Comprehensive Compilation and Comparison of TEM Sample Preparation Methods! (Part 1)

Transmission Electron Microscope (TEM) is one of the most widely used electron microscopes and, together with scanning electron microscopy, constitutes the "two pillars" of electron microscopy. The operating principle of TEM involves imaging and structural analysis through electrons transmitted through the sample. Due to the weak penetrating power of electrons, factors such as sample thickness, conductivity, magnetism, and dispersion directly impact the quality of the test results. Therefore, compared to the preparation of samples for scanning electron microscopy, TEM sample preparation is more complex and delicate. It should be tailored to the characteristics of different materials, employing appropriate sample preparation methods. In this article, we will introduce and compare various commonly used sample preparation methods in transmission electron microscopy.

Introduction to Sample Loading Tools

TEM commonly uses sample loading tools in the form of rods, referred to as sample rods. Different sample rods are available for various testing requirements (Figure 1). The samples are mounted on support grids, typically made of copper, and placed at the front end of the sample rod. Each sample rod can accommodate a maximum of 1-2 copper grids. The advantage of using sample rods for loading is that the tools are compact, occupying minimal space. They can be positioned in the upper half of the objective lens, contributing to an improvement in electron microscope resolution. However, the drawback is that a limited number of sample grids can be loaded simultaneously, and changing samples requires breaking the vacuum in the sample chamber, resulting in lower efficiency.

Figure 1: TEM Sample Rod

Support grids mainly include ultra-thin carbon films, microgrid copper grids, pure carbon films, double-mesh support films, etc. The materials for metal support grids vary and may include Cu (copper), Ni (nickel), Be (beryllium), nylon, among others. When choosing support grids, it is important to select a material composition different from that of the sample to be analyzed.

Figure 2:The Tip Structure of the Sample Rod and Types of Support Grids

Sample Requirements

"The transmission electron microscope (TEM) has a high magnification, a limited observation range, and high sensitivity. Therefore, in order to obtain clear, accurate, and stable signals, there are also high requirements for the samples used in TEM. TEM samples can be categorized into powder samples, thin film samples, surface replicas, and extraction replicas.

Powder Samples: Mainly used for observing the morphology of powder materials, measuring particle size, and analyzing structure and composition.
Thin Film Samples: Used to study the internal organization, structure, composition, dislocation configuration, density, and phase orientation relationships within the sample.
Surface Replicas and Extraction Replicas: Used for observing metallographic structures, fracture morphology, strain patterns, second-phase morphology, distribution, and structure.

In general, the sample requirements for TEM are as follows:

The sample should generally be a solid with a thickness less than 100 nm.
The sample should not be sucked out and attached to the pole piece under the electromagnetic field of the electron microscope.
The sample should remain stable in high vacuum.
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 principles of sample preparation for TEM are simplicity, non-destruction of the sample surface, and obtaining the largest observable thin section possible. Depending on the nature of the sample to be tested, commonly used sample preparation methods include powder sample preparation and bulk sample preparation.

Sample Preparation Methods for Powder Samples

Solution Dispersion-Drop Method

Sample Requirements: For small-sized samples such as nanomaterials or samples with thin edge thickness and low edge position thickness for microparticle samples, the solution dispersion-drop method can be directly used for the preparation of TEM samples."

Step 1: Choose a suitable support film based on the sample characteristics and testing requirements. The support film is a thin layer loaded in the middle of a copper grid, used to carry the sample. The support film typically needs to have (1) good mechanical strength, resistance to high-energy electron bombardment; (2) not to display its own structure at high magnifications, with small particle size to enhance sample resolution; (3) good chemical stability, conductivity, and thermal conductivity. Commonly used support films include microgrid films, FIB microgrid films, pure carbon microgrid films, porous carbon films, Quantifoil regular porous films, C-flat pure carbon porous support films, etc.

Step 2: Select a dispersant. Typically, anhydrous ethanol is used based on the nature of the sample.

Step 3: Disperse the powder in the dispersant using ultrasound or stirring to create a suspension. Note: The powder density in the suspension should not be too high.

Step 4: Load the sample onto the support film using either the drop or pick-up method. Drop method: Use tweezers to hold a copper grid with the support film, and drop a few drops of the suspension onto the support film. Maintain the clamping state until it dries. Pick-up method: Use tweezers to hold a retrieval grid immersed in the solution, picking up droplets. This method loads the sample on both sides of the support film.

Step 5: Once the droplets on the support film are thoroughly dried, the sample preparation is complete, and electron microscopy observation can be carried out.

Adhesive Powder Mixing Method

Sample Requirements: Generally used for magnetic powder samples and situations where the observation magnification is not high.

Step 1: Drop a solution of collodion on a clean glass slide, then add a small amount of powder on the collodion-coated slide and stir evenly. Place another glass slide on top, press the two glass slides together, and suddenly pull them apart. Wait for the film to dry.

Step 2: Use a blade to cut the film into small squares. Insert the glass slide diagonally into a beaker filled with water, and move the slide up and down. The film will gradually peel off.

Step 3: Use a copper grid to retrieve the square film, and it is ready for TEM observation.