In transmission electron microscopy (TEM) analysis, the primary and crucial step in obtaining a high-quality image that can be interpreted reasonably is sample preparation. Inappropriate sample thickness, poor conductivity, or damage introduced during sample preparation can directly lead to abnormal electron beam penetration, image distortion, and even sample scrapping.

TEM sample requirements
① The sample should generally be a solid with a thickness less than 100 nm;
② The sample will not be sucked out and attached to the pole shoe under the action of electron microscopy electromagnetic field;
③ The sample can maintain stability in high vacuum;
④ If the sample does not contain moisture or other volatile substances, it should be dried first.

TEM sample preparation method
There are roughly four types of TEM specimens used for material research:

1.Powder particles
2.Block samples, including ceramics, metals, etc
3.Biological samples
4.A replica film obtained by replicating the surface or fracture morphology of a material using a replication method.

First. Preparation of powder samples

Dispersion: Use an ultrasonic disperser to disperse the powder to be observed into a suspension in a solution (without interacting with the powder). Drip a few drops onto the electron microscope copper mesh covered with carbon support film using a dropper. After drying (or absorbing with filter paper), it becomes a powder sample for electron microscopy observation.
Points to note

① Selection of Copper Mesh

Ordinary carbon support film: suitable for low magnification observation, the carbon film lining will be more obvious at high magnification
Microgrid: The sample can be mounted at the edge of the hole, and the part inside the hole can be observed without a back bottom to improve the contrast of imaging
Ultra thin carbon film: ultra small particles such as quantum dots; two-dimensional materials
Dual network support film: used for magnetic samples to prevent them from adsorbing onto the pole shoes of transmission electron microscopy;

② Selection of solvents

Polarized samples: dispersed in water and ethanol
Non polar sample: acetone dispersion

③ Other precautions

Ultrasonic dispersion time: Ultrasonic power is more important than time, and samples should be prepared immediately after ultrasonic dispersion. For samples with severe agglomeration, solvents can be added for dispersion without damaging the sample;
Block sample: If the effect of sintering and grinding is not good, it is recommended to use the block sample preparation method;
Magnetic samples: require dual network support film or pre demagnetization.

Second. Preparation of bulk samples

The preparation of bulk samples usually involves electrolytic double spraying, ion thinning FIB、 Prepare thin film samples below 100nm using ultra-thin or frozen slicing, and then conduct testing.

① Electrolytic dual spray

The electrolytic double spray method has a simple process, easy operation, and low cost; The central thin area has a large range, making it easy for electron beams to penetrate; But it is required that the sample is conductive, and once it is made, the sample must be immediately removed and rinsed multiple times in distilled water, otherwise the electrolyte will continue to corrode the thin area, damage the sample, and even render the sample useless. If the sample cannot be observed under an electron microscope in a timely manner, it should be stored in glycerol, acetone, or anhydrous alcohol.

② Ion thinning

Principle: Ar ion beam bombards the sample at a certain inclination angle (5-30) to thin it;
Object: brittle materials such as ceramics and intermetallic compounds, which require a long time, usually around ten hours or even longer, and have low work efficiency;
Applicable conditions: The ion thinning method can be applied to various materials; The temperature is high during the thinning process and is not suitable for heat sensitive materials.

Focused lon beam (FIB) is a microscopic cutting instrument that uses an electric lens to focus an ion beam into a very small size. At present, the ion beam used in commercial systems is a liquid-phase metal ion source, and the metal material is gallium (Ga), because gallium has a low melting point, low vapor pressure, and good oxidation resistance; The use of an external electric field (Suppressor) on a liquid metal ion source can form small tips of liquid gallium, and a negative electric field (Extractor) can be used to pull the gallium at the tip, resulting in the emission of a gallium ion beam. The beam is focused by an electric lens, and the size of the ion beam can be determined by a series of aperture changes. After a second focusing, it is directed to the surface of the specimen, and physical collision is used to achieve the purpose of cutting.

④ Ultra-thin slice

Ultrathin sectioning is a section used for electron microscopy observation. Due to the low ability of electrons to penetrate tissues, ultra-thin slices (generally 80-100nm thick) are required for electron microscopy observation, mainly for the preparation of biological samples, polymer materials, micro nano particles, rubber and other materials.

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