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Le graphène est un matériau bidimensionnel cristallin, forme allotropique du carbone dont l'empilement constitue le graphite. Cette définition théorique est donnée par le physicien en 1947. Par la suite, le travail de différents groupes de recherche permettra de se rendre compte que la structure du graphène tout comme ses propriétés ne sont pas uniques et dépendent de sa synthèse/extraction (détaillée dans la section Production). Le graphène n'a pu être isolé et caractérisé qu'en 2004 par le groupe d'Andre Geim, du département de physique de l'université de Manchester, qui a reçu pour cela, avec Konstantin Novoselov, le prix Nobel de physique en 2010. La structure théorique du graphène a été identifiée comme constituant l'élément structurel de base d’autres formes allotropiques, comme le graphite, les nanotubes de carbone (forme cylindrique) et les fullerènes (forme sphérique). Les principales qualités sur lesquelles la recherche en science des matériaux/chimie en Europe est centr

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The students understand the relevant experimental and theoretical concepts of the nanoscale science. The course move from basic concepts like quantum size effects to hot fields such as spin transport for data storage applications (spintronics), carbon electronics, or nanocatalysis.

This course will focus on the electron transport in semiconductors, with emphasis on the mesoscopic systems. The aim is to understand the transport of electrons in low dimensional systems, where even particles with statistics different than fermions and bosons will be discussed.

This course gives the basics for understanding nanotechnology from an engineer's perspective: physical background, materials aspects and scaling laws, fabrication and imaging of nanoscale devices.

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Electrical Properties of Chemically Derived Graphene

Ravi Shankar Sundaram

Graphene, an atomically thin sheet of carbon, is the most recent endeavor for the application of carbon nanostructures in conventional electronics. The envisioned creation of devices completely carved out of graphene could lead to the revolution of electronic circuitry. However, the most established technique to obtain high quality graphene sheets, i.e, by micromechanical cleavage cannot be easily upscaled, serving as an impediment towards technological applications. The present thesis is dedicated to the study of graphene prepared via an alternative scalable, high yield and cost effective method which involves the chemical reduction of graphene oxide. The first part of this thesis describes in detail the atomic structure of graphene obtained by this method. Raman spectroscopy was used for this purpose followed by Transmission Electron Microscopy (TEM) and Near Edge X-Ray and Fine Structure (NEXAFS) measurements locally probe these sheets with atomic resolution. This revealed a highly disordered structure of this material, where regions with perfect crystallinity are separated by defect clusters. These defective patches were found to contain remnant oxygenated functional groups. Electrical characterization of chemically derived graphene sheets yielded ambipolar transfer characteristics similar to that of micromechanically cleaved graphene, albeit with moderate performance. The presence of a significant amount of disorder precludes ballistic transport and the charge carriers traverse through the sheet predominantly by thermally assisted variable range hopping in combination with a field driven tunneling mechanism. This is clearly evidenced in the low temperature two-probe conduction measurements performed on devices fabricated from these sheets. Metal contacts in graphene devices play a crucial role in limiting the applicability of these materials. Although the performance of graphene based devices as prepared today is sufficient for a variety of applications, a clear understanding and engineering of contacts is important for utilization of these materials towards high end applications. In the latter part of this thesis a detailed study of contacts is presented; firstly on graphene grown via CVD on copper to understand the physics in play at a simpler carbon-metal interface, followed by a photocurrent microscopy study to understand the more complex interface between metal and reduced graphene oxide and its implications on device characteristics. A strong difference between the two kinds of graphene is found with metal contacts to high quality graphene creating a potential barrier at the interface owing to a charge transfer between them, whereas in the case of the chemically synthesized version contacts are of a non invasive nature stable upto 300 °C. In addition, Raman spectroscopy and Scanning Photocurrent Microscopy investigation of graphene obtained by CVD under the gold, away from the contact edge provided interesting insights towards the phenomena occuring at the gold graphene interface, thus highlighting the importance of understanding and engineering adequate contacts before the successful transition of graphene based devices into the industry. Finally, a novel chemical vapor treatment of graphene oxide was employed to enhance its electrical characteristics. Exposure of reduced graphene oxide to ethylene vapor at 800 °C results in an improvement of conductivities by 2 orders of magnitude lagging behind that of pristine graphene. Electrical and structural characterization revealed that this increase in conductivity occurred inspite of the presence of a considerable amount of defects.

EPFL2011

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Electronic transport in graphene nanoribbon junctions

Kristians Cernevics

Graphene nanoribbons (GNRs) - one-dimensional strips of graphene - share many of the exciting properties of graphene, such as ballistic transport over micron dimensions, strength and flexibility, but more importantly, they exhibit a tunable band gap that depends on the atomic structure. Recent advances of fabricating atomically precise bottom-up GNRs with unprecedented control over their atomic structure have attracted interest from the field of nanoelectronics. However, a big part of the future success of GNRs depends on the ability to produce and integrate GNR junctions into complex next-generation devices. Much more effort is needed in both perfecting the production techniques and improving the theoretical understanding of these exciting nanostructures. This thesis is, therefore, devoted to exploring electronic transport properties in graphene nanoribbon junctions and unraveling their underlying structure-property relationships. Using tight-binding models, density functional theory and Green's function method, we determine the electronic properties of both experimentally synthesized and theoretically proposed junctions.In the first part of the thesis, we examine width-modulated GNR nanostructures and discover a subtle interplay between the localized states in the scattering region and the continuum of states in the leads. We show that depending on the size of the scattering region, we observe contrasting behavior on the electronic transport properties. Next, we expand the width-modulated region and show that a width-dependent transport gap opens in the presence of a quantum dot, thereby yielding built-in one-dimensional metal-semiconductor-metal junction. Part II of this thesis is dedicated to a joint experimental and theoretical effort in order to reveal the detrimental effect of

bite'' defects, resulting upon the cleavage of phenyl groups of precursor molecules. We explore their effect on the electronic transport from first-principles calculations and show how conduction is disrupted at the band edges. We then generalize our theoretical findings to other nanoribbons in a systematic manner, thus establishing guidelines to minimize the detrimental role of such defects. Later, we show that strategically placed

bite'' defects can selectively modify electronic transport properties and apply this concept to construct two prototypical components for nanoelectronics. Whereas, in Part III we employ high-throughput screening of over 400000 angled junctions in order to find potential candidates for interconnects in logic circuits and determine design rules based on structure-property relationships. We discover that the bipartite symmetry of graphene lattice and the presence of resonant states, localized at the junction, play an important role in determining the transport properties of angled junctions. Besides, we also provide a web application that allows easy design and calculation of electronic properties of GNR junctions. Finally, the last chapter of the thesis involves developing a more realistic model for transport calculations by including finite length and contact effects in order to reduce the gap between the experimental and theoretical results.

EPFL2022

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Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

Since their discovery, graphene and other 2D materials have become a subject of intense research in condensed matter physics. Especially the vast possibilities of combining those materials into heterostructures are promising for the discovery of novel physical phenomena. The heterostructures accessible through state-of-the-art techniques have suitably clean interfaces between the layers. Graphene has gained a lot of attention due to its remarkable mechanical stability and extremely high electron mobility. However, the zero bandgap of graphene is a major bottleneck for implementing this 2D material into most electronic applications. The ability to tune the properties of graphene by proximity effects has shifted the focus of graphene research towards the combination of graphene with other van der Waals materials. The main objective of the present thesis was to explore for two different types of graphene-based van der Waals heterostructure whether fingerprints of electronic proximity effects can be traced in their low temperature magnetotransport properties. The first part of the thesis deals with heterostructures of graphene and Bi2Te2Se (BTS). The strong spin-orbit coupling of BTS makes it a three-dimensional topological insulator with topological surface states which are protected by time-reversal symmetry. At the same time, BTS is promising to exert a pronounced spin-orbit proximity effect on graphene, and thereby to open a band gap and/or introduce a spin texture in the latter. The graphene/BTS heterostructures were fabricated by the direct growth of BTS on graphene to ensure a clean interface between both materials and correspondingly a good electronic coupling between them. Analysis of the weak localization effect observed in the magnetoconductivity revealed the presence of enhanced SOC in the proximitized graphene. The second part of the thesis focuses on heterostructures wherein graphene is combined with a-RuCl3 which has recently gained a lot of attention as a potential quantum spin liquid system. Previous studies on graphene/a-RuCl3 heterostructures found an unusual temperature evolution of the quantum oscillation amplitude, whose origin remained unclear. The magnetotransport data collected in this thesis point toward two possible origins for this behavior, namely spin fluctuations associated with the magnetic transition into an antiferromagnetic phase at the Néel temperature of approximately 7 K, and the hybridization-induced formation of heavy (flat) bands, both of which are likely to depend on the a-RuCl3 layer thickness. In addition, heterostructures comprising an a-RuCl3 monolayer were found to display a unusual gate dependence of the quantum oscillations, further corroborating the importance of the a-RuCl3 thickness.

EPFL2020

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Nanotube de carbone

thumb|Représentation d'un nanotube de carbone. (cliquer pour voir l'animation tridimensionnelle). thumb|Un nanotube de carbone monofeuillet. thumb|Extrémité d'un nanotube, vue au microscope électroniq

Carbone

Le carbone est l'élément chimique de et de Il possède trois isotopes naturels :

C et C qui sont stables ;
C qui est radioactif de demi-vie ce qui permet de dater des éléments utilisant du car

Nanotechnologie

Les nanosciences et nanotechnologies (d’après le grec , « nain »), ou NST, peuvent être définies au minimum comme l’ensemble des études et des procédés de fabrication et de manipulation de structures

This course gives the basics for understanding nanotechnology from an engineer's perspective: physical background, materials aspects and scaling laws, fabrication and imaging of nanoscale devices.

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