Closer to reality
The group presents further developments: a new aberration corrected environmental scanning transmission electron microscope (AC ESTEM). The main advance is to extend the “environmental” methodology to scanning TEM (= STEM) studies. Experiments can be carried out at pressures of several Pascals whilst retaining atomic resolution and full TEM functionality. Using the new technology, the scientists were able to show Pt atom migration during sintering and a restructuring of Pt clusters at elevated temperatures and pressures – which would have been impossible to observe using conventional TEM. This promises new insights into catalytic and other systems under conditions that approach ambient pressures. On-going developments are designed to increase the gas pressure at the sample.
The aberration correction of the system is particularly beneficial in dynamic in-situ experiments because there is rarely the opportunity to record for subsequent data reconstruction a full through focal series of images. It is instead necessary to extract the maximum possible information from each single image frame in a continuously changing sequence. It is also essential to limit the electron dose to ensure minimally invasive conditions, to control secondary effects such as contamination, and to avoid introducing additional mechanisms not related to the real catalyst chemistry, e.g. through gas ionization by the beam.
In contrast to their previous TEM work, which illuminated a thin specimen with a relatively broad electron beam, in STEM a focused electron probe is rastered across the sample to create an image pixel-by-pixel. Donald MacLaren from Glasgow University (UK) summerizes the main advantages of the methodology: an STEM image compiled using electrons scattered through high angles is directly-interpretable and uncomplicated by the diffraction effects that tend to dominate TEM images of crystalline materials. Exquisite three-dimensional and atom-resolved studies of nanoparticle surfaces are delivered which could, e.g., help to identify the active sites of a supported metal catalyst. Furthermore, additional signals can be collected during rastering, such as x-rays or inelastically-scattered electrons, providing comprehensive functional maps.
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