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Self-assembly forces in crystals

Posted May 18, 2017

Rocks, shells, bones — they’re all made of crystals that grow from smaller crystals that attach to each other. A better understanding of how small crystals join themselves together might improve energy storage and other applications that rely on crystallization.

As particles move around in solution, ions drive them to approach each other. The same ion-induced forces adjust the particles so their faces are arranged as an ideal crystal. Then solvent leaves the gap between the particles, and a defect-free crystal is formed.

Two researchers at the Department of Energy’s Pacific Northwest National Laboratory used a computer model to study self-assembly in a titanium oxide mineral called anatase. Maria Sushko and Kevin Rosso designed a model that re-created the conditions in which the smaller crystals grow. Then they measured the attraction and repulsion between simulated nanocrystals to dig out the forces responsible for crystal self-assembly, publishing in the journal Nanoscale.

Based on their simulations, the researchers described how the anatase nanocrystals self-assemble: As two nanocrystals float near each other, ions within the liquid environment drive them towards each other. At the same time, water between them prevents them from immediately slamming into each other. Wavering there between attraction and repulsion, the nanocrystals make slight adjustments in their positions. When almost or perfectly aligned, and after attractive forces win out and allow the crystals to touch, chemical forces take over and the nanocrystals fuse together.

Look closely enough, and you’ll see ingenious patterns everywhere in nature. Scientists and engineers have long understood this, but mimicking Mother Nature in building such patterns—especially highly ordered crystal structure—has proven challenging. Recently, Maria Sushko and Kevin Rosso at Pacific Northwest National Laboratory (PNNL) significantly advanced understanding by clarifying the driving forces behind particle-based crystal growth with their new computational approach. They learned that crystal growth depends on the subtle balance of interactions between atoms, ions, molecules, and particles. Their discovery holds significant promise for creating materials to address energy challenges.

In natural crystal-growth processes, nanoparticle building blocks attach along specific crystal faces. Studying these examples, researchers were inspired to contemplate how they might create similar crystal structures for a range of practical applications including energy storage. Armed with a greater understanding of the fundamental processes underlying the pathways of crystal growth, researchers could control these processes to synthesize new materials with precise detail. In their research, Sushko and Rosso found that coordinated motion of ions close to nanoparticle surfaces drive the way nanoparticles arrange into matching crystal shapes and structures. They discovered that ions in solution can direct the rotation of nanoparticles into a matching crystal orientation-mimicking nature’s pattern precisely-to produce perfect crystals.

Why It Matters: The PNNL researchers’ discovery provides key fundamental insights into geochemical processes leading to mineral formation, and helps to create complex, hierarchical, single-crystal structures in the lab. It also holds promise for eventually creating innovative materials for consumer electronics, batteries, and more. According to Sushko, their new computational approach creates “a new paradigm in knowledge-based synthesis of highly ordered three-dimensional crystal structures” for a range of practical applications in catalysis and energy-storage technologies.

Methods: Rosso and Sushko developed a new multi-scale computational model that encompasses the essential forces acting between atoms, molecules, and particles. Their approach spans the length scales from Angstrom to half a micron and is fully transferable to a wide range of systems. The method is rooted deeply in quantum mechanics and provides a parameter-free approach for modeling experimentally relevant systems.

What’s Next? Their new computational approach is a major step towards developing a comprehensive theory of particle-based crystallization. Future research will extend the model to include a broader range of macroscopic forces, such as magnetic and electric polarization. The model will be also further applied to other materials to get insight into various crystallization pathways.

Source: PNNL


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