Researchers in the US and South Korea have for the first time managed to image the process of nanocrystal growth at the atomic scale. Their technique, which involves placing the crystals inside a liquid cell bound by graphene sheets and imaging them with a transmission electron microscope, has revealed new and unexpected growth stages as they were occurring. The method might be used to study a variety of nanomaterials in solution, and even biological samples in their natural liquid environments.
The transmission electron microscope (TEM), which was first introduced in the 1930s, produces images at a significantly higher resolution than an optical microscope as it works using electron beams instead of light. However, liquids are notoriously difficult to image with a TEM because they need to be hermetically encapsulated in a solid material (usually silicon nitride or silicon oxide) to prevent them from evaporating, since the microscope operates under vacuum conditions. Such capsules, or liquid cells as they are known, can have membranes that are up to 100 nm thick. This is far too thick to penetrate successfully using an electron beam and means that objects can only be imaged with a spatial resolution of a few nanometres at best.
Now, Jungwon Park at the University of California, Berkeley and colleagues at the Lawrence Berkeley National Laboratory and KAIST in South Korea have shown that capsules fabricated from graphene can be used as see-through “windows” for liquid cells. The sub-nanometre walls of the capsule are effectively transparent because graphene is a sheet of carbon just one atom thick. Therefore, the graphene does not scatter the electron beam but instead lets it pass through. Graphene is also very strong and impermeable, as well as being chemically non-reactive, and so helps protects the sample in the liquid cell from the high-energy electrons in the microscope beam.
The researchers filled the graphene capsules with a solution containing platinum nanocrystals and studied the capsule using an aberration-corrected form of TEM. Park explains that his group was able to see particle nucleation and growth on the very high-resolution angstrom-scale (0.1 nm). He says that his team also observed new and unexpected stages of nanocrystal growth as they happened, in particular how certain nanocrystals frequently coalesce along the same crystallographic direction, modify their shape and form facets on their surfaces.
Park believes that electron-microscopy experiments using graphene liquid cells could be used to image a wide range of nanomaterials, such as nanoparticles, nanostructures and even biological samples in liquid. But the group’s next step will be to use its graphene liquid cells to study how nanocrystals other than platinum grow. “Watching real-time chemical reactions in liquids is a dream for chemists and physicists, and we now hope to study a variety of nanoparticles growing in solution using our liquid-phase electron-microscopy technique,” says Park.
The technique is described in Science.
