In a paper published today in Nature Synthesis , a team from the lab of University of Chicago Pritzker School of Molecular Engineering (UChicago PME) and Chemistry Department Prof. Paul Alivisatos explores the role of cation exchange in one of chemistry and material science's central challenges: How covalent materials undergo structural change at the nanoscale. This important post-synthetic lattice-reconstruction strategy for discovering novel nanocrystals was pioneered by Alivisatos in a 2004 Science paper.
This greater understanding of how materials transform could have applications for designing and building semiconductors, unraveling complex chemical processes or creating previously unimagined material architectures, for example, in this work, nanocubes of indium arsenide (InAs) and gallium arsenide (GaAs).
Surprising results
"We observed that under the right conditions, one kind of atom can be replaced with another in a very tiny crystal, even as all the atoms remain in the ordered positions. Once the switch starts at a single point on the surface, the remaining atom swaps cascade across the crystal in an orderly way," said Alivisatos, who is also president of the University of Chicago. "It's fun to imagine how atoms move in this systematic way, and we are hopeful that this approach can be used to expand the range of nanocrystals that can be fabricated for a wide range of uses."
One of their most surprising results was that, although the cubic copper arsenide template – like any cube – has six sides, cation exchange could be initiated from as few as one of them, starting a reaction that ripples through the cube.
"The kinetic-controlled cation exchange reaction can, in principle, start from all six surfaces," said Binyu Wu, the first author of this work and a UChicago PME PhD candidate. "However, once it starts from one surface, the rate of starting from the other surfaces will become much, much slower."
There are many reasons for this symmetry-breaking in structure, including the reduction of copper vacancies on the nanocubes' surface. The results shed new light on how the rate of chemical reactions (kinetics) can have a bigger role in the structure of some materials than the energy transformations within those reactions (thermodynamics).
The type of chemical bonds that create nanocrystals are also at play.
"In general, in the ionic nanocrystal, the thermodynamics would be the main driving force – we can call it a destination – that determine the final structure," Wu said. "But in the covalent system, the chemical bond reconstruction might be more irreversible, so it is more dominated by the reaction kinetics instead of the thermodynamics."
A cube of spheres
A new simulation shifting cubes into spheres sheds light on the forces that set nanocrystals' structures at the nanoscale, and could change how researchers build new materials from the atomic level up.
The team applied a cellular automaton computational model to explore the science, building a clear, simple model for future researchers to envision these minute changes. Wu believes this is the first time a cellular automaton model has been applied to the cation exchange reactions of nanocrystals.
"We always hope to make our research more accessible to a general audience, and we made it by designing a cool model and explaining in a fun way," Wu said. "The nanocrystal is made by many, many building blocks. It's like a Lego. And we replace the building blocks from the cube with spheres to represent the symmetry and stacking change of the lattice within the nanocrystals."
One major advantage that this model has over complex computational systems like density functional theory (DFT) simulations or all-atom simulations is its simplicity.
"Now we can just use 14 KB of source code package to finish all the simulation," Wu said.
The team's new model mimicked and simplified the form copper arsenide nanocubes took after cation exchange swapped out the copper to create, respectively, InAs and GaAs.
"In our experiment, we captured the symmetry change of the lattice, where a cubic lattice becomes a hexagonal one," Wu said. "So, we used a cellular automaton to mimic this process with these features. We found that a huge cube can also be made by many, many spheres."
This simplified model of cubes and spheres is easier for computers to simulate and researchers to grasp, but still delivers nuanced, accurate results.
In addition to the advance in fundamental science, these new insights and the simple, intuitive model the UChicago team used to garner them could change how future researchers synthesize new nanomaterials to handle the world's biggest challenges.
The researchers would like to acknowledge Giulia Galli, the Liew Family Professor of Electronic Structure and Simulations in UChicago PME and the Department of Chemistry. and Joseph S. Francisco, President's Distinguished Professor of Chemistry at the University of Pennsylvania.
Citation: "Kinetic-controlled transformations of group-III arsenide nanocubes," Wu et al, Nature Synthesis, March 11, 2026. DOI: 10.1038/s44160-026-01015-6
Funding: NSF Center for Chemical Innovation (CCI), Multimodal Observations for Single Atom Imaging of Chemistry (MOSAIC) under award No. CHE2420536.
