Neutrons and Nanoscience

ORNL's world-class neutron sources will enable advances in nanomagnetism, membranes, and catalysis.

Like a child playing with toy building blocks, researchers are creating new materials, objects, and devices from nanoscale blocks as small as individual atoms. These creations have unique properties that, when understood, will enable the design of new "functional" materials to carry out specific tasks—a type of "nanotechnology." The properties of materials and devices are dictated by the arrangement, or structure, of their building blocks, and by the way these nanoscale blocks move over time. The ability of neutrons to measure the positions and motions of atoms and molecules makes these nuclear particles essential research tools in nanoscience and nanotechnology.

Not surprisingly, the Department of Energy elected to build the agency's first nanoscience center at Oak Ridge National Laboratory to give researchers direct access to the world's most powerful probes of nanostructured materials located in the neutron scattering facilities at the High Flux Isotope Reactor and the Spallation Neutron Source. Coupled with the Laboratory's unique high-performance computing capability, these "neutron microscopes" can provide unique "images" at the nanoscale of the properties of materials synthesized at DOE's Center for Nanophase Materials Sciences.

Neutrons cover all the length scales of interest in nanoscience, from the atomic structure of individual building blocks to the configuration of assembled, functional structures, making them essential tools for the elucidation of the structure and function of nanostructured materials. At the same time neutron probes enable researchers to determine the motions, or dynamics, of nanoscale building blocks over a wide range of time scales ranging from ultrafast structural relaxation to slow diffusion processes—a remarkable time scale that spans 10 orders of magnitude.

Moreover, because they have a magnetic moment, neutrons behave like small magnets, making them extremely sensitive to magnetic structure on the nanoscale. Advanced techniques allow researchers to align these small neutron magnets and use them to probe both the structure and temporal variations of magnetic particles in nanomaterials such as nanomagnets, molecular magnets, and electronic devices. Significantly, neutrons travel easily through most materials without causing damage. Researchers can take measurements deep inside the surface, and in situ under a wide range of environments, including vacuum, high or low temperature, high pressure, or intense magnetic fields. Critical measurements can be made even during the synthesis and subsequent processing of the material.

A variety of examples demonstrate the value of neutrons to nanoscience.

Nanomagnetism

For years, information has been transmitted by means of electric currents or charge flowing through electronic circuits. Today, however, the electronics industry has adopted technologies that use electron "spin"—electrons acting like small spinning tops—rather than charge to store and transmit information, enabling faster devices and higher-density storage media. Examples include magnetic random access memories and the read heads on modern computer hard disks. These so-called "spintronic" devices rely on the relative alignment of the electron spins to the magnetic direction in nanostructured thin-film devices, which determines the way they can flow through the device.

Polarized neutron reflection studies provide unique information on magnetic direction in deeply buried layers, and laterally, across the surface. Such information is required to understand and further develop these devices. Future advances in instrumentation using the intense neutron beams at SNS will allow these studies to be extended to understanding the dynamical processes that give rise to switching phenomena and the interactions of magnetic nanoparticles—important steps on the path to developing quantum computers.

Membranes

Another area of interest to nanotechnology where neutron studies will play a crucial role is the understanding of the structure and dynamical properties of complex fluids confined in nanometer-scale architectures. Such fluids include polymers, surfactants, and small-molecule liquids that exhibit novel properties when confined in one, two, or three dimensions. In particular, controlled synthesis of membrane structures will provide an interface to biological systems and allow researchers to probe, modify, or mimic live cells, cell components, and molecular structures relevant to biology.

The control of the passage of materials and information across cell membranes is one of the most critical and little understood phenomena for biological function, and hence life itself. Because of their sensitivity to light atoms, neutrons are ideal probes for determining membrane structure. In the past, neutron scattering studies have been used to determine, for example, the structure of pneumolysin, the toxin from Pneumococcus bacteria which causes pneumonia, middle ear infections, and meningitis and promotes cell death by creating pores in the cell membrane.

Ongoing neutron studies are aimed at determining protein crystal structures and understanding how membranes fuse and transmit information. In the future, using a unique combination of synthesis of materials at CNMS and neutron characterization techniques at HFIR and SNS, researchers coming to ORNL will be able to study protein and lipid interactions in membranes suspended over specifically synthesized nanostructured scaffolds, which may be adapted to promote controlled insertion of proteins, molecules, or functional structures into the membrane.

Of course, a full understanding of biological systems requires additional information on their dynamics. By definition living organisms move, and do so on time scales spanning from vibrations to slow folding processes, all of which are vital to function. Neutrons come to the rescue once again, easily covering the relevant time domain. Novel instrumentation under development for SNS will allow simultaneous measurements of membrane structures 10 to 1000 nanometers wide and their movements ranging from picoseconds to microseconds.

Biosensors, microfluidic devices, and structural templates for tissue engineering and drug delivery are evidence that the development of improved structures that mimic biological functions in a controlled manner have increasingly important applications in everyday life.

Catalysis

Many commercial products are made possible by chemical reactions driven by catalysts. Because most catalytic reactions take place on surfaces, the increased surface area afforded by nano-sized particles is of great interest. Neutrons provide invaluable information on the sizes and surface structures of nanoparticle catalysts and adsorbed molecules. Researchers seek to determine how these adsorbed molecules change during contact with a catalyst surface and relate these changes to the catalyzed chemical processes. Relating the structures of both the adsorbed species and catalytic particle is crucial to understanding the catalytic process. These techniques have led to the understanding of the role of nanostructured cerium oxide particles in platinum-based catalytic converters for automobiles.

However, understanding the structure alone may not be sufficient to fully interpret a functioning catalyst. The catalytic process is dynamic; adsorbed molecules undergo rapid transformations including diffusion, rotation, and vibration, all of which have a role in the catalytic process. Fortunately, neutrons also can be used to characterize dynamic processes and relate them to structural properties. Once again, all this data can be obtained under real operating conditions. Neutrons have been used to study processes that deactivate or poison catalysts. As our understanding of the chemical processes involved improves, so does the demand for more detailed information. In the future polarized neutron techniques will allow researchers to follow spin-dependent processes involving paramagnetic molecules and obtain an even more detailed understanding of the chemistry of catalytic reactions.

While efficient catalysts are important in our search for clean, environmentally friendly energy sources, the role of neutrons to support this goal does not stop at catalysts. Neutrons are also used to study how well carbon nanotubes can store hydrogen, a primary need for an efficient hydrogen-based economy, and how well various membranes work in advanced fuel cells.

Neutrons offer an important key to unlocking the secrets of nature at the nanoscale. Discovering these secrets will accelerate the development of nanotechnology, which promises new energy, health, and environmental solutions in the years ahead.—Ian S. Anderson, SNS Experimental Facilities Division Director

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