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Methods of Preparation for the Microscope TEM

Transmission electron microscopy (TEM) is a technique used to examine and characterize materials at very high resolutions. A beam of high energy electrons is aimed at the sample. Some of these electrons are transmitted through the sample and are focused onto a detector where an image is recorded. Other signals can be recorded, for example from electrons that are not transmitted through the sample and scattered back towards the beam source. This provides additional information that helps researchers characterize materials. High-quality images require samples to be very carefully prepared using the appropriate technique.

Sample Thickness
- The type of TEM image you want to record dictates the manner in which you must prepare your sample. However, in all cases, the sample must be thin enough to allow electrons to pass through the sample and form an image on the detector. Samples are typically 10 to 300 nanometers (nm) thick. When studying elemental composition, thinner samples are typically used. For routine imaging, a compromise between image resolution and sample stability is required, and a medium thickness of around 100 nm is used. Samples around 300-nm thick are used to record information about the shape of the sample's surface.
Powdered Samples
- Powdered samples are the easiest and fastest type of sample to prepare for TEM imaging. If the sample is not already powdered, it is crushed and ground into a fine powder -- for example, using a pestle and mortar -- or if the sample consists of nanoparticles, then grinding is not necessary. The powder is then dissolved in a relatively volatile solvent to give a solution of the sample. This is then dropped onto a circular copper grid 3 mm in diameter using a Pasteur pipette. The solvent is left to evaporate leaving the sample on the grid. The copper grid is mounted on a sample holder and inserted into the TEM.
Mechanical Polishing
- Sometimes it is not possible to grind up your sample and dissolve it in a solution -- for example, when you want to look at a specific region of the sample. Instead, you can machine a piece of your sample and then cut it into slices. These slices are then mounted in a sample holder and mechanically thinned to the required thickness. Mechanical thinning is achieved by grinding the mounted sample to the required thickness using a dimple wheel or planar grinder. The former has the added benefit of making the sample's edges thicker than the center and prevents the sample from buckling.
Electropolishing and Ion-Beam Milling
- Two more advanced techniques are called electropolishing and ion-beam milling. Electropolishing uses the sample as an electrode in an electrochemical cell. The oppositely charged electrode acts as an "electrolyte jet" and coats the sample to the required thickness. This technique has the advantage of enabling you to coat the sample with an additional layer, such as a carbon, to prevent accumulation of charge during imaging and improve the quality of the image recorded. Ion-beam milling blasts atoms from the surface of the sample until it is of the required thickness using an intense beam of heavy ions.

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