Transmission electron microscopy of specimens and processes in liquids
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troduction Transmission electron microscopy (TEM) is such a powerful technique for obtaining high-resolution spatial and chemical information about solid samples that it has always been tempting to apply it to liquid samples as well. During the past decade, this goal has been realized, and TEM of liquid samples has become a widely used microscopy method.1,2 Implementing TEM of liquid samples required overcoming two technological challenges. First, most liquids do not exhibit a sufficiently low vapor pressure to remain stable in the vacuum environment of the TEM. Second, it is necessary to control the physical location of the liquid to create a geometry compatible with TEM. Early pioneers of electron microscopy recognized these two issues in the 1940s.3,4 Their early work set the stage for two experimental strategies, which are referred to as open- and closed-cell (or chamber) approaches; these still form the basis of electron microscopy in liquids (Figure 1). In open-cell microscopy,3,5 the pressure around the sample is controlled using differential pumping to maintain a vacuum in the rest of the system that is good enough to operate the microscope (Figure 1a). In closed-cell microscopy,4,6 the liquid
is enclosed between two closely spaced electron-transparent membranes (Figure 1b). By physically confining the liquid within a thin layer, imaging is possible in transmission mode through the windows and liquid. Despite this impressive progress, electron microscopy of liquids did not become widely popular because of the engineering challenges involved. Closed cells used relatively thick windows (made, for example, of carbon) and thick liquid layers, so they generally did not provide a resolution that was much better than that of light microscopy. In open-cell TEM, it was difficult to control the liquid/vapor environment, and the analogous open-cell use of environmental scanning electron microscopy (ESEM) for studying liquids remained underappreciated because the lower beam energy (and other factors) prevented high-resolution imaging. Biologists turned to rapid freezing to prepare samples for imaging, while materials scientists found other problems to solve. The use of electron microscopy for high-resolution and in situ investigations of liquids had to await advances in microfabrication to provide solutions to the technical challenges.
Frances M. Ross, IBM T.J. Watson Research Center, USA; [email protected] Chongmin Wang, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, USA; [email protected] Niels de Jonge, INM-Leibniz Institute for New Materials, Germany; [email protected] doi:10.1557/mrs.2016.212
• VOLUMECore • www.mrs.org/bulletin 2016 Materials Research Society MRS BULLETIN 41 • terms OCTOBER Downloaded© from http:/www.cambridge.org/core. Washington University St. Louis, on 30 Nov 2016 at 13:50:22, subject to the Cambridge of 2016 use, available at http:/www.cambridge.org/core/terms. http://dx.doi.org/10.1557/mrs.2016.212
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TRANSMISSION ELECTRON MICROSCOPY OF SP
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