Transmission electron microscopes (TEMs) are the big guns of the microscope world, capable of peering far beyond the diffraction limit of conventional optical microscopes to resolve sub-nanometer details and probe the atomic structure of materials.  The most sophisticated aberration-corrected transmission electron microscopes offer the highest resolution achieved by any microscopes yet developed. 
Thanks to its superior resolving power and arsenal of imaging modes, transmission electron microscopy is a hugely powerful tool for investigating the properties of engineering materials.
Transmission Electron Microscopy Fundamentals
Transmission electron microscopy was first developed by Max Knoll and Ernst Ruska in Berlin in the 1930s. Their 1933 microscope offered a magnification of 12,000 times and a resolution of 50nm – far better than even the best optical microscopes of the time. It didn’t take long for microscope manufacturers to catch on, and the first commercial TEMs were available only a few years later.
Transmission electron microscopy is fundamentally based on the use of electrons instead of light to probe samples. Conventional optical (“light”) microscopes are “diffraction limited” due to the relationship between the wavelength and energy of light. This puts their maximum resolution at around 200nm – any higher and the light would carry sufficiently high energy to damage the sample. 
However, electrons can feasibly achieve much shorter wavelengths without risking sample damage – so, by illuminating samples with an electron beam instead of light, transmission electron microscopy enables us to image structures thousands of times smaller than those visible through light microscopes.
Transmission electron microscopy is so-called because electrons are transmitted through a sample to form an image – this means that samples are typically very thin sections (<100nm) or films. Electrons interact with the sample on their way through, forming an image which can then be magnified and focused onto an imaging device.
Benefits of Transmission Electron Microscopy
Transmission electron microscopy is prized not only for its resolution and magnification, but for a vast range of operating modes which together offer a detailed interrogation of material properties.
Conventional TEM imaging enables the morphology of a sample to be imaged at up to 50,000,000 times magnification. Other imaging modes include Z-contrast, whereby the atomic numbers of different atoms can be seen; and crystallographic contrast, which enables visualization of crystal orientation and structure within a sample. 
Tilting the sample and collecting multiple images enables the construction of 3D images showing surface topography. 
Modified transmission electron microscopy techniques include Electron Energy Loss Spectroscopy (EELS), in which transmitted electrons are separated according to their energy. This can be used to uncover information about the manner in which electrons interact with the sample, revealing things like sample thickness, local electronic band structure and elemental composition of the sample. [7,8]
Analysis of Engineering Materials via Transmission Electron Microscopy
The unprecedented level of insight offered by transmission electron microscopy makes it highly valuable in fields such as materials science, nanotechnology and semiconductor research.
TEM has far-reaching applications in virtually any area where analysis of micro- or nanoscale morphology is important. These include imaging of nanoparticles, investigation of grain structures in ceramic materials, and characterization of semiconductor devices. 
At RQM+ Lab Services, we use TEM in conjunction with other advanced analytical techniques to provide comprehensive analysis and testing services for all industries. To find out more about our TEM expertise or any of the other services we offer, contact us today to set up a consultation.
References and Further Reading
1. Imaging dislocation cores – the way forward: Philosophical Magazine: Vol 86, No 29-31. https://www.tandfonline.com/doi/abs/10.1080/14786430600776322?journalCode=tphm20.
2. Pennycook, S. J., Varela, M., Hetherington, C. J. D. & Kirkland, A. I. Materials Advances through Aberration- Corrected Electron Microscopy. MRS BULLETIN 31, 8 (2006).
3. Introduction to Conventional Transmission Electron Microscopy – Google Books. https://www.google.co.uk/books/edition/Introduction_to_Conventional_Transmissio/77V3Dq7RgSIC?hl=en&gbpv=1&printsec=frontcover.
4. The Diffraction Barrier in Optical Microscopy. Nikon’s MicroscopyU https://www.microscopyu.com/techniques/super-resolution/the-diffraction-barrier-in-optical-microscopy.
5. Lloyd, G. E. Backscattered electron techniques. Mineralogical Magazine 3–19 (1987).
6. Ercius, P., Xin, H., Intaraprasonk, V., Muller, D. & Gignac, L. 3D Imaging of Nanostructures Using Electron Tomography, and the Impact of Aberration Correctors. 37.
7. Khanal, S. R. et al. Characterization of small-scale surface topography using transmission electron microscopy. Surf. Topogr.: Metrol. Prop. 6, 045004 (2018).
8. Brydson, R. et al. Electron energy-loss spectroscopy (EELS) and the electronic structure of titanium dioxide. Solid State Communications 64, 609–612 (1987).
9. Pennycook, S. J. & Nellist, P. D. Scanning Transmission Electron Microscopy: Imaging and Analysis. (Springer Science & Business Media, 2011).