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On Dec 6th, we held a day long workshop to solicit user input on the new directions that the SSRL materials science scattering program should take to optimally address the needs of material and chemical sciences and maximally utilize the improved source characteristics that the SPEAR3 upgrade provides. The workshop was held on the Stanford campus and was attended by about 30 participants from industry, national labs, and universities. The workshop began with a short presentation by Apurva Mehta (SSRL) detailing a proposal developed by the staff for SSRL’s scattering program and was followed by 7 formal scientific presentations from users and staff as well as several impromptu presentations from attendees. These talks delineated numerous interesting scientific thrusts that SSRL should consider when developing its future capabilities.
The first scientific presentation was given by Alan Pelton (NDC/J&J) on the importance of measuring texture using 2D x-ray detectors and specifically its application in better understanding mechanical properties of advanced materials, such biomedical stents, MEMS devices, and thin film structures. He emphasized the desirability of real time data reduction for area detectors.
Scott Fendorf (Stanford University) then talked about the importance of using x-ray scattering to study in-situ solid-phase transformations of complex environmental materials, such as disordered iron oxides, during chemical reactions. Of particular importance is the ability to simultaneously identify multiple crystalline phases in complex multicomponent mixtures (e.g., soils) under hydrated conditions. Scott pointed out that defining complex material reactants and products in natural systems and measuring their kinetics is a current frontier in molecular environmental science and that the relevant time scales for these reactions are seconds to several hours. Scott highlighted how quick 2D diffraction (i.e., recording the data with a CCD or image-plate area detector) is essential to these investigations, because it provides time resolution on the scale of seconds, enables diffraction imaging (i.e., rastering the sample) and facilitates texture analysis.
In the discussion session, Bruce Clemens (Stanford University) pointed out that for nanocrystalline materials the range of stability of certain phases/polymorphs changes significantly, otherwise stable phases completely disappear from the phase diagram, and new phases appear and dominate the properties of nanomaterials. It is, therefore, essential to determine phase diagrams for nanomaterials in detail. A diffraction facility where a large number of samples can be screened quickly, perhaps in a semi-automated fashion on combinatorial sample libraries, will be a big advantage to study of nanocrystals and structures.
Micheal Chabinyc (Palo Alto Research Center) discussed some of the extensive X-ray scattering work done at SSRL on organic thin films, which have potential applications as transistors and solar cells. He noted that previous work had showed the connection between film structure and morphology and electronic properties. He then observed that it would be desirable to have annealing cells that could reach 250 C but where both heating and cooling could be achieved quickly (few seconds). The ability to achieve lower temperatures (70K) was also desirable, but less important. Agreeing with Pelton, he said that real time data reduction for area detectors was important.
Jason Hancock (Stanford University) from Martin Greven’s group presented some thoughts on using x-ray scattering to probe spin, charge and lattice ordering to tease out the complex interactions that govern the physics of correlated electron systems. Jason showed how measuring weak scattering, such as diffuse scattering and superstructure reflections, can yield information about Jahn–Teller distortions, charge texture and nanoscale disorder in high Tc superconductors. He further emphasized how measuring intensities of these weak reflections under resonance conditions and as a function of x-ray polarization (Psi meaurements) provides a definitive signature of orbital order and other subtle electronically and magnetically driven “hidden” ordering.
Ian Fisher (Stanford University) continued the theme of correlated electron materials. He discussed using single crystal diffraction to study electronic instabilities such as charge density waves in rare earth tellurides, and the interplay between charge, orbital and spin degrees of freedom in 5d magnetic compounds. A better understanding of these materials will aid in understanding the physics of more complicated transition metal compounds, such high Tc superconductors and CMR manganites. Ian noted that it was important to have cryostats capable of achieving 3-4 K and furnaces that can reach about 300 deg C. Both Jason and Ian emphasized that magnetic X-ray scattering was a useful probe of strongly correlated materials both for phase transitions and polarization analysis. The different polarization dependence of scattering cross-section for spin and orbit magnetic order in x-ray scattering complements neutron scattering. Such measurements require several degrees of freedom to align the crystal in the appropriate scattering conditions and to maintain the scattering condition but vary the angle between the diffraction and the polarization vectors.
Steve Conradson (Los Alamos National Lab) emphasized the importance of using total scattering and pair-distribution (PDF) analysis to study inhomogeneities such as nano-scale ordering between different unit cells, which are important because these non-uniformities contribute to the properties of materials. Steve anticipates that total-scattering PDF will be important for a range of materials including advanced nuclear energy materials, nanomaterials for solar energy production and heterogeneous catalysis, and for contaminant transport in the environment, For such studies, it is necessary to use high energy (at least 30 keV), focused x-ray beams and to record data using an area detectors. Such a capability is within the grasp of existing beam line facilities (e.g., BL 10-2) with proper optimization of hutch instrumentation.
Mike Toney (SSRL) was the last speaker for the morning and he discussed surface, interface and thin film scattering. He showed that X-ray scattering can be used to determine surface and interface atomic structure and that this structure determines surface reactivity. Such studies must be conducted in-situ and are important for catalysis and geochemistry. Mike also showed that it is important for thin film and interface studies to have a flexible scattering spectrometer to allow optimal sample alignment (e.g., grazing incidence angle, but large exit angle).
A delicious boxed lunch was served during the last two scientific talks. The afternoon session continued with several informal scientific talks by attendees. The first of this was given by Paul McIntyre (Stanford University). Paul talked about his research on nanowires, where he wishes to probe the nanowire and catalyst structure during wire growth, and core-shell wires, where he wants high resolution measurements of the strain state in the core and shell. Bruce Clemens made a brief comment on importance of texture measurement during growth of thin films, such growth of Atomic Layer Deposition (ALD). Paul McIntyre and Arturas Vailionis (Stanford University) expanded upon Bruce’s scientific thrust by suggesting inclusion of in-situ growth studies of thin films and multi-layers via other techniques such as CVD. Jennie Acrivos (SJSU) showed how interesting phase decompositions can be studied via angle dependent inelastic scattering. At this point Sean Brennan (SSRL) pointed out that all of the scientific thrusts suggested so far are exciting and rapidly developing fields; some of these, such as angle dependent inelastic scattering, very high energy scattering experiments and in-situ thin-film growth studies are brightness hungry experiments and perhaps SSRL can better utilize its resources by concentrating on flux density hungry experiments.
Based on the discussion following the science sessions, we think that SSRL scattering program should modify and design beam lines to facilitate and provide technical and scientific support for the following types of measurements and scientific thrusts.
The beam line will be designed so that a small point detector can also be used and scanned using the 2theta arm, which is non-operational during the use of the 2D detector. Further, the design will be such that the data collection can be easily and quickly changed between the two detectors.
The diffractometer will accommodate several different temperature stages, including a small unit that spans room temperature (range: 70 -700K). Further, the sample mounting will be modular so other user-designed sample chambers, including various types of reactors (from flow reactors to electrochemical cells, to high temperature hydrogen reactors), strain rigs and kinematic stages for combinatorial libraries. The sample mounting will be identical on both the wiggler beam lines (see below) and will be compatible with the bend magnet beam line, so that most stages can be used on any of the three scattering stations.
The diffractometer will have a cryostat that goes down to ~ 10 K and a furnace that reaches 1200K as standard equipment that users can request. It may also have a He flow cryostat that can reach 1.4 K. The sample holder design is such that this will accommodate most of the stages designed for the other wiggler beam line (e.g., 70-700 K stage, electrochemical cells).
The current analyzer setup will be made more robust. In addition to the currently available Soller slits for the other leg of the 2theta arm, this arm will be modified to accommodate the linear PSD and the small 2D detector.
The current sample mounting arrangement on BL2-1 is different from that of a 4 circle or a Kappa diffractometer. A transition piece will be designed so that many of the reactors and sample chambers designed for the two wiggler beam lines can be used, with minor changes, on this beam line.
The data collection platform for the large area detector will be linked with a fast write link to a high capacity data storage server. The platform for the point detector will also have capability of controlling the linear PSD and the small 2D detector, and combining several of the individual image frames into a composite diffraction map.
There will also be data reduction and analysis software linked to the 2D data collector for preliminary processing of data (converting the images from pixel space to chi-omega and Qxy-Qz diffraction space). The analysis software will be designed to interact with other higher level data processing, such as strain calculator, or a texture analysis module, which frequent beam line users and experts in the field will be encouraged to develop.