The effect of interstitial clusters and vacancies on the STM image of graphite

Making use of the tight-binding Green's function technique, we have calculated the STM images of graphite with surface and sub-surface defects, while taking into account the relaxation of the lattice due to defects. We have demonstrated that two different physical mechanisms may result in the formation of hillocks in the STM images: buckling of the graphite surface due to interstitials between the uppermost graphite layers and the enhancement of the electron density of states close to the Fermi energy on the carbon atoms in the vicinity of vacancies. Our results indicate that small hillocks may originate both from the interstitial clusters and from the vacancies. By contrast, however, large hillocks in excess of 10 \AA~ in diameter can be caused only by interstitial clusters.Comment: Submitted to Surface Scienc

Clusters of interstitial carbon atoms near the graphite surface as a possible origin of dome-like features observed by STM

Formation of clusters of interstitial carbon atoms between the surface and second atomic layers of graphite is demonstrated by means of molecular dynamics simulations. It is shown that interstitial clusters result in the dome-like surface features that may be associated with some of the hillocks observed by STM on the irradiated graphite surface.Comment: 7 pages, 7 eps figures, submitted to Surface Scienc

Electron Wave Function in Armchair Graphene Nanoribbons

By using analytical solution of a tight-binding model for armchair nanoribbons, it is confirmed that the solution represents the standing wave formed by intervalley scattering and that pseudospin is invariant under the scattering. The phase space of armchair nanoribbon which includes a single Dirac singularity is specified. By examining the effects of boundary perturbations on the wave function, we suggest that the existance of a strong boundary potential is inconsistent with the observation in a recent scanning tunneling microscopy. Some of the possible electron-density superstructure patterns near a step armchair edge located on top of graphite are presented. It is demonstrated that a selection rule for the G band in Raman spectroscopy can be most easily reproduced with the analytical solution.Comment: 7 pages, 4 figure

Study of Au/n- ZnSe contact by ballistic electron emission microscopy

Ballistic Electron Emission Microscopy (BEEM) has been used to characterise the Au/n-ZnSe contact. A mean statistical BEEM threshold of 1.63eV is in good agreement with literature. Metal - Insulator- Semiconductor (MIS) structures are invoked to explain the Schottky barrier height dispersion and the observed shift of BEEM thresholds to higher values.Ballistic Electron Emission Microscopy (BEEM) has been used to characterise the Au/n-ZnSe contact. A mean statistical BEEM threshold of 1.63eV is in good agreement with literature. Metal - Insulator- Semiconductor (MIS) structures are invoked to explain the Schottky barrier height dispersion and the observed shift of BEEM thresholds to higher values

u/n-Si(100) contact homogeneity studied by direct and reverse ballistic electron emission microscopy and spectroscopy

The Au/n-Si(100) contact has been studied using reverse ballistic electron emission microscopy and spectroscopy. Two types of localised collector currents have been observed; one, positive corresponding to electron injection into Si, and the other, negative, associated with hole injection into the semiconductor. The comparative trial of BEEM and reverse BEEM images from the same area shows this difference to be linked to the interface structure. Effects of surface roughness on the observed contrasts are also discussed.The Au/n-Si(100) contact has been studied using reverse ballistic electron emission microscopy and spectroscopy. Two types of localised collector currents have been observed; one, positive corresponding to electron injection into Si, and the other, negative, associated with hole injection into the semiconductor. The comparative trial of BEEM and reverse BEEM images from the same area shows this difference to be linked to the interface structure. Effects of surface roughness on the observed contrasts are also discussed

Etude par Microscopie a Effet Tunnel de surfaces de graphite implante et autres applications

SIGLEINIST T 76530 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Méthode de Boltzmann sur réseaux hybride : application aux écoulements compressibles complexes

Les travaux présentés dans ce manuscrit s'inscrivent dans une démarche de modélisation des systèmes physiques et décrivent une méthode numérique innovante pour la résolution des équations de conservation relatives à la mécanique des fluides dans le cas des écoulements compressibles. La méthode de Boltzmann sur réseaux est introduite en tant qu'outil de modélisation limité aux écoulements incompressibles. Pour répondre à cette problématique, une dérivation rigoureuse d'un modèle compressible hybride n'incluant que les plus proches voisins est présentée. Ce modèle est ensuite testé sur plusieurs applications présentant une complexité croissante grâce à l'ajout d'ingrédients numériques. La mise en place de conditions aux limites, d'une méthode capture des chocs, de raffinements de maillages et d'un modèle de turbulence ont ainsi permis la simulation d'une aile Onera M6 entourée par un écoulement compressible et turbulent. Basé sur les caractéristiques du système d'Euler et sur un couplage fort avec les flux numériques de masse et de quantité de mouvement issus de la méthode de Boltzmann sur réseaux, un schéma conservatif pour l'équation d'énergie est dérivé. Ce travail se termine par une étude du traitement des conditions aux limites liées à la méthode compressible. Les défauts de la méthode d'interpolation des variables macroscopiques sur les nœuds proches des parois initialement proposée sont mis en avant et certaines pistes d'amélioration sont proposéesThe work presented in this manuscript is part of an approach to modeling physical systems and describes an innovative numerical method for solving fluid mechanics conservation equations in the scope of compressible flows. The Lattice Boltzmann method is presented as a modeling tool that is restricted to incompressible flows. To address this issue, a rigorous derivation of a hybrid compressible model that includes only the nearest neighbors is presented. This model is then tested on a variety of applications with increasing complexity as numerical ingredients are added. With the addition of boundary conditions, a shock-capturing method, mesh refinements, and a turbulence model, the simulation of a Onera M6 wing surrounded by a compressible and turbulent flow was possible. A conservative scheme for the energy equation is derived based on the characteristics of the Euler system and a strong coupling with the fluxs of mass and momentum derived from the Lattice Boltzmann method. The study of compressible boundary conditions concludes this work. The shortcomings of the initially proposed method of interpolating macroscopic variables on nodes close to walls are highlighted, and some areas for improvement are suggeste

STUDY OF A PRESSURE-BASED HYBRID LATTICE BOLTZMANN METHOD FOR THE SIMULATION OF COMPRESSIBLE FLOWS

International audienceThe Lattice Boltzmann Method (LBM) is an alternative technique for the simulation and mod-elling of fluid flows based on the Boltzmann equation. One of its advantages is its ability tohandle very complex geometries with massively parallel computing. The LBM has achieved greatsuccess in simulating nearly incompressible and isothermal fluid flows, but it restricts its appli-cation range. A particularly active topic of investigation is its extension to more complex flows(e.g. multi-phase, thermal, compressible). We propose a pressure-based prediction-correction hybrid LBM model compatible with nearest-neighbor lattices (D2Q9 and D3Q19) with asingle time relaxation process, to simulate subsonic and transonic compressible flows withoutshock.The approach is hybrid: mass and momentum conservation equations are computed usinga LBM solver while an entropy conservation equation is solved via a finite difference approach.Following, an adequate forcing term is added to reproduce a correct viscous stress tensorand hybrid recursive regularized approach is used to stabilize the solution. Discretization ofthe entropy equation with viscous heat dissipation, in the finite difference part of the solver, isstudied to improve accuracy of the scheme and to reduce the cost of calculations.Validation of this new method is carried out on a number of canonical cases, systematicallychallenging the coupling between velocity, pressure and temperature, including pressure wavepropagation, and thermal Couette flows