A short introduction to electron microscopy
It was not until the early 1920s that Ernst Ruska and Max Knoll developed the first electron microscope; an invention for which Ruska received the Nobel Prize for Physics in 1986. Almost 40 years later, Richard Feynman gave his famous lecture titled “There is Plenty of Room at the Bottom” (Caltech, December 29th 1959). Many consider this talk the birthplace of modern nanotechnology and a huge push for electron microscopy. Early observations using electron microscopy were dominated by physicists and biologists.
However, at about the same time of Feynman’s lecture, geoscientists, and in particular mineralogists, started to use transmission electron microscopes to study Earth materials. Some early work on minerals included mica (Amelinckx, 1952), clay minerals (Honjo and Mihama, 1954) and feldspar (1963). In 1965 McLaren and Phakey at the University of Cambridge published the first series of papers on the deformation behaviour of quartz using transmission electron microscopy.
In the 1950s, electron microprobe analysis began developing rapidly. The first commercial scanning electron microscope and electron microprobe were available in the 1960s. In 1973 John Venables collected the first electron backscatter diffraction pattern in a scanning electron microscope, although it wasn’t until 1994 that the first, automated electron backscatter diffraction (EBSD) map was published (Kunze, 1994), initiating a new era in analysing deformed Earth materials.
In the early 2000s, electron microscopy was again revolutionised with the commercial introduction of aberration-corrected lenses. This technological breakthrough allowed, among many other things, the detailed study of graphene. Although aberration-corrected TEM is still under-utilised in Earth sciences, numerous studies have shown its potential (e.g., Marquardt et al. 2011). One of the biggest breakthroughs for Earth-material research came with the commercialisation of focused ion beam scanning electron microscopy. This allowed the extraction of site-specific samples from complex materials and revolutionised sample preparation of ultra-thin transmission electron microscopy samples to investigate the multiscale microstructural phenomena.
The latest developments include automated mineralogy, high-resolution EBSD for strain mapping and big data-driven machine-learning approaches to quantifying the complexity of Earth materials.
Electron microscopy is a cornerstone of modern Earth-materials research and, to close with the words of Richard Feynman, “There is plenty of room at the bottom”; also for the understanding of Earth materials.
How it works
Electron microscopy is an analytical technique that uses an incident electron beam as a source of “illumination”. The electron beam exhibits a shorter wavelength, thus providing a higher resolution compared to a visible light microscope. The instrument consists of a high-energy electron emitter, with an acceleration voltage ranging between 1 and 30 kV. A series of Electromagnetic lenses focus the electron beam and control magnification. The electrons interact with the sample that is mounted on a stage capable of three-dimensional movement in a vacuum chamber capable of pressures below 10-7 Pa. The electrons interact with the sample and produce various signals to be detected, each providing different information of the sample. The most commonly used signals for the analysis of geological materials include Secondary Electrons (SE), Backscattered Electrons (BSE), Cathodoluminescence (CL), Energy Dispersive X-ray spectroscopy (EDX/EDS), and Electron Backscattered Diffraction (EBSD) (Egerton, 2005).
- SEs originate from inelastic interaction between the primary electron beam and the electrons from the surface, or the near surface regions. The ejected SEs are detected by an Everhart–Thornley detector, which is the most commonly used SE detector. SEs are very useful for providing topographic information of the sample due to edge effects, where more detected electrons seem to leave the sample at and edge than at a surface.
- BSE, as the name suggests, scatters back the incident electrons. Under the right conditions, the incident electrons interact with the nucleus of the atoms in the sample and their trajectories deviate. Heavier elements with larger nuclei can deflect electrons more strongly, making them appear brighter in images. BSE is useful for detecting chemical variation in your sample.
- CL is an optic electromagnetic effect where photons are released after excitation of defect centres in the sample. CL is an effective method to analyse the chemical and structural variations of the material.
- EDX/EDS relies on incident electrons to remove inner shell electrons. The removal of an electron causes a charge imbalance, which is filled in by an outer shell electron. The jump from outer to inner shell is more energetically favourable and goes paired with the emission of characteristic X-rays. These characteristic X-rays can be used to determine major element composition, based on the unique energy peaks in the X-ray spectra.
- EBSD also relies on the backscattering of electrons mentioned before. As backscattered electrons leave the sample, they are elastically diffracted and lose energy as they interact with the surrounding atoms. The diffracted electrons leave the sample at specific scattering angles, related to the crystal’s internal structure. These scattering angels can be measured and indexed, providing information about the material’s crystal structure, crystallographic orientation and phase. EBSD can be used to investigate plastic deformation, average misorientation, grain size, and crystallographic texture.
References
Amelinckx, S. (1952). Screw dislocations in mica. Nature, 169(4301), 580-580.
Egerton, R. F. (2005). Physical principles of electron microscopy (Vol. 56). New York: Springer.
Honjo, G., & Mihama, K. (1954). A study of clay minerals by electron-diffraction diagrams due to individual crystallites. Acta Crystallographica, 7(6-7), 511-513.
Kunze, K. ; Heidelbach, Florian ; Wenk, H.-R. ; Adams, B. L.: Orientation imaging microscopy of calcite rocks. In: Bunge, Hans-Joachim (ed.): Textures of Geological Materials. – Oberursel : DGM-Informationsgesellschaft , 1994 . – pp. 127-146.
Marquardt, K., Ramasse, Q. M., Kisielowski, C., & Wirth, R. (2011). Diffusion in yttrium aluminium garnet at the nanometer-scale: Insight into the effective grain boundary width. American Mineralogist, 96(10), 1521-1529.
McLaren, A. C., & Phakey, P. P. (1965). Dislocations in quartz observed by transmission electron microscopy. Journal of Applied Physics, 36(10), 3244-3246.
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