A graphene electron lens

International audienceAn epitaxial layer of graphene was grown on a pre patterned 6H-SiC(0001) crystal. The graphene smoothly covers the hexagonal nano-holes in the substrate without the introduction of small angle grain boundaries or dislocations. This is achieved by an elastic deformation of the graphene by ~0.3% in accordance to its large elastic strain limit. This elastic stretching of the graphene leads to a modification of the band structure and to a local lowering of the electron group velocity of the graphene. We propose to use this effect to focus two-dimensional electrons in analogy to simple optical lenses

Structure and Stability of Si(114)-(2x1)

We describe a recently discovered stable planar surface of silicon, Si(114). This high-index surface, oriented 19.5 degrees away from (001) toward (111), undergoes a 2x1 reconstruction. We propose a complete model for the reconstructed surface based on scanning tunneling microscopy images and first-principles total-energy calculations. The structure and stability of Si(114)-(2x1) arises from a balance between surface dangling bond reduction and surface stress relief, and provides a key to understanding the morphology of a family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published in Phys. Rev. Let

Decay of isolated surface features driven by the Gibbs-Thomson effect in analytic model and simulation

A theory based on the thermodynamic Gibbs-Thomson relation is presented which provides the framework for understanding the time evolution of isolated nanoscale features (i.e., islands and pits) on surfaces. Two limiting cases are predicted, in which either diffusion or interface transfer is the limiting process. These cases correspond to similar regimes considered in previous works addressing the Ostwald ripening of ensembles of features. A third possible limiting case is noted for the special geometry of "stacked" islands. In these limiting cases, isolated features are predicted to decay in size with a power law scaling in time: A is proportional to (t0-t)^n, where A is the area of the feature, t0 is the time at which the feature disappears, and n=2/3 or 1. The constant of proportionality is related to parameters describing both the kinetic and equilibrium properties of the surface. A continuous time Monte Carlo simulation is used to test the application of this theory to generic surfaces with atomic scale features. A new method is described to obtain macroscopic kinetic parameters describing interfaces in such simulations. Simulation and analytic theory are compared directly, using measurements of the simulation to determine the constants of the analytic theory. Agreement between the two is very good over a range of surface parameters, suggesting that the analytic theory properly captures the necessary physics. It is anticipated that the simulation will be useful in modeling complex surface geometries often seen in experiments on physical surfaces, for which application of the analytic model is not straightforward.Comment: RevTeX (with .bbl file), 25 pages, 7 figures from 9 Postscript files embedded using epsf. Submitted to Phys. Rev. B A few minor changes made on 9/24/9

Nanoscience and nanotechnology in Provence-Alpes-Côte d'Azur

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Cross characterization by scanning electron microscopy and atomic force microscopy of Ag islands grown on Si (111) 7×7

Two types of microscopy, scanning electron (SEM) and atomic force microscopy (AFM), have been used in a comparative way to study very big (10 - 40 μm) Ag islands grown on a Si(111)√3 x √3 interface. The signals used for image formation are of very different nature; secondary electrons emitted from the sample, in SEM and, in AFM, light deflected from the system probing the surface. In the present study, both microscopes were used in similar magnification ranges. A rather irregular morphology of the islands and a surface topography with large inhomogeneities on the top face were observed. The size distribution of the islands was found to be very wide. AFM and SEM, for minor differences, basically give access to the same results.Deux types de microscopies, la microscopie électronique à balayage (SEM) et la microscopie à force atomique (AFM), ont été utilisées d'une façon comparative pour étudier des ilots d'Ag de très grande taille (10 - 40 μm) formés sur l'interface Si(111)√3 x √3-Ag. Les images sont de nature très différente dans les deux cas: Electrons secondaires émis par l'échantillon en SEM, lumière réfléchie par la sonde balayant la surface en AFM. Les deux microscopes ont été utilisés dans des gammes de grossissement comparables. Une morphologie plutôt irrégulière et une topographie de surface démontrant de grandes inhomogenéités ont pu être mises en évidence sur les îlots dont la distribution de taille est par ailleurs très large. Les résultats obtenus en AFM et SEM sont, mis à part des différences mineures, principalement identiques