After the electrons have passed through the specimen and been scattered to varying degrees, the information is converted into a macroscopic image. The simplest way of doing this is by simply magnifying the image or diffraction pattern until it is of the required size for analysis. This is the basis of conventional TEM.
Alternatively, if a very fine beam of electrons is rastered across the sample, the amount of scattering from each point may be measured separately and successively, and an image gradually built up. This technique, requiring no lenses after the specimen, is called scanning TEM (STEM).
The projection system magnifies the images or diffraction patterns formed from the specimen, projecting them onto the viewing screen, where the electron density is converted into light-optical images for the microscopist to see.
Beneath all the lenses is a phosphorescent screen that glows when it is struck by electrons, displaying the image or diffraction pattern. The screen is viewed through a lead-glass window (to protect the users from X-rays generated in the microscope).
The information contained in a TEM micrograph is solely due to the difference in the flux of electrons through each point in the image - the contrast. The electron microscopist must understand the reasons for contrast in order to gather information from the sample. We shall deal briefly with the main sources of contrast in the following:
- Mass absorption contrast
- On passing through matter, a beam of electrons is gradually attenuated. The degree of attenuation increases with the thickness of the specimen and its mass, so variations of mass and thickness across the sample give rise to contrast in the image.
- Diffraction contrast
- Diffraction of electrons from Bragg planes causes a change in their direction of travel (elastic scattering). Hence, contrast can arise between adjacent grains or between different regions near the core of a dislocation.
- Phase contrast
- Scattering mechanisms often cause a change in the phase of the scattered electrons, as well as a change in direction. Interference between electrons of different phase which are incident on the same part of the image will cause a change in intensity and give rise to contrast. This is normally only visible at high magnifications and for microscopes that can achieve atomic resolution (HRTEMs).
Instead of recording the image from a sample all at once, we can illuminate a very small segment of the sample at one time and record the magnitude of electron scattering from the point. This can by done rapidly, and an image is built up in the same way as on a television screen by scanning the beam across the sample. This technique is called scanning transmission electron microscopy (STEM).
Since the whole image is not collected and focussed at the same moment, no lenses are needed after the sample. Instead, a set of annular detectors is used. The spatial resolution of this technique is given by the size of the electron beam at the specimen surface (controlled by the gun and condenser system). An advantage in image formation is that electrons scattered through large angles (Rutherford scattering) may be detected using a high-angle annular dark-field (HAADF) detector and a fourth mechanism of contrast exploited. At large angles the intensity of scattering,
I ∝ Z x where x~2.
STEM HAADF images display compositional contrast, and can be used to quantitatively assess elemental composition up to the atomic scale.
Using a STEM in conjunction with analytical detectors it is possible to collect compositional maps of specimens, for example by energy dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS). EM is used for high resolution chemical analysis of specimens.