Image credit: Jeff Whyte/Shutterstock.com Light microscopes were developed to magnify and examine microscopic structures. The phenomenon of light refraction is one of the main defects of optical microscopes. The clarity with which a microscopic specimen can be seen is limited by the refraction of light. Electron microscopes were developed to overcome the disadvantage that light diffraction imposes on optical microscopy.
Diffraction of light
The bending of light as it travels through a hole or around the edge of an object is known as diffraction of light. Figure 1 illustrates how diffraction causes light to scatter when traveling through a slit. Projecting the diffraction pattern on a screen results in the formation of bright and dark lines.
Figure 1. Light diffraction imaging. Image Credit: Ilamaran Sivarajah The image of the slit, or of an object, can be formed by inserting a convex lens or an objective lens into the path of the diffracted beam. The lens must be able to collect all refracted light rays to produce a clear, sharp image. The image will be blurry if the lens cannot collect all the refracted light. Red light exhibits the most diffraction in the visible light spectrum due to its longer wavelength. Light with longer wavelengths diffracts more than light with shorter wavelengths. As shown in Figures 2(a) and 2(b), the images created by red light are consequently more blurred than those created by UV or blue light.
Figure 2. Illustration of an image of an object formed after focusing refracted light rays from (a) a red laser and (b) a blue laser using a lens. Image Credit: Ilamaran Sivarajah Due to the limitation caused by the refraction of light, optical microscopes cannot capture images of extremely small samples with a high level of detail. Depending on the wavelength of the light, the maximum sample size that can be imaged with an optical microscope is 200-250 nanometers.
Wave-particle duality
Louis de Broglie, a French physicist, proposed the wave-particle duality of matter in 1924. According to the wave-particle duality hypothesis, all matter behaves as both a particle and a wave. Wave-particle duality was experimentally confirmed in 1927 using electron beams as a diffraction source. A diffraction pattern was created on a screen when a beam of charged electrons was directed through a slit. Diffraction, however, is a phenomenon previously thought to occur only in waves. This experiment proved that electrons exhibit dual wave-particle behavior. De Broglie’s idea was therefore validated, earning him the 1929 Nobel Prize in Physics.
The invention of the Electron Microscope
Ernst Ruska, a physicist from Germany, created the first electron microscope in 1933. Compared to photons, which are particles of light, electrons have a much shorter wavelength. When electrons are used instead of light, the diffraction limit defined by the wavelength of the light is removed. Since electrons are charged particles, the diffraction of electron beams is captured by magnetic lenses. An electron microscope produces images with a resolution that is 1000 times greater than that of an optical microscope. Electron microscopes were developed as Transmission Electron Microscopes (TEM) and later as Scanning Electron Microscopes (SEM), which provided additional scanning capabilities with magnetic coils, detectors and circuits. Scanning transmission electron microscopes are an advanced version of electron microscopes that use the technology of both TEM and SEM.
Transmission Electron Microscope (TEM)
Figure 3(a) illustrates the fundamental working principle of a TEM. An electron gun creates an electron beam source. Electromagnetic lenses can be used to direct the beam path of moving electrons because they create magnetic fields. A condenser lens, an objective lens, and a projector lens are all parts of the TEM. As illustrated in figure 3(a), they are used to direct the electron beam. Uranyl acetate is commonly used to stain samples in TEM. The high electron density in the sample is produced by uranyl acetate and helps improve image contrast.
Scanning Electron Microscope (SEM)
Using a modified technology called SEM, high-resolution images of the sample are projected onto a detector. Similar to TEM, SEM guides the beam using objectives and an electromagnetic condenser as well as an electron gun as a source (figure 3(b)). In the SEM, a second scanning coil is used in place of an objective lens. The electron beam can be moved along the plane of the coil in two dimensions by the scanning coil. The object being scanned has a heavy metal coating, such as gold, platinum or tungsten. The presence of heavy metals on the surface of the object causes the colliding electrons to be scattered again. Electron detectors collect the backscattered electrons and software is used to create high-resolution images.
Figure 3. Schematic diagrams of (a) Transmission Electron Microscopy (TEM) and (b) Scanning Electron Microscopy (SEM). Image Credit: Ilamaran Sivarajah
Impact of STEM on Nanotechnology
A new era of scientific advances has been made possible by STEM, especially in nanotechnology. With atomic or subnanometer spatial resolution, STEM techniques can be used in imaging, spectroscopy, and refraction. Data from nanomaterials can be collected simultaneously or sequentially for in-depth analysis. In addition to its use for nanomaterial characterization, STEM can be combined with cutting-edge technologies for nanomaterial engineering and manipulation. Due to the use of a field emission gun and deflection correctors, sub-nanometer or sub-angstrom electron detectors are available in STEM instruments. This ensures advanced capabilities for studying the sizes, shapes, defects, surface structures and electronic states of nanoparticle systems. In 1986, Ernst Ruska won the Nobel Prize in Physics for the development of the electron microscope. Cryogenic electron microscopy (cryo-EM), a later variant of the electron microscope, also led to the 2017 Nobel Prize in Chemistry.
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References and further reading
Liu J. (2005) Scanning transmission electron microscopy and its application to the study of nanoparticles and nanoparticle systems. J Electron Microsc (Tokyo). Jun;54(3):251-78. Brodusch N, Demers H, Gauvin R. (2018) Imaging with a Commercial Electron Backscatter Diffraction Camera (EBSD) in a Scanning Electron Microscope: A Review. Journal of Imaging. 4(7):88. Golding, C., Lamboo, L., Beniac, D. et al. (2016) The scanning electron microscope in microbiology and the diagnosis of infectious diseases. Sci Rep 6, 26516 https://doi.org/10.1038/srep26516 Disclaimer: The views expressed here are those of the author expressed in his capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork, owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.
