Colloidal semiconductor nanocrystals (NCs) offer remarkable tunability of optoelectronic properties through synthetic control of their size, shape, and composition. An emerging strategy is the synthesis of III-V nanocrystals in molten salts, where high temperatures and unique redox chemistry enable the production of highly crystalline binary, ternary, and quaternary semiconductor NCs. A notable example of the compositional complexity that can be achieved by these methods is the cation exchange of InP1-yAsy in a gallium-containing molten salt to yield In1-xGaxP1-yAsy NCs. However, the process by which Ga atoms migrate through the NC lattice, and the resulting elemental distributions, was not fully understood.
STROBE scientists, together with collaborators at the University of Chicago, employed advanced scanning transmission electron microscopy (STEM) methods to reveal the atomic structure and elemental distribution of NCs taken throughout the cation exchange process. High angle annular dark field (HAADF-STEM) imaging revealed only subtle changes in NC morphology, with the tetrahedral shape and zinc blende crystal structure largely maintained throughout the exchange. Energy dispersive x-ray spectroscopy (STEM-EDS) was used to map the elemental distribution of the NCs. To overcome the beam sensitivity and low STEM-EDS signal resulting from the NCs, extensive experimental optimization and novel image processing methods were applied to produce elemental maps representative of individual In1-xGaxP1-yAsy NCs. The elemental maps revealed that Ga first exchanges as the surface of NCs, before diffusing inward. The In/Ga distribution becomes more, but never completely homogeneous, resulting in a compositional gradient that persists at long annealing times. Modeling in COMSOL suggests that the rate of diffusion becomes progressively slower as the cation exchange progresses.
These findings demonstrate that cation exchange occurs through a diffusion-limited process, resulting in a graded elemental distribution and incomplete exchange even at long annealing times. The intentional use of graded elemental distributions in II-VI NCs for superior optoelectronic performance suggests that diffusion-limited exchange may be desirable for producing high-performance III-V NCs. Furthermore, the image processing methods developed in this work may be generally useful for the analysis of NCs by methods like EDS or electron energy loss spectroscopy.