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Nanocrystal Growth Observed

Imaging: Using electron microscopy, researchers reveal mechanism of nanoparticle fusion

by Lauren K. Wolf
May 28, 2012 | A version of this story appeared in Volume 90, Issue 22

Credit: Zina Deretsky
Nanocrystal growth can occur when smaller particles approach one another, align their crystal lattices (indicated by red and gray atoms in the artist’s rendition), and fuse.
Some nanocrystals grow via a process, depicted in this graphic, in which smaller particles approach one another, align their crystal lattices (as indicated by red and gray atoms), and fuse.
Credit: Zina Deretsky
Nanocrystal growth can occur when smaller particles approach one another, align their crystal lattices (indicated by red and gray atoms in the artist’s rendition), and fuse.

Two research groups have independently monitored the real-time dynamics of nano­crystal growth via electron microscopy, observing particles form and then attach to one another in solution (Science, DOI: 10.1126/science.1219185 and 10.1126/science.1219643). Such particle-level knowledge of nanocrystal growth should help scientists better control the synthesis and properties of nanomaterials.

Scientists have been kicking around two theories of how nanocrystals grow, says Haimei Zheng, leader of one of the groups to make the measurements. According to the classic theory of crystal growth, nano­particles form when individual atoms stick to high-energy crystal faces, she says. A second theory posits that small nanoparticles initially form, approach one another, align their crystal faces, and finally attach to create larger nanoparticles.

“It’s been conjectured for some time that some nanostructures come about during synthesis through the process of oriented attachment,” which is the second theory, says James J. De Yoreo, leader of the other research team. “But no one had ever before demonstrated beyond a shadow of a doubt that it actually happens.”

The technique that enabled the two groups to observe nanocrystals growing is liquid-cell transmission electron microscopy. The researchers built their own customized cells, consisting of sealed sets of ultrathin silicon nitride windows that allow electrons to pass through sample solutions sandwiched in between.

Zheng and De Yoreo, who are both researchers at Lawrence Berkeley National Laboratory, say their groups discussed preliminary observations but didn’t work together or coordinate submission of the papers.

That’s how science sometimes works, says Christopher B. Murray, a chemist at the University of Pennsylvania who was not involved in either study. “There’s a grand challenge and multiple groups apply a technique to meet it,” he adds.

What’s remarkable about the work, Murray continues, is that the groups observed distinctly different nanomaterials forming via similar processes. They saw that “attachment doesn’t occur just by particles sticking together at a first point of contact,” he says. “Instead, it’s a subtle process of rotation and orientation, followed by fusion and the ‘healing’ of defects.”

With the liquid-cell TEM method, Zheng’s group watched Pt3Fe nanorods and nanowires grow, and De Yoreo’s team monitored the formation of iron oxy­hydroxide nanoparticles.

Both teams report that as small particles approach each other during synthesis, they orient so that their crystal lattices match. Once this happens, the tiny crystals accelerate toward one another and join. If small defects form during fusion, the particles shift afterward to fix them.

Zheng says she will next study how chemical potential affects nanoparticle growth. And De Yoreo wants to further examine the driving force that causes the nanocrystals to lurch toward one another during attachment.



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