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Exfoliation of graphene in 2004 initiated intensive attention on layered two-dimensional (2D) materials. So far, except for graphene, a large family of 2D materials has been reported, such as transition metal dichalcogenides (TMDCs), hexagonal boron nitride, black phosphorene, metal nitrides/carbides and their van der Waals heterostructures, etc. The large number of species, unique electrical and optical properties and versatile functionalities render 2D materials as promising materials for electronic, optoelectronic and photovoltaic devices. This thesis investigates ultrafast charge dynamics in 2D material system based on real-time time-dependent density functional theory method within Ehrenfest framework. This work is divided into three major parts. In first part, we investigate the ultrafast interlayer charge transfer process in the graphene/WS2 heterostructure. Our results demonstrate that photo-induced holes transfer from WS2 to graphene more efficient than electrons. The ultrafast charge dynamics arises from the coupling to nuclear vibrations and its amplitude and polarity show a strong dependence on the external electric fields. Further analysis reveals that carrier dynamics in the heterostructure is the result of competition between interlayer and intralayer relaxation process, which is governed by the couplings between carriers and their acceptor states. This work establishes a firm correlation between the charge dynamics and couplings between states in 2D heterostructures, and provide practical methodology to manipulate carrier dynamics at heterointerfaces. In second part, we study the carrier multiplication (CM) phenomenon in six monolayer TMDCs MX2 (M = Mo, W; X = S, Se, Te). Our results present that CM is observed in all six TMDCs. The threshold energy of CM can be substantially reduced to 1.75 bandgap (Eg) via couplings to phonon modes. Since electron-phonon couplings can result in significant changes in electronic structures, even trigger semiconductor-metal transition, and eventually assist CM beyond threshold limit. Chalcogen vacancies can further decrease the threshold due to sub-gap defect states. In particular for WS2, CM occurs with excitation energy of only 1.51Eg. Our results identify TMDCs as attractive candidate materials for efficient photovoltaic devices with the advantages of high photo-conductivity and phonon-assisted CM characteristic. In third part, we report the effect of doping levels on CM in graphene. Our calculation results indicate that doping level can introduce remarkable differences in CM conversion efficiency in graphene. Specifically, the CM quantum yield can be promoted from 1.41 to 1.89 when the Fermi level rising from 0.40 eV to 0.78 eV via n-doping. Consistently, time- and angle-resolved photoemission spectroscopy measurements on n-doped graphene present the same correlation between doping levels and CM conversion efficiency. Our results provide a practical strategy to promote the performance of graphene as a photovoltaic material by tuning doping levels.
This thesis focuses on the exploration of nontrivial spin dynamics in graphene-based devices and topological materials, using realistic theoretical models and state-of-the-art quantum transport methodologies. The main outcomes of this work are: (i) the analysis of the crossover from diffusive to ballistic spin transport regimes in ultraclean graphene nonlocal devices, and (ii) investigation of spin transport and spin dynamics phenomena (such as the (quantum) spin Hall effect) in novel topological materials, such as monolayer Weyl semimetals WeTe2 and MoTe2. Indeed, the ballistic spin transport results are key for further interpretation of ultraclean spintronic devices, and will enable extracting precise values of spin diffusion lengths in diffusive transport and guide experiments in the (quasi)ballistic regime. Furthermore, the thesis provides an in-depth theoretical interpretation of puzzling huge measured efficiencies of the spin Hall effect in MoTe2, as well as a prediction of a novel canted quantum spin Hall effect in WTe2 with spins pointing in the yz plane.
Monoelemental 2D materials called Xenes have a graphene-like structure, intra-layer covalent bond, and weak van der Waals forces between layers. Materials composed of different groups of elements have different structures and rich properties, making Xenes materials a potential candidate for the next generation of 2D materials. 2D Monoelemental Materials (Xenes) and Related Technologies: Beyond Graphene describes the structure, properties, and applications of Xenes by classification and section. The first section covers the structure and classification of single-element 2D materials, according to the different main groups of monoelemental materials of different components and includes the properties and applications with detailed description. The second section discusses the structure, properties, and applications of advanced 2D Xenes materials, which are composed of heterogeneous structures, produced by defects, and regulated by the field. Features include: Systematically detailed single element materials according to the main groups of the constituent elements Classification of the most effective and widely studied 2D Xenes materials Expounding upon changes in properties and improvements in applications by different regulation mechanisms Discussion of the significance of 2D single-element materials where structural characteristics are closely combined with different preparation methods and the relevant theoretical properties complement each other with practical applications Aimed at researchers and advanced students in materials science and engineering, this book offers a broad view of current knowledge in the emerging and promising field of 2D monoelemental materials.
Strongly correlated materials manifest some of the most intriguing behaviors found in condensed matter physics. However, their understanding remains a challenge because they cannot be described using standard theoretical approaches, as such systems are complex and typically have many competing degrees of freedom. Many strongly correlated systems are realized in layered or highly anisotropic materials, and they behave as effectively two-dimensional systems. When the dimensionality is reduced, fluctuations due to the competition between degrees of freedom are more pronounced and easier to observe. Anomalous transport properties are one of the hallmarks of strongly correlated materials; thus, charge transport measurements have proven remarkably effective for their study. This thesis focuses on three very different two-dimensional (2D) materials using charge transport to understand the origin of some of the observed behaviors. The electron system (ES) in Si metal-oxide-semiconductor field-effect transistors (MOSFETs) is a model strongly correlated system with only the interplay of Coulomb interactions and disorder. The insight from this simple system can help to build a more general picture of strongly correlated materials. Second, atomically thin layers of WSe2 produce a unique 2D system to study the universality of the correlated phenomena observed in Si MOSFETs. Finally, the Cu-O planes of La-214 compounds with charge and spin stripe order are complex materials, which behave effectively as 2D, with the interplay of many orders. By varying the dopant of the La-214 compound, the origin of the correlated behaviors can be probed. In the first two materials, the 2D MIT is controlled by applying a gate voltage. The existence of the 2D MIT is supported by an abundance of experimental data but remains poorly understood. Within experimental systems, both electron-electron interactions and disorder are present; however, the theory of their interplay is not fully developed. Studies performed on highly disordered Si MOSFETs suggest the importance of Coulomb interactions for the glassy dynamics observed at low electron densities. Therefore, the first study discussed in this thesis explores relaxations of conductivity in a strongly disordered 2DES with screened Coulomb interactions after the system is quenched-revealing the necessity of long-range Coulomb interactions for the existence of the collective (glassy) relaxation dynamics. Additionally, we have shown that, in the 2DES of the Si MOSFET weakly thermally coupled to the environment, abnormally long relaxations are observed in the presence of short-range interactions, suggesting that the system is in the proximity to a many-body-localized phase. Thus our results also demonstrate a promising new platform for exploring the breakdown of thermalization and MBL in real materials. Evidence of quantum criticality associated with the MIT has been almost exclusively studied in Si MOSFETs. Our second study reveals quantum criticality in WSe2 field-effect transistors showing that transition metal dichalcogenides are viable systems for the low-temperature investigation of the 2D MIT. Our scaling analysis found that the critical exponents agree with those found in low-disordered Si MOSFETs in the presence of local magnetic moments. These findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides. The final study focuses on fluctuations of charge order (CO) and the magnetoresistance (MR) of stripe-ordered La-214 cuprates. The dynamics of CO are thought to be relevant for the unconventional properties of the normal state and high-temperature superconductivity. We report observations of dynamic charge stripes close to the charge order (and structural) transition in response to temperature perturbations but absent in magnetic field in La1.875Ba0.125CuO4. These dynamic behaviors are only observed when the transition is approached from the charge-ordered state. Additionally, a comparative analysis of the MR of several La-214 single crystals is presented to establish which behaviors are characteristic of the family of materials as opposed to dopant specific. Together, these three studies contribute to understanding the complex interplay of orders found in strongly correlated materials.
Spintronic 2D Materials: Fundamentals and Applications provides an overview of the fundamental theory of 2D electronic systems that includes a selection of the most intensively investigated 2D materials. The book tells the story of 2D spintronics in a systematic and comprehensive way, providing the growing community of spintronics researchers with a key reference. Part One addresses the fundamental theoretical aspects of 2D materials and spin transport, while Parts Two through Four explore 2D material systems, including graphene, topological insulators, and transition metal dichalcogenides. Each section discusses properties, key issues and recent developments. In addition, the material growth method (from lab to mass production), device fabrication and characterization techniques are included throughout the book. Discusses the fundamentals and applications of spintronics of 2D materials, such as graphene, topological insulators and transition metal dichalcogenides Includes an in-depth look at each materials system, from material growth, device fabrication and characterization techniques Presents the latest solutions on key challenges, such as the spin lifetime of 2D materials, spin-injection efficiency, the potential proximity effects, and much more
This book reviews the structure and electronic, magnetic, and other properties of various MoS2 (Molybdenum disulfide) nanostructures, with coverage of synthesis, Valley polarization, spin physics, and other topics. MoS2 is an important, graphene-like layered nano-material that substantially extends the range of possible nanostructures and devices for nanofabrication. These materials have been widely researched in recent years, and have become an attractive topic for applications such as catalytic materials and devices based on field-effect transistors (FETs) and semiconductors. Chapters from leading scientists worldwide create a bridge between MoS2 nanomaterials and fundamental physics in order to stimulate readers' interest in the potential of these novel materials for device applications. Since MoS2 nanostructures are expected to be increasingly important for future developments in energy and other electronic device applications, this book can be recommended for Physics and Materials Science and Engineering departments and as reference for researchers in the field.
Defects in Two-Dimensional Materials addresses the fundamental physics and chemistry of defects in 2D materials and their effects on physical, electrical and optical properties. The book explores 2D materials such as graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMD). This knowledge will enable scientists and engineers to tune 2D materials properties to meet specific application requirements. The book reviews the techniques to characterize 2D material defects and compares the defects present in the various 2D materials (e.g. graphene, h-BN, TMDs, phosphorene, silicene, etc.). As two-dimensional materials research and development is a fast-growing field that could lead to many industrial applications, the primary objective of this book is to review, discuss and present opportunities in controlling defects in these materials to improve device performance in general or use the defects in a controlled way for novel applications. Presents the theory, physics and chemistry of 2D materials Catalogues defects of 2D materials and their impacts on materials properties and performance Reviews methods to characterize, control and engineer defects in 2D materials