Electron microscopy techniques are extensively employed in material science for their excellent spatial resolutions and the variety of imaging modalities available, including 3D image reconstruction.1,2
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In transmission electron microscopy (TEM), where the electron beam passes through the sample to be directly imaged on the detector below, it is often necessary to support the thin samples on a grid.
In many ways, an electron microscopy grid serves the same purpose as a microscope slide in standard optical microscopy techniques, providing support for the sample while not interfering with the imaging process.
Most TEM grids are round or square and have a mesh-like structure, allowing electrons to pass through the holes.
Additional support may be used on top of the grid for very small samples, especially if the holes are large enough to allow the sample to fall through. This approach is also important for more flexible samples, as a lack of support might induce additional strain on the material.
When choosing an electron microscopy grid, consideration must be given to whether the material is ferromagnetic and could potentially distort the electric fields used to focus the electron beam, ensuring that the grid will not interfere with the imaging process or obscure any part of the sample of interest.
Materials Used in Electron Microscopy Grids
Common materials for TEM grids include copper and silicon nitride.3,4 Copper is non-ferromagnetic, so it does not distort the fields associated with the objective lens. It can also support a variety of materials, including polymers.
However, the reactivity of copper can be problematic for some samples, so it may be better to use a more inert material.
Silicon nitride is highly versatile for both material and biological samples.4,5 One advantage of silicon nitride membranes or windows is that they are sufficiently robust. This means that direct write techniques can be used to deposit substrates, or cellular material can be grown directly on the substrate.
Liquid phase TEM measurements often employ silicon nitride windows to contain the sample within the electron microscope.6 For such measurements, the sample cell needs to be vacuum-tight, and a small spacer is placed between the two windows to give the cell sufficient volume for sample storage.
Additional silicone supports are sometimes required as the assemblies can be very fragile.6
Where reactivity is a concern, inert materials like platinum or rhodium may be a good choice. Along with the more traditional gridded mesh designs, some materials like gold and platinum are also available on ‘holey’ films, which can be useful for performing resolution checks in the experiment.7
Holey or lacey films, sometimes placed on top of more rigid grids, provide additional sample support on the grid. Carbon or graphene are also popular materials for such films.
Manufacturing Processes for Grids
Traditional metal grids for TEM are usually made using electroforming processes, as the grid must be highly regular in structure. Electroforming is a process that uses electrodeposition to grow metal parts into a mandrel, or model, to achieve a particular shape.
Other materials, such as aluminum, stainless steel, titanium, and molybdenum, are typically formed into grids through an etching process.
An etching process with micron-level accuracy or better is necessary for making good quality grids, and focused ion beam (FIB) methods can be used to both etch or clean the grid, as well as make any necessary sample preparation steps before measurement.8
Applications and Importance of Grids in Electron Microscopy
Support materials and grids are essential for any electron microscopy measurement. This is because TEM samples are usually too thin and fragile to be handled easily without support.
Sample preparation is vital for obtaining high-quality images in TEM measurements and avoiding issues with thicker sample regions that lead to poor electron beam transmission, resulting in low signal levels and noisy images.9 The wrong choice of materials can also lead to distortions of the electron beam.
The grid materials need to be considered carefully to avoid unwanted interactions or reactions between the substrate and the grid, which may affect the sample morphology and, therefore, the representativeness of the measurement under standard conditions.
In many fields of electron microscopy, including cryogenic electron microscopy (cryo-EM), finding improved workflows for growing samples directly onto supports or grids is now an integral part of sample preparation.10 Techniques like micropatterning can help with this.
Grids and foils can also be marked, or the samples can be deposited in specific patterns to help users locate the sample. While TEM provides excellent spatial resolution, scanning and locating the sample of interest can be difficult and time-consuming—grids can serve as a guide for this.
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References and Further Reading
- Wang, ZL. (2003). New Developments in Transmission Electron Microscopy for Nanotechnology. Advanced Materials. doi.org/10.1002/adma.200300384
- Watanabe, M. (2013). Microscopy Hacks: Development of various techniques to assist quantitative nanoanalysis and advanced electron microscopy. Journal of Electron Microscopy. doi.org/10.1093/jmicro/dfs085
- Sharma, V., Sundaramurthy, A. (2022). Surface modification of bare copper grids with charged polymers: A simple alternative for carbon-coated copper grids in transmission electron microscopy. Materials Letters. doi.org/10.1016/j.matlet.2021.131204
- Wang, H., Luo, J., Schäffel, F., Rümmeli, MH., Briggs, GAD., Warner, JH. (2011). Carbon nanotube nanoelectronic devices compatible with transmission electron microscopy. Nanotechnology, 22(24). doi.org/10.1088/0957-4484/22/24/245305
- Ring, EA., Peckys, DB., Dukes, MJ., Baudoin, JP., De Jonge, N. (2011). Silicon nitride windows for electron microscopy of whole cells. Journal of Microscopy. doi.org/10.1111/j.1365-2818.2011.03501.x
- Ross, FM. (2015). Opportunities and challenges in liquid cell electron microscopy. Science. doi.org/10.1126/science.aaa9886
- Yoshida, H., Omote, H., Takeda, S. (2014). Oxidation and reduction processes of platinum nanoparticles observed at the atomic scale by environmental transmission electron microscopy. Nanoscale. doi.org/10.1039/c4nr04352a
- Li, J., Malis, T., Dionne, S. (2006). Recent advances in FIB-TEM specimen preparation techniques. Materials Characterization. doi.org/10.1016/j.matchar.2005.12.007
- Mayer, J., Giannuzzi, LA., Kamino, T., Michael, J. (2007). TEM Sample Preparation and FIB-Induced Damage. MRS Bulletin. doi.org/10.1557/mrs2007.63
- Sibert, BS., Kim, JY., Yang, JE., Wright, ER. (2021). Micropatterning transmission electron microscopy grids to direct cell positioning within whole-cell cryo-electron tomography workflows. Journal of Visualized Experiments. doi.org/10.3791/62992
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