Fundamental Principle
The essence of the revolution of electron microscopy is the utilization of the wave-particle duality of electrons. Resolution according to the Abbe limit of diffraction: the shortest wavelengths result in the greatest resolution; shorter wavelengths have a finer resolution. It is the ability to use electrons with wavelengths as short as 0.0025 nm (compared with a wavelength of 400-700 nm used in light microscopy) that allows electron microscopes to surpass the inherent resolution limits in optical microscopy. It is functioning under high vacuum to suppress the scattering of electrons by air molecules, and special preparation and handling procedures are needed for the samples.
Main Types and Their Operation
Scanning Electron Microscope (SEM)
The SEM uses a pattern of focused electron beams to scan a specimen to generate detailed three-dimensional surface images. When the primary electrons strike the sample surface, they produce a number of signals: secondary electrons are emitted by surface atoms, and topographic contrast is provided by these electrons; backscattered electrons are reflected by deeper layers, and elemental analysis is possible using EDS. SEM possesses outstanding field curvature, and rough surfaces are virtually completely focused, and it allows relatively solid samples of moderate preparation.
Transmission Electron Microscope (TEM)
TEM is based on transmission principles where a beam of coherent, narrow, but broad electrons is directed through a thin specimen (usually less than 100 nm). Thickness or rather more dense atoms in the sample, causes more electrons to scatter; thus, the transmitted beam forms a contrast. The resultant two-dimensional projection image shows the internal structure at an almost atomic scale. The new types of TEM are selected area electron diffraction (SAED) to obtain crystallographic data and high-resolution TEM (HRTEM) to see the crystal lattices. The preparation of samples is very intensive and usually entails ultramicrotomy, ion milling or electrochemical thinning.
Materials Science and Engineering
EMs are used to describe the structure of alloys of metals, phase separations in polymers, micro-cracks in ceramics, and the interfaces in composite materials. Failure analysis determines the source of fracture in engineering parts, whereas semiconductor inspection ascertains the fabrication of the chips at the nanometre level. The study of catalysis is focused on the size, distribution, and support of nanoparticles.
Strengths
The unmatched resolution allows one to see a few microns up to atoms in the same instrument family. Outstanding depth of field in SEM delivers sharp images on harsh surfaces. Multi-modal analysis is the imaging that is combined with elemental (EDS/WDS), chemical (EELS), and crystallographic (diffraction) measurements. Trace elements and minor phases are detected by high sensitivity. Digital imaging eases the quantitative analysis of a sample, such as particle sizing or stereological measurements.
Challenges
The access is restricted by the high cost of capital (100,000-10+ million) and high maintenance demand. TEM sample preparation is a lengthy and usually destructive process. The needs of vacuums do not allow any observation of any liquid processes or living organisms without special equipment. The specialized training is necessary in interpretation, especially in TEM images, which depict complex two-dimensional projections of three-dimensional objects.
