Structural and Compositional Evolution of Colloidal In(1-x)Ga(x)P(1-y)As(y) Nanocrystals during Cation Exchange Revealed by Electron Microscopy.

Cation exchange in molten salts has emerged as a method to synthesize colloidal III-V quantum dots with tunable composition and excellent optoelectronic properties. However, the atomistic details of how cation exchange occurs and the elemental distribution in the products remain elusive. In this work, we use scanning transmission electron microscopy to reveal the chemical and structural evolution of InGaPAs nanocrystals made by cation exchange of InPAs nanocrystals in a gallium-containing molten salt. The starting material and InGaPAs nanocrystals at multiple time points in the reaction were characterized by aberration-corrected HAADF-STEM and STEM-EDS. HAADF-STEM reveals subtle changes in size and shape, but the tetrahedral morphology and zinc-blende crystal structure are maintained throughout the reaction. We overcome the technical challenges of beam damage and low signals in STEM-EDS through a combination of experimental optimization and image processing to yield elemental composition maps representative of individual nanocrystals. This analysis revealed that the particles have Ga-rich surfaces at the early reaction stage. As the reaction proceeds, Ga permeates the lattice, but the elemental distribution remains graded with In enrichment in the core even after 16 h. To investigate the origin of the observed elemental distributions, we use finite element analysis to model the cation exchange as a diffusion-limited process, which suggests that cation exchange becomes progressively more difficult as In atoms are replaced by Ga. We discuss the implication of the graded composition on optical properties. More broadly, the image processing techniques developed here could be applied to other compositionally complex nanocrystals.