It is well-known that stacking of hard spheres results in close-packed structures. However, until recently, it was not clear which of the various possible phases (cubic, hexagonal, mixed, or random) was the stable one. We have performed a microscopy characterization of solid crystals made of monodisperse SiO2 nanometric spheres. It was found that, for a wide range of particle diameters, the cubic phase is the only one present. This largely serves to confirm recent theoretical calculations by L. V. Woodcock which conclude that the cubic phase is the most stable one. This opens new prospects in the application of colloidal crystals to photonic band gap engineering.

We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si(100) substrate with 1.2 keV Ar ions at normal incidence. Atomic Force and High-Resolution Transmission Electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 hours) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to the sputtering yield dependence on the local surface curvatureComment: 4 two-column pages (revtex4), 3 figures (higher quality copies in the printed jrnl. version

A simple method is presented for measuring the surface diffusion coefficients of Au and Pt atoms at electrodispersed electrodes of the same metals in contact with 0.5M H2SO4. The technique is based upon the time dependence of the surface roughness factor of electrodispersed metal overlayers. The method requires a model for the surface roughness of the metal structure. The model is deduced from microscopic measurements by a STM integrated into a conventional SEM microscope. This allows the relationship between the roughness factor and the area of the surface structure to be obtained. For Au and Pt in contact with an electrolyte solution, the values of our diffusion coefficients are higher than those reported in vacuum at the same temperature.

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.

Chemical vapor deposition (CVD) is a widely used technique to grow solid materials with accurate control of layer thickness and composition. Under mass-transport-limited conditions, the surface of thin films thus produced grows in an unstable fashion, developing a typical motif that resembles the familiar surface of a cauliflower plant. Through experiments on CVD production of amorphous hydrogenated carbon films leading to cauliflower-like fronts, we provide a quantitative assessment of a continuum description of CVD interface growth. As a result, we identify non-locality, non-conservation and randomness as the main general mechanisms controlling the formation of these ubiquitous shapes. We also show that the surfaces of actual cauliflower plants and combustion fronts obey the same scaling laws, proving the validity of the theory 6 2 over seven orders of magnitude in length scales. Thus, a theoretical justification is provided, which had remained elusive so far, for the remarkable similarity between the textures of surfaces found for systems that differ widely in physical nature and typical scales. Contents

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The conditions required to electroetch nanometer-sized craters in flat gold substrates with a scanning tunneling microscope operating in air are identified. Reproducible nanometer-scale modifications of the substrate are possible. Letters and complex symbols with linewidths as small as 2 nm have been written. Experiments show that a good tunneling tip is not destroyed by the writing process.

A direct scanning tunneling microscopy ex-situ determination on the nanometer scale of the topography of electrochemically highly activated platinum electrodes is presented. A correlation between catalytic activity and surface microtopography becomes evident. This result gives support to a structural model for the activated electrode surface. In the model, a volume with a pebble-like structure allows electrocatalytic processes to occur practically free of diffusion relaxation contributions under usual voltammetric conditions. Catalytic activity and surface roughness are of the outmost importance in heterogeneous catalysis, including electrocatalysis. The term roughness usually implies the existence of both macropores (macroroughness), which to a great extent are responsible for additional diffusional relaxation,' and micropores (microroughness), which concern the effective catalytic area.2 Despite the close relationship between microroughness and catalytic activity, many real systems involve complex macro-and micropore structures which make the direct determination of microroughness a difficult task. A new approach to overcome this drawback is forseen by using metal surfaces which offer large catalytic activity, negligible micropore diffusional relaxation, and distribution of active sites very close to that of the starting materiaL3 This is the case, among others, with platinum electrodes in acid solutions, which have been subjected to a relatively fast square potential cycling, over a potential range such that a hydrous metal oxide multilayer is formed and immediately afterwards is electroreduced to yield a substantially increased active area. The new surface