Cleaning graphene : a first quantum/classical molecular dynamics approach

Graphene outstanding properties created a huge interest in the condensed matter community and unprecedented fundings at the international scale in the hope of application developments. Recently, there have been several reports of incomplete removal of the polymer resists used to transfer as-grown graphene from one substrate to another, resulting in altered graphene transport properties. Finding a large-scale solution to clean graphene from adsorbed residues is highly desirable and one promising possibility would be to use hydrogen plasmas. In this spirit, we couple here quantum and classical molecular dynamics simulations to explore the kinetic energy ranges required by atomic hydrogen to selectively etch a simple residue, a CH3 group, without irreversibly damaging the graphene. For incident energies in the 2-15 eV range, the CH3 radical can be etched by forming a volatile CH4 compound which leaves the surface, either in the CH4 form or breaking into CH3+H fragments, without further defect formation. At this energy, adsorption of H atoms on graphene is possible and further annealing will be required to recover pristine graphene.Comment: 9 figures, 27 page

Black Silicon with high density and high aspect ratio nanowhiskers

Physical properties of black Silicon (b-Si) formed on Si wafers by reactive ion etching in chlorine plasma are reported in an attempt to clarify the formation mechanism and the origin of the observed optical and electrical phenomena which are promising for a variety of applications. The b-Si consisting of high density and high aspect ratio sub-micron length whiskers or pillars with tip diameters of well under 3 nm exhibits strong photoluminescence (PL) both in visible and infrared, which are interpreted in conjunction with defects, confinement effects and near band-edge emission. Structural analysis indicate that the whiskers are all crystalline and encapsulated by a thin Si oxide layer. Infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder induced LO-TO optical mode coupling can be an effective tool in assessing structural quality of the b-Si. The same phonons are likely coupled to electrons in visible region PL transitions. Field emission properties of these nanoscopic features are demonstrated indicating the influence of the tip shape on the emission. Overall properties are discussed in terms of surface morphology of the nano whiskers

Mechanical cleaning of graphene using in situ electron microscopy

Avoiding and removing surface contamination is a crucial task when handling specimens in any scientific experiment. This is especially true for two-dimensional materials such as graphene, which are extraordinarily affected by contamination due to their large surface area. While many efforts have been made to reduce and remove contamination from such surfaces, the issue is far from resolved. Here we report on an in situ mechanical cleaning method that enables the site-specific removal of contamination from both sides of two dimensional membranes down to atomic-scale cleanliness. Further, mechanisms of re-contamination are discussed, finding surface-diffusion to be the major factor for contamination in electron microscopy. Finally the targeted, electron-beam assisted synthesis of a nanocrystalline graphene layer by supplying a precursor molecule to cleaned areas is demonstrated

A Quasi Method of Characteristics with Applications to Fluid Lines with Frequency Dependent Wall Shear and Heat Transfer

Professor Streeter has given a fine summary of the basic numerical techniques for unsteady flows, presuming that equation One exception, well known to Professor Streeter and included in several of his references, is the simpler case of laminar rather than turbulent friction for low frequency excitation. Only minor variations in the equations are necessary. A much greater departure from equation 4 Numbers in brackets designate Additional References at end of discussion. at intermediate frequencies in turbulent flow. Apparently because of a little-understood resonance of ring vortices, the step response of a tube may contain significant oscillations. Wavelengths of the complicated patterns are about 25 and 50 diameters. (Further information is forthcoming in a thesis by Margolis.) The report also discusses the details of numerical application of the quasi method of characteristics to large amplitude transients, with illustrations. Readers should know that the paper and this discussion represent a highly selected rather than comprehensive review of the important literature on numerical methods for unsteady flow calculations in channels and tubes. T. P. Propson 6 The author has conducted a thorough review of the most popular techniques currrently employed to numerically evaluate the effect of transient flows in liquid piping systems. His discussion of the relative advantages and disadvantages of both the characteristics (explicit) and centered implicit method is excellent; of particular interest to the writer were the author's comments relative to the occurrence of instabilities and inaccuracies occasionally encountered during application of the implicit techniques. Recent unpublished work by the writer has confirmed these problems. When frictional effects are very important, the writer would suggest that equations (64) It may be shown that the error introduced into the integration of the friction term by these finite-difference equations is usually about one-half of that introduced by equations (30) and (31)

Spectroscopie d'absorption UV et VUV dans les plasmas réactifs : application au domaine de la microélectronique

International audienc

Pourquoi et comment est ce que les parois des réacteurs contrôlent la chimie des plasmas basse pression ?

International audienc

Probing radical kinetics in the afterglow of pulsed discharges by absorption spectroscopy with LED: application to BCl radical

International audienc