Keywords: transmission electron microscope TEM、 Imaging principle, sample preparation, high-resolution imaging

As the "perspective eye" of the microscopic world, transmission electron microscopy (TEM) has become an indispensable tool in the fields of materials science, life sciences, and nanotechnology since its invention by Ernst Ruska in 1932. The core principle is based on electron beam replacing visible light and achieving nanoscale resolution imaging through an electromagnetic lens system. This article will analyze the basic structure, imaging principle, sample preparation technology, and cutting-edge applications of TEM.

1、 The core structure and imaging principle of TEM

The TEM system consists of an electron gun, a condenser lens, an objective lens, an intermediate lens, a projection lens, and a fluorescent screen. The electron beam emitted by the electron gun is accelerated by an accelerating voltage and focused into a parallel light source through a condenser lens, forming a transmitted electron signal after penetrating the ultra-thin sample. The objective lens completes one magnification, and the intermediate mirror and projection mirror relay magnification, ultimately presenting the image on a fluorescent screen or CCD camera. Its resolution can reach 0.2 nanometers, far exceeding the 200 nanometer limit of optical microscopes.

During the imaging process, the interaction between the electron beam and the sample generates transmission, scattering, and diffraction signals. High resolution TEM (HRTEM) directly observes atomic arrangement through lattice fringe images and structural images, while electron diffraction modes (such as selected area diffraction SAD) are used to analyze crystal structure. For example, in semiconductor material research, HRTEM can analyze lattice defects and interface structures, providing a basis for optimizing device performance.

2、 Sample preparation technology: key to nanoscale imaging

The quality of TEM imaging is highly dependent on the thickness of the sample. The material film samples need to be prepared to a thickness of 10-100 nanometers using an ion thinning instrument or ultra-thin slicing mechanism; Biological samples require negative staining or freezing fixation techniques. For example, TEM observation of virus particles requires embedding the sample in resin and ultra-thin slicing, while imaging of metal nanoparticles relies on the preparation of thin regions using an electrolytic dual spray apparatus.

During the sample preparation process, it is necessary to avoid contamination and mechanical damage. The ion thinning instrument uses a low-energy ion beam to bombard the surface of the sample, gradually thinning it to a thickness that can be penetrated by an electron beam; Cryofixation technology uses liquid nitrogen to rapidly freeze samples, preserving the natural conformation of biomolecules.

3、 Frontier Applications: From Materials Science to Life Sciences

In materials science, TEM is used to analyze the phase transition process of battery electrode materials. For example, during the charging and discharging process of lithium-ion battery cathode materials, HRTEM can observe dynamic changes in lattice parameters, revealing the mechanism of capacity decay.

In the field of life science, the three-dimensional structure of S protein of COVID-19 was analyzed by TEM and freeze electron microscopy. Through the single particle reconstruction algorithm, scientists have reconstructed atomic models with a resolution of 3.5 angstroms from tens of thousands of two-dimensional projections, providing key targets for vaccine design.

4、 Technological Challenges and Future Trends

Although TEM technology has matured, it still faces challenges such as complex sample preparation and electron beam damage. Future development directions include:

In situ TEM technology: Integrating heating and stretching devices inside the electron microscope cavity to observe the phase transition behavior of materials in real-time under extreme conditions.

Spherical aberration correction technology: By introducing a multipole corrector, the resolution is improved to sub angstroms, achieving direct imaging of individual atoms.

Artificial intelligence assisted analysis: using deep learning algorithms to automatically identify lattice defects and phase information, improving data analysis efficiency.