A team of scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley has developed a technique for encapsulating liquids of nanocrystals between layers of graphene so that chemical reactions in the liquids can be imaged with an electron microscope. With this technique, movies can be made that provide unprecedented direct observations of physical, chemical and biological phenomena that take place in liquids on the nanometer scale. The results were published in Science.
"Watching real-time chemical reactions in liquids at the atomic-scale is a dream for chemists and physicists," says Jungwon Park, a member of the team who holds joint appointments with Berkeley Lab's Materials Sciences Division and UC Berkeley's chemistry department. "Using our new graphene liquid cell, we're able to capture a small amount of liquid sample under a high vacuum condition for taking real-time movies of nanoparticle growth reactions. Since graphene is chemically inert and extremely thin, our liquid cell provides realistic sample conditions for achieving high resolution and contrast."
The research was done as a collaboration between the research groups of Paul Alivisatos, director of Berkeley Lab and UC Berkeley's Larry and Diane Bock Professor of Nanotechnology, and Alex Zettl, who holds joint appointments with Berkeley Lab's Materials Sciences Division and UC Berkeley's Physics Department where he directs the Center of Integrated Nanomechanical Systems.
In using a beam of electrons rather than a beam of light for illumination and magnification, electron microscopes can "see" objects hundreds and even thousands of times smaller than what can be resolved with an optical microscope. However, electron microscopes can only operate in a high vacuum as molecules in the air disrupt the electron beam. Since liquids evaporate in high vacuum, liquid samples must be hermetically sealed in special solid containers - called cells - with a viewing window before they can be imaged in an electron microscope.
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Until now, such liquid cells have featured viewing windows made from silicon nitride or silicon oxide. While this has permitted studies of some nanoscale phenomena in liquids, the silicon-based cell windows are too thick to allow strong penetration by the electron beam and this has limited resolution to only a few nanometers. In addition to not allowing true atomic-resolution, the thick silicon-based cell windows also appear to perturb the natural state of the liquid or sample suspended in the liquid.
"Graphene is single carbon atom in thickness, making it one of the thinnest known membranes," says Park, a member of the Alivisatos' research group. "It does not scatter the electron beam but lets it pass through. Furthermore, graphene is also very strong and impermeable, as well as being chemically non-reactive, and this helps protects the sample in the liquid cell from the high-energy beam of an electron microscope."
To make their graphene liquid cell, the Alivisatos-Zettl collaboration encapsulated a platinum growth solution between two laminated graphene layers that were suspended over holes in a conventional transmission electron microscope (TEM) grid. The graphene was grown on a copper foil substrate via chemical vapor deposition and then directly transferred onto a gold TEM mesh with a perforated amorphous carbon support. The platinum growth solution was pipetted directly atop two graphene-coated TEM grids facing in opposite directions.
"Upon wetting the system, the solution wicks between the graphene and amorphous carbon layers, allowing one of the graphene sheets to detach from its associated TEM grid," says co-author Kim, a member of the Zettl research group. "Because the van derWaals interaction between graphene sheets is relatively strong, liquid droplets ranging in thickness from six to 200 nanometers can be securely trapped in a pocket or blister between the graphene sheets."
To test their graphene liquid cells, the collaborators used the world's most powerful electron microscope, the TEAM I at the National Center for Electron Microscopy (NCEM), which is housed at Berkeley Lab. TEAM stands for Transmission Electron Aberration-corrected Microscope and the TEAM I instrument is capable of producing images with a half-angstrom resolution, which is less than the diameter of a single hydrogen atom. With TEAM I and their new graphene liquid cells, the Alivisatos-Zettl collaboration was able to directly observe at the highest resolution possible to date and with minimal sample perturbation, the growth of nanocrystals of platinum, one of the best metal catalysts in use today.
Jong Min Yuk, Jungwon Park, Peter Ercius, Kwanpyo Kim, Daniel J. Hellebusch, Michael F. Crommie, Jeong Yong Lee, A. Zettl, A. Paul Alivisatos: High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells, Science 6 April 2012: Vol. 336 no. 6077 pp. 61-64 , DOI: 10.1126/science.1217654
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Keywords: Chemical Imaging Electron Microscopy Graphene Graphene Liquid Cells Lawrence Berkeley National Laboratory Nanocrystals TEM Transmission Electron Aberration-corrected Microscope University of California at Berkeley