Nanocrystal forces exposed


Rebecca Pool

Tuesday, September 12, 2017 - 11:45
Image: When matching sides are aligned, nanocrystals can fuse seamlessly, growing larger and larger. [EMSL]
Claiming a world first, US-based researchers have measured and modeled the the van der Waals attraction that causes two nano-sized crystals to twist, align, slam into each other, and snap together.
These latest insights into crystal self-assembly point the way to new materials for energy storage and production.
Researchers studying crystal formation and growth have long known that nanocrystals suspended in a liquid medium often aggregate to form larger crystals.
However, such aggregation is not a simple piling up of each crystal; when two crystals approach, they mutually rotate, align and snap together.
To better understand crystal behaviour, researchers from Pacific Northwest National Laboratory, the University of Pittsburgh and the Environmental Molecular Sciences Laboratory combined environmental TEM with nanocrystal force probes to measure the van der Waals attraction between rutile titanium dioxide nanocrystals.
Dr Kevin Rosso from PNNL and colleagues imaged the contact point in-situ with environmental transmission electron microscopy, which allowed the interparticle distances to be measured accurately.
In this way the researchers could watch the crystals interact while simultaneously measuring the torque between two crystals under different conditions and relative orientations.
Analyses revealed that the van der Waals force between the two crystals depended on the molecular coverage of water on the crystal surface as well as the relative orientation of the crystal.
"At tens of nanometers of separation, the attraction is weak and shows no dependence on azimuthal alignment or surface hydration," highlights Rosso. "At separations of approximately one hydration layer, the attraction is strongly dependent on azimuthal alignment and systematically decreases as intervening water density increases."
Crucially, the researchers' method has revealed the relationship between the nanocrystals' orientations, surface hydrations, and interactions, with results suggesting that there is enough force to generate a torque between the crystals to ensure a complementary interaction. 
Research is published in Science.
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