Abstract
High resolution secondary electron microscopy (HRSEM) in dedicated scanning transmission electron microscopy (STEM) instruments has proved very useful for characterizing supported catalysts. In order to understand the contrast mechanisms of HRSEM images of small particles, clean samples and UHV environments are needed since the emission of secondary electrons (SEs) can be significantly influenced by contamination on the particle surface. In this paper we report results on imaging small metal particles by HRSEM in a UHV STEM instrument. In addition, we show that with computer assistance digitized HRSEM images of small metal particles can be used to estimate the average inelastic mean free path and the mean escape depth of the collected SEs.The Vacuum Generators UHV STEM HB-501S, codenamed MIDAS (a Microscope for Imaging, Diffraction and Analysis of Surfaces), was used for these experiments. Detailed descriptions of the MIDAS system have been published previously. The collection efficiency of SEs approaches 100% through the use of magnetic ‘parallelizers’ situated inside the objective pole pieces.
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