Chemical and bonding analysis of liquids using liquid cell electron microscopy
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Introduction Liquid cell transmission electron microscopy (TEM) has a long history in quantitative materials science and life science microscopy and has evolved significantly since the early work by prominent pioneers of the field, such as Ruska and Marton, in the early 1930s1 and 1940s.2–4 Early liquid cell TEM required dedicated instruments, where the liquid sample was either directly exposed to the vacuum (open cell) or separated by a thick window layer (usually 100s-of-nanometerthick metal foil, such as Al, or thick plastic layers) capable of withstanding the pressure differential (closed cell). Over the last 20 years, these approaches have been significantly improved and were used, for instance, to successfully characterize the growth of Si nanowires from a liquid phase5 and identify the dynamics of Cu plating on Au.6 However, both open- and closed-cell approaches severely limit sample choice, either to low-vapor pressure liquids that can withstand the vacuum in the TEM, or high-contrast samples with sufficient signal to overcome the thick liquid and window layers. Resolution was also limited due to multiple scattering. Furthermore, conventional analytical approaches, such
as energy-dispersive x-ray spectroscopy (EDS) or electron energy-loss spectroscopy (EELS), were also precluded in dedicated liquid cell instruments due to the small differential pumping apertures in an open-cell microscope and the thick window layers blocking most of the photons or inelastically scattered electrons in a closed-cell configuration. The development of Si/SiNx in situ heating/biasing holders in the early 2000s7 (see timeline of selected liquid cell development in Figure 1) spurred the implementation of liquid cells using modular side-entry stages in conventional and aberration-corrected TEMs. The resolution of these in situ liquid cell experiments, starting in 2009, reached 1 nm as the result of significant reduction in the SiNx window layer thickness (∼25–50 nm) and the dramatic reduction in the liquid layer thickness.8 In addition to higher resolution imaging of colloidal nanoparticles in solution, the ability to flow and mix solvents also allowed for observations of growth dynamics and imaging of whole cells in a liquid environment. The need for more analytical capabilities was met by redesigning the window shape to support ultrathin SiNx
Peter Ercius, National Center for Electron Microscopy, Molecular Foundry Division, Lawrence Berkeley National Laboratory, USA; [email protected] Jordan A. Hachtel, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, USA; [email protected] Robert F. Klie, University of Illinois at Chicago, USA; [email protected] doi:10.1557/mrs.2020.230 • VOLUME Core • mrs.org/bulletin © 2020 Materials Research Society Auckland University of Technology, on 15 Sep 2020 at 03:07:51, subject MRS BULLETIN 45 • terms SEPTEMBER 2020 Downloaded from https://www.cambridge.org/core. to the Cambridge of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2020.230
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