P. F. Lyman

Lead adsorption at the calcite-water interface: Synchrotron x-ray standing wave and x-ray reflectivity studies

Abstract By combining synchrotron X-ray standing wave (XSW) measurements with synchrotron X-ray reflectivity measurements, we have determined: (1) the precise three-dimensional location within the calcite unit cell of submonolayer Pb ions adsorbed at the calcite (104) surface from dilute aqueous solutions, and (2) the precise one-dimensional location of these unit cells relative to the calcite surface. Our XSW measurements, using three separate calcite Bragg reflections for triangulation, show that most adsorbed Pb ions occupy Ca sites in the calcite lattice with an ordered coverage of 0.05 equivalent monolayers, while the remaining Pb ions are disordered with a coverage of 0.03 equivalent monolayers. Our X-ray reflectivity measurements show that the ordered Pb ions occur primarily (>70%) in the surface atomic layer of calcite. Atomic force microscopy (AFM) was used to characterize the topography of the calcite (104) surface under conditions similar to the X-ray experiments. The quantitative morphological information obtained by AFM was used to develop realistic models of the calcite surface. The calculated X-ray reflectivities for these model surfaces were compared with the measured X-ray reflectivities. The new combined X-ray method that we have developed can be used to determine the atomic-scale structure of other metals adsorbed at mineral-water interfaces. Such high-resolution structural determinations are essential before detailed conceptual and theoretical models can be further developed to understand and predict the behavior of dissolved metals in mineral-water systems.

High-resolution structural study of zinc ion incorporation at the calcite cleavage surface

The atomic-scale structure of Zn{sup 2+} incorporated at the CaCO{sub 3} (10{ovr 1}4) surface by adsorption from solution was determined by X-ray standing wave triangulation and surface extended X-ray absorption fine structure spectroscopy. At low coverage (approximately 0.1 ML), Zn{sup 2+} substitutes for Ca{sup 2+} in the surface layer. Structural relaxation of the adjacent in-plane CO{sup 2-}{sub 3} ions in the host surface is shown by the reduced nearest-neighbor distance of Zn-O relative to Ca-O. Relaxation of the Zn{sup 2+} ion in the out-of-plane direction is shown by the displacement of its lattice position from the ideal Ca{sup 2+} position. These relaxations, resulting in a local lattice buckling feature at the Zn{sup 2+} adsorption site, can be fully explained as the combined effect of the electrostatic relaxation of the nearest-neighbor anions in response to the smaller size of Zn{sup 2+}, and the bonding asymmetry due to surface truncation.

X-ray standing wave investigation of the surface structure of selenite anions adsorbed on calcite.

The adsorption of selenite ions (SeO2−3) from a dilute aqueous solution onto a freshly-cleaved calcite (1014) surface was studied with the X-ray standing wave (XSW) technique. The complex ion SeO2−3 is found to selectively adsorb at the CO2−3 site via ionic exchange, forming a two-dimensional solid-solution of the form Ca(SeO3)x(CO3)1 − x at the interface. The calcite (1014), (0006) and (1120) Bragg reflections were used to triangulate the Se position with respect to the calcite lattice. The local surface structure at the SeO2−3 adsorbate site, derived from the XSW results, is consistent with a model in which the base of the SeO2−3 trigonal pyramid aligns with (and replaces) the CO2−3 equilateral triangular group. The SeO2−3 adsorption saturated at a coverage of 0.02 monolayers. Under identical chemical conditions, selenate (SeO2−4) adsorption was inhibited.

Structure of a passivated Ge surface prepared from aqueous solution

The structure of a passivating sulfide layer on Ge(001) was studied using X-ray standing waves and X-ray fluorescence. The sulfide layer was formed by reacting clean Ge substrates in (NH 4 ) 2 S solutions of various concentrations at 80°C. For each treatment, a sulfide layer containing approximately two to three monolayers (ML) of S was formed on the surface, and an ordered structure was found at the interface that contained approximately 0.4 ML of S. Our results suggest the rapid formation of a glassy GeS x layer containing 1.5‐2.5 ML S residing atop a partially ordered interfacial layer of bridge-bonded S. The passivating reaction appears to be self-limited to 2‐3 ML at this reaction temperature.

Local structure of Sn/Si(001) surface phases

Abstract The surface structures of the (6 × 2), c(8 × 4) and (5 × 1) phases of Sn Si (001) were studied using the X-ray standing wave technique. Using the (004) and (022) Bragg reflections, we find that the (6 × 2) and c(8 × 4) phases are composed of highly buckled SnSn ad-dimers located 1.58 A above the bulk-like Si(004) surface atomic plane. The Sn atoms occupy two distinct sites with a vertical separation of 0.68 A, resulting in a dimer buckling angle of approximately 14°. Occupation of second-layer sites by Sn in the (5 × 1) phase, and even in the high-coverage region of the c(8 × 4) phase, changes the Sn spatial distribution normal to the surface, which we attribute to unbuckling and/or breaking of the dimers in the first layer.

The use of X-ray standing waves and evanescent-wave emission to study buried strained-layer heterostructures

Abstract A heteroepitaxial structure consisting of 1 ML InAs grown on GaAs(0 0 1) and then capped with 25 A GaAs was examined by the X-ray standing wave (XSW) method. By monitoring the In L fluorescence while scanning through the GaAs(0 0 4) Bragg reflection, the perpendicular strain within the InAs layer was directly measured to be 7.7%, which is in good agreement with macroscopic elasticity theory (7.3%). We also demonstrate that, combined with the evanescentwave-emission effect, the XSW method can be used to measure the strain-induced displacement of the cap layer in the growth direction.

Surfactant‐mediated epitaxy of metastable SnGe alloys

An effective method for molecular beam epitaxial construction of metastable, pseudomorphic SnGe/Ge(001) heterostructures is presented. This method exploits a surfactant species, Bi, to alter Sn surface‐segregation kinetics. Using the x‐ray standing wave technique, we demonstrate not only that Bi segregates to the growth surface more strongly than Sn, but that it also dramatically suppresses the segregation mobility of Sn. The limited Sn diffusivity, which is believed to stem from the full coordination of subsurface Sn atoms, allows the epitaxy of well‐ordered, metastable SnGe heterostructures.

Surface And Interface Studies At APS Endstation 5ID-C

We have instrumented the 5ID‐C hutch of DND‐CAT at the Advanced Photon Source (APS) for x‐ray surface spectroscopy and scattering. A wide variety of experiments have already been carried out at 5ID‐C, including studies on semiconductors, oxides, metals, nanodots, and biomolecular adsorption. We describe the beamline optics, data collection hardware and software, and both diffractometers, a five‐circle kappa diffractometer for open‐air and liquid‐cell experiments, and a psi‐circle diffractometer with an integrated ultrahigh vacuum (UHV) chamber, paying particular attention to the requirements of an XSW measurement.

Adsorbate structure and substrate relaxation for the Sb Si(001)-(2 × 1) surface

Abstract X-ray standing wave measurements were undertaken to study the bonding position of Sb adatoms on the Sb-saturated Si(001)-(2 × 1) surface. Using the (004) and (022) Bragg reflections, we find that the Sb atoms form dimers, and that the center of the Sb ad-dimers lies 1.64 A above the bulk-like Si(004) surface atomic plane. When combined with a previous determination of the SbSi and SbSb bond lengths, our results show that the surface Si plane is contracted inward by 0.10 A upon saturation with Sb. Our in-plane results are compared to two structural models consisting of dimers whose bonds are parallel to the surface plane and whose centers are either shifted or unshifted (parallel to the dimer bond direction) relative to the underlying substrate planes. We thus find two special cases consistent with our data: one with symmetric (unshifted) dimers having a dimer bond length of 2.81 A, and the other with midpoint-shifted dimers, having a bond length of 2.88 A and a lateral shift of 0.21 A.

UHV surface-analysis endstation with x-ray scattering and spectroscopic capabilities

The design of a versatile ultrahigh vacuum (UHV) endstation for use at the Advanced Photon Source is described. The capabilities of the endstation include x-ray scattering and x-ray spectroscopic techniques for the investigation of surfaces, interfaces, and thin films. The UHV analytical chamber also includes facilities for surface preparation, thin film growth, and standard (non-x-ray) surface analyses. The endstation, which is inspired by previous successful implementations for surface scattering, incorporates several novel design features to facilitate the use of both scattering and spectroscopic techniques, and also allows the examination of small samples. Its capabilities include x-ray reflectivity and crystal truncation rod studies, grazing-incidence x-ray diffraction, x-ray standing waves, surface extended x-ray absorption fine structure, x-ray holography, and x-ray photoelectron spectroscopy.