W.J. Huisman

Layering of a liquid metal in contact with a hard wall

When a liquid makes contact with a solid wall, theoretical studies indicate that the atoms or molecules will become layered adjacent to the wall, giving rise to an oscillatory density profile. This expectation has not, however, been directly verified, although an oscillatory force curve is seen for liquids compressed between solid surfaces. Here we present the results of an X-ray scattering study of liquid gallium metal in contact with a (111) diamond surface. We see pronounced layering in the liquid density profile which decays exponentially with increasing distance from the wall. The layer spacing is about 3.8 å, which is equal to the repeat distance of (001) planes of upright gallium dimers in solid α-gallium. Thus it appears that the liquid near thewall assumes a solid-like structure similar to the α-phase, which is nucleated on freezing at lower temperatures. This kind of ordering should significantly influence flow, capillary osmosis, lubrication and wetting properties,, and is likely to trigger heterogeneous nucleation of the solid.

X-ray diffraction studies of potassium dihydrogen phosphate (KDP) crystal surfaces

We have studied the surface atomic structure of KDP crystals using X-ray scattering. These crystals were grown from an aqueous solution and we have done measurements both ex situ and in situ. The ex situ measurements were performed in vacuum or in air. In order to be able to do in situ measurements, we designed and built a crystal growth chamber which is compatible with X-ray diffraction experiments. The atomic arrangement of the two naturally existing faces of KDP has been determined. Preliminary results are presented of measurements performed during growth. Furthermore, the influence of metal impurities on the atomic structure of the growing interface is examined.

Surface atomic structure of the (3×3)R30°-Sb reconstructions of Ag(111) and Cu(111)

Abstract We present an X-ray diffraction structure analysis of the ( 3 × 3 )R30° reconstructions of Ag(111)-Sb and Cu(111)-Sb surfaces. We find these structures to be very similar. Contrary to previous reports, we find that all top layer atoms reside at stacking fault positions. Each ( 3 × 3 )R30° surface unit cell contains one substitutional Sb atom. We determined the out-of-plane relaxations of the top layer atoms and the in-plane distortions in the second layer. For coverages below 1/3 monolayer, the Sb atoms are embedded randomly at fcc positions in the top surface layer.

The effect of Sb on the nucleation and growth of Ag on Ag(100)

Abstract We have investigated the homoepitaxial growth of Ag(100) at room temperature by means of surface X-ray diffraction. The out-of-phase specular reflection shows intensity oscillations during growth indicating layer-by-layer growth. From transverse in-plane profile scans the island-size distribution is determined at various stages during the growth. Pre-deposition of Sb on the clean surface improves the layer-by-layer growth by enhancing the interlayer transport. The lineshape of the peak profiles indicates that the preferred terrace size has disappeared in this case, showing that the randomly deposited Sb atoms act as nucleation centers for the formation of Ag islands.

A new x-ray diffraction method for structural investigations of solid-liquid interfaces

A synchrotron x-ray diffraction method is presented for structural investigations of interfaces between low-Z substrates and heavier liquids. The method, similar to methods used in neutron scattering, is based on illuminating the interface through the solid substrate. The backgrounds arising from bulk scattering and the signal-to-background ratio are estimated and compared with experimental results. An ultrahigh vacuum (UHV) setup is described in which the atomic arrangement and roughness of clean interfaces can be studied in situ. Our first results illustrate the possibilities for both out-of-plane and in-plane diffraction studies. The specular reflectivity of the Ga/diamond(111)-2×1 interface was measured for perpendicular momentum transfers up to 2.2 A−1. In an in-plane study of Ga/Si(111)-7×7 the in-plane structure factor of Ga liquid within a depth of ∼50 A was compared to the structure factor of the bulk liquid.

Interface structure of Si(111)-(√3 × √3)R30°-ErSi2 − x

The interface structure of epitaxial ErSi2 − x (x ≈ 0.3) on Si(111) has been measured by means of surface X-ray diffraction. The silicide consists of a stack of alternating Si and Er planes. The Si planes resemble compressed substrate bilayers, in which a regular network of vacancies releases the compressive strain. This vacancy network gives rise to a (√3 × √3)R30° reconstruction of the silicide, in which Si atoms are displaced towards the vacancies, and Er atoms are displaced away from the vacancies. The Si-Si bond length in the silicide is 2.36±0.02 A, and the nearest-neighbour ErSi distance is 2.95±0.02 A. The positions of the ato the interface region are identical to those found for two-dimensional erbium silicide on Si(111), and first-layer Er atoms are located on T4 sites on the substrate. Our preparation method (reactive deposition epitaxy) leads to rough silicide films. Nevertheless, we are able to extract the atomic occupancies of eight individual layers. All silicide layers are reconstructed, but only the silicide layer at the interface shows long-range order of the vacancy network.

Structure and morphology of the as-polished diamond(111)-1 × 1 surface

Abstract We present an X-ray diffraction study of the polished, unreconstructed diamond(111) surface. By measuring the distribution of diffracted intensities along the ( hk )=(10) crystal truncation rod, we have determined the structure of the unreconstructed surface. We find that after heating to 650°C, the polished diamond (111)-(1 × 1) surface exhibits a single-bond cleavage plane terminated by hydrogen atoms and, for 15% of the surface area, by adsorbed oxygen, most likely in the form of hydroxyl groups. We observe relaxations in the first layers, most notably a 3% contraction between the first and second bilayer. AFM results are presented that confirm the morphology found in X-ray analysis. Medium-energy ion scattering was used to determine the oxygen coverage.

Evidence for tilted chains on the diamond (111)-(2 × 1) surface

We present the first X-ray diffraction structure analysis of the reconstructed diamond (111) surface. The atomic geometry is obtained from the distribution of diffracted intensities along integer-order rods of Bragg scattering perpendicular to the surface. Of all the geometries considered, an atomic arrangement featuring tilted chains of π-bonded surface atoms with distortions in subsurface layers is found to provide the best agreement between the calculated and measured structure factors. The tilted-chain topology explains the origin of the observed surface band gap, but it is at variance with recent theoretical predictions based on the local density approximation.

Structure of the diamond (111)-(2 × 1) surface revisited with ion scattering

Abstract We present a medium-energy ion scattering study of the reconstructed diamond (111)-(2 × 1) surface. Measured blocking patterns were compared with patterns calculated for a variety of structure models. We find that a π-bonded chain model featuring tilted chains best describes our data. The tilt explains the observed electronic band gap at the surface, but is at variance with the results of recent energy minimization calculations based on the local density approximation. Our results are in disagreement with a previous ion scattering study of the reconstructed diamond (111) surface.

Modelling the atomic density across a solid-liquid interface

Abstract From the specular X-ray reflectivity from an interface between two media one may derive the local atomic density distribution. In this work, models for the density profile across a solid–liquid interface are introduced. We concentrate on models in which the liquid is stratified near the solid wall. The calculations are compared with the measured X-ray reflectivity from the Ga–diamond(111)-2×1 interface. It appears that the liquid near the diamond surface already assumes a solid-like structure similar to that of the α-Ga phase.