Scanning transmission electron microscopy

A scanning transmission electron microscope (STEM) is a type of transmission electron microscope. With it, the electrons pass through the specimen, but, as in scanning electron microscopy, the electron optics focus the beam into a narrow spot which is scanned over the sample in a raster.

The rastering of the beam across the sample makes these microscopes suitable for analysis techniques such as mapping by energy dispersive X-ray (EDX) spectroscopy, electron energy loss spectroscopy (EELS) and annular dark-field imaging (ADF). These signals can be obtained simultaneously, allowing direct correlation of image and quantitative data.

By using a STEM and a high-angle detector, it is possible to form atomic resolution images where the contrast is directly related to the atomic number. This is in contrast to the conventional high resolution electron microscopy technique, which uses phase-contrast, and therefore produces results which need interpretation by simulation.

History
The first STEM was built in 1938 by Baron Manfred von Ardenne, working in Berlin for Siemens. However, the results were inferior to that of TEM at the time, and von Ardenne only spent two years working on the problem. The microscope was destroyed in an air raid in 1944, and von Ardenne did not return to the field after WWII

The technique did not become developed until the 1970s, with Albert Crewe at the University of Chicago developing the field emission gun and adding a high quality objective lens  to create the modern STEM, and demonstrated the ability to image atoms using ADF

Crewe and coworkers at the University of Chicago developed the cold field emission electron source and built a STEM able to visualize single heavy atoms on thin carbon substrates (Crewe et al., 1970).

Atomic resolution chemical analysis using the STEM was first reported in 1993.

Biological Application
The first application of this method to the imaging of biological molecules was demonstrated soon thereafter (Wall, 1971). At Brookhaven National Laboratory, STEM1 was designed (and built) by Joe Wall. The motivation for STEM imaging of biological samples is particularly to make use of dark-field microscopy, where the STEM is more efficient than a conventional TEM, allowing high contrast imaging of biological samples without requiring staining. The method has been widely used to solve a number of structural problems in molecular biology (Wall and Hainfeld, 1986; Hainfeld and Wall, 1988; Wall and Simon, 2001).