In the growth of crystals, do nanoparticles act as "artificial atoms" forming molecular¹-type building blocks that can assemble into complex structures? This is the contention²（争论） of a major but controversial theory to explain nanocrystal growth. A study by researchers at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) may resolve the controversy³ and point the way to energy devices of the future. Led by Haimei Zheng, a staff scientist in Berkeley Lab's Materials Sciences Division, the researchers used a combination of transmission electron microscopy（显微镜学） and advanced liquid cell handling techniques to carry out real-time observations of the growth of nanorods from nanoparticles of platinum⁴ and iron. Their observations support the theory of nanoparticles acting⁵ like artificial atoms during crystal growth.

"We observed that as nanoparticles become attached they initially⁶ form winding⁷ polycrystalline chains," Zheng says. "These chains eventually align⁸ and attach end-to-end to form nanowires that straighten and stretch into single crystal nanorods with length-to-thickness ratios up to 40:1. This nanocrystal growth process, whereby nanoparticle chains as well as nanoparticles serve as the fundamental building blocks for nanorods, is both smart and efficient."

Zheng is the corresponding author of a paper describing this research in the journal Science. The paper is titled "Real-Time Imaging of Pt3Fe Nanorod Growth in Solution." Co-authors are Hong-Gang Liao, Likun Cui and Stephen Whitelam.

If the near limitless potential of nanotechnology is to even be approached, scientists will need a much better understanding of how nano-sized particles can assemble into hierarchical（分层的） structures of ever-increasing organization and complexity⁹. Such understanding comes from tracking nanoparticle growth trajectories¹⁰ and determining the forces that guide these trajectories.

Through the use of transmission electron microscopy and liquid observation cells, scientists at Berkeley Lab and elsewhere have made significant progress in observing nanoparticle growth trajectories, including the oriented attachment¹¹ of nanoparticles -- the chemical phenomenon that starts the growth of nanocrystals in solution. However, these observations have typically been limited to the first few minutes of crystal growth. In their study, Zheng and her colleagues were able to extend the time of observation from minutes to hours.

"The key to studying the growth of colloidal¹² nanocrystals with different shapes and architectures is to maintain the liquid in the viewing window long enough to allow complete reactions," Zheng says. "We dissolved molecular precursors¹³ of platinum and iron in an organic solvent¹⁴（溶剂） and used capillary¹⁵ pressure to draw the growth solution into a silicon-nitride liquid cell that we sealed with epoxy. The sealing of the cell was especially important as it helped keep the liquid from turning viscous¹⁶ over time. Previously¹⁷, we'd often see the liquids become viscous and this would prevent the nanoparticle interactions that drive crystal growth from taking place."