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Axe. 1990. X-ray specular reflection studies of silicon coated by organic monolayers (alkylsiloxanes). Physical Review B 41(2): 1111-1128.

The evolution of the surface structure of a wetting film on a rough surface as a function of the film thickness has been studied by x-ray specular reflection and surface diffuse scattering. For thin films (;$ 60 A) the liquid surface is characterized by static undulations induced by the roughness of the substrate; however, with increasing film thickness the structure is dominated by thermally induced capillary waves. The data are quantitatively described by a model with exclusively van der Waals liquid-substrate interactions.

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 Å. 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 correlation lengths and integrated peak areas are significantly reduced relative to complete monolayers, the peak positions of the incomplete monolayers are comparable to those of complete monolayers. Given the lower average areal density of incomplete monolayers, this finding implies that incomplete monolayers are inhomogeneous.

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X-ray reflectivity measurements were used to determine the structure of the Pt(001)/solution interface as a function of the applied potential in alkaline and acid electrolytes. Unlike the Pt(001)/vacuum interface, the electrolyte interface was found to be unreconstructed at all potentials. The lattice spacing between the first and second layer was found to be a function of potential in the region where approximately a monolayer of H was adsorbed. The expansion was up to 2.5% of a lattice spacing (0.05 A) in alkaline electrolyte, and up to 1% of a lattice spacing in acid electrolytes.PACS numbers: 78.70.Ck, The nature of the metal/solution interface has in the past few years become a topic of much investigation after the development of scanning tunneling microscopy and synchrotron x-ray scattering techniques. In particular, several recent studies have investigated both the potential dependent adsorption of metals onto single crystal surfaces [1] and potential dependent reconstructions of surfaces [2][3][4]. In the latter case, anion adsorption and desorption (controlled by potential) appear to play a critical role in the removal and formation of the reconstruction. These studies have concentrated on chemically quite inactive surfaces (such as silver and gold) where contamination problems are relatively insignificant and the energy associated with adsorption is quite small. In contrast, platinum surfaces have generally much higher interaction energies with most adsorbates than is the case for gold, and thus adsorption may have a more profound effect on the surface structure and issues of cleanliness are of crucial importance. X-ray scattering studies have shown that the vacuum interface of the Pt(001) surface at room temperature is reconstructed, with the top layer well represented by an incommensurate-distorted-hexagonal overlayer which is rotated approximately ±0.8° from the [110] direction [5]. The vacuum interface reconstruction is removed, however, on exposure to hydrogen [6], water vapor, and several other adsorbates [7]. Ex situ low-energy electron diffraction (LEED) studies of the electrolyte interface [8] strongly suggest that the reconstruction is unstable in electrolyte at any potential, although this has not previously been conclusively demonstrated in situ. The interaction of H with Pt is very different from the interaction of H with Au. In electrolyte, from voltammetric studies and thermodynamic calculations an adsorbed state of H is known to exist on Pt at potentials positive of H2 evolution [9]. No such state exists at the Au /electrolyte interface. Even though the H/metal interaction is one of the most fundamental to physical chemistry, it is very difficult to study (especially in situ), and the details of the bonding are still a subject of controversy [10].In this Letter we describe our x-ray scattering experiments to probe the Pt(001) interface with aqueous alkaline and acid electrolytes. In all cases the Pt was found to be unreconstructed at all potentials within the region of thermodynamic sta...