Science and Technology at the Center
Cutting-edge scientific research is conducted at the NCNR in diverse areas of condensed-matter science: hard and soft condensed-matter physics, including biological physics, chemistry, and biology. The number of participants continues to increase, representing 142 universities, 32 government organizations and national laboratories, and 46 U.S. corporations; national laboratory and university researchers comprise 80 percent of the participants. The proposal pressure continues to be high, as does the quality as well as the number and impact of the publications. Overall, the metrics of assessment in the areas of research conducted during the past year, publication quality, and impact continue to be impressive.
Continuous improvements in the resolution and sensitivity of measurement techniques—combined with increasingly effective integration of theoretical analysis of data—and the availability of functional sample environments (rheometry, magnetic fields, humidity) are essential for state-of-the-art studies of the structure and properties of condensed matter. The problems investigated by NCNR researchers during the past year are diverse, covering both fundamental and applied topics. They include the following: materials for batteries, solar cells, fuel cells, hydrogen storage, magnetocaloric materials, polymer nanocomposites, drug delivery, properties of magnetic nanoparticles, structure and dynamics of lipid bilayers, superconductors, metals, and electronic ceramics. Publications resulting from research performed at the NCNR appear most frequently in four journals, Physical Review B, Physical Review Letters, Macromolecules, and Langmuir, covering hard and soft condensed matter. It is anticipated that, with the increasing emphasis on biology, the publications in more biologically relevant journals will reflect this new trend.
The research described in the oral and poster presentations for the panel is at the forefront, reflecting the output of highly competent researchers. The following sections discuss the assessment of the research reviewed in these areas: (1) soft matter—synthetic polymer systems and biological systems; (2) hard matter (energy, magnetic properties); (3) engineering (structural materials); (4) chemical physics; (5) earth science; and (6) fundamental physics.
The soft-matter research conducted at the NCNR in the area of polymers is at the leading edge, covering diverse areas of structure, dynamics, and functionality, particularly in the area of energy, including research on fuel cells and solar cells. NCNR researchers effectively exploited the strengths of the facilities by carefully identifying and investigating problems for which the use of neutrons was necessary and critical. In some cases, neutron scattering provided information that was otherwise difficult, or impossible, to ascertain. In other cases, the information gained using neutrons provided
complementary information, thereby providing important new insight into the properties of complex systems.
In an example involving a commonly studied organic solar cell, a thin-film blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), neutron reflectometry was used to determine the distribution of PCBM in the film, thereby providing insight into the efficiency of these materials. The depth distribution of PCBM had not been determined before these studies were conducted. With regard to polymer thin films, neutrons are uniquely suited for understanding the interactions between chains grafted to a substrate and free chains in a melt, or solution. The role of molecular weight and the degree of interpenetration between the brush layer and free chains, and the connection to dynamics illustrated by these studies are new and previously not predicted or anticipated. An understanding of these so-called brush-brush interactions is central to the understanding of interactions in a range of systems, including colloidal suspensions and brush coated nanoparticles in polymer nanocomposites. Information gained from the neutron measurements of the structure of hydrogels, formed using triblock copolymers, combined with information from other structural probes, provided important new insight into the structure of these complex nanostructured, technologically important materials. Neutron scattering thus far has proven to be one of the single most important probes toward understanding the structure and dynamics of polymer nanocomposites (PNCs). The uses of PNCs range from structural, to biomedical, to sensors and organic electronics. The primary challenges are associated with understanding and controlling the structure of these materials. Combinations of SANS, ultra-small angle neutron scattering (USANS), and inelastic neutron scattering (INS)/quasielastic neutron scattering (QENS) are able to discern much of the short-range and long-range structure and relaxations of these technologically important materials. This was illustrated rather well by the work conducted by the NCNR over the past year and by high-quality publications from the facility in this area.
Previous NRC assessment reports have encouraged the NCNR to place a greater emphasis on addressing questions of interest in contemporary biological science. Recommendations have included the employment of direct hires to enhance the NCNR staff, the development of new partnerships—for example, a joint hire with NIST’s Chemical Science and Technology Laboratory (now CSTL’s Biochemical Science Division [BSD])—and better utilization of the large research enterprise of the National Institutes of Health, located near the NCNR facility.
The NCNR has demonstrated a significant increase in efforts over those of previous years to address questions of biological significance. For example, the chief of the BSD discussed the application of SANS and neutron-reflectivity measurements and NCNR expertise in these areas in tackling biological problems of relevance to the BSD and its mission of the advancement of measurement science in biomolecular structure. In addition, a joint proposal by the NCNR and the BSD to support a deuteration and isotope-labeling facility was submitted. Although the proposal is not yet funded, the impact of such a facility for biological studies would greatly benefit not only BSD and NCNR scientists but the broad array of potential NCNR researchers who could better utilize
neutron scattering techniques given this capability—especially to investigate biomolecular structure and dynamics.
Such collaborative projects between the NCNR and the BSD are expected to be facilitated by a joint position focused on membrane protein biophysics (a focus area for the advancement of measurement science in biomolecular structure) that is currently being advertised. The expectation is that the primary focus of the person hired would be neutron scattering. A strong candidate has been identified, but a firm hire has not been made. As this joint hire has been “on the books” for more than 2 years, the renewed vigor in the pursuit of a mid- to senior-career research leader for the position is encouraging.
Finally, complementary techniques available at the BSD and the Center for Advanced Research in Biotechnology—for example, nuclear magnetic resonance, x-ray crystallography, and synthesis capabilities—could also be better leveraged to advance biological research at the NCNR by staff and external users.
The soft-matter group is to be commended for a number of developments over the past year. First, the SASSIE software package has the potential to greatly facilitate the analysis of SANS (as well as small-angle x-ray scattering [SAXS], reflectometry, and electron microscopy) measurements of protein structure. The arduous tasks of data analysis and the determination of a physically realistic structure have stymied such measurements previously. A user-friendly and robust analysis package, if fully realized, will have a significant, broad impact and likely lead to a much more vigorous utilization of SANS by the biological community.
Second, NCNR scientists have developed a routine method for fabricating cushioned biological membranes. Indeed, the NCNR reported that this new platform can be viewed as a readily available sample environment for any user of the facility. Again, such enabling capabilities are a necessity for meeting the needs of nontraditional neutron scattering users and for demonstrating the greater emphasis that the NCNR is making to reach out to the broad biological research community.
The hard-matter group has established important leadership in studies of high-Tc superconductors. Here the close synergy between theory and experiment has led to major breakthroughs during the past year, in particular in the advances made in the pnictide superconductors. The calculations show that the magnetism of iron is key to understanding the superconductivity in these new materials. The work has included important high-pressure studies in which transitions were found experimentally (i.e., within the range of techniques available at the NCNR) and in excellent agreement with theory. These studies show the importance of maintaining a strong program of sample environments in hard-matter experiments. Recent papers, with their extraordinary number of citations, represent one of the great success stories of the past year. The close interactions between theory and experiment demonstrated by this group serve as a model for other groups and programs within the NCNR.
Hard-matter studies also include high-quality measurements on ferroelectrics. Like the oxide superconductor studies, these experiments take advantage of the important role of neutrons for diffraction studies of oxides, particularly as a function of temperature, in which cases detailed studies of ferroelectric transitions can be carried out.
Recent measurements on single crystals are providing important new insight into the origin of the very high dielectric response of lead zirconate titanate (PZT). The work is important for the NIST mission and could lead to the development and application of new classes of transducer materials for data memory, medical devices, sonar, and energy storage.
Another strength of the NCNR has been in neutron scattering investigations of the structures and dynamics of hydrogen-rich materials. The hard-matter group has exploited this strength over the years to explore the incorporation of hydrogen in various solids, most recently in the development of new hydrogen-storage materials, as a function of pressure and temperature. This work has also benefited strongly from a close association between theory and experiment, most recently for graphene-based materials.
ENGINEERING (STRUCTURAL MATERIALS)
The study of texture development in metals is a good example of the use of NCNR capabilities for tackling industrial challenges important for the NIST mission, in this case for the automobile industry’s need for light, high-strength vehicles. The work involves the development of new approaches for inverting stresses from strains in polycrystalline metals. The effort could take advantage of important advances that have been made for texture development with submicron and nanoscale diffraction x-ray imaging techniques.
A number of important problems of technological importance were studied under the topic of chemical physics. The research spanned areas of ion transport and mobility in materials for energy storage, water dynamics under hydrophobic confinement, and hydrogen storage. Achieving an understanding the interaction of hydrogen with different chemical entities poses major scientific and technological challenges. The research on hydrogen interactions with coordinated metals (specifically, metal-organic frameworks) combines first-principles calculations with inelastic scattering to learn about the short-range structure and dynamics. These studies take advantage of the unique strengths of inelastic neutron scattering. The work on ion transport in polyelectrolytes was interesting; the information gleaned from neutrons provided information about the short-range structure and dynamics, which was essential and could not have been otherwise obtained.
Earth and environmental science applications of NCNR capabilities include a study of the interface between bedrock and soils using SANS and USANS. This study made good use of NCNR capability for addressing nanoscale heterogeneity in complex materials, in this case for understanding the transitional regime between soils and bedrock. The use of focusing optics described below (see the section entitled “Other Comments and Broader Issues”) would open up capabilities for a broader range of earth science problems (such as measurements on earth materials at high pressures and temperatures, and examination of polyphase assemblages—that is, rocks and geological
fluids). Only one earth science article was published in the 2009 NCNR annual report,3 which describes accomplishments for 2009. Applications in earth, environmental, and planetary science could be a growth area for the future.
NCNR’s continuing support of basic physics experiments is laudable. The aCORN—“a CORelation in Neutron decay”—experiment to measure the electron-antineutrino correlation in neutron beta decay probes the nature of the electroweak force at the most elementary level. This is a compelling example of using the unique capabilities of the NCNR to make a significant improvement in a fundamental measurement. This work also complements experiments done at other facilities to test the limits of the Standard Model of fundamental particles and interactions. The NCNR is encouraged to support this and other similar experiments.
OTHER COMMENTS AND BROADER ISSUES
The large beam and low flux relative to spallation neutron sources preclude certain types of measurements at the NCNR. Fiber focusing methods are being used to focus beams to increase flux on samples. This approach is being used, for example, on the prompt-gamma neutron activation analysis (PGNAA) experiments. There have also been important developments in other neutron focusing methods. For example, Kirkpatrick-Baez mirrors that can focus beams to below 100 microns have been developed at the ORNL. These could be built and used at the NCNR, opening up a broad range of new experimental capabilities for measurements on small length scales—for example, for heterogeneous materials, and properties of samples that are necessarily small, as well as samples in extreme environments of pressure, temperature, and magnetic or electric fields. Other important developments in techniques allow larger samples to be studied under extreme environments, thereby increasing the range of opportunities for scattering studies using the flux available at the NCNR. The NCNR should remain aware of these methods and remain positioned to serve a growing user community in extreme environments.
Neutron and x-ray scattering are complementary techniques. Thus, having an x-ray program in-house based on conventional x-ray (for example, rotating anodes) forms an essential component of the support laboratory for sample characterization at the NCNR. NIST scientists in general and NCNR scientists in particular could benefit from greater involvement in advanced synchrotron sources. In fact, competing facilities in Europe benefit from having both neutron and synchrotron sources on the same site (the ISIS Spallation Neutron Source and Diamond Light Source in England, the ILL and the European Synchrotron Radiation Facility in France), whereas the United States no longer has this capability. However, new opportunities are becoming available in the United States with the creation of new x-ray sources, most notably the future National Synchrotron Light Source (NSLS-II) at the Brookhaven National Laboratory. NIST could
National Institute of Standards and Technology, 2009 NIST Center for Neutron Research Accomplishments and Opportunities, NIST Special Publication 1105, December 2009. Available at http://www.ncnr.nist.gov/AnnualReport/. Accessed August 2010.
take advantage of these developments by being a part of, or in fact by leading, the construction of NIST-managed beam lines (e.g., for biology, hard matter, imaging, etc.).
The recent report on crystal growth from the National Research Council made strong recommendations for investments in single-crystal growth in this country because of its importance for technological advances.4 The United States was once a leader in this area but now lags behind numerous other countries (notably in Asia). NIST could be an ideal place to develop this effort. If the effort were embraced as a part of the NIST mission, the NCNR would be very well positioned to take advantage of such renewed focus by providing a crucial tool for the characterization of new single-crystal materials, as demonstrated by the recent work on PZT.
Finally, the plans of the NCNR for the development of new techniques, improvements in resolution, and data collection will place the NCNR in a highly competitive position worldwide. Future instruments to be added include very small angle neutron scattering (vSANS), the new CANDoR, and the Materials Diffractometer. With the addition of new instruments and improved capabilities, the number of users will undoubtedly increase. Moreover, the scope, diversity, and intellectual depth of problems that users can examine within the broad and diverse field of condensed-matter science will improve considerably.