We report the direct observation of internal layering in thin (ϳ45 90 Å) liquid films of nearly spherical, nonpolar molecules, tetrakis(2-ethylhexoxy)silane, using synchrotron x-ray reflectivity. The Patterson functions have secondary maxima indicating layer formation, and model-independent fitting to the reflectivity data shows that there are three electron density oscillations near the solid-liquid interface, with a period of ϳ10 Å (consistent with the molecular dimensions). The oscillation amplitude has a strong inverse dependence on the substrate surface roughness. [S0031-9007(99)

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Divalent ions dissolved in the aqueous subphase of fatty acid Langmuir monolayers have two types of effects on the structure of the organic film. The first and more familiar effect is to induce a structure similar to the high-pressure "S" phase on pure water, even at low pressures; ions of the first type include Ni 2+ (aq), Ba 2+ -(aq), Co 2+ (aq), and Cu 2+ (aq). The presence of ions of the second type results in the appearance of superlattice structures: we see a 1 × 2 superlattice with Mn 2+ (aq) and a 2 × 2 superlattice with Mg 2+ (aq), and it is known that Cd 2+ (aq) and Pb 2+ (aq) also cause the formation of superlattices. Out-of-plane (Bragg rod) scans indicate that Mn 2+ (aq) and Mg 2+ (aq) interact with the headgroups so strongly that the organic film buckles, with a periodic out-of-plane density modulation (amplitude ∼2.5 Å). In addition, a thin (∼4 Å) ordered inorganic layer forms in the subphase under the Langmuir film.

A spin-coated thin polystyrene film on a silicon single crystal has been studied using the X-ray reflectivity technique. Signature of layering as a function of depth, due to confinement, in this polystyrene film could be detected from the reflectivity profile using a new analysis scheme. Small variations of electron density across the depth of a thin film can be determined from reflectivity data using this scheme with no a priori distribution of electron density profile.

We have examined the self-assembly process of octadecyltrichlorosilane on silicon using x-ray reflectivity. By comparing the commonly used "interrupted-growth" characterization technique with results obtained in situ, we have determined that quenching the growth and then rinsing and drying the sample introduces free area into the film, presumably by removal of non-cross-linked (physisorbed) molecules. Reintroduction of a quenched and rinsed film to solvent does not restore the thickness of the film to its previous value. We have also performed in situ growth studies over a range of concentrations. For all concentrations, we observe growth of islands of vertical molecules. The growth follows Langmuir kinetics, except at short times for low concentration solutions.