ABSTRACT
The resolution of a light microscope discussed previously is directly proportional to the wavelength. To improve the resolution, light with a shorter wavelength should be used. Scientists began experimenting with ultraviolet light, which has a shorter wavelength than visible light. Due to the difficulty in generation and maintaining coherence, it was not commercially successful.
Meanwhile, the French physicist Louis de Broglie proved a traveling electron has both wave and particle duality similar to light. He was awarded a Nobel Prize in 1929 for this work.
An electron wave with higher energy will have lower wavelength and vice versa. Thus, improving the resolution would involve increasing the energy. The wavelength of electrons is considerably shorter than that of visible light and hence very high-resolution images can be obtained. Visible light has a wavelength of 400 - 700 nm. Electrons, on the other hand, have a wavelength of 0.0122 nm for an accelerating voltage of 10 kV.
Ernst Ruska and Max Knoll created the first electron microscope (EM) with the ability to magnify objects 400 times. Upon further work, Ruska improved its resolution beyond the resolution of optical micro- scopes and hence made the EM an indispensable tool for microscopists. The EM used today does not measure a single characteristic property but rather measures multiple characteristics of the material. The one common thing among all of them is the electron beam.
In Section 16.2, we discuss some of the physical principles that need to be understood regarding EM. We begin with a discussion on the properties of the electron beam and its ability to produce images at high resolution. We introduce the interaction of electrons with the matter and various particles and waves that are generated as a consequence. The fast moving electron beam from the electron gun passes through the material to be imaged. During its transit through the material, the electron interacts with the atoms in that material. We integrate the two basic principles and discuss the construction of an EM. We also discuss specimen preparation and general precautions when preparing the specimen.
