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Posted: May 15, 2017
Nanosize crystals grow by twisting, aligning and snapping together
(Nanowerk News) Crystalline nanoparticles suspended in a liquid medium often combine to form large crystals. However, the fundamentals governing this behavior remain unclear. A new study to measure and model the interaction force, called the van der Waals force, between two nanosize crystals provides clues for understanding this behavior ("Direction-specific van der Waals attraction between rutile TiO2 nanocrystals").
When matching sides are aligned, smaller crystals can fuse seamlessly (snap together), growing larger and larger. (Image: Environmental Molecular Sciences Laboratory)
Crystal growth from a liquid medium may follow a range of processes, one of them is the assembly of small crystals in a self-regulated fashion. The discovery of the interaction between the orientation of the crystals and the force between the two approaching crystals provides insights into how crystals self-assemble, an activity that occurs in a wide range of cases in nature, from rocks to shells to bones.
Scientists who study crystal formation and growth have long known that nanosized crystals suspended in a liquid medium often aggregate to form large crystals. However, such aggregation is not a simple piling up of each nanosize crystal. Instead, when two nanosize crystals approach, they mutually rotate, align and snap together.
To better understand the nanosize crystals’ behavior, a team of researchers from Pacific Northwest National Laboratory; University of Pittsburgh; and EMSL, the Environmental Molecular Sciences Laboratory, developed a new approach. They combined an environmental transmission electron microscope at EMSL, a Department of Energy Office of Science User Facility, with nanocrystal force probes, which allowed the team to watch crystals interact in a life-like situation while simultaneously measuring the interaction force (torque) between two nanosize crystals.
The team also used focused ion beam, called FIB, an a scanning electron microscope at EMSL to fabricate the nanosize crystal with a designated crystal surface, and they measured the attracting force between the two nanosize crystals under moist conditions and at different relative orientations.
The team discovered that the attraction force, or van der Waals force, between the two crystals depended on molecular coverage of water on the crystal surface as well as the relative orientation of the crystal.
The in situ measurement indicates the maximum amount of force at perfect alignment of two crystals, while force is minimal at the largest disorientation. Due to force dependence on the orientation, approaching nanocrystals strive to match orientation.