the thickness-dependent magnetism of Cr2Ge2Te6 has been studied and the exfoliation and oxidation study of CuCrP2S6 has been carried out. Through the tunable electronic properties of graphene, specifically its ability to move between electron and hole side by applying back gate voltage, we demonstrate how the quantum Hall effect in graphene can proove a physical realization of non-Hermitian topological phase. By systematically measuring the resistance matrices of graphene in the quantum Hall regime and inverting them to the conductance matrices, it has been shown that the quantum Hall phase in graphene corresponds to the conductance matrix in Hatano-Nelson model, with the electron and hole side exhibiting chirality or non-chirality. For Cr2Ge2Te4, the thickness-dependent magnetism has been studied through exfoliation and succesfully reached a thickness down to 4 nm. Using the magnetooptical Kerr method (MOKE), distinct variations in the hysteresis curve across different thicknesses have been observed. Notably, 8nm thick part of the flake exhibit highly coercive hysteresis, while down-to-a few layer part show no signal. For CuCrP2S6, a systematic degradation study has been conducted. Flakes were observed over the course of the year. AFM analysis confirmed the thickness remained stable for the first month, with no noticeable degradation. However, after a year, significant degradation was visible in optical images, indicating that Cu-CrP2S6 oxidizes in air over a long time scale.