Ulrich Dahmen of the Lawrence Berkeley National Laboratory and colleagues obtained their result by confining an extremely thin liquid layer between two even thinner solid windows made from a pair of 100 µm thick silicon wafers to observe how platinum nanocrystals grow. The researchers fabricated an experimental cell that is transparent to the electron beam in a TEM. They drive crystal growth by reducing platinum cations with an electron beam and follow the motion of individual particles while measuring their position, size and shape in real time using a video. They found that each nanocrystal can grow either in a "normal" way – atom by atom – or by randomly merging with other nanocrystals.
The merger growth mode is not considered in classical models of nanocrystal growth that only assume a discrete nucleation stage followed by growth by single molecule addition. Dahmen and colleagues say that being able to distinguish between the different growth modes in this way could allow researchers to synthesize nanocrystals with more complex shapes and tailor-made physical properties for use in catalysts, for example.
Intuitively, coalescence might be expected to produce a broad distribution of particle sizes. However, Dahmen's team found that the particles had a fairly monodisperse, or narrow, size distribution. "This 'size-focusing' occurs because large particles that have formed by coalescence seem to 'wait' for the smaller particles to catch up," explained Dahmen. "We can observe this surprising behaviour by following the trajectory and evolution of individual particles."
The observations provide new input for theoretical models of nanocrystal growth that could lead to better predictions and alternative strategies for making monodisperse nanoparticles, adds Dahmen.
The technique might also come in useful for general single-particle studies because the novel liquid cell set-up will allow a whole range of new experiments on the growth and interactions of nanoparticles in liquids. "By following the fate of individual particles, we can now see in real-time events that could only be deduced from still photos at the end of a reaction," Dahmen told nanotechweb.org.
The team is currently using its set-up to learn more about how particles diffuse as liquid droplets dry, something that could provide important information for self-assembly. The researchers also hope to understand more about fundamental branching mechanisms during particle growth, processes that are essential for making complex nanoparticles. "We are also looking to further improve our technique to obtain even higher resolution and gain better control of local experimental conditions, like liquid flow, composition and temperature," revealed Dahmen.
The work was reported in Science.