Scanning Transmission Electron Microscopy: Z‐Contrast Imaging

As its name suggests, the scanning transmission electron microscope is a combination of the scanning electron microscope and the transmission electron microscope. Thin specimens are viewed in transmission, while images are formed serially by the scanning of an electron probe. In recent years, electron probes have become available with atomic dimensions, and, as a result, atomic resolution images may now be achieved in this instrument. The nature of the images obtained in scanning transmission electron microscopy (STEM) can differ in significant ways from those formed by the more widespread conventional transmission electron microscopy (CTEM). The key difference lies in their modes of image formation; the STEM instrument can be configured for almost perfect incoherent imaging whereas CTEM provides almost perfect coherent imaging. The latter technique is generally referred to as high-resolution electron microscopy (HREM), though both methods now provide atomic resolution. STEM provide advantages for the imaging of materials with electrons. A probe of atomic dimensions illuminates the sample, and a large annular detector is used to detect electrons scattered by the atomic nuclei. The large angular range of this detector performs the same function as Lord Rayleigh's condenser lens in averaging over many optical paths from each point inside the sample. This renders the sample effectively self-luminous; i.e., each atom in the specimen scatters the incident probe in proportion to its atomic scattering cross-section. With a large central hole in the annular detector, only high-angle Rutherford scattering is detected, for which the cross-section depends on the square of the atomic number (Z); hence this kind of microscopy is referred to as Z-contrast imaging. An incoherent image provides the most direct representation of a material's structure and, at the same time, improved resolution. In an incoherent image, there are no fixed phase relationships, and the intensity is given by a straightforward convolution of the electron probe intensity profile with a real and positive specimen object function. With Rutherford scattering from the nuclei dominating the high-angle scattering, the object function is sharply peaked at the atomic positions and proportional to the square of the atomic number. A monolayer raft of atoms is scanned by the probe, and each atom scatters according to the intensity in the vicinity of the nucleus and its high-angle cross-section. This gives a direct image with a resolution determined by the probe intensity profile. Later it is shown how crystalline samples in a zone axis orientation can also be imaged incoherently. These atomic resolution images show similar characteristics to the incoherent images familiar from optical instruments such as the camera; in a Z-contrast image, atomic columns do not reverse contrast with focus or sample thickness. Columns of Ga can be distinguished directly from columns of As simply by inspecting the image intensity. Detailed image simulations are therefore not necessary. This article focuses on Z-contrast imaging of materials at atomic resolution. Keywords: scanning transmission electron microscopy; competitive techniques; related techniques; principles; probe formation; incoherent scatting; dynamical diffraction; atomic resolution spectroscopy; practical aspects; data analysis; strain contrast; sample preparation; specimen modification; problems