ABSTRACT
The quantum mechanical concept of particle-wave duality is the basis for electron microscopy. An electron wave with an energy of 300 keV, and with a de Broglie wavelength of ca. 2 pm, is the analogue of visible light with a wavelength of 500 nm. An electron wave has all the wave-like attributes such as phase, amplitude, coherence, luminosity, and direction of propagation. It might be thought that the point-to-point resolution of an electron microscope should therefore be of magnitude comparable to the electron wavelength, 2 pm, in accord with the diffraction limit, which is the limiting factor for optical microscopy. Unfortunately, electromagnetic lens aberrations conspire to degrade the diffraction-limited resolution by a factor of 102, to 0.1 nm, for the latest generation of aberration-corrected transmission electron microscopy (TEM) instruments. There are other similarities between the imaging mechanisms for optical and electron microscopy, such as diffuse scattering contrast (known as ‘mass thickness contrast’ for TEM) in transmission and phase contrast. On the other hand, there are some differences. For instance, a TEM cannot generate contrast in the backscatter mode, (but backscatter imaging is nevertheless very useful and popular in scanning electron microscopy (SEM)). Since the scattering mechanisms for electrons are different from those of photons, and because the wavelengths of keV electrons are comparable to interatomic spacings, spatially resolved structural information can be obtained readily in the diffraction mode.
