TRANSMISSION ELECTRON MICROSCOPY OF VACUUM SENSITIVE, RADIATION SENSITIVE, AND STRUCTURALLY DELICATE MATERIALS
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The transmission electron microscope (TEM) is a powerful tool for characterizing the nanoscale and atomic structure of materials, offering insights into their fundamental physical properties. However, TEM characterization requires very thin samples of material to be placed in a high vacuum environment, and exposed to electron radiation. The high vacuum will induce some materials to evaporate or sublimate, preventing them from being accurately characterized, radiation may damage the sample, causing mass loss, or altering its structure, and structurally delicate samples may collapse and break apart when they are thinned for TEM imaging. This dissertation discusses three different projects in which each of these three difficulties pose challenges to TEM characterization of samples.
Firstly, we outline strategies for minimizing radiation damage when characterizing materials in TEM at atomic resolution. We consider types of radiation damage, such as vacancy enhanced displacement, that are not included in some previous discussions of beam damage, and we consider how to minimize damage when using new imaging techniques such as annular bright-field scanning TEM. Our methodology emphasizes the general principle that variation of both signal strength and damage cross section must be considered when choosing an experimental electron beam voltage to minimize damage.
Secondly, we consider samples containing sulfur, which is prone to sublimation in high vacuum. TEM is routinely used to attempt to characterize the sulfur distribution in lithium-sulfur battery electrodes, but sublimation artifacts can give misleading results. We demonstrate that sulfur sublimation can be suppressed by using cryogenic TEM to characterize sulfur at very low temperatures, or by using the recently developed airSEM to characterize sulfur without exposing it to vacuum.
Finally, we discuss the characterization of aging cadmium yellow paint from early 20th century art masterpieces. The binding medium holding paint particles together bends and curls as sample thickness is reduced to ~100 nm, making high resolution characterization challenging. We acquire lattice resolution images of the pigment particles through the binder using high voltage zero-loss energy filtered TEM, allowing us to measure the pigment particle size and determine the pigment crystal structure, providing insight into why the paint is aging and how it was synthesized.
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Hanrath, Tobias