March 2004 NSLS-II Workshop: Scattering Breakout Session

The scattering breakout session at the NSLS-II workshop had 25 attendees. The distribution was Stony Brook University (7), other academic institutions (10), BNL (3), other government labs (2), and industry (3). The three invited speakers presented their views on the needs of their respective fields of interest through the description of recent research results, problems or challenges encountered, and ideas about what studies they would like to pursue and capabilities they would like to see at NSLS-II.

In his talk, "3-2-1-0: Extending X-ray Scattering Studies from Bulk Crystals to Single Nanoparticles," Paul Fuoss of Argonne National Laboratory stated that his interest in nanoscale systems was not in the systems themselves, but in the fundamental properties of physics they expose. An example of this is changes in structure due to the constraints imposed by reduced dimensionality. He presented results from several recent studies, including the determination of which of two competing models describes the microstructure of "amorphous" SiGe2 thin films. Using GIXS to measure the diffraction from films of varying thickness, he was able to identify the structure as a random covalent network rather than microcrystalline.

A study of the dynamics of the in-situ oxidation of copper was also described, where beam effects at 8 keV caused adiabatic temperature changes that changed the time-resolved redox behavior of the system. Using higher energy photons (24 keV) greatly reduced this effect. Dr. Fuoss also mentioned interest in future studies of phase-change memory chalcogenides used as the medium in rewritable CDs, and nanosized single TiO₂ clusters. He calculated that a 1000-molecule TiO₂ cluster would produce 148 photons/sec at an NSLS-II 1:1 focus beamline.

Some of the capabilities Dr. Fuoss would like to see in a new synchrotron are:

the ability to use higher energy photons (tens of keV) to reduce the energy-dumping effects of the beam on susceptible samples,
small, bright beams to do time-dependent studies on nanoscale clusters and thin film features,
high beam stability so that the x-rays, probes such as lasers, and the sample are co-located throughout the experiment,
innovative sample manipulation techniques, such as micromachines and optical traps,
temperature and overall environmental stability at experimental stations, and
advances in detectors to quickly and efficiently gather all photons scattered from a sample.

Paul Fenter, a geoscientist at Argonne National Lab, described a great number of x-ray techniques covering a wide area of energy - momentum transfer space and converged on several that combine both spectroscopic and scattering measurements in order to obtain elemental or chemical as well as structural information about a sample. Dr. Fenter's research emphasis is on liquid-solid interfaces in environmental and catalytic systems. In his talk "X-ray Scattering as an Elemental and Chemical Probe of Solid-Fluid Interfaces," he gave several examples of experiments performed at the APS using sophisticated variations of x-ray standing wave (XSW) and reflectivity (XR) techniques.

Resonant Anomalous X-ray Reflectivity (RAXR) was used to locate the Pt atoms in Pt(NH3)4 at a quartz-water interface. Taking energy scans through the Pt LIII absorption edge at several Q values in the saddle area between Bragg peaks provided a model-independent technique for finding 2 distinct locations for Pt at the interface. In X-ray Standing Wave Imaging, again, a model-independent determination of 3D atomic positions can be obtained by measuring the amplitudes and phases of several reflections. This technique was used to identify the type and positions of a number of adsorbed impurity ions in Rutile.

For both of these techniques high stability is essential. In addition, the use of a small, bright beam would enable the study of more "real-world" samples having rough surfaces and heterogeneous compositions. At NSLS-II, Dr. Fenter would like to see beamlines that have combined spectroscopy and scattering capabilities in order to obtain (ideally) 1Å resolution, nanoscale spatial sensitivity, and elemental and chemical sensitivity. He would also like to have a large space in the hutch upstream of the sample to accommodate several post-monochromators, so that an appropriate selection of crystals can be put in place to match the incident beam characteristics to those of the sample. Microbeams would allow migration away from ideal crystals for XSW work and more precise diffraction peak profiling and position determination for GIXS on rough, uneven samples.

In general, samples of the future will be more difficult: smaller in size, amorphous, thin, liquid, heterogeneous, etc.; and researchers will be trying to do harder measurements: dynamics, in-situ structural changes, coherent beam imaging, local structure, nucleation and growth from vapor, etc. To quote Dr. Fuoss, we "…need to see it well enough to make sure (we) know everything that's going on".

The third invited talk was on "X-ray Diffraction Studies of Strongly Correlated Systems in High Magnetic Fields: Current and Future Research Opportunities," by Valery Kiryukhin of Rutgers University. Professor Kiryukhin stated that 10% of recent PRLs and PRBs are on topics related to magnetic fields. Although originally studied using neutron scattering, magnetic systems are actually well suited to x-rays. X-rays allow for higher resolution, elemental sensitivity, measurement of valence electron density, charge and orbital order, small samples, surfaces and interfaces, extreme environments, and magnetic scattering. Several examples of challenging experiments were described. The CuGeO₃ spin-Peierls compound is a model 1D magnet system that can be described by theory that is exactly solvable. The effect of small amounts of impurities on this system under high magnetic field changes the character of the transition, removes long-range order, and forms lattice modulations with soliton domains having short correlation lengths. These disordered lattices require high fluxes, small beams, and high magnetic fields for study.

Another rich system is Colossal Magneto Resistive (CMR) materials. Films of the manganite composition (A_1-xB_xMnO₃) exhibit transitions between insulating paramagnetic and conducting ferromagnetic phases under applied field. The properties of the thin films differ from those of the bulk, and it is difficult to study transition mechanisms and the structures of correlated regions, or to get a good microscopic description of such things as percolative phase transitions. Other highly correlated systems such as cuprates and nickelates would be interesting to study as well.

Professor Kiryukhin would like to see at least one ID beamline at NSLS-II dedicated to magnetic studies with a high-field DC magnet permanently installed or on rails. While there was some discussion about the relative merits of DC vs. higher-field pulsed magnets, the latter has not been developed yet for x-ray studies. While high flux is vital for all of these studies, high-brightness nanobeams would be useful for studying heterogeneous samples and systems having small (2 - 3 nm) correlation lengths.

A general discussion after coffee break was led by several short presentations by audience members. Mark Croft of Rutgers presented some transmission Energy Dispersive X-ray (EDX) results on strain profiling of crack-tips in steels, and depth-dependent stress distribution in shot- and laser-peened materials. He would like to see a wiggler-based facility beamline dedicated to this technique with a pinhole for collimation, x, y, and z translation, and energy-dispersive multielement detectors. Detector and collimator designs were stressed, so that as many scattered photons as possible can be collected over a wide range of reciprocal space. Temperature stability also was mentioned again.

Yong-Jin Han from Bell Labs/Lucent Technologies talked about his work on growth of nanocrystals of calcite on self-organized organic templates of alkylthiols. His research would be helped by lower energy x-rays, in the Ca and Mg edge regions, as well as small, bright beams to determine orientation, structure, and elemental distribution in the calcite nanocrystals.

Joe Strzalka at University of Pennsylvania presented work done at the APS on fatty acid Langmuir monolayers using time-resolved resonant XR to determine the position of selected elements in a model-independent way. Time-resolved studies can be performed using a CCD area detector and curving the sample to distribute the reflectivity angles across the detector in one dimension, limiting detection to one strip of the detector, and line-shifting the data in exposed pixels to build up a time-resolved data set without being limited by detector readout time. One bottleneck to liquid surface experiments is that waiting for the surface to settle limits the speed of data acquisition. A vibration-free environment would also be critical to nanobeam experiments.

General comments from the audience, in addition to seconding those of the speakers, included requests for lots of lab space (with pure water sources, chemical-handling space, and refrigerators) close to the beamlines, more thought about allocation of bending magnet beamlines, and a low energy (1.8 - 4 keV) beamline. One interesting suggestion was to consider whether VUV beamlines could have the beam deflected up so that they could be built on an overhead mezzanine. Another was to possibly make a large deflection fixed energy beamline by taking it off a wiggler to the inside of the ring rather than the outside. The need for a very rapid (10^-3 - 10^-6 sec) time-resolved in-situ diffraction beamline for study of such systems as catalysts and microelectronics materials growth and transformations was stated. This would require high flux and fast area or linear detectors. Two superconducting wiggler beamlines with emphasis on materials science were suggested. Some concern was also stated that only 4 ID beamlines have not been accounted for in the core beamline description. This leaves little prime territory for non-facility groups that may have outstanding ideas and resources.

Last Modified: January 31, 2008
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