ALS beamline 12.2.2

Widely used in catalysis, electronics, photonics, and sensing applications, metallic nanocrystals have properties that are highly dependent on their size, shape, and structure. Defects and structural transformations can benefit certain properties while impeding others. Using chemical methods, it’s possible to synthesize nanocrystals with defined shapes, sizes, and structures for specific applications, but our understanding of the stability of these synthesized structures under operational stresses is limited.

Here, researchers tested the stability of 3.9 and 6 nm gold nanocrystals under stress applied using a diamond anvil cell. They tracked structural changes in the nanocrystals using in situ x-ray diffraction (XRD) at Beamline 12.2.2 of the Advanced Light Source (ALS). By observing changes in the width and position of XRD peaks, the researchers measured elastic strain, defects, and structural transformations.

For the 3.9 nm nanocrystals, peaks (200) and (220) got wider with pressure cycling and remained at higher values after unloading. This indicated the formation of stacking faults, which were shown using molecular dynamics (MD) simulations to be formed via surface-nucleated partial dislocations. For the 6 nm nanocrystals, all peak positions recovered completely with pressure cycling except one, the (200) peak. This peak position was significantly affected by the degree of twinning in the nanocrystal. The irreversible change indicated that the initial multiply twinned nanocrystal transformed into a single-crystalline nanocrystal. Using theoretical calculations and MD simulations, the researchers showed how this transformation is thermodynamically feasible under high pressure. The results indicate that large-scale structural transformation is possible in nanocrystals and must be considered when designing structures at the nanoscale.