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

Development of X-ray facilities for materials research at the Advanced Photon Source. Final technical report for period AUGUST 15, 1996 - AUGUST 14, 2000

The P.I. and his research team successfully used the funds from the DOE Instrumentation grant entitled, 'Development of X-Ray Facilities for Materials Research at the Advanced Photon Source,' to design, build, test, and commission a customized surface science x-ray scattering spectroscopy chamber. This instrumentation, which is presently in use at an APS x-ray undulator beam line operated by the DuPont-Northwestern-Dow Collaborative Access Team, is used for x-ray measurements of surface, interface, thin film and nano-structures

Structure and ad-atom thermal vibrational amplitude for the Ga/Si(001) 2x1 surface

Fundamental and higher-order harmonic x-ray standing wave measurements were combined to directly measure the lattice positions and thermal vibrational amplitudes of Ga adatoms on the Si(001) surface. In this room temperature study, surface structural parameters such as ad-dimer bond lengths were also determined with much higher accuracy than previous studies

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

Rapid atom-efficient polyolefin plastics hydrogenolysis mediated by a well-defined single-site electrophilic/cationic organo-zirconium catalyst

Polyolefins comprise a major fraction of single-use plastics, yet their catalytic deconstruction/recycling has proven challenging due to their inert saturated hydrocarbon connectivities. Here a very electrophilic, formally cationic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic sulfated alumina support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions, with catalytic onset as low as 90 °C/0.5 atm H2 with 0.02 mol% catalyst loading. For polyethylene, quantitative hydrogenolysis to light hydrocarbons proceeds within 48 min with an activity of > 4000 mol(CH2 units)·mol(Zr)−1·h−1 at 200 °C/2 atm H2 pressure. Under similar solventless conditions, polyethylene-co−1-octene, isotactic polypropylene, and a post-consumer food container cap are rapidly hydrogenolyzed to low molecular mass hydrocarbons. Regarding mechanism, theory and experiment identify a turnover-limiting C-C scission pathway involving ß-alkyl transfer rather than the more common σ-bond metathesis

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