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X-ray Nanoimaging

Washington, D.C., 9 April, 2013 — Wenge Yang of the Geophysical Laboratory and his team have made a major breakthrough in measuring the structure of nanomaterials under extremely high pressures. For the first time, they developed a way to get around the severe distortions of high-energy X-ray beams that are used to image the structure of a gold nanocrystal. The technique could lead to advancements of new nanomaterials created under high pressures and a greater understanding of what is happening in planetary interiors. 

Yang explained: “The only way to see what happens to such samples when under pressure is to use high-energy X-rays produced by synchrotron sources. Synchrotrons can provide highly coherent X-rays for advanced 3-D imaging with tens of nanometers of resolution. This is different from incoherent X-ray imaging used for medical examination that has micron spatial resolution. The high pressures fundamentally change many properties of the material.”

The team found that by averaging the patterns of the bent waves—the diffraction patterns—of the same crystal using different sample alignments in the instrumentation, and by using an algorithm  developed by researchers at the London Centre for Nanotechnology, they can compensate for the distortion and improve spatial resolution by two orders of magnitude.

“The wave distortion problem is analogous to prescribing eyeglasses for the diamond anvil cell to correct the vision of the coherent X-ray imaging system,” remarked Ian Robinson, leader of the London team.

The researchers subjected a 400-nanometer (.000015 inch) single crystal of gold to pressures from about 8,000 times the pressure at sea level to 64,000 times that pressure, which is about the pressure in Earth’s upper mantle, the layer between the outer core and crust.

The team conducted the imaging experiment at the Advanced Photon Source, Argonne National Laboratory. They compressed the gold nanocrystal and found at first, as expected, that the edges of the crystal become sharp and strained. But to their complete surprise, the strains disappeared upon further compression.  The crystal developed a more rounded shape at the highest pressure, implying an unusual plastic-like flow.

“Nanogold particles are very useful materials,” remarked Yang. “They are about 60% stiffer compared with other micron–sized particles and could prove pivotal for constructing improved molecular electrodes, nanoscale coatings, and other advanced engineering materials. The new technique will be critical for advances in these areas.”

“Now that the distortion problem has been solved, the whole field of nanocrystal structures under pressure can be accessed,” said Robinson. “The scientific mystery of why nanocrystals under pressure are somehow up to 60% stronger than bulk material may soon be unraveled.”

The research was published in the April 9, 2013, issue of Nature Communications found here.

Figure Captions: Top Overall schematic of the experimental setup. A large opening panoramic diamond-anvil cell is used to compress the studied crystal, positioned at the rotation centre of the diffractometer. An X-ray sensitive charge-coupled device is placed at 1 m away to collect far-field diffraction patterns. The insert scanning-electron microscopy (SEM) picture shows typical gold nanoparticles distributed on a silicon substrate. The zoomed-in figure of the DAC shows the sample environment. Bottom: Strain distribution in a 400 nm single crystal gold particle subjected to 1.7 GPa, detected by the recently developed coherent x-ray diffraction imaging (CXDI) technique. (a)Crystal shape and surface truncated (100) and (111) planes, (b) and (c) are the top and bottom views, (d-f) are side views with 120 degrees rotated along the surface normal (111) direction. The color represents the phase shift (lattice strain) ranged from -pi/4 to pi/4.