Quantum Hall Effect in Fractal Graphene: Growth and Properties of Graphlocons

Highly dendritic graphene crystals up to 0.25 mm in diameter are synthesized by low pressure chemical vapor deposition inside a copper enclosure. With their sixfold symmetry and fractal-like shape, the crystals resemble snowflakes. The evolution of the dendritic growth features is investigated for different growth conditions and surface diffusion is found to be the growth-limiting step responsible for the formation of dendrites. The electronic properties of the dendritic crystals are examined down to sub-Kelvin temperatures, showing a mobility of up to 6300 cm$^2$V$^{-1}$s$^{-1}$ and quantum Hall oscillations are observed above 4T. These results demonstrate the high quality of the transport properties despite their rough dendritic edges

Ultrafast optoelectronics in 2D materials and their heterostructures

Understanding and improving how light is converted into electricity in materials is one of the central goals in the field of optoelectronics. This basic physical process lies at the heart of several major technologies, including solar cells and ultrafast optical communication systems, which have pervaded and shaped the world we live in. As technological performance approaches the limit of conventional materials, the need for optoelectronic platforms presenting novel properties and superior characteristics, in terms of speed, efficiency and wavelength range, is rapidly growing. Two-dimensional (2D) layered materials, such as graphene and transition metal dichalcogenides (TMDs), have recently emerged as prime candidates optoelectronic applications. This new class of one-atom-thick materials has sparked huge interest due to their exceptional electrical and optical properties, which can be very different from those of their bulk counterpart. Since the first isolation of graphene in 2004, the library of 2D materials has grown considerably and now comprises many other crystals covering a wide range of complementary properties. Assembling these 2D building blocks into vertical heterostructures opens up exciting possibilities for designing artificial materials with atomic-layer precision. The resulting van der Waals heterostructures (vdWH), in addition to combining the properties of their constituent layers, provide a rich playground for studying photophysical phenomena and implementing novel photodetection schemes. The goal of this thesis is to explore the optoelectronic response of devices based on 2D materials and vdWHs in order to understand the dynamic processes governing their photocurrent generation mechanisms, and thereby facilitate the design of high-performance photodetectors. From the broad library of 2D materials, we focus our attention on the three that have attracted the most interest: graphene, TMDs and hexagonal boron nitride (hBN). These materials possess entirely distinct optoelectronic properties. For instance, graphene, a semimetal, displays a broadband photoresponse dominated by hot carriers, whereas the optical response of semiconducting TMDs is governed by strong excitonic effects. Studying how these light-matter interactions induce an electric signal in actual devices poses many experimental challenges. Besides the fabrication of high-quality devices, one of the main difficulties is to assess the ultrafast electronic processes involved in the photocurrent generation. To this end, we developed a versatile time-resolved photocurrent measurement technique (TRPC) which combines electronic detection with subpicosecond optical excitation. This thesis comprises three introductory and technical chapters and four experimental chapters, each covering a different 2D material system. Chapter 1 gives an overview of the electronic and optical properties of graphene, TMDs, hBN and their vdHWs, with an emphasis on the dynamics of their photocarriers. Chapter 2 presents the basic photodetection concepts and measurement techniques relevant for this thesis, along with a review of the main photocurrent generation mechanisms observed in 2D materials. Chapter 3 describes techniques to fabricate state-of-the-art devices based on 2D materials and vdWHs. Chapter 4 presents an in-depth study on the photocurrents generated laterally in various graphene devices and an investigation of the ultrafast heating dynamics of the hot carriers driving this process. Chapter 5 explores the interlayer transport of photocarriers in graphene-based vdWHs and demonstrates the possibility of extracting hot carriers vertically. Chapter 6 shows that vdWHs made of thin TMD crystals can possess both a high efficiency and a fast photoresponse, and examines the processes that limit their performance. Finally, Chapter 7 reports on the dissociation of excitons under in-plane electric fields in monolayer WSe2, and on the related Stark and Franz-Keldysh effects.Materiales bidimensionales (2D) en capas, tales como el grafeno y los dicalcogenuros de metales de transición (TMD), han surgido recientemente como candidatos principales para aplicaciones fotónicas y optoelectrónicas. Esta nueva clase de materiales de un átomo de espesor ha despertado un enorme interés debido a sus propiedades eléctricas y ópticas excepcionales, que pueden ser drásticamente diferentes de las de su contrapartida tridimensional (3D). Desde la primera exfoliación de grafeno en 2004, la biblioteca de materiales 2D se ha expandido considerablemente y ahora comprende muchos otros cristales cubriendo una amplia gama de propiedades complementarias. Montar estos bloques de construcción 2D en heteroestructuras verticales abre un abanico de posibilidades emocionantes para el diseño de materiales artificiales con precisión atómica. Las heteroestructuras de Van der Waals (vdWH) que resulta, además de combinar las propiedades de sus capas constituyentes, proporcionan un amplio terreno de juego para estudiar fenómenos fotofísicos e implementar nuevos esquemas de fotodetección. El objetivo de esta tesis es explorar la respuesta optoelectrónica de dispositivos basados en materiales 2D y sus heteroestructuras para comprender los procesos dinámicos que rigen sus mecanismos de generación de fotocorriente y facilitar así el diseño de fotodetectores con mayor rendimiento. De la amplia biblioteca de materiales 2D, nos centramos en los tres que más han llamado la atención: el grafeno, los TMDs y el nitruro de boro hexagonal (hBN). Estos materiales poseen propiedades optoelectrónicas totalmente distintas. Por ejemplo, el grafeno, un semimetal, muestra una fotorrespuesta de banda ancha dominada por portadores calientes, mientras que la respuesta óptica de los TMD semiconductores se rige por fuertes efectos excitónicos. Estudiar cómo la interaccion luz-materia inducen una señal eléctrica en dispositivos reales plantea muchos desafíos experimentales. Además de la fabricación de dispositivos de alta calidad, una de las principales dificultades es determinar los procesos electrónicos ultra-rápidos que intervienen en la generación de fotocorriente. Para ello, aplicamos y desarrollamos una técnica versátil de medición de fotocorriente de resolución en tiempo (TRPC) que combina la detección electrónica con la excitación óptica subpicosegunda. Esta tesis comprende tres capítulos introductorios y técnicos y cuatro capítulos experimentales, cada uno de los cuales abarca un sistema material 2D diferente. El Capítulo 1 da una visión general de las fascinantes propiedades electrónicas y ópticas del grafeno, TMDs, hBN y sus heteroestructuras, con énfasis en la dinámica de sus fotoportadores. El Capítulo 2 presenta los conceptos básicos de fotodetección y las técnicas de medición relevantes para esta tesis, junto con una revisión de los principales mecanismos de generación de fotocorriente observados en materiales 2D. El Capítulo 3 describe las técnicas empleadas para fabricar dispositivos de última generación y alta calidad basados en materiales 2D y vdWHs. El Capítulo 4 presenta un estudio en profundidad sobre la fotocorriente generada lateralmente en varios dispositivos de grafeno y sobre la dinámica de calentamiento ultrarrápida de los portadores calientes que impulsan dicho proceso. El Capítulo 5 explora el transporte intercapa de fotoportadores en heteroestructuras a base de grafeno y demuestra la posibilidad de extraer portadores calientes verticalmente. El Capítulo 6 muestra que las heteroestructuras hechas de cristales finos de TMD pueden poseer tanto alta eficiencia como una rápida fotorrespuesta, del orden de algunos picosegundos, y se examinan los procesos dinámicos que limitan su rendimiento. Por último, el Capítulo 7 informa sobre la ionización por efecto túnel de los excitones bajo altos campos eléctricos en el plano de la monocapa WSe2, y sobre los efectos relacionados Stark y Franz-Keldysh

Weak Localization in Graphene: Theory, Simulations, and Experiments

We provide a comprehensive picture of magnetotransport in graphene monolayers in the limit of nonquantizing magnetic fields. We discuss the effects of two-carrier transport, weak localization, weak antilocalization, and strong localization for graphene devices of various mobilities, through theory, experiments, and numerical simulations. In particular, we observe a minimum in the weak localization and strong localization length reminiscent of the minimum in the conductivity, which allows us to make the connection between weak and strong localization. This provides a unified framework for both localizations, which explains the observed experimental features. We compare these results to numerical simulation and find a remarkable agreement between theory, experiment, and numerics. Various graphene devices were used in this study, including graphene on different substrates, such as glass and silicon, as well as low and high mobility devices

Ultrafast nonlinear optical response of Dirac fermions in graphene

The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we investigate the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.Peer ReviewedPostprint (published version

Experimental review of graphene

This review examines the properties of graphene from an experimental perspective. The intent is to review the most important experimental results at a level of detail appropriate for new graduate students who are interested in a general overview of the fascinating properties of graphene. While some introductory theoretical concepts are provided, including a discussion of the electronic band structure and phonon dispersion, the main emphasis is on describing relevant experiments and important results as well as some of the novel applications of graphene. In particular, this review covers graphene synthesis and characterization, field-effect behavior, electronic transport properties, magneto-transport, integer and fractional quantum Hall effects, mechanical properties, transistors, optoelectronics, graphene-based sensors, and biosensors. This approach attempts to highlight both the means by which the current understanding of graphene has come about and some tools for future contributions.Comment: Equal contributions from all author

Petahertz optical response in graphene

The temporal dynamics of charge carriers determines the speed with which electronics can be realized in condensed matter, and their direct manipulation with optical fields promises electronic processing at unprecedented petahertz frequencies, consisting in a million-fold increase from state of the art technology. Graphene is of particular interest for the implementation of petahertz optoelectronics due to its unique transport properties, such as high carrier mobility with near-ballistic transport and exceptionally strong coupling to optical fields. The back action of carriers in response to an optical field is therefore of key importance towards applications. Here we investigate the instantaneous response of graphene to petahertz optical fields and elucidate the role of hot carriers on a sub-100 fs timescale. Measurements of the nonlinear response and its dependence on interaction time and field polarization allow us to identify the back action of hot carriers over timescales that are commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.Comment: 6 pages, 4 figure

Dissociation of two-dimensional excitons in monolayer WSe₂

In two-dimensional semiconductors excitons are strongly bound, suppressing the creation of free carriers. Here, the authors investigate the main exciton dissociation pathway in p-n junctions of monolayer WSe2 by means of time and spectrally resolved photocurrent measurements

Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure

Ultrafast electron thermalization - the process leading to Auger recombination, carrier multiplication via impact ionization and hot carrier luminescence - occurs when optically excited electrons in a material undergo rapid electron-electron scattering to redistribute excess energy and reach electronic thermal equilibrium. Due to extremely short time and length scales, the measurement and manipulation of electron thermalization in nanoscale devices remains challenging even with the most advanced ultrafast laser techniques. Here, we overcome this challenge by leveraging the atomic thinness of two-dimensional van der Waals (vdW) materials in order to introduce a highly tunable electron transfer pathway that directly competes with electron thermalization. We realize this scheme in a graphene-boron nitride-graphene (G-BN-G) vdW heterostructure, through which optically excited carriers are transported from one graphene layer to the other. By applying an interlayer bias voltage or varying the excitation photon energy, interlayer carrier transport can be controlled to occur faster or slower than the intralayer scattering events, thus effectively tuning the electron thermalization pathways in graphene. Our findings, which demonstrate a novel means to probe and directly modulate electron energy transport in nanoscale materials, represent an important step toward designing and implementing novel optoelectronic and energy-harvesting devices with tailored microscopic properties.Comment: Accepted to Nature Physic