Coherent diffraction imaging of nanoscale strainevolution in a single crystal under high pressure - Dr. Wenge Yang
When materials are constrained to nanoscale dimensions, they exhibit interesting properties that are different from their bulk counterparts, known as “the Nano-size Effect”. Surfaces and interfaces in nanomaterials play a pervasive role in their properties. For example, gold nanocrystals with average size of 30 nm, show 60% stiffer compared with other micron-sized particles. Recently, a research group, led by Dr. Wenge Yang, a scientist of HPSTAR and Carnegie Institution of Washington, studied the Nano-size Effect in gold nanoparticles under very high pressures. They used a Diamond Anvil Cell (DAC), to apply pressures up to 6.4 GPa. DAC, which has been around for about half a century, is a refined version of the Opposed Anvil Machine built by Professor P. W. Bridgman who won the Nobel prize for his work in high pressure physics in 1946. In a DAC, the sample is put between two diamond anvils, and a blend of fluids such as a mixture of methanol and ethanol is used as the pressure-transmitting medium to apply a uniform hydrostatic pressure. This design enables the DAC to create in-situ pressures up to hundreds of GPa (several million times higher than atmospheric pressure). “When coupled with laser heating, one can simulate the real condition that exists in the core of earth and even other planets.” Dr. Yang said.
Figure 1. Schematic of Coherent X-ray Diffraction Imaging (CXDI).
This research, published in the April 2013 issue of Nature Communications, is one of the first works that shows nanometer resolution imaging of a material in real conditions, and in real time. Other nanometer resolution imaging techniques such as transmission electron microscopy, require the measurement to be done in ultra high vacuum due to scattering of the electron beam in otherwise ambient conditions. The method used by Dr. Yang and coworkers, called Coherent X-ray Diffraction Imaging (CXDI), uses the high energy, highly coherent x-ray beam at the Advanced Photon Source (APS) in Argonne National Laboratory (Figure 1). High energy x-ray is the only source capable of nanoscale resolution that can penetrate through the surrounding materials under high pressure. When the x-ray beam reaches the crystals, it interacts with the electronic structure of the crystal and gets scattered. This phenomenon is called diffraction. The diffracted x-ray beam carries vital information about the electronic structure of the crystal, which is related to its physical properties, such as strain, and is recorded by an x-ray sensitive area detector.
Figure 2. 3-D strain distribution in a 400 nm single crystal gold particle subjected to 1.7 GPa. (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 -π/4 to π/4.
Dr. Yang and his team used a computer modeling system to convert this information to 3-D models of the nanocrystals (Figure 2). They were able to observe the evolution of strain in gold nanoparticles while the applied external pressure was increased from 0.8 GPa to 6.4 GPa step by step. Until now, scientist could only observe the Nano-size Effect in particles smaller than a few tens of nanometers but this work shows that the Nano-size Effect can be observed at a much larger scale, in 400 nm gold particles. According to Dr. Yang, “Nano-size effects do exist in larger crystals. You can see them when you zoom in and do good microscopy”. Their next step will be to look at even more interesting materials like core-shell structures during deformation to study the boundary effects. “It took us about two years to develop the coherent diffraction tool to work in high pressure. It is harder to take the first step, our next steps will be much easier.” he said.