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Closer Look at Nanoparticle Growth Through a Graphene Window

A liquid sample encapsulated by two graphene monolayers. Below, Transmission electron micrographs of colloidal platinum nanoparticles forming in solution inside a graphene cell.
A team co-headed by Alex Zettl and Paul Alivisatos of the Materials Sciences Division has invented an elegant technique for encapsulating liquid samples in pockets of graphene and imaging the contents with high-resolution electron microscopy. This technique makes it possible to image a broad range of solution-phase phenomena, such as nanoparticle growth and protein folding, with unprecedented resolution.

Many important phenomena occur in liquids but to study them at the nanometer scale with electron microscopes, scientists must encapsulate liquid samples in tiny cells with windows of silicon nitride or quartz. These windows interfere with electron transmittance, limiting image resolution to a few nanometers – too large to see individual atoms.

Alivisatos and his team instead created a new kind of liquid cell: a graphene pocket just a few hundred nanometers across, fabricated by growing two layers of graphene on a flat surface and letting a drop of liquid wick between the layers. Using a transmission electron microscope at the National Center for Electron Microscopy, the team imaged platinum nanoparticles growing from solution in a graphene cell with sub-nanometer resolution and discovered a host of unexpected phenomena. This technique opens the door to understanding solution-phase phenomena at the atomic level.

To understand this mechanism, which has been debated for decades, Zheng and coworkers took real-time transmission electron microscopy images of rod-shaped platinum-iron nanoparticles growing in solution. Their images reveal that small nanoparticle "blocks" first join into a crooked chain and then straighten out to form single-crystal rods.

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"High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells," Jong Min Yuk, Jungwon Park, Peter Ercius, Kwanpyo Kim, Daniel J. Hellebusch, Michael F. Crommie, Jeong Yong Lee, A. Zettl, and A. Paul Alivisatos, Science 336 61 (2012). DOI:10.1126/science.1217654