Stress Induced Structural Transformations in Au Nanocrystals

TL;DR

This study demonstrates that high-pressure can induce displacive structural transformations in gold nanocrystals, revealing reversible polymorphic changes driven by stress and surface dynamics, with implications for nanomaterial stability.

Contribution

It provides experimental evidence of displacive motion mediated transformations in nanocrystals under high pressure, supported by in-situ microscopy, simulations, and energy calculations.

Findings

High-pressure induces reversible transformation from multiply twinned to single crystalline nanocrystals.

Surface recrystallization and twin boundary motion govern the recovery process.

Defects nucleate from high-stress regions, destabilizing twin boundaries.

Abstract

Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell that is driven by displacive motion. In-situ X-ray diffraction and transmission electron microscopy were used to detect the transformation of multiply twinned nanocrystals into single crystalline nanocrystals. High-pressure single crystalline nanocrystals were recovered after unloading, however, the nanocrystals quickly reverted back to multiply twinned state after redispersion in toluene solvent. The dynamics of recovery was captured using transmission electron microscopy which showed that the recovery was governed by surface recrystallization and rapid twin boundary motion. We show that this transformation is energetically favorable by calculating the pressure-induced change in strain energy. Molecular dynamics simulations showed that defects nucleated from a region of high stress region in the interior of the nanocrystal, which make twin boundaries unstable. Deviatoric stress driven Mackay transformation and dislocation/disclination mediated detwinning are hypothesized as possible mechanisms of high-pressure structural transformation.