Studies of Catalytic Materials with In Situ Transmission Electron Microscopy

Abstract

Despite the omnipresence of optical illusions, the old adage of 'seeing is believing' remains in widespread use, never more so than in electron microscopy. However, although it is indeed important to be careful with interpretation of electron microscopy (EM) data, as illustrated by Figure 1.5, it is an incredibly powerful tool in many fields of nanotechnology. Due to their short de Broglie wavelength, high-energy electrons do not suffer from diffraction limitations seen in optical microscopes when one is trying to observe nanomaterials. This allows for significantly higher spatial resolution and visualization of nanoscale morphology and even atomic structures with the right equipment. 78 In STEM, the electron beam is focused into a very small spot (typically 50-200 pm) and raster-scanned over the field of view. Depending on the electrons of interest, one or more electron detectors are placed below the sample. For bright-field STEM (BF-STEM), the detector is again located directly in the path of the unscattered beam, in the optical axis, and detects electrons that have not been scattered significantly. However, it is also possible to detect the scattered electrons only, using an annular detector located outside of the direct path. Because in this mode the parts that scatter most electrons are the brightest and the background is dark, this mode is called darkfield STEM (DF-STEM).As mentioned previously, in (bright field) TEM, a camera detects electrons that have not been scattered to any significant extent and all pixels are measured at the same time. As a result, if electrons are scattered before interacting with the sample, they will only contribute to the background noise. On the other hand, if scattered after interaction, the electrons might hit the camera in a slightly different location, causing sample blurring. Therefore, minimizing scattering after interaction with the sample is crucial and achieved by placing the sample on the bottom chip.