Sequential adsorption of a cationic polyelectrolyte and individual sheets of the silicate mineral hectorite has allowed controlled, stepwise formation of multilayered films on silicon wafers. Each component adsorbs rapidly by an ion-exchange mechanism, and x-ray diffractometry indicates structural order even in films with thicknesses greater than 0.2 micrometer. The large lateral extent of the silicate sheets (about 25 to 35 nanometers) allows each layer to cover any packing defects in the underlying layer, thus preserving structural order in the growing film. With careful choice of component materials, this method should allow for the preparation of multilayered films with a variety of technologically important properties.

Alkanethiolates in self-assembled monolayers on gold oxidize in air, in the dark, to form sulfinates and sulfonates. The kinetics of oxidation, however, vary depending on the morphology of the underlying gold, with the rate of oxidation increasing dramatically with a decrease in the size of the grains and the amount of Au(111) on the surface. This difference in kinetics of oxidation is sufficiently great that it may provide insight into discrepancies among previous reports in the literature regarding the inertness of these SAMs in air. The oxidized products also desorb readily, and these species decompose under prolonged X-ray irradiation in ultrahigh vacuum.

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Thin gold films placed in contact on compliant elastomeric poly(dimethylsiloxane) supports weld together. This ;;cold welding'' is remarkable both for the low loads required and for the fact that it occurs under ambient laboratory conditions, conditions in which the gold surfaces are covered with films of weakly adsorbed organic impurities. These impurities are probably displaced laterally during the welding. Welding can be prevented by the presence of a self-assembled gold(I) alkylthiolate monolayer on the gold surfaces. The welded contacts have low electrical resistivity and can be made thin enough to transmit light. This system is a promising one with which to study interaction between interfaces.

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Selective preparation of self-assembled monolayers (SAMs) on gold was accomplished by electrolysis of alkylthiosulfates (Bunte salts) in tetrahydrofuran in the presence of tetrabutylammonium tetrafluoroborate as the supporting electrolyte. Tetrafluoroborate ions inhibited the spontaneous chemisorption of alkylthiosulfates to form monolayers, so that monolayers formed only on gold electrodes at a sufficiently oxidative potential. This oxidative potential was applied using square-wave pulses, and as expected, the degree of completeness of the monolayers depended upon both the voltage and the period during which it was applied. Using this method, monolayers could be formed quickly, in only twenty 5-s pulses. X-ray photoelectron spectroscopy and infrared spectroscopy supported our hypothesis that this process involves formation of stable gold−thiolate bonds at the gold surface and that (bi)sulfate is a side-product under our experimental conditions. Unlike the preparation of SAMs from alkanethiols, clean gold is not required for forming complete monolayers from alkylthiosulfates. Preliminary results indicate that monolayers of alkanethiolates can also be formed by reductive electrolysis of corresponding alkylthiosulfates in aqueous solution.

Silica films with controlled thickness and refractive index have been formed by the sequential adsorption of a cationic polyelectrolyte and silica sols. The conditions used to prepare the sol were varied, and allowed films with refractive indices as low as 1.16 to be obtained. The sequential adsorption technique allows the thickness of these films to be controlled in increments of 5-10 nm, depending on the desired refractive index. Scanning electron microscopy revealed that a low packing density of constituent silica particles was responsible for the low indices of these films. The as-adsorbed films are thermally robust; calcination at 500 degrees C resulted in only very small decreases in film thickness (by < or =1.8%) and refractive index (to as low as 1.14). After calcination, the silica films remained hydrophilic and sorbed water vapor from the atmosphere. As a result, the refractive indices of these films increased with increasing relative humidity (RH). The dependence of the refractive index on RH was eliminated by treating the calcined films with trimethylchlorosilane.