Types of electron microscopy:

SEM (Scanning Electron Microscopy):

SEM is one of the most widely used techniques used in characterization of nanomaterials and nanostructures. The resolution of the SEM approaches a few nanometers, and the instruments can operate at magnifications that are easily adjusted from ~10 to over 300,000. Not only topographical information SEM also provides chemical composition information near the surface.


Figure: Schematic diagram of Scanning Electron Microscope.

In a typical SEM, a source of electrons is focused into a beam, with very fine spot size of ~5 nm and having energy ranging from a few hundred eV to 50 KeV, which is rastered over the surface of the specimen by deflection coils. As the electrons strikes and penetrate the surface, a number of interactions occur that result in the emission of electrons and photons from the sample, and SEM images are produced by collecting the emitted electrons on a cathode ray tube (CRT). Various SEM techniques are differentiated on the basis of what is subsequently detected and imaged, and the principle images produced in the SEM are of three types: secondary electron images, back scattered electron images and elemental X-ray maps.

When a high energy primary electron interacts with an atom, it undergoes either inelastic scattering (deflecting the electrons with loss of energy) with atomic electrons or elastic scattering (deflecting the electrons with no loss of energy) with the atomic nucleus.

In an inelastic collision with an electron, the primary electron transfers part of its energy to the other electron. When the energy transferred is large enough, the other electron will emit from the sample. If the emitted electron has the energy of less than 50 eV, it is referred to as secondary electron. Back scattered electrons are the high energy electrons that are elastically scattered and essentially possess the same energy as the incident or primary electrons. The probability of backscattering increases with the atomic number of the sample material. Although backscattering images can not be used for elemental identification, useful contrast can develop between regions of the specimen that differ widely in atomic number, Z.

An additional electron interaction in the SEM is that the primary electron collides with and ejects a core electron from an atom in the sample. The excited atom will decay to its ground state by emitting either a characteristic X-ray photon or an Auger electron, both of which have been used for chemical characterization.

Combining with chemical analytical capabilities, SEM not only provides the image of the morphology and microstructures of bulk and nanostructured materials and devices, but can also provide detailed information of chemical composition and distribution.