Issue Date: April 9, 2012
Imaging Liquids With Graphene
Solution-phase processes can be probed with atomic-level resolution in a transmission electron microscope (TEM) by encapsulating a liquid sample in a nanometer-sized graphene “blister,” according to a study published in Science by researchers at the University of California, Berkeley (DOI: 10.1126/science.1217654). The study advances liquid-phase imaging capabilities and provides a technique that may allow researchers to probe complex biochemical and biological systems with exceptional resolution.
Liquid specimens are inherently incompatible with the vacuum environment of a TEM. Despite that challenge, microscopists began devising methods for handling and analyzing these kinds of samples decades ago.
Currently, TEM analysis of liquids relies on sealed microfabricated sample cells featuring silicon nitride or silicon oxide windows. Researchers have used such cells to probe nanoparticle dynamics in chemical and electrochemical reactions. But because of the thickness of the windows (up to 100 nm) and the nature of the silicon materials, the sample cells restrict electron transmission, which reduces imaging sensitivity and limits resolution to a few nanometers. In addition, the window materials tend to interact with the species in solution, which perturbs the liquid’s natural state.
To sidestep those problems, the research team, which includes UC Berkeley’s Jong Min Yuk, Jungwon (Justin) Park, Alex Zettl, and A. Paul Alivisatos, came up with a way to replace silicon-based TEM sample cells with unconventional ones made from graphene. They found that pipetting liquid droplets onto graphene-coated specimen grids causes the graphene to detach from the grids and encapsulate the droplets securely in nanosized blisters.
The advantages of graphene as a sample cell material derive from its thinness and carbon’s low atomic number. Because of those properties, graphene, which is nonreactive, hardly scatters the TEM’s electron beam and functions as an inert, transparent cell window.
The team, which also includes Jeong Yong Lee of South Korea’s Korea Advanced Institute of Science & Technology (KAIST), tested the new technique by using it to probe platinum nanocrystal growth in colloidal solutions of platinum (II) acetylacetonate. They report that the method generates atomic-level-resolution images and can discern and track nanoparticles as small as 0.1 nm in diameter in real time. They also report that the Pt crystals grow via particle coalescence, structural reshaping, surface faceting, and other processes that could not be resolved in earlier studies partly owing to bonding interactions between the nanoparticles and silicon nitride cell windows.
“This approach opens new domains of research in the physics and chemistry in the fluid phase,” remarks Christian Colliex, a research director at the Solid State Physics Laboratory in Orsay, France, in an accompanying commentary in Science. He adds that further exploration of liquid-phase analysis will show how this new generation of sample cells may be useful for biochemical and biological problems.
- Chemical & Engineering News
- ISSN 0009-2347
- Copyright © American Chemical Society