Universal classification of twisted, strained and sheared graphene moir\'e superlattices

Moir\'e superlattices in graphene supported on various substrates have opened a new avenue to engineer graphene's electronic properties. Yet, the exact crystallographic structure on which their band structure depends remains highly debated. In this scanning tunneling microscopy and density functional theory study, we have analysed graphene samples grown on multilayer graphene prepared onto SiC and on the close-packed surfaces of Re and Ir with ultra-high precision. We resolve small-angle twists and shears in graphene, and identify large unit cells comprising more than 1,000 carbon atoms and exhibiting non-trivial nanopatterns for moir\'e superlattices, which are commensurate to the graphene lattice. Finally, a general formalism applicable to any hexagonal moir\'e is presented to classify all reported structures.Comment: 14 pages, 6 figure

Few layers graphene on 6H-SiC(000-1): an STM study

We have analyzed by Scanning Tunnelling Microscopy (STM) thin films made of few (3-5) graphene layers grown on the C terminated face of 6H-SiC in order to identify the nature of the azimuthal disorder reported in this material. We observe superstructures which are interpreted as Moir\'e patterns due to a misorientation angle between consecutive layers. The presence of stacking faults is expected to lead to electronic properties reminiscent of single layer graphene even for multilayer samples. Our results indicate that this apparent electronic decoupling of the layers can show up in STM data.Comment: 20 page

Convergent Fabrication of a Nanoporous Two-Dimensional Carbon Network from an Aldol Condensation on Metal Surfaces

We report a convergent surface polymerization reaction scheme on Au(111), based on a triple aldol condensation, yielding a carbon-rich, covalent nanoporous two-dimensional network. The reaction is not self-poisoning and proceeds up to a full surface coverage. The deposited precursor molecules 1,3,5-tri(4'-acetylphenyl) first form supramolecular assemblies that are converted to the porous covalent network upon heating. The formation and structure of the network and of the intermediate steps are studied with scanning tunneling microscopy, Raman spectroscopy and density functional theory.Comment: 1 Scheme, 5 Figure

Electronic structure of epitaxial graphene layers on SiC: effect of the substrate

Recent transport measurements on thin graphite films grown on SiC show large coherence lengths and anomalous integer quantum Hall effects expected for isolated graphene sheets. This is the case eventhough the layer-substrate epitaxy of these films implies a strong interface bond that should induce perturbations in the graphene electronic structure. Our DFT calculations confirm this strong substrate-graphite bond in the first adsorbed carbon layer that prevents any graphitic electronic properties for this layer. However, the graphitic nature of the film is recovered by the second and third absorbed layers. This effect is seen in both the (0001)and $(000\bar{1})$ 4H SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. It causes the graphene to be doped and gives rise to a gap opening at the Dirac point after 3 carbon layers are deposited in agreement with recent ARPES experiments (T.Ohta et al, Science {\bf 313} (2006) 951)

Propriétés électroniques et structurales du graphène sur carbure de silicium

Le graphène est un plan unique d'atomes de carbone formant une structure en nid d'abeilles. Dans le cas idéal, le graphène possède des propriétés physiques étonnantes, comme une structure électronique en cône de Dirac . Depuis 2004, il est connu qu'on peut obtenir ce matériau bidimensionnel à partir de la graphitisation du carbure de silicium (SiC). Sur la base de calculs ab initio et d'expériences de microscopie à effet tunnel (STM), nous avons entrepris de sonder les propriétés électroniques et structurales du graphène sur SiC et de déterminer en quoi elles sont similaires ou au contraire différentes du graphène idéal. Ce manuscrit commence par une introduction générale sur la thématique du graphène et se poursuit par une description des deux méthodes utilisées durant ce travail. Il vient ensuite l'exposé de nos résultats obtenus pour le graphène sur la face terminée Si et celle terminée C des polytypes hexagonaux du SiC. Nous avons montré notamment que le premier plan de carbone généré sur la face terminée Si se comporte comme un plan tampon, lequel permet aux autres plans qui le recouvrent d'avoir une structure électronique de type monoplan/multiplan de graphène. D'autres aspects liés à la nature complexe de l'interface comme la présence d'états localisés ou l'existence d'une forte structuration du plan tampon sont également discutés. Pour une surface terminée C suffisamment graphitisée, nos travaux révèlent l'existence d'un désordre rotationnel entre les plans de graphène successifs qui se manifeste sous forme de Moiré sur les images STM. Nous montrons par des calculs ab initio qu'une simple rotation permet de découpler électroniquement les plans de graphène.This work is devoted to theoretical and experimental studies of graphene on silicon carbide. Graphene consists of a single carbon plane arranged on a honeycomb lattice. Because of this peculiar lattice, ideal graphene exhibits outstanding electronic properties such as massless Dirac fermions close to the Fermi energy. From experimental point of view, a convenient way to synthetize graphene is the thermal decomposition of the hexagonal faces of a silicon carbide (SiC) crystal. In the present work, electronic and structural properties of graphene on SiC are studied by ab initio calculations and scanning tunneling microscopy. They are compared to the ideal graphene case. After an introduction on the graphene topic, we describe ab initio calculations (based on density functional theory) and scanning tunnelling microscopy which are the two methods used in this thesis. Then we present our different results for graphene on both surfaces of SiC (Si- and C-face, respectively). For the Si-face, we demonstrate the existence of a strong interaction between the substrate and the first carbon layer. This prevents any graphitic electronic properties for this layer (buffer layer). However, the graphitic nature of the film is recovered by the second and the third absorbed layers. For the C-face, we have analyzed by STM thin films made of few graphene layers. We observe superstructures which are interpreted as Moiré patterns due to a misorientation angle between consecutive layers. Ab initio calculations are used to demonstrate that a twisted bilayer presents a band structure with the linear dispersion characteristic of isolated graphene.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Etude de l'interface graphène - SiC(000-1) (face carbone) par microscopie à effet tunnel et simulations numériques ab initio

Le graphène est un cristal bidimensionnel composé d'atomes de carbone arrangés sur un réseau en nids d'abeille. Ce matériau présente des propriétés électroniques intéressantes tant au niveau fondamental qu'en vue d'applications avec notamment une structure de bande exotique en cône de Dirac et de grandes mobilités de porteurs. Sa fabrication par graphitisation du SiC est particulièrement adaptée aux applications électroniques. Nous avons étudié ce système par microscopie à effet tunnel (STM) et simulations numériques ab initio avec comme objectif la caractérisation au niveau atomique de l'interface graphène - SiC(000-1) (face carbone) et l'étude de l'impact du substrat sur la structure électronique du graphène. Après un chapitre introductif à la thématique du graphène, suivi d'un chapitre présentant les deux techniques utilisées au cours de ce travail, nous présentons nos échantillons faiblement graphitisés obtenus sous ultra-vide. Nous avons identifié deux types d'interfaces, les reconstructions natives de la surface du SiC(000-1) appelées (2x2)C et (3x3), sur lesquelles reposent les ilots monoplan de graphène, avec un fort désordre rotationnel donnant lieu à des figures de moiré sur les images STM. Nous montrons par imagerie STM et spectroscopie tunnel que l'interaction graphène/(3x3) est très faible. Nous étudions ensuite le cas d'interaction plus forte graphène/(2x2) successivement du point de vue des états du graphène et des états de la reconstruction, dans l'espace direct et réciproque, de façon expérimentale et théorique. Enfin, nous considérons l'effet de défauts observés par STM à l'interface des ilots sur (2x2), modélisés par des adatomes d'hydrogène, sur le dopage et la structure de bande électronique du graphène.Graphene refers to a two-dimensional crystal made of carbon atoms arranged on a honeycomb lattice. This material presents interesting electronic properties regarding fundamental physics as well as industrial applications, such as an exotic low-energy band structure and high charge carrier mobility. Its fabrication through the graphitization of SiC is a promising method for electronics. We studied this system using scanning tunnelling microscopy (STM) and ab initio numerical simulations with the aim of characterizing the graphene - SiC(000-1) (carbon face) interface and studying the impact of the substrate on graphene's electronic structure. After an introduction to the graphene topic and a description of our investigation techniques, we present our lightly graphitized samples obtained under ultra-high vacuum. We identify two interface structures, the native SiC(000-1) surface reconstructions named (2x2)C and (3x3), on top of which lie graphene monolayer islands with a high rotational disorder leading to various moiré patterns on STM images. Using STM, we show that the graphene/(3x3) interaction is very weak. We then study the stronger graphene/(2x2) interaction successively from the point of view of the graphene and the reconstruction states, in the direct and reciprocal space, using both our experimental and theoretical methods. Finally, we consider the impact of interfacial defects observed by STM through graphene/(2x2) islands and modelled with hydrogen adatoms on the electronic band structure and doping of grapheneSAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Graphene buffer layer on Si-terminated SiC studied with an empirical interatomic potential

International audienceThe atomistic structure of the graphene buffer layer on Si-terminated SiC is investigated using a modified version of the environment-dependent interatomic potential. The determination of the equilibrium state by the conjuguate gradients method suffers from a complex multiple-minima energy surface. The initial configuration is therefore modified to set the system in specific valleys of the energy surface. The solution of minimal energy forms a hexagonal pattern composed of stuck regions separated by unbonded rods that release the misfit with the SiC surface. The structure presents the experimental symmetries and a global agreement with an ab initio calculation. It is therefore expected that the interatomic potential could be used in classical molecular dynamics calculations to study the graphene growth

Ab initio study of boron-hydrogen complexes in diamond and their effect on electronic properties

7 pagesInternational audienceThe atomic and electronic structures of neutral and charged (+ and −) boron-hydrogen complexes in diamond are studied by means of density-functional theory calculations. The stability of the different configurations is discussed and used to derive ionization energies. For neutral B-H complex, H in puckered position along B-C axis is energetically more favorable. No density of states is then found within the diamond band gap but this configuration gives rise to an acceptor level at Ev+4.44 eV. B-H2 most stable structure gives a donor level at Ec−2.80 eV. Other geometries are also investigated to explain the discrepancy observed between recent electronic structure calculations. © 2008 The American Physical Societ

Deep hole traps in boron-doped diamond

International audienceDeep hole traps in boron-doped diamond epitaxial layers are studied by means of several types of deep-level transient spectroscopy and density-functional theory calculations. Standard deep-level transient spectroscopy and highresolution isothermal transient spectroscopy permit to identify nine deep hole traps. Their capture cross-sections and ionization energies are systematically determined. In parallel, the ionization energies of donor and acceptor levels related to boron- and/or hydrogen-related complexes in diamond are assessed by ab initio calculations in this work and summarized with others from the literature including native defects. Tentative assignments of the measured deep hole traps to the calculated ones are proposed