Quasiparticle Energy and Excitons in Two-Dimensional Structures

Two-dimensional materials, such as graphene-related structures, transition metal dichalcogenides, are attracting enormous interest in nowadays condensed matter physics. They not only serve as ideal testbeds for rich physics in reduced-dimensional electron systems but are also of particular importance in nanoelectronic technology. Their electronic, transport, and optical properties are largely determined by the nature of excited states, such as quasiparticles and excitons. Understanding how these excited states emerge from a many-electron system is an intriguing intellectual process, which gives insight into experimental observation and sheds light on manipulating the materials\u27 properties. From this aspect, it is highly desirable to introduce many-body perturbation theories, which do not rely on data from experiments, to study these excited-state properties and their relations to experimental measurements.In thisthesis, I will present a comprehensive study on a variety of two-dimensional materials using first-principles calculation with many-body effects taken into account. Particular attention is given to the impact of electrical gating, stacking order, and doping on the quasiparticle and excitonic properties

Novel effects of strains in graphene and other two dimensional materials

The analysis of the electronic properties of strained or lattice deformed graphene combines ideas from classical condensed matter physics, soft matter, and geometrical aspects of quantum field theory (QFT) in curved spaces. Recent theoretical and experimental work shows the influence of strains in many properties of graphene not considered before, such as electronic transport, spin-orbit coupling, the formation of Moir\'e patterns, optics, ... There is also significant evidence of anharmonic effects, which can modify the structural properties of graphene. These phenomena are not restricted to graphene, and they are being intensively studied in other two dimensional materials, such as the metallic dichalcogenides. We review here recent developments related to the role of strains in the structural and electronic properties of graphene and other two dimensional compounds.Comment: 75 pages, 15 figures, review articl

Electronic Structure of Novel Two-dimensional Materials and Graphene Heterostructures

Today a well-equipped library of two-dimensional materials can be synthesized or exfoliated, ranging from insulating hexagonal boron nitride, to semi-metallic graphene, and metallic as well as superconducting transition metal dichalcogenides and many others. Due to strong intra-layer covalent bondings, but weak inter-layer Van-der-Waals interactions, these layered materials can be stacked in a Lego-like fashion to artificial heterostructures which do not occur in nature. Thereby, these novel systems offer the possibility to combine specific properties of each of its constituents to tailor the heterostructure's properties on demand which might allow for completely new device classes. In fact, these kind of systems are already constructed and studied in labs around the world. In order to guide these efforts, we need an in-depth understanding of these complex heterostructures starting with its smallest components, namely the different two-dimensional materials and their mutual interactions. To this end, we study electronic and optical properties of novel two-dimensional materials in this thesis. In more detail, we here aim to investigate functionalized graphene, graphene heterostructures and doped or optically excited molybdenum disulfide (MoS$_2$) monolayers for which we combine \abinitio based models with many-body or multi-scale approaches. The first part is devoted to functionalized graphene and is subdivided into the investigation of disorder-induced optical effects of fluorographene and into a detailed study of the Coulomb interaction in graphene heterostructures in form of multilayer graphene, intercalated graphite and few-layer graphene within a dielectric environment. In the case of fluorographene we use a multi-scale approach to study the effects of realistic disorder patterns to the optical conductivity. Thereby, we provide important insights into the role of non-perfect fluorination of graphene. Regarding the graphene heterostructures we present a novel approach to easily and reliably derive Coulomb-interaction matrix elements in these structures. This method is used to study the robustness of bilayer graphene's ground state to changes in its dielectric surrounding. In the second part of the thesis we study a variety of many-body effects that arise in doped and optically excited MoS$_2$ monolayers. Once again, by deriving simplified yet accurate models from first-principles we are able to investigate many-body excitations like plasmons or excitons as well as many-body instabilities like superconductivity or charge-density wave phases. Regarding the latter, we are able to extend the electron-doping phase diagram of MoS$_2$ by the formation of a charge-density-wave phase and reveal its potential coexistence with the superconducting state. In the field of many-body excitations we study in detail excitonic line shifts upon optical excitations and we precisely describe different types of plasmonic excitations under electron or hole doping in MoS$_2$. Finally, we make use of the fundamental properties of the many-body interactions in layered materials in order to externally induce heterojunctions within homogeneous semiconducting monolayers by non-local manipulations of the Coulomb interaction

Graphene for Electronics

Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a two-dimensional (2D) honeycomb lattice. Graphene's unique properties of thinness and conductivity have led to global research into its applications as a semiconductor. With the ability to well conduct electricity at room temperature, graphene semiconductors could easily be implemented into the existing semiconductor technologies and, in some cases, successfully compete with the traditional ones, such as silicon. This reprint presents very recent results in the physics of graphene, which can be important for applying the material in electronics

Present and future of surface-enhanced Raman scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article

Present and Future of Surface-Enhanced Raman Scattering.

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article

The 2021 ultrafast spectroscopic probes of condensed matter roadmap

In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light–matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.Peer ReviewedPostprint (published version