Investigation of the mechanism of pressure-induced solid-solid structural transformations in semiconductor nanocrystals

The mechanism of the pressure-induced 4- to 6-fold coordinated structural transformation in CdSe nanocrystals was investigated by applying computer-generated X-ray diffraction (XRD) simulations to experimental XRD data. The transition from the low-pressure wurtzite phase to the high-pressure rocksalt phase in CdSe nanocrystals was followed experimentally using high-pressure XRD, and the resulting patterns were analyzed for crystallite size, planar disorder, and shape. Spherical and rod-shaped CdSe nanocrystals containing between 1000 and 20,000 atoms were synthesized, characterized, and taken through the pressure cycle in order to investigate the effects of total volume, shape, and aspect ratio on the transformation. Spherical 4.5 nm CdSe nanocrystals in particular were investigated in great detail, resulting in the proposal of a possible structural transformation mechanism based on the XRD simulation results. Simulations showed that after the sample had completed one hysteresis loop, the overall domain size was preserved but stacking disorder in the recovered phase was greatly increased. In addition the average shape of the crystals in the high-pressure phase was shown to be an elongated, flattened one. Analysis of the diffraction from subsequent transformation cycles on the same sample showed the results to be reproducible through multiple cycles. This information led to the proposal of a martensitic-type mechanism for the transformation involving the sliding of the close-packed planes in the 4-fold coordinated structure to yield the close-packed planes of the 6-fold coordinated structure. The high-pressure behavior of larger spheres and rods with various aspect ratios were also investigated.