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

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

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.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

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

Ripples in epitaxial graphene on the Si-terminated SiC (0001) surface

International audienceInteraction with a substrate can modify the graphene honeycomb lattice and thus alter its out- standing properties. This could be particularly true for epitaxial graphene where the carbon layers are grown from the SiC substrate. Extensive ab initio calculations supported by Scanning Tunneling Microscopy experiments demonstrate here that the substrate indeed induces a strong nanostruc- turation of the interface carbon layer. It generates an apparent 6x6 modulation different from the interface 6√3×6√3R30 symmetry used for the calculation. The top carbon layer roughly follows the interface layer morphology. This creates soft 6x6 ripples in the otherwise graphene-like hon- eycomb lattice. The wavelength and height of the ripples are much smaller than the one found in exfoliated graphene. Their formation mechanism also differs: They are due to the weak interaction with the interface layer and not to a roughening of the plane due to the instability of a strictly two-dimensional crystal

Interface structure of graphene on SiC: an ab initio and STM approach

High temperature treatment of SiC surfaces is a well-established technique for producing graphene directly on top of an insulating substrate. In this domain an important question is the influence of the substrate on the atomic and electronic structure of the graphene layers. This requires a detailed investigation of the interactions at the graphene-SiC interface. Surface science techniques and ab initio calculations are well suited for that purpose. In this paper, we present a brief review of the recent investigations performed in this domain by scanning tunnelling microscopy (STM) and ab initio simulations. It is largely based on the work performed in our group, but it also provides a survey of the literature in these fields. Both the so-called Si and C face of the hexagonal 6H(4H)SiC{0 0 0 1} substrates will be considered, as they show markedly different types of behaviour

Graphene-substrate interaction on 6H-SiC(0001¯): A scanning tunneling microscopy study

International audienceThe early stage of graphene formation on the 6H-SiC 0001¯ surface is investigated by scanning tunneling microscopy. Islands made of a single graphitic plane form above regions of the substrate that show either a 2 2 C or a 3 3 reconstruction. The orientations of the single-layer domains present a broad distribution of rotation angles with respect to the substrate. The atomic structures of the 3 3 and 2 2 C substrate reconstructions are preserved under the first carbon layer. Low bias images reveal a graphitic structure, which indicates that the interaction between the first carbon plane and the SiC surface is comparatively much weaker on the C face than on the Si face of the substrate. The coupling is stronger on the 2 2 C surface reconstruction than on the 3 3 one, where an almost ideal graphene structure is found close to the Fermi level

Scanning Tunneling Microscopy investigation of the graphene/6H-SiC(000N1) (3 3) interface

73.20.-r 68.55.-aInternational audienceThe early stages of the graphitization of the 6H-SiC(000N1) (3 3) surface in an Ultra-High Vacuum (UHV) are investigated by means of scanning tunneling microscopy (STM). Different kinds of graphitic islands are found on the surface. One kind (called G_3 3) consists of a single graphene-like carbon plane covering the initial (3 3) reconstructed substrate surface. We observe a broad distribution of rotation angles between the substrate and the carbon plane revealed by superstructures of different directions and periods. Low bias images show that the graphene structure is preserved close to the Fermi level. The data indicate a weak substrate-graphene coupling for the G_3 3 islands