Scanning Electron Microscopes Vs Transmission Electron Microscopes
Carbon Nanotubes imaged by TEM (L) and SEM (R)]
Electron microscopy has revolutionized the imaging of nanostructures. With its excellent sub-Angstrom spatial resolution, electron microscopies can be used to image objects as small as atoms on a surface.1 For high-precision manufacturing of nanoscale structures, such high spatial resolution is essential for quality control and checking for the presence of any defects in the structure that may impede performance.
The reason electron microscopy has such high spatial resolution in comparison to traditional optical techniques is due to the much shorter wavelength of an electron compared to that of a photon. In optical microscopes, this limits the apparatus to a spatial resolution of several hundred nanometers.
Two of the most popular electron microscopy methods make use of scanning electron microscopes (SEM) and transmission electron microscopes (TEM). Scanning electron microscopes and transmission electron microscopes require slightly different sample preparation and recover different imaging information but many of the basic operating principles are similar between the techniques.
Basics of Electron Microscopy
Most electron microscopes are composed of the electron source, typically a high voltage electron gun, a series of focusing optics, the sample holder, and a detector. High brightness electron guns produce the electron beam that is then focused onto the sample. The focusing optics in an electron microscope play an important role as changing these conditions affects the depth of field and illumination area of the electron beam. After interacting with the specimen, the scattered electrons are collected in various geometries to reconstruct an image.
What is the Difference Between SEM and TEM?
The positioning of the detector and the type of electrons detected is one of the key differences between scanning electron microscopes and transmission electron microscopes.2 In a transmission electron microscope, the electron beam passes straight through the sample where the change in the electron transmission is detected.
Transmission electron microscopy is ideal for thin layer samples (< 150 nm) and can be sensitive to not just the surface of the sample but the inner regions as well. It is commonly used for highly crystalline structures and nanotechnology applications but can be used for looking at tissues and bacteria as well.3
Scanning electron microscopes instead detect the backscattered and secondary electrons from the sample instead of just detecting the incident beam. When combined with raster stages to translate the sample, a scanning electron microscope can be used to reconstruct a full structural image.
Both scanning electron microscopes and transmission electron microscopes are ‘label-free’ microscopy techniques and image the sample as it is. However, for imaging with scanning electron microscopes, one of the big challenges is ensuring the sample can withstand the high vacuum conditions and is sufficiently clean to capture the incredibly detailed images the technique is capable of.
JEOL Microscopes
JEOL offer an extensive range of scanning electron microscopes and transmission electron microscopes.2 Contact JEOL today to find out what the right technique for your application is and how their instrumentation can benefit you.
References
- Smith, D. J. (2008). Ultimate resolution in the electron microscope? Materials Today, 11, 30–38. https://doi.org/10.1016/S1369-7021(09)70005-7
- JEOL (2022) Scanning Electron Microscopes, https://www.jeolusa.com/PRODUCTS/Scanning-Electron-Microscopes-SEM, accessed April 2022
- Arenal, F. L., A., D., & R., M. (2015). Advanced transmission electron microscopy. Springer