Important Knowledge for TEM Analysis

Transmission Electron Microscope (TEM)

The Transmission Electron Microscope (TEM) uses a high-energy electron beam with a very short wavelength to penetrate thin samples and obtain various types of information. As electrons pass through the sample, they collide with atoms in the material, altering their energy and movement direction. Different sample structures interact with electrons in different ways, forming specialized images on the objective lens' back focal plane or image plane, allowing researchers to study the internal structure of the sample.

History of TEM

In 1931, German physicist Ernst Ruska and his mentor developed the world's first transmission electron microscope. In 1986, Ruska was awarded the Nobel Prize in Physics along with Gerd Binnig and Heinrich Rohrer, the inventors of the scanning tunneling microscope.

Basic Structure of TEM

1. Electron Optical System: Comprising the column and sample chamber, including illumination, imaging, observation, and recording components.

2. Vacuum System: Consisting of various vacuum pumps and vacuum display systems.

3. Power Supply System: Providing electrical power, safety, and control functions.

The electron optical system is the core component that determines the microscope’s resolution. The vacuum system ensures the necessary high vacuum environment for the electron optics. The power supply system stabilizes the voltage and current of the magnetic lenses, ensuring operational precision.

Contrast Formation in TEM Images

1. Mass-Thickness Contrast

Contrast in electron microscopy refers to the variation in light intensity observed on a fluorescent screen or camera, depending on the electron intensity projected onto different regions.

For amorphous samples, the number of atoms an incident electron encounters as it passes through the material increases with the sample thickness or atomic density. The stronger the Coulomb field of the atomic nucleus, the more electrons are scattered outside the objective aperture, reducing the number of electrons contributing to the final image.

As a result, differences in thickness or density between adjacent regions of a sample produce variations in transmitted electron intensity, leading to mass-thickness contrast. This contrast is primarily determined by the number of scattered electrons and is fundamental to interpreting amorphous material images (morphology contrast).

2. Diffraction Contrast

For thin crystalline samples, when sample regions have uniform thickness and density with similar average atomic numbers, mass-thickness contrast alone may not yield satisfactory image contrast. In such cases, diffraction contrast is used.

The basis of diffraction contrast is that different microregions of a thin crystal interact with an incident electron beam at different orientation angles or have different crystalline structures. As a result, the degree to which each region satisfies Bragg’s diffraction condition varies, leading to differences in diffraction intensity.

By allowing only the transmitted beam or a specific diffracted beam to pass through the objective aperture, contrast is formed on the image plane, generating diffraction contrast.

3. Phase Contrast

For thin crystalline samples, if the objective aperture allows both the transmitted beam and one or more diffracted beams to contribute to imaging, the interference of these coherent diffraction phases produces lattice fringe images and atomic structure images.

• Lattice fringe images correspond to the projection of atomic planes in the crystal.

• Atomic structure images represent the two-dimensional projection of atomic or molecular potential fields.

Increasing the number of diffracted beams participating in imaging enhances the level of structural detail that can be observed.

Phase contrast images have higher resolution than diffraction contrast images, revealing details below 1.5 nm. Hence, phase contrast imaging is also referred to as high-resolution TEM (HRTEM), which provides crucial information on crystal structures.

4. Atomic Number (Z) Contrast

Z-contrast imaging, also known as atomic resolution Z-contrast imaging, is a type of phase contrast imaging based on incoherent phase scattering. It provides higher resolution than conventional HRTEM imaging.

Z-contrast imaging uses a fine-focused, high-energy electron beam to scan the sample surface, with a ring-shaped annular detector (STEM mode) positioned to detect only high-angle Rutherford scattered electrons while excluding the central transmitted beam. This reduces the contribution of Bragg diffraction to the image, ensuring that image brightness is proportional to the square of the atomic number (Z²). This technique is called High-Angle Annular Dark-Field (HAADF) imaging in STEM mode (STEM-HAADF).

Key characteristics of STEM (Scanning Transmission Electron Microscopy):

1. A high-brightness electron probe with a diameter of less than 0.2 nm, usually generated by a field emission electron gun.

2. Combines the functionalities of both a transmission electron microscope (TEM) and a scanning electron microscope (SEM).

Sample Preparation for TEM

1. Powder Sample Preparation

For TEM observation, powder samples must be evenly distributed on a copper grid support film. Since TEM requires electrons to pass through the sample, a specialized perforated copper grid is used to hold the material.

• A thin (20–30 nm) organic support film (Formvar film) is often pre-applied to the copper grid.

• To enhance stability and conductivity, a thin carbon layer (carbon support film) is further deposited onto the Formvar film.

• Fine powder samples are then placed on the carbon support film without blocking the grid holes.

2. Thin Film Sample Preparation

Thin film samples can be prepared using techniques such as vacuum sputtering, magnetron sputtering, solution solidification, electrospinning, and ultrathin sectioning.

If a film can be directly deposited on a bare copper grid or carbon support film, no additional preparation is required.

If direct deposition is not feasible, the film can be floated on a liquid surface and then transferred to a copper grid for TEM observation.

General requirements for thin film samples:

• The film must be thin (< 200 nm) to allow electron penetration.

• The film must be firmly attached to the copper grid.

• The film must be firmly attached to the copper grid.

3. Bulk Metal and Inorganic Nonmetallic Sample Preparation

For bulk materials, samples must be thinned to below 200 nm using methods such as mechanical thinning, ion milling, or electrochemical polishing.

• Electropolishing is crucial for preparing metallic thin films and requires optimization of electrolyte composition, voltage, current, and temperature.

• Ion milling is particularly suitable for ceramics, silicon, oxides, and carbides.

4. Cross-Sectional Sample Preparation

For multilayer films deposited on substrates, cross-sectional TEM (XTEM) samples are challenging to prepare. The process involves:

1. Cutting the sample into a semi-cylindrical shape and embedding it in an epoxy resin.

2. Thinning and polishing the sample down to 50–100 μm.

3. Ion milling the sample to a final thickness of 50–500 nm.

5. Focused Ion Beam (FIB) Sample Preparation

FIB uses a high-energy ion beam to etch and extract specific microregions for TEM observation. It enables rapid preparation of site-specific TEM samples, particularly for semiconductor devices.

6. Polymer and Biological Sample Preparation

• Polymers are sectioned into thin films (< 100 nm) using an ultramicrotome with a glass or diamond knife.

• Biological tissues require fixation, dehydration, embedding, ultrathin sectioning (60–100 nm), and heavy metal staining to enhance electron scattering contrast.

7. TEM-EDS (Energy Dispersive X-ray Spectroscopy)

TEM can be equipped with an EDS detector to perform elemental composition analysis. EDS detects characteristic X-rays emitted from the sample upon electron beam interaction, allowing for both qualitative and quantitative microanalysis of elemental distribution.