Electron-beam-driven chemical processes during liquid phase transmission electron microscopy
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Introduction Microscopic observations of nanomaterials, soft matter, and biological systems in their functional state and native environment is critical to gain mechanistic understanding of their behavior and function. While great strides have been made in the fields of super resolution optical microscopy, highresolution transmission electron microscopy (HRTEM), and cryo-electron microscopy (cryo-EM), these techniques lack either the spatial resolution for imaging unlabeled samples or are unable to distinguish real-time dynamics of liquid phase systems on the molecular and nanoscale. Liquid phase (or liquid cell) transmission electron microscopy (LP-TEM) has emerged as a technique uniquely capable of visualizing the dynamics of liquid phase nanostructured materials and biological systems with atomic to few nanometer-scale resolution.1–3 LP-TEM utilizes hermetically sealed nanoreactors enclosed by electron transparent membranes, typically either silicon nitride or graphene, to contain a thin liquid layer in the TEM.4 Radical formation was initially exploited in LP-TEM studies that utilized the electron beam as the stimulus to drive nanocrystal formation in liquid.5 Heating,6 electrochemical biasing,7,8 and mixing of chemical reagents9 have been
used to initiate reactions during LP-TEM, but each of these requires complex capabilities built into the microfabricated sample devices. They can also be hard to control; mixing, for example, is limited by diffusion and not representative of macroscopic turbulent mixing. A significant fraction of LP-TEM studies therefore continue to utilize the electron beam as the main stimulus to drive nanoscale dynamics due to its conven ience, despite a relatively poor understanding of the electronbeam-driven chemical processes in the sample. On the other hand, the impact of electron-beam damage on the sample must be considered in LP-TEM experiments where beam-induced reactions are undesirable. Controlling and mitigating electronbeam-induced chemical processes during LP-TEM is critical to enabling quantitative data to be obtained and provide confidence that the observed phenomena are representative of ensemble-scale materials behavior. In this article, we describe recent advances in understanding electron-beam-driven chemical processes during LP-TEM. These are summarized in Figure 1. We first describe approaches for modeling the chemical changes produced by the beam, especially near solid–liquid interfaces. We then discuss beam-driven processes involving inorganic metal nanoparticles, polymers, nanoparticle capping ligands, and
Taylor J. Woehl, Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, USA; [email protected] Trevor Moser, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, USA; [email protected] James E. Evans, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, USA; [email protected] Frances M. Ross, Department of Materials Science and Engineering, Massachusetts Instit
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