in an SEMThe most common methods of obtaining images in an electron microscope involve selecting different directions for the electrons that have been transmitted through a sample, and/or electrons of different energies. There are a very large number of methods of doing this, although not all are very common.
Secondary electrons In a SEM the signals result from interactions of the electron beam with atoms within the sample. The most common mode is to use the
secondary electrons (SE) to produce images. Secondary electrons have very low energies, on the order of 50
eV, which limits their
mean free path in solid matter to a few
nanometers below the sample surface. which is a type of collector-
scintillator-
photomultiplier system. The signal from secondary electrons tends to be highly localized at the point of impact of the primary electron beam, making it possible to collect images of the sample surface with a resolution of better than 1
nm, and with specialized instruments at the atomic scale. The brightness of the signal depends on the number of secondary electrons reaching the
detector. If the beam enters the sample perpendicular to the surface, then the electrons come out symmetrically about the axis of the beam. As the angle of incidence increases, the interaction volume from which they cone increases and the "escape" distance from one side of the beam decreases, resulting in more secondary electrons being emitted from the sample. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with a well-defined, three-dimensional appearance that is similar to a reflected light image. One variant of this is called
weak-beam dark-field microscopy, and can be used to obtain high resolution images of defects such as dislocations.
High resolution imaging In high-resolution transmission electron microscopy (also sometimes called high-resolution electron microscopy) a number of different diffracted beams are allowed through the objective aperture. These interfere, leading to images which represent the atomic structure of the material. These can include the incident beam direction, or with scanning transmission electron microscopes they typically are for a range of diffracted beams excluding the incident beam. and the effect of the
contrast transfer function of the microscope. There are many other imaging variants that can also to lead to atomic level information.
Electron holography uses the interference of electrons which have been through the sample and a reference beam.
4D STEM collects diffraction data at each point using a scanning instrument, then processes them to produce different types of images.
X-ray microanalysis '' Most of these peaks are
K-alpha and
K-beta lines. One peak is from the L shell of iron. X-ray microanalysis is a method of obtaining local chemical information within electron microscopes of all types, although it is most commonly used in scanning instruments. When high energy electrons interact with atoms they can knock out electrons, particularly those in the
inner shells and
core electrons. These are then filled by
valence electron, and the energy difference between the valence and core states can be converted into an
x-ray which is detected by a spectrometer. The energies of these x-rays is somewhat specific to the atomic species, so local chemistry can be probed.
EELS Similar to X-ray microanalysis, the energies of electrons which have transmitted through a sample can be analyzed and yield information ranging from details of the local electronic structure to chemical information.
Electron diffraction Transmission electron microscopes can be used in
electron diffraction mode where a map of the angles of the electrons leaving the sample is produced. The advantages of electron diffraction over
X-ray crystallography are primarily in the size of the crystals. In X-ray crystallography, crystals are commonly visible by the naked eye and are generally in the hundreds of micrometers in length. In comparison, crystals for electron diffraction must be less than a few hundred nanometers in thickness, and have no lower boundary of size. Additionally, electron diffraction is done on a TEM, which can also be used to obtain other types of information, rather than requiring a separate instrument. There are many variants on electron diffraction, depending upon exactly what type of illumination conditions are used. If a parallel beam is used with an aperture to limit the region exposed to the electrons then sharp diffraction features are normally observed, a technique called
selected area electron diffraction. This is often the main technique used. Another common approach uses conical illumination and is called
convergent beam electron diffraction (CBED). This is good for determining the symmetry of materials. A third is
precession electron diffraction, where a parallel beam is spun around a large angle, producing a type of average diffraction pattern. These often have less multiple scattering.
Other electron microscope techniques •
Cathodoluminescence - analysing photons due to the electron beam •
Charge contrast imaging - using how charging varies with position to produce images •
CryoEM - using frozen samples, almost always biological samples •
EBSD - using the back-scattered electrons, typically in a SEM •
TKD - using the
Kikuchi lines in diffraction patterns •
ECCI - using contrast due to electron channelling •
EBIC - measuring the current produced as a function of the position of a small beam •
Electron tomography - methods to produce 3D information by combining images •
FEM - technique to interrogate nanocrystalline materials •
Immune electron microscopy - the use of electron microscopy in immunology •
Geometric phase analysis - a method to analyze high-resolution images •
Serial block-face scanning electron microscopy - a way to produce 3D information from many images •
WDXS - higher precision detection of x-rays to analyze local chemistry == Aberration corrected instruments ==