Graphene: Status and Prospects

Graphene is a wonder material with many superlatives to its name. It is the thinnest material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have the smallest effective mass (it is zero) and can travel micrometer-long distances without scattering at room temperature. Graphene can sustain current densities 6 orders higher than copper, shows record thermal conductivity and stiffness, is impermeable to gases and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a bench-top experiment. What are other surprises that graphene keeps in store for us? This review analyses recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.Comment: pre-edited version of the review published in Science Please note that only 40 references are allowed by the magazine. Sorr

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

A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene

Colloquium: Graphene spectroscopy

Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.Comment: 36 pages, 16 figure

Electronic transport in two dimensional graphene

We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.Comment: Final version as accepted for publication in Reviews of Modern Physics (in press), 69 pages with 38 figure

Edge states, magnetisation and topological domain walls in graphene

We studied the edge states and their roles in conductivity and magnetism of graphene nanoribbions and flakes. we studied the Aharonov-Bohm effect in graphene nanodisks and rings. We described the quantum oscillations of the magnetization of graphene flakes. we have examined the snake-like states of transport electrons in the configurations of graphene ribbons with a domain wall in the centre

Strain engineering of graphene

The focus of this thesis is on using mechanical strain to tailor the electronic properties of graphene. The first half covers the electro-mechanical coupling for graphene in different configurations, namely a hexagonal Y-junction, various shaped bubbles on different substrates, and with kirigami cuts. For all of these cases, a novel combination of tight-binding electronic structure calculations and molecular dynamics is utilized to demonstrate how mechanical loading and deformation impacts the resulting electronic structure and transport. For the Y-junction, a quasi-uniform pseudo magnetic field induced by strain restricts transport to Landau-level and edge-state-assisted resonant tunneling. For the bubbles, the shape and the nature of the substrate emerge as decisive factors determining the effectiveness of the nanoscale pseudo magnetic field tailoring in graphene. Finally, for the kirigami, it is shown that the yield and fracture strains of graphene, a well-known brittle material, can be enhanced by a factor of more than three using the kirigami structure, while also leading to significant enhancements in the localized pseudo magnetic fields. The second part of the thesis focuses on dissipation mechanisms in graphene nanomechanical resonators. Thermalization in nonlinear systems is a central concept in statistical mechanics and has been extensively studied theoretically since the seminal work of Fermi, Pasta, and Ulam (FPU). Using molecular dynamics and continuum modeling of a ring-down setup, it is shown that thermalization due to nonlinear mode coupling intrinsically limits the quality factor of nanomechanical graphene drums and turns them into potential test beds for FPU physics. The relationship between thermalization rate, radius, temperature and prestrain is explored and investigated

Fabrication and characterization of Quantum Materials: Graphene heterostructures and Topological Insulators

[ES]La tesis empieza con una descripción de la Sala blanca de Salamanca y de su equipamiento, instalado durante los primeros años de mi doctorado. Sigue una detallada explicación de los procesos de fabricación de dispositivos en grafeno y otros materiales bidimensionales. En particular el sistema de trasferencia y la realización de contactos de borde tienen un rol fundamental en la realización de dispositivos de alta calidad. En nuestros dispositivos de grafeno encapsulado en nitruro de boro hexagonal hemos observado efecto Hall cuántico (QHE) a temperatura ambiente bajo la aplicación de altos campos magnéticos. El QHE en nuestros dispositivos de alta movilidad tiene características diferentes del QHE en dispositivos de grafeno de baja movilidad. Hemos también estudiado el transporte balístico y casi balístico en constricciones de grafeno con media y alta movilidad. En particular en las constricciones de mayor movilidad hemos introducido un método de definición de la constricción a bajas temperaturas, por la primera vez aplicado a dispositivos de grafeno y que nos han permitido obtener bordes con muy baja rugosidad. Esto ha permitido obtener un comportamiento balístico cerca del ideal y la observación de cuantización de la conductancia. En la última parte de la tesis reportamos medidas de transporte en pozos cuánticos de InAs/GaSb con diferente configuración de bandas (aislante, invertida y crítica). En la muestra crítica hemos encontrado una resistencia longitudinal anormal que hemos justificado con la posible formación de un excitón en bajas temperaturas.[EN]Starting from a detailed description of the Clean Room facilities, installed during this thesis work, we report the fabrication processes based on graphene and other 2D materials in detail. In hBN-encapsulated graphene the Quantum Hall Effect (QHE) at room temperature and high magnetic field was observed. We found different features in the QHE respect a previous work on lower mobility graphene on silicon oxide (Novoselov et al. Science 315 1379 2007). A detalied study of transport properties in graphene nanoconstrictionsis also reported. In particular in encapsulated graphene we introduced a new cryo-etching method to obtain low roughness edges nanocostrictions, in which quantized conductance was observed. In the last part of the thesis we report transport measurements on InAs/GaSb double quantum wells with different bandgap configurations (inverted, normal or critical)

Low-Dimensional Materials at the Nanoscale: Transition Metal Chalcogenides, Carbon Nanomaterials and Organic Semiconductors

The overall theme of this dissertation is the electronic transport and electromechanical study of low dimensional materials at the nanoscale. The dissertation is divided into three parts based on the class of materials: I. collective ground states in ultrathin materials, II. carbon nanomaterials based nanomechanical resonators and III. organic semiconductors. In part I, the superconductivity and charge density waves in transition metal chalcogenides are introduced. Crystal synthesis of transition metal chalcogenides by chemical vapor transport is presented. The materials have quasi-low dimensional crystal structure: either quasi-two dimensional (e.g. NbSe2, TaS2, WTe2, FeSe) or quasi-one dimensional (e.g. NbSe3, TaS3, (NbSe4)3I). Monolayer NbSe2, grown by molecular beam epitaxy, shows a superconducting transition at Tc=2K and is studied down to 50mK with magnetic fields. The sliding charge density waves in NbSe3 nanoribbons are studied with narrowband noise, which directly probes the order parameter. A proposal to scale down the contactless conductivity measurement technique for nanoscale samples with lithographically fabricated planar coils is presented.In part II, microstructures of suspended carbon nanotubes and graphene are studied as nanomechanical resonators. Carbon nanotubes are clamped on one end and the other end is free to enable field emission. The field emission provides a means of electrical readout. Fabrication of carbon nanotube field emitting mechanical resonators on an integrated platform are explored. The platform is designed to allow the study of the nanomechanical motion across multiple characterization techniques. Graphene nanomechanical resonators are studied as a first step in the development of a microactuator-based platform to control strain fields in graphene. In particular, non-uniaxial strains for large pseudo-magnetic field effects are intended.In part III, organic nanowire formation with DPP-TPA molecules for use in photovoltaics is explored. The nanowire’s charge carrier mobility is characterized in a field effect transistor. In addition, the use of rubrene single crystals for the study of photophysics at the interface with novel acceptor molecules is explored

Tuning the properties of group III-As in the thinnest limit: a theoretical study of single layer and 2D-heterostructures

El presente trabajo aporta nuevos conocimientos teóricos a la investigación de los materiales bidimensionales, conformados por elementos del grupo III-As (BAs, GaAs, InAs). Igualmente sirve de fundamento para futuras investigaciones en materiales bidimensionales, tanto teóricas como experimentales. En esta tesis se lleva a cabo una investigación de primeros principios a partir de la teoría Funcional de la Densidad (DFT) para sintonizar las propiedades físicas de los materiales del grupo III-V en el límite más delgado utilizando: multicapas bidimensionales, tensión, funcionalización con hidrógeno y adsorción de metales de transición.DoctoradoDoctor en Ciencias Naturale