High-resolution images of microscopic samples can be obtained experimentally using scanning transmission electron microscopy (STEM). It is an effective method to examine the complex structure of nanomaterials used in the development of nanotechnology.
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Light microscopes were developed to magnify and examine microscopic structures. The phenomenon of light diffraction is one of the main defects of optical microscopes. The amount of clarity with which a microscopic specimen can be seen is limited by the diffraction of light. Electron microscopes were developed to overcome the drawback imposed by the diffraction of light in 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 spread as it travels through a slit. Projecting the diffraction pattern onto a screen results in the formation of bright and dark lines.
Figure 1. Illustration of light diffraction. Image Credit: Ilamaran Sivarajah
The image of the slit, or of an object, can be generated by inserting a convex lens, or objective, into the path of the diffracted beam. The lens must be able to collect all the light rays that have been diffracted to get a clear and sharp image. The image will be blurry if the lens cannot collect all the diffracted light.
Red light exhibits the greatest diffraction in the visible light spectrum due to its longer wavelength. Light with longer wavelengths diffracts more than light with shorter wavelengths. As can be seen in Figures 2(a) and 2(b), images created with red light are therefore more blurred than those created with UV or blue light.
Figure 2. Illustration of an image of an object formed after diffracted light rays from (a) a red laser and (b) a blue laser are focused by a lens. Image Credit: Ilamaran Sivarajah
Due to the restriction caused by light diffraction, 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 captured by optical microscopy is 200 to 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 confirmed experimentally in 1927 by 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 that until then was only perceived to be exhibited by waves. This experiment showed that electrons exhibit dual wave-particle behavior. Therefore, De Broglie’s idea was validated, earning him the Nobel Prize in Physics in 1929.
The invention of the electron microscope
Ernst Ruska, a German physicist, 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 set by the wavelength of light is removed.
Because electrons are charged particles, the diffraction of electron beams is captured by magnetic lenses. An electron microscope produces images with a resolution 1000 times higher than that of an optical microscope.
Electron microscopes were built as transmission electron microscopes (TEM) and later as scanning electron microscopes (SEM), which provided additional scanning capabilities with magnetic coils, detectors, and circuitry. Scanning transmission electron microscopes are an advanced version of electron microscopes that use both TEM and SEM technology.
Transmission Electron Microscope (TEM)
Figure 3(a) shows the fundamental working principle of a TEM. An electron gun generates an electron beam source. Electromagnetic lenses can be used to direct the beam path of moving electrons because they generate magnetic fields. A condenser lens, an objective lens, and a projector lens are all parts of the TEM. As shown in Figure 3(a), they are used to direct the electron beam. Uranyl acetate is typically used to stain specimens in TEM. Uranyl acetate produces a high electron density in the sample and helps improve image contrast.
Scanning Electron Microscope (SEM)
Using a modified technology called SEM, high-resolution images of the specimen are projected onto a detector. Similar to TEM, SEM guides the beam using objective lenses and an electromagnetic capacitor as well as an electron gun as a source (Figure 3(b)). In SEM, a second scanning coil is used instead of an objective. The electron beam can be moved along the plane of the coil in two dimensions by the scanning coil. The item being scanned has a heavy metal coating, such as gold, platinum, or tungsten. The presence of heavy metals on the object’s surface 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. Schematics of (a) Transmission electron microscope (TEM) and (b) Scanning electron microscope (SEM). Image Credit: Ilamaran Sivarajah
Impact of STEM in nanotechnology
A new era of scientific advances was made possible by STEM, especially in nanotechnology. With atomic or subnanometer spatial resolution, STEM techniques can be used in imaging, spectroscopy, and diffraction. Data on nanomaterials can be collected simultaneously or sequentially for in-depth analysis. In addition to being used 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 aberration correctors, sub-nanometer or sub-angstrom electron probes are available in STEM instruments. This ensures advanced capabilities to study 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 variation of the electron microscope, also resulted in the 2017 Nobel Prize in Chemistry.
Continue reading: Using ZnO Nanowires for 4D STEM Technology
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). June;54(3):251-78.
Brodusch N, Demers H, Gauvin R. (2018) Imaging with a commercial electron backscatter diffraction (EBSD) camera in a scanning electron microscope: a review. Image Magazine. 4(7):88.
Golding, C., Lamboo, L., Beniac, D. et al. (2016) The Scanning Electron Microscope in Microbiology and Infectious Disease Diagnosis. Science Rep 6, 26516 https://doi.org/10.1038/srep26516
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