Workshop on measurement needs for local-structure determination in inorganic materials

Journal of Research of the National Institute of Standards and Technology, Nov-Dec, 2008 by Igor Levin, Terrell Vanderah

The functional responses (e.g., dielectric, magnetic, catalytic, etc.) of many industrially-relevant materials are controlled by their local structure--a term that refers to the atomic arrangements on a scale ranging from atomic (sub-nanometer) to several nanometers. Thus, accurate knowledge of local structure is central to understanding the properties of nanostructured materials, thereby placing the problem of determining atomic positions on the nanoscale--the so-called "nanostructure problem"--at the center of modern materials development. Today, multiple experimental techniques exist for probing local atomic arrangements; nonetheless, finding accurate comprehensive, and robust structural solutions for the nanostructured materials still remains a formidable challenge because any one of these methods yields only a partial view of the local structure. The primary goal of this 2-day NIST-sponsored workshop was to bring together experts in the key experimental and theoretical areas relevant to local structure determination to devise a strategy for the collaborative effort required to develop a comprehensive measurement solution on the local scale. The participants unanimously agreed that solving the nanostructure problem--an ultimate frontier in materials characterization--necessitates a coordinated interdisciplinary effort that transcends the existing capabilities of any single institution, including national laboratories, centers, and user facilities. The discussions converged on an institute dedicated to local structure determination as the most viable organizational platform for successfully addressing the nanostructure problem. The proposed "institute" would provide an intellectual infrastructure for local structure determination by (1) developing and maintaining relevant computer software integrated in an open-source global optimization framework (Fig. 2), (2) connecting industrial and academic users with experts in measurement techniques, (3) developing and maintaining pertinent databases, and (4) providing necessary education and training.

Key words: diffraction; local structure; measurements; microscopy; nanostructure; spectroscopy.

Accepted: August 12, 2008

Available online: http://www.nist.gov/jres

1. Introduction

The functional responses (e.g., dielectric, magnetic, catalytic, etc.) of many industrially-relevant materials are controlled by their local structure--a term that refers to the atomic arrangements on a scale ranging from atomic (sub-nanometer) to several nanometers. Examples include nanostructured materials having atomic order limited by the nanoscale dimensions of their constituent particles or layers (e.g., nanoparticles, mesoporous materials, nanometer-thick layers in thin film architectures) and nanostructured bulk materials exhibiting deviations from the average periodicity arising from local chemical and/or displacive order (e.g., dielectrics, relaxor ferroelectrics, thermoelectrics). The existing or envisioned applications for these materials impact the entire industrial spectrum including the multi-billion electronic, energy, automotive, and health sectors. The 2009 budget request for the U.S. National Nanotechnology Initiative alone is $1.5 billion to promote understanding of the unique phenomena and processes at the nanometer scale, and to expedite the use of this knowledge to advance practical applications. Accurate knowledge of local structure is critical for understanding the properties of nanostructured materials, thereby placing the problem of determining atomic positions on the nanoscale--the so-called "nanostructure problem"--at the center of modern materials development.

Unfortunately, traditional structure-solving approaches that assume long-range structural periodicity and rely upon Bragg reflections observed by x-ray/neutron diffraction methods fail on the local scale. Today, multiple experimental techniques exist for probing local atomic arrangements. A partial list includes various spectroscopic tools (e.g., based on x-ray absorption, Raman, nuclear magnetic resonance), rapidly emerging total x-ray/neutron scattering, and a suite of diffraction and imaging techniques implemented in modern transmission electron microscopes. Recent revolutionary advances in synchrotron, neutron, and electron microscopy instrumentation radically expanded the technical capabilities of many of these methods by providing critical high-energy/high-brightness probes. At the same time, modern theoretical calculations, especially those based on first principles, have become increasingly successful in predicting local atomic arrangements and their associated measurement responses (e.g., diffuse x-ray scattering or Raman spectra). Nonetheless, finding accurate comprehensive, and robust structural solutions for nanostructured materials still remains a formidable challenge because any one of the existing methods yields only a partial view of the local structure. The problem is further complicated by the substantial fractions of atoms in these materials associated with surfaces (external or internal). Additionally, the information is often needed at several length scales (e.g., from < 1 nm to 10 nm). A recent review article in Science (1) proposed a plausible path for solving the local-structure problem by integrating complementary information from multiple measurement techniques and theory into a global structure optimization framework that could be used freely by experimentalists and theorists alike. The article emphasized that the development of this framework requires a coherent focused interdisciplinary effort, which is currently missing.