Today, the transmission electron microscope has attracted the attention of many researchers for advanced research in various fields of science due to its high resolution and magnification. In the following article, an attempt has been made to compare two types of transmission electron microscope and transmission electron microscope with high resolution after a brief introduction, so that dear researchers can choose their type of analysis with full knowledge.

1- Introduction of TEM

By using the interaction of high-energy electrons with the sample, Transmission Electron Microscopy can create images with a resolution of one nanometer and a magnification of up to a million times. This type of microscope has the ability to take images from inside the sample, and for this reason, the microstructural features and crystal structure of the material and its crystal defects can be identified with high accuracy. The components of this device are shown in Figure 1.

Figure 1) Transmission electron microscope and its components

The main component of electron microscopes is their electron gun, which exists in two types of ion emission and field emission and produces electron beam. After forming the electron beam, electromagnetic lenses are used in the microscope chamber to focus the electron beam. Since the quality of the created image is highly dependent on the electron beam beam and its scattering should be avoided as much as possible, the vacuum pump is another important component of this microscope. The created vacuum also prevents the oxidation of the sample and contamination of the electron gun.
After hitting the sample, the electrons either leave the sample without changing their direction, or they deviate from the original path by hitting different obstacles such as lattice atoms, crystal defects, etc., which are the basis of image formation in the electron microscope. It is transitory. Different imaging modes are divided into three groups: normal imaging, dark field and bright field. In normal imaging, all transmitted rays, both deflected and undeviated, are involved in image formation. This type of imaging has a lower resolution than the other two types. Imaging in bright field mode is used only from rays that have not deviated from their path, and other rays have been removed with the help of built-in shutters. The valves are adjustable and adjusted according to the desired sample and output. The higher the density of the sample, the lower the intensity of the transmitted light and the darker the image. Dark field mode imaging is generally used when the goal is to investigate a specific phenomenon in the sample, such as a crystalline defect, a change in the composition, or to investigate a component of the composite. Unlike the bright field mode, this mode examines only the deflected rays. Figure 2 shows the transmission electron microscope image in both bright field and dark field modes

The basis of image formation in these microscopes is that the electron-sensitive screen, which is embedded in the lower part of the microscope, changes color due to the impact of passing electrons and the image is formed. Objective and eyepiece lenses are used to enlarge the image and are located behind the sample chamber. Apart from the mentioned components, it is possible to use tools for quantitative and qualitative analysis of the constituent elements of the sample along with the electron microscope.

Figure 2) Transmission electron microscope image of bright field (left) and dark field (right).

Advantages of using transmission electron microscopy

• Preparation of two-dimensional microscopic images with high magnification and resolution
• The ability to analyze crystallography of very fine material components and study crystal defects
• Determining the particle size of powders in nanometer dimensions, checking morphology
• Determining the growth direction of crystalline materials and crystal plates
• Determining the Burgers vector of dislocations and stacking defect energy
• Determining the grain boundaries
• Similarity check
• Fuzzy transformations

Limitations of transmission electron microscopy

• Hard preparation and time on sample
• Heavy and expensive equipment
• The samples must have the ability to pass electrons through them
• Examining a very small portion of the sample

2- Introduction of HR-TEM

High-Resolution Transmission Electron Microscopy is a special mode of transmission electron microscopy imaging that is capable of photographing the atomic structure of the sample. In fact, the most important difference between TEM and HRTEM is its advanced imaging system, which makes it possible to detect the properties of materials such as semiconductors, metals, and nanoparticles at the atomic scale. This type of microscope is also used to observe phase contrast in the sample.
In order to increase the resolution of the images of this type of microscope, it is necessary to remove the electrons passing through the sample that had inelastic scattering because they reduce the quality of the image. To achieve this goal, an energy filter is embedded under the sample, which allows only rays with a certain energy to pass. Figure 3 shows the image taken of tin nanoparticles using HRTEM. The magnification in these images is such that the distance between the atomic planes is also specified. By calculating the distance between the plates, one can obtain information about the atomic arrangement and the phase structure of the material.

As a result, if dear researchers are looking to observe the atomic structure of matter at very high magnifications, or if the dimensions of their particles are smaller than 5 nm, it is better to use HRTEM.

Advantages of using high-resolution transmission electron microscopy

• Very high resolution and magnification due to the small wavelength of the electron beam compared to the light beam
• The ability to analyze crystallography of very fine material components and study crystal defects

Limitations of using high-resolution transmission electron microscopy

• The possibility of damage to the sample by high-energy rays
• Dependence of resolution on quality of equipment and electron beam
• Allocating the results to a limited part of the sample