A Guide to Scanning Transmission Electron Microscopy (STEM)
High resolution images of microscopic samples can be obtained experimentally using Scanning Electron Transmission Microscopy (STEM). It is an effective method for examining the intricate structure of nanomaterials used in developing nanotechnology.
Image Credit: Jeff Whyte/Shutterstock.com
Light microscopes were developed to enlarge and examine microscopic structures. The phenomenon of light diffraction is one of the optical microscopes' main flaws. The amount of clarity with which a microscopic sample can be viewed is constrained by light diffraction. Electron microscopes were developed to overcome the drawback imposed by light diffraction in optical microscopy.
The bending of light as it travels through a hole or around the edge of an object is known as light diffraction. Figure 1 illustrates how diffraction causes light to spread out when it travels through a slit. The projection of 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 an object, can be generated by inserting a convex lens, or an objective, into the diffracted beam path. The lens must be able to collect all of the light rays that have been diffracted for a clear, sharp image to be produced. The image will be hazy if the lens cannot collect all of the diffracted light.
Red light exhibits the most diffraction in the visible light spectrum because of 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 consequently blurrier than those created with UV or blue light.
Figure 2. Illustration of an image of an object formed after the diffracted light beams from (a) a red laser and (b) a blue laser are focused using a lens. Image Credit: Ilamaran Sivarajah
Because of 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 light, the maximum sample size that can be imaged by optical microscopy is 200-250 nanometers.
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 both like a particle and like a wave.
The wave-particle duality was experimentally confirmed in 1927 by employing 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 proved that electrons exhibit dual wave-particle behavior. De Broglie's idea was therefore validated, winning 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 light particles, electrons have a far shorter wavelength. When electrons are utilized instead of light, the diffraction limit set by the light's wavelength 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 higher than that of an optical microscope.
Electron microscopes were constructed 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 utilizes the technology of both TEMs and SEMs.
Transmission Electron Microscope (TEM)
Figure 3(a) depicts the fundamental operating 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 depicted in figure 3(a), they are employed to direct the electron beam. Uranyl acetate is typically used to stain the samples in TEM. High electron density in the sample is produced by uranyl acetate and aids in improving image contrast.
Scanning Electron Microscope (SEM)
With the help of a modified technology called SEM, high-resolution images of the specimen are projected onto a detector. Similar to the TEM, the SEM guides the beam using objective lenses and an electromagnetic condenser as well as an electron gun as a source (figure 3(b)). In SEM, a second scanning coil is employed in place of an objective. The electron beam can be moved along the coil's plane 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 back scattered. Electron detectors gather the back-scattered electrons, and software is used to create high resolution images.
Figure 3. Schematic diagrams of (a) Transmission Electron Microscope (TEM) and (b) Scanning Electron Microscope (SEM). Image Credit: Ilamaran Sivarajah
Impact of STEM in Nanotechnology
A new age of scientific advancements was made possible by STEM, especially in nanotechnology. With atomic or sub-nanometer spatial resolution, STEM techniques can be used in imaging, spectroscopy and diffraction. Data from nanomaterials can be collected simultaneously or successively for in-depth analysis. In addition to being used for the characterization of nanomaterials, STEM can be combined with cutting-edge technologies for nanomaterials 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 for studying the sizes, shapes, defects, surface structures and electronic states of nanoparticle systems.
In 1986, Ernst Ruska won the physics Nobel Prize for developing the electron microscope. Cryogenic electron microscopy (cryo-EM), a later variation of the electron microscope, also led to the 2017 Nobel Prize in chemistry.
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. https://doi.org/10.1093/jmicro/dfi034
Brodusch N, Demers H, Gauvin R. (2018) Imaging with a Commercial Electron Backscatter Diffraction (EBSD) Camera in a Scanning Electron Microscope: A Review. Journal of Imaging. 4(7):88. https://doi.org/10.3390/jimaging4070088
Golding, C., Lamboo, L., Beniac, D. et al. (2016) The scanning electron microscope in microbiology and diagnosis of infectious disease. Sci Rep 6, 26516 https://doi.org/10.1038/srep26516
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