Summary and Comparison of Sample preparation methods for TEM Transmission Electron Microscopy testing

Focused ion beam function

The Transmission electron microscope (TEM) is one of the most widely used electron microscopes and, together with scanning electron microscopy technology, constitutes the "two pillars" of electron microscopy. The testing principle of TEM is to use electrons passing through the sample for imaging and structural analysis. Due to the weak penetration ability of electrons, the characteristics of the sample such as thickness, conductivity, magnetism and dispersibility have a direct impact on the quality of the test results. Therefore, compared with the sample preparation for scanning electron microscopy, the sample preparation for transmission electron microscopy is more complex and precise. For different materials, appropriate sample preparation methods should be adopted based on their characteristics. In this article, we will introduce and compare various sample preparation methods commonly used in transmission electron microscopes for everyone.

Introduction to sample loading tools

The commonly used sample loading tool for TEM is made into a rod shape and is called a sample rod. Different sample rods are available for different testing requirements (Figure 1). The sample is carried in a support mesh (usually a copper mesh) and placed at the front end of the sample rod. Each sample rod can hold a maximum of 1 to 2 copper meshes. The advantage of using a sample rod to load samples lies in the fact that the sample loading tool is small in size and occupies less space. It can be set at the upper half of the objective lens, which is conducive to improving the resolution of the electron microscope. The drawback is that too many sample meshes cannot be placed simultaneously. When changing samples, the vacuum in the sample chamber must be disrupted once, which is relatively inefficient.

Figure 1:TEM sample rod

Support nets mainly include ultra-thin carbon films, micro-grid copper nets, pure carbon films, and double-load mesh support films, etc. There are various materials for metal support nets, such as Cu, Ni, Be, nylon, etc. When choosing, it is necessary to pay attention to selecting support nets with components different from the samples to be analyzed.

Figure 2:Sample support net

Sample Requirements

The magnification of TEM is high, its observation range is limited, and its sensitivity is high. Therefore, in order to obtain clear, accurate and stable signals, the requirements for TEM samples are also very high. The sample types of TEM are classified into powder samples, thin film samples, surface replicas and extraction replicas. Style: (1) Powder pattern: Mainly used for the morphology observation of powder materials, the determination of particle size, and the analysis of structure and composition, etc.(2) Thin-film specimens: The internal tissue, structure, composition, dislocation configuration and density, phase orientation relationship, etc. of the sample. (3) The surface replica and extraction replica: microstructure observation and fracture morphology, deformation stripes, the second phase morphology, distribution and structure, etc. In general, the TEM sample request is as follows: (1) The sample should be general thickness of less than 100 nm solid; (2) Samples in electron microscopy will not be sucked out, under the influence of electromagnetic field on pole shoe; (3) The sample can remain stable in high vacuum. (4) It should not contain moisture or other volatile substances. If the sample contains moisture or other volatile substances, it should be dried first.

Sample Preparation Method

The sample preparation principle of TEM is: simplicity, no damage to the sample surface, and obtaining the largest possible observable thin area. According to the different properties of the samples to be tested, the commonly used sample preparation methods are divided into powder sample preparation method and block sample preparation method.

The sample preparation method for powder samples:

Dispersion, dripping solution method of sample request: for nanoparticles materials such as small size of the sample or the edge of the micron particle samples with thinner thickness and position of the lower edge, the thickness of the dispersion, dripping solution method directly can be used for TEM sample preparation.

Step 1: Select the appropriate support film based on the characteristics of the sample and the testing requirements. The supporting film is a thin film loaded in the middle of the copper mesh and can be used to carry samples. The supporting film usually needs to have quite good mechanical strength and be resistant to high-energy electron bombardment; It should not display its own tissue at high magnification and its own particle size should be small to improve the resolution of the sample. It has good chemical stability, electrical conductivity and thermal conductivity. The commonly used supporting membranes mainly include microgrid membranes, FIB microgrid membranes, pure carbon microgrid membranes, porous carbon membranes, Quantifoil regular porous membranes, C-flat pure carbon porous supporting membranes, etc.

Step 2: Select the dispersant. The choice is based on the nature of the sample, and anhydrous ethanol is usually adopted.

Step 3: Use ultrasonic waves or stirring to disperse the powder in the dispersant to form a suspension. Note: The powder density in the suspension should not be too high.

Step 4: Load the sample onto the support membrane. There are two methods: dropping and scooping. Drops of sample: use tweezers clamping covered with copper network support membrane, dropping a few drops suspending liquid dropper on the support membrane, keep the clamping state to dry; Make: use tweezers clamping load network make droplets immersed in solution, this method can be in samples of support membrane on both sides of the load. Step 5: to support the droplet fully after drying can be completed on the membranes of the sample preparation, then can be electron microscope.

Rubber powder mixing method:Sample requirements: Generally used for magnetic powder samples and when the observation magnification is not high. Step 1: Drop the collodion solution on a clean glass plate, then put a little powder on the collodion solution of the glass plate and stir well. Press on another glass plate, grind the two glass plates together and suddenly pull them apart. Wait for the membrane to dry. Step 2: Use a blade to cut the diaphragm into small squares. Insert the glass sheet diagonally into the water cup and insert it up and down on the water surface. The diaphragm will gradually fall off. Step 3: The square membrane can be scooped out with a copper mesh and then used for TEM observation.