Electron Microscopy Imaging Techniques in Environmental and Geological Science
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Nanoparticles and nanodomains are omnipresent in low-temperature environments at the Earth's surface, including the hydrosphere and the atmosphere. This part of the Earth is often referred to as the “critical zone” because this zone experiences the greatest impact from human activities. The relatively low temperatures ( < 100°C) of the critical zone are conducive to the formation and survival of nanoparticles. Many trace elements are toxic (Pb, Hg, and As), and their release and mobilization at the nanoscale can cause serious health effects. However, other trace elements (e.g., Se) are essential for life forms and have an important impact on the success of different organisms in different ecological systems. In addition, the speciation and distribution of trace elements have important implications for geologic processes, such as the formation of ore deposits, environmental contaminations, and geologic age dating using isotopes (e.g., U, Th, and Pb). Trace elements in the critical zone are frequently present as dispersed nanoparticles, or in nanodomains that contain the elements at relatively high concentration. Finally, at the nanoscale, the chemical, thermal, and electronic properties of materials can change dramatically from bulk values as a function of size.
For our understanding of nanoparticles in environmental and geological sciences to continue to develop, our ability to directly study the chemical and structural forms of these particles at continually shorter length scales becomes evermore critical. This brief review of traditional analytical techniques illustrates a few of these techniques, their applications, and their limitations. The remainder of this article will focus on the application of high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and associated high-resolution analytical techniques by presenting four examples. These examples will focus on heavy trace elements of importance in the fields of geological and environmental sciences.