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X‐ray reflection (both specular and off‐specular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers (n‐C18H37SiO1.5) formed by self‐assembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 A. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid n‐alkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermal‐expansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational corr...

X-ray specular reflectivity has been used to characterize the structure of silicon--silicon-oxide surfaces coated with chemisorbed hydrocarbon monolayer films (alkylsiloxanes). Using synchrotron radiation the reflectivity was followed over 9 orders of magnitude, from grazing incidence to an incident angle of \ensuremath{\varphi}\ensuremath{\approxeq}6.5\ifmmode^\circ\else\textdegree\fi{}, or q=(4\ensuremath{\pi}/\ensuremath{\lambda})sin(\ensuremath{\varphi})=0.8 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, allowing a spatial resolution of features approximately \ensuremath{\pi}/0.8\ensuremath{\approxeq}4.0 A\r{} along the surface normal. Analysis was performed by fitting the data to reflectivities calculated from models of the surface electron density and by calculating Patterson functions directly from the data. Although the measured reflectivities could be equally well described by different sets of model parameters, the electron densities calculated from these different parameters were remarkably alike. Inspection of the electron densities allowed identification of a layer of ${\mathrm{SiO}}_{2}$ (\ensuremath{\approxeq}17-A\r{} thick), a layer of head-group region where the alkylsiloxane adsorbs to the ${\mathrm{SiO}}_{2}$, and the hydrocarbon layer. Fitting the data also required that the various interfaces have different widths. The fact that the same local hydrocarbon density of 0.85 g/${\mathrm{cm}}^{3}$ was observed for both fully formed and partially formed monolayers with alkane chains of varying length excluded a model in which the partially formed monolayer was made up of separated islands of well-formed monolayers. Measurements before and after chemical reaction of a monolayer in which the alkyl chain was terminated by an olefinic group demonstrated the ability to use x-ray reflectivity to characterize chemical changes. The effects of radiation damage on these types of measurements are described.

X-ray scattering from the solid/liquid interface of a Au(001) single crystal in 0.1 M potassium hydroxide and 0.1 M perchloric acid electrolytes has been measured. In both solutions the Au surface reconstruction consists of an incommensurate-hexagonal monolayer with a structure similar to the clean annealed vacuum surface at room temperature. In KOH electrolyte the reconstruction is aligned along the [110] direction, unlike the vacuum and perchloric acid interfaces, where the reconstruction is rotated [plus minus](0.5--0.8)[degree] from the [110] direction. In KOH solution the reconstruction forms rapidly at electrode potentials below [minus]0.25 V (versus the saturated calomel electrode) and is lifted in a narrow potential region (40 mV) above [minus]0.10 V, very close to the estimated potential of zero charge (PZC) for the reconstructed surface. In perchloric acid solution the reconstruction forms much more slowly at potentials just below the PZC of the unreconstructed surface, and is stable up to approximately 0.30 V, which is just positive of the PZC. In KOH solution the stability of the reconstruction is controlled primarily by the interaction of the surface with hydroxyl anions. The controlling mechanism in perchloric acid is unclear.

The wetting of silicon wafers and silicon wafers coated with alkylsiloxane monolayers by saturated vapors of cyclohexane and methanol were studied using x-ray specular reflection. Differentially heating the substrate surface relative to the temperature of a liquid reservoir was used to probe the disjoining pressure as a function of the film thickness and surface chemistry. Uncoated silicon wafers wet completely. The variations in film thickness with \ensuremath{\Delta}T are explained exclusively in terms of the nonretarded van der Waals forces for films 10--120 \AA{} thick. Wafers coated with methyl terminated alkylsiloxane monolayers wet incompletely, with a microscopic film 1--3 \AA{} thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -${\mathrm{CH}}_{3}$ to -${\mathrm{CH}}_{2}$OH converts the surface from incompletely to completely wet. Surfaces coated with partial monolayers of methyl terminated alkylsiloxane of greater than 50% coverage are incompletely wet by cyclohexane, with the monolayers ``swelling'' to a thickness close to that of fully extended alkane chains through incorporation of cyclohexane into the film structure. The data are consistent with a first-order transition to complete wetting upon reduction of the alkylsiloxane coverage below approximately 50%. The importance of the surface atomic layer in the promotion or suppression of complete wetting is explained in the context of van der Waals forces.