Shape Transition in the Epitaxial Growth of Gold Silicide in Au Thin Films on Si(111)

Growth of epitaxial gold silicide islands on bromine-passivated Si(111) substrates has been studied by optical and electron microscopy, electron probe micro analysis and helium ion backscattering. The islands grow in the shape of equilateral triangles up to a critical size beyond which the symmetry of the structure is broken, resulting in a shape transition from triangle to trapezoid. The island edges are aligned along $Si[110]$ directions. We have observed elongated islands with aspect ratios as large as 8:1. These islands, instead of growing along three equivalent [110] directions on the Si(111) substrate, grow only along one preferential direction. This has been attributed to the vicinality of the substrate surface.Comment: revtex version 3.0, 11 pages 4 figures available on request from [email protected] - IP/BBSR/93-6

Structure determination of the (1×2) and (1×3) reconstructions of Pt(110) by low-energy electron diffraction

The atomic geometry of the (1×2) and (1×3) structures of the Pt(100) surface has been determined from a low-energy electron-diffraction intensity analysis. Both structures are found to be of the missing-row type, consisting of (111) microfacets, and with similar relaxations in the subsurface layers. In both reconstructions the top-layer spacing is contracted by approximately 20% together with a buckling of about 0.17 Å in the third layer and a small lateral shift of about 0.04 Å in the second layer. Further relaxations down to the fourth layer were detectable. The surface relaxations correspond to a variation of interatomic distances, ranging from -7% to +4%, where in general a contraction of approximately 3% for the distances parallel to the surface occurs. The Pendry and Zanazzi-Jona R factors were used in the analysis, resulting in a minimum value of RP=0.36 and RZJ=0.26 for 12 beams at normal incidence for the (1×2) structure, and similar agreement for 19 beams of the (1×3) structure. The (1×3) structure has been reproducibly obtained after heating the crystal in an oxygen atmosphere of 5×10-6 mbar at 1200 K for about 30 min and could be removed by annealing at 1800 K for 45 min after which the (1×2) structure appeared again. Both reconstructed surfaces are clean within the detection limits of the Auger spectrometer. CO adsorption lifts the reconstruction in both structures. After desorption at 500 K the initial structures appear again, indicating that at least one of the reconstructions does not represent the equilibrium structure of the clean surface and may be stabilized by impurities

Pro-autophagic signal induction by bacterial pore-forming toxins

Pore-forming toxins (PFT) comprise a large, structurally heterogeneous group of bacterial protein toxins. Nucleated target cells mount complex responses which allow them to survive moderate membrane damage by PFT. Autophagy has recently been implicated in responses to various PFT, but how this process is triggered is not known, and the significance of the phenomenon is not understood. Here, we show that S. aureus α-toxin, Vibrio cholerae cytolysin, streptolysin O and E. coli haemolysin activate two pathways leading to autophagy. The first pathway is triggered via AMP-activated protein kinase (AMPK). AMPK is a major energy sensor which induces autophagy by inhibiting the target of rapamycin complex 1 (TORC1) in response to a drop of the cellular ATP/AMP-ratio, as is also observed in response to membrane perforation. The second pathway is activated by the conserved eIF2α-kinase GCN2, which causes global translational arrest and promotes autophagy in response to starvation. The latter could be accounted for by impaired amino acid transport into target cells. Notably, PKR, an eIF2α-kinase which has been implicated in autophagy induction during viral infection, was also activated upon membrane perforation, and evidence was obtained that phosphorylation of eIF2α is required for the accumulation of autophagosomes in α-toxin-treated cells. Treatment with 3-methyl-adenine inhibited autophagy and disrupted the ability of cells to recover from sublethal attack by S. aureus α-toxin. We propose that PFT induce pro-autophagic signals through membrane perforation–dependent nutrient and energy depletion, and that an important function of autophagy in this context is to maintain metabolic homoeostasis

Emission microscopy and surface science

Basic concepts for the development of various forms of emission microscopy are discussed. It is proposed that all types of charged particles leaving a surface can be used to image the surface using a technique of emission microscopy. Angular distributions of all types of charged particles can be viewed by projection of the back oocal plane of an emission lens. Most familiar forms of spectroscopy used in surface science and involving charged particles can be combined, in principle, with a technique of emission microscopy. The following techniques are discussed: low-energy electron microscopy (LEEM) and diffraction (LEED), Auger electron emission microscopy and angular-resolved Auger electron spectroscopy (ARAES), electron energy-loss emission microscopy (EELS microscopy), secondary electron emission microscopy, thermionic emission electron microscopy, electron-stimulated desorption ion microscopy and electron-stimulated desorption ion angular distributions (ESDIAD), inverse photoemission microscopy, UV photoelectron emission microscopy and diffraction, X-ray photoelectron emission microscopy and diffraction, scanning photoelectron emission microscopy at atmospheric pressure and above, photon-stimulated desorption ion microscopy and photon-stimulated desorption ion angular distributions, ion neutralization electron microscopy, Penning ionization electron emission microscopy, secondary ion mass spectroscopy microscopy (SIMS microscopy), thermal ionization emission microscopy, exo-emission microscopy and positron emission microscopy

Chemical Reaction Fronts on Platinum Surfaces

In many chemical reactions catalysed on platinum surfaces it is necessary that two reactants be adsorbed simultaneously. Often one reactant is so strongly adsorbed that it blocks the adsorption of the second; such a reaction is said to be self-poisoned. An example is the oxidation of carbon monoxide, where carbon monoxide forms a strongly adsorbed monolayer which effectively blocks the adsorption and decomposition of oxygen. Photoelectron microscopy shows, however, that oxygen can penetrate the carbon monoxide film at special defect sites, typically inclusions or microdust particles, on the platinum. From these special adsorption sites the oxygen rapidly reacts with neighbouring adsorbed carbon monoxide. Reaction fronts initiate at these sites and rapidly propagate across the surface. A second type of self-poisoning occurs in decomposition reactions for which vacant surface sites are necessary; for instance, the decomposition of nitric oxide in the presence of hydrogen. A monolayer film of nitric oxide poisons the reaction not by blocking the adsorption of hydrogen, but rather by preventing the dissociation of nitric oxide which requires a neighbouring unoccupied surface site. Empty sites are provided on impurity particles which weakly adsorb nitric oxide and initiate reaction fronts. Impurity sites also initiate reaction fronts when graphite is removed from platinum by oxidation. In order to avoid self-poisoning in catalytic reactions, these studies suggest that special adsorption sites should be introduced artificially to provide vacant sites by adsorbing only weakly the reactants causing self-poisoning

ANALYSIS OF FIELD EMITTER SURFACES BY VERY HIGH RESOLUTION AUGER ELECTRON SPECTROSCOPY

Impurity segregation on Pt was studied using very high resolution Auger electron spectroscopy analysis of annealed field emitter surfaces. Using elemental analysis obtained from the Auger studies to complement information obtained from field emission microscopy studies, the lateral distribution of segregated surface impurities was determined at the sub-micron level