STEM-HAADF

Scanning Transmission Electron Microscopy (STEM) is an invaluable tool for the characterization of nanostructures. It provides elemental composition and crystal information at atomic scale. The Scanning TEM works on the same principle as a normal scanning electron microscope by focusing an electron beam into a very small spot which is scanned over the TEM sample. The difference with SEM is that one has to prepare ultrathin specimens of 200 nm or less (except for nanoparticles) so that the accelerated beam of electrons passes through the sample. This electron beam interacts with the TEM sample and the transmitted beam is collected by a detector. This can either be a CCD chip (when operating in TEM mode) or a High Angle Annular Dark Field (HAADF) detector.

HR-TEM

High Resolution STEM image

When operating in Transmission Electron Microscopy mode, dense areas as well as areas that contain heavy elements appear dark due to more scattering of the electrons within the sample. In STEM mode the opposite is realized, making the HAADF detector very sensitive to differences of the element irradiated based on the atomic number (Z-contrast). See the example images below of the metal oxide catalysts.
catalyst STEM HAADF metal oxide

STEM imaging can be combined with several material analysis techniques like Electron Energy Loss Spectroscopy (EELS), Energy Filtered TEM (EFTEM) and Energy Dispersive X-ray (EDX). EELS is a technique that provides elemental information on a nanometer scale when coupled with TEM. The energy of the incident electrons is altered as they pass through the sample. This Energy Loss can be characterised using EELS to provide elemental identification. Compared to EDX, EELS provides improved signal to noise, spatial resolution (down to 1 nm), energy resolution (<1 eV for EELS) and sensitivity to the lower atomic number elements. In the example at the right side Energy Filtered TEM is used to elucidate the diffusion of Ti through the Pd layer.

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