Scanning Transmission Electron Microscopy:Z‐Contrast Imaging

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Abstract Scanning transmission electron microscope (STEM) is a combination of the scanning electron microscope and the transmission electron microscope (TEM). Thin specimens are viewed in transmission, while images are formed serially by the scanning of an electron probe. In recent years, through the successful correction of Q2 the intrinsically high aberrations of electron lenses it has become possible to form probes well below atomic dimensions, and as a result, atomic resolution images showing sub‐angstrom resolution may now be routinely achieved in this instrumentThe nature of the images obtained in the STEM can differ in significant ways from those formed by the conventional TEM, in particular, multiple signals can be acquired in the STEM including bright field, annular dark field (ADF) images, and spectroscopic images using either electrons that have lost energy by exciting specific inner shell or valence band levels or by detecting emitted characteristic x‐rays. A key difference between STEM and TEM imaging is that the ADF image available on the STEM represents almost perfect incoherent imaging, while the bright field image represents almost perfect coherent imaging, equivalent to the image provided by a conventional high‐resolution TEM. The latter technique has often been simply referred to as high‐resolution electron microscopy (HREM). This article focuses on Z‐contrast imaging of materials at atomic resolution and associated techniques such as electron energy loss spectroscopy (EELS).