Conductive nanodots on the surface of irradiated CaF2

CaF2(111) single crystal surfaces have been irradiated with swift heavy ions under oblique angles resulting in chains of nanosized hillocks. In order to characterize these nanodots with respect to their conductivity we have applied non-contact atomic force microscopy using a magnetic tip. Measurements in UHV as well as under ambient conditions reveal a clearly enhanced electromagnetic interaction between the magnetic tip and the nanodots. The dissipated energy per cycle is comparable to the value found for metals, indicating that the interaction of the ion with the target material leads to the creation of metallic Ca nanodots on the surface

Magic Islands and Barriers to Attachment: A Si/Si(111)7x7 Growth Model

Surface reconstructions can drastically modify growth kinetics during initial stages of epitaxial growth as well as during the process of surface equilibration after termination of growth. We investigate the effect of activation barriers hindering attachment of material to existing islands on the density and size distribution of islands in a model of homoepitaxial growth on Si(111)7x7 reconstructed surface. An unusual distribution of island sizes peaked around "magic" sizes and a steep dependence of the island density on the growth rate are observed. "Magic" islands (of a different shape as compared to those obtained during growth) are observed also during surface equilibration.Comment: 4 pages including 5 figures, REVTeX, submitted to Physical Review

HICS: Highly charged ion collisions with surfaces

The layout of a new instrument designed to study the interaction of highly charged ions with surfaces, which consists of an ion source, a beamline including charge separation and a target chamber, is presented here. By varying the charge state and impact velocity of the projectiles separately, the dissipation of potential and kinetic energy at or below the surface can be studied independently. The target chamber offers the use of tunable metal-insulator-metal devices as detectors for internal electronic excitation, a timeof-flight system to study the impact induced particle emission and the possibility to transfer samples in situ to a UHV scanning probe microscope. Samples and detectors can be prepared in situ as well. As a first example data on graphene layers on SrTiO3 which have been irradiated with Xe36+ are presented. Key words: highly charged ions, sputtering, AFM, grapheneComment: 4 pages, 4 figures, conference proceeding to 17th Internat. Workshop for Ion Surf. Collision

Physics of Organelle Membrane Bridging via Cytosolic Tethers is Distinct From Cell Adhesion

Tremendous progress has been made recently in imaging the contacts between intra-cellular organelles, which are thought to be mediated by soluble tethers. However, they are still difficult to study in cellulo, and reconstituting them in vitro is a standing challenge. Here we take a mimetic approach to study Giant unilamellar vesicles (GUVs) and supported lipid bilayers (SLBs) interacting via single- (or double-) stranded DNA sequences of two different lengths. Like intra-cellular tethers which may reside in the cytosol when unbound, the DNA-tethers are soluble, but can insert into the membrane with the help of cholesterol moieties found at their extremities. Tether-exchange between the bulk “cytosol” and the GUV/SLB membrane leads to a novel statistical ensemble in which the entire system equilibrates together, rather than individual GUVs behaving as separate closed systems. As a consequence, adhesion between the GUV and the SLB is driven by collective entropic effects amplified by tether shape changes associated with membrane bridging. A direct experimental consequence is an unusual dependence on tether-concentration, which becomes an important control parameter at low concentrations, while tether length/flexibility are less important. The establishment of this fundamentally different interaction between two membranes suggests that in physiological conditions, the regulation of contact formation inside cells may be very different from the case of the much studied ligand-receptor mediated cell adhesion

Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations

An incident fast ion in the electronic stopping regime produces a track of excitations which can lead to particle ejection and cratering. Molecular Dynamics simulations of the evolution of the deposited energy were used to study the resulting crater morphology as a function of the excitation density in a cylindrical track for large angle of incidence with respect to the surface normal. Surprisingly, the overall behavior is shown to be similar to that seen in the experimental data for crater formation in polymers. However, the simulations give greater insight into the cratering process. The threshold for crater formation occurs when the excitation density approaches the cohesive energy density, and a crater rim is formed at about six times that energy density. The crater length scales roughly as the square root of the electronic stopping power, and the crater width and depth seem to saturate for the largest energy densities considered here. The number of ejected particles, the sputtering yield, is shown to be much smaller than simple estimates based on crater size unless the full crater morphology is considered. Therefore, crater size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see: http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded 5/16/01, with minor text changes + new figure

Microscopie par effet tunnel et application a l'etude de surfaces ordonnees de silicium (111)

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Atomic structures of Si() surface during silane UHV-CVD

International audienc

Ultrafast Photothermal Release of DNA from Gold Nanoparticles

We report results on ultrafast photothermal release of DNA from gold nanoparticles. We show that dehybridization of oligonucleotide duplex anchored on a gold nanoparticle surface occurs during a single laser pulse, leading to the release of single-strand DNA in solution. Breaking of the Au–S bond anchoring the duplex and the release of thiolated DNA are also evidenced. Our findings show that the size distribution of the nanoparticles plays a major role in the control of both phenomena. We establish a criterion regarding the size distribution of nanoparticles that allows full release of DNA without breaking of the anchoring thiol bonds