228 research outputs found
X-Ray Standing Wave Studies of Ad-Dimers on Si(001)
X-ray standing waves generated by dynamical Bragg diffraction were used as an element-specific structural probe for investigating Ga and Sb adsorption on Si(001). Using the (004) and (022) reflections, we precisely measured Ga and Sb ad-dimer bond lengths and ad-dimer heights above the bulk-extrapolated Si(001) surface. The room temperature [001] thermal vibration amplitudes of Ga and Sb adatoms on Si(00l) were also directly determined by combining the fundamental (004) and high-order harmonic (008) X-ray standing wave measurements. These high-resolution measurements reveal important quantitative structural information regarding the dimerized surface structures, and provide a stringent test for structural models proposed by various theoretical calculations. In this paper, we also give an over-view of the X-ray standing wave technique and its application in investigating surface structure and dynamics
Epitaxial Stabilization of Ultrathin Films of Rare-Earth Nickelates
We report on the synthesis of ultrathin films of highly distorted EuNiO3
(ENO) grown by interrupted pulse laser epitaxy on YAlO3 (YAO) substrates.
Through mapping the phase space of nickelate thin film epitaxy, the optimal
growth temperatures were found to scale linearly with the Goldschmidt tolerance
factor. Considering the gibbs energy of the expanding film, this empirical
trend is discussed in terms of epitaxial stabilization and the escalation of
the lattice energy due to lattice distortions and decreasing symmetry. These
findings are fundamental to other complex oxide perovskites, and provide a
route to the synthesis of other perovskite structures in ultrathin-film form.Comment: 7 pages, 3 figure
Local structure of Sn/Si(001) surface phases
Abstract The surface structures of the (6 x 2), c(8 x 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 x 2) and c(8 × 4) phases are composed of highly buckled Sn-Sn ad-dimers located 1.58 i above the bulk-like Si(004) surface atomic plane. The Sn atoms occupy two distinct sites with a vertical separation of 0.68 ~,, 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
Adsorption-Site Investigation of Rb/Cu(111) Using the X-Ray Standing-Wave Method
We have carried out a back reflection x-ray standing-wave investigation to study the adsorbate-substrate bond length and bonding site for the system Rb/Cu(111). Our low-energy electron diffraction study showed an ordered (2×2) phase for one monolayer of Rb on Cu(111) (θ=0.25). By measuring both the (111) and (111¯) Bragg reflections at this coverage, we determine the Rb adsorption to be in the top site. We observe a high degree of order in the Rb overlayer perpendicular to the surface, with large disorder parallel to the surface
X-ray standing wave and reflectometric characterization of multilayer structures
Microstructural characterization of synthetic periodic multilayers by x-ray
standing waves have been presented. It has been shown that the analysis of
multilayers by combined x-ray reflectometry (XRR) and x-ray standing wave (XSW)
techniques can overcome the deficiencies of the individual techniques in
microstructural analysis. While interface roughnesses are more accurately
determined by the XRR technique, layer composition is more accurately
determined by the XSW technique where an element is directly identified by its
characteristic emission. These aspects have been explained with an example of a
20 period Pt/C multilayer. The composition of the C-layers due to Pt
dissolution in the C-layers, PtC, has been determined by the XSW
technique. In the XSW analysis when the whole amount of Pt present in the
C-layers is assumed to be within the broadened interface, it l eads to larger
interface roughness values, inconsistent with those determined by the XRR
technique. Constraining the interface roughness values to those determined by
the XRR technique, requires an additional amount of dissolved Pt in the
C-layers to expl ain the Pt fluorescence yield excited by the standing wave
field. This analysis provides the average composition PtC of the
C-layers .Comment: 12 pages RevTex, 10 eps figures embedde
CO oxidation over ZnO films on Pt(1 1 1) at near-atmospheric pressures
Well-ordered ultrathin ZnO(0 0 0 1) films were grown on Pt(1 1 1) in a layer-by-layer mode. The reactivity of the films as a function of the film thickness and coverage was examined by the CO oxidation reaction at near-atmospheric pressures. At low temperatures (∼450 K), CO2 production is found to be much higher on the films of partial coverage than on dense ZnO(0 0 0 1) films and bare Pt(1 1 1). Under reaction conditions, monolayer islands and an entire monolayer film transform into two-monolayers-thick islands, which dominate the surface of the active catalysts. The results provide an adequate structural model for elucidating the reaction mechanism on the oxide/metal boundary at technologically relevant conditions
Strain-Driven Mn-Reorganization in Overlithiated LixMn2O4 Epitaxial Thin-Film Electrodes
Lithium manganate LixMn2O4 (LMO) is a lithium ion cathode that suffers from the widely observed but poorly understood phenomenon of capacity loss due to Mn dissolution during electrochemical cycling. Here, operando X-ray reflectivity (low- and high-angle) is used to study the structure and morphology of epitaxial LMO (111) thin film cathodes undergoing lithium insertion and extraction to understand the inter-relationships between biaxial strain and Mn-dissolution. The initially strain-relieved LiMn2O4 films generate in-plane tensile and compressive strains for delithiated (x 1) charge states, respectively. The results reveal reversible Li insertion into LMO with no measurable Mn-loss for 0 1) reveals Mn loss from LMO along with dramatic changes in the intensity of the (111) Bragg peak that cannot be explained by Li stoichiometry. These results reveal a partially reversible site reorganization of Mn ions within the LMO film that is not seen in bulk reactions and indicates a transition in Mn-layer stoichiometry from 3:1 to 2:2 in alternating cation planes. Density functional theory calculations confirm that compressive strains (at x = 2) stabilize LMO structures with 2:2 Mn site distributions, therefore providing new insights into the role of lattice strain in the stability of LMO
Shape Transition in the Epitaxial Growth of Gold Silicide in Au Thin Films on Si(111)
Growth of epitaxial gold silicide islands on bromine-passivated Si(111)
substrates has been studied by optical and electron microscopy, electron probe
micro analysis and helium ion backscattering. The islands grow in the shape of
equilateral triangles up to a critical size beyond which the symmetry of the
structure is broken, resulting in a shape transition from triangle to
trapezoid. The island edges are aligned along directions. We have
observed elongated islands with aspect ratios as large as 8:1. These islands,
instead of growing along three equivalent [110] directions on the Si(111)
substrate, grow only along one preferential direction. This has been attributed
to the vicinality of the substrate surface.Comment: revtex version 3.0, 11 pages 4 figures available on request from
[email protected] - IP/BBSR/93-6
- …