Frontiers of in situ electron microscopy
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Introduction In situ transmission electron microscopy (TEM) is a fast-growing and fascinating area of research that has drawn tremendous attention from various fields ranging from materials science to chemistry and biology. As a powerful and indispensable tool for nanomaterials characterization, in situ TEM provides great opportunities to characterize dynamic changes in size, shape, interface structure, electronic state, and chemical composition in materials at and below the nanoscale. In situ TEM has benefited from advances in electron microscopy instrumentation that have achieved spatial resolutions in the subnanometer range, energy resolution in the sub-electron-volt range, and sensitivity to individual atoms. It is now possible to image the atomic structure of materials in real time under various external stimuli while simultaneously measuring relevant properties. A variety of in situ TEM holders have been developed to enable imaging and measurements under applied heat, stress, optical excitation, and magnetic or electric fields, and the development of environmental cells allows experiments to be performed in different gaseous and liquid environments. Developments in in situ TEM combined with aberrationcorrected high-resolution imaging, electron energy-loss spectroscopy (EELS), and energy dispersive x-ray spectroscopy have enabled many discoveries in dynamic materials processes at the atomic level that were not previously possible.1–6
With the development of controlled-environment TEM, environmental TEM (ETEM), direct observations of the structural evolution of catalytic nanoparticles under dynamic reaction conditions has been realized. The further requirements of achieving better spatial and energy resolution of dynamic measurements under relatively high gas pressures while minimizing electron-beam effects provide a framework for the advancement of ETEM. In recent years, a number of breakthroughs have occurred in the development of ETEM for imaging liquid samples.2,7,8 To introduce liquids into the high vacuum of a TEM instrument, either a microfabricated liquid-cell enclosure or an open-cell configuration using low-vapor-pressure ionic liquids has been utilized. These technical breakthroughs have yielded a plethora of achievements in imaging dynamic growth of colloidal nanoparticles,2,7,9 electrochemical processes relevant to batteries,10,11 and biological materials in liquid environments.8,12 These studies have paved the way to characterize chemical reactions and dynamic processes of materials under working conditions in real time. With the continuing development of instrumental capabilities, in situ TEM experiments can be performed to study material behavior under various external stimuli such as electrical and magnetic fields13,14 and mechanical stress.15 In situ TEM has been applied to visualize domain dynamics during ferroelectric and magnetic switching,14,16
Haimei Zheng, Materials Sciences Division, Lawrence Berkeley National Laboratory, USA; [email protected] Ying Shirley Meng, Department of NanoEngineering
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