Transmission electron microscopy is a microscopy technique which used a beam of highly accelerated electron to transmit through a very thin sample (< 100 nm) to form an image. The image formed based on the different interactions between the sample and the electron as the beam is transmitted through the sample which is represented in Figure 1a and 1b. If the sample is crystalline, a part the electron beam undergoes diffraction by the atoms and the rest can pass through. By collecting either of the transmitted beam or the diffracted beam imaging in different modes can be done such as Bright field and Dark field mode as shown in Figure 1.

Different modes of imaging in TEM.

In the Figure 2, images corresponding to different modes are represented. In the bright field image (Figure 2b), transmitted electron beam is selected with the aperture, and the scattered electrons are blocked. As the transmitted electron beams are selected, the areas with higher mass appears with dark contrast whereas the areas with lower mass appear with bright contrast. As can be seen from the Figure 1b, which is a selected area diffraction pattern (SADP), the bright most spot in the middle represents to the transmitted beam, whereas the weak spots are represents the planes in the sample from where the electron beams are diffracted. When the aperture is placed in the transmitted beam by tilting the beam itself, imaging can be done from the regions from where the beams are transmitted through. When, the imaging is done by placing the aperture over the diffracted spots just adjacent to the transmitted beam spot, only the diffracted beam is allowed through the aperture. Hence, the areas where there are no electron scattering and (e.g, the areas around the sample) will be black, while the areas with materials will appear bright (Figure 2c). This is referred to as dark field imaging mode. In the weak beam dark field imaging, the specimen is tilted to excite higher angle diffraction spots, and the aperture is placed on weak diffraction spots (preferably 3rd spot).

Figure 2. (a) Selected area diffraction pattern (SADP) from a T_iB₂ grain (b) Bright field-TEM image of T_iB₂ (c) Dark field-TEM image of (a) (d) Weak beam dark field-TEM image of (a).

In the crystalline materials, specific diffraction angles are present per the Bragg’s law, thus the interference between the electron beams which are diffracted from different planes (which satisfy the Bragg’s condition), generates a periodic pattern as per the sample’s atomic structure as shown in Figure 4.

Weak Beam Dark Field (WBDF):
(WBDF) is a technique used to enhance the visibility of lattice defects like dislocations, stacking faults, and precipitates. The image contrast in WBDF is governed by the diffraction vector (g). A higher-order reflection (e.g., 3g) is selected to make the image more sensitive to strain fields and local distortions. By tilting the electron beam slightly off the exact Bragg condition, WBDF improves spatial resolution and contrast, making it highly sensitive to strain fields and local distortions. The weak beam condition is achieved by tilting the electron beam slightly off the exact Bragg condition and selecting a high-order diffraction reflection (e.g., g + 3g). This enhances the contrast of lattice distortions. The technique relies on the dynamical diffraction theory, where only weakly scattered electrons contribute to the image, reducing background noise and enhancing defect contrast. The g·b = 0 (where b is the burgers vector) invisibility criterion determines whether a defect is visible in the WBDF image, making it useful for analyzing dislocation structures, stacking faults, and strain fields in materials like semiconductors, ceramics, and alloys.

zone-axis: The zone axis is a specific crystallographic direction in a sample through which the electron beam is aligned, allowing multiple lattice planes to satisfy the Bragg condition simultaneously. When the electron beam is oriented along a zone axis, it produces a selected area diffraction pattern (SADP), which consists of a regular array of diffraction spots corresponding to the crystallographic planes. SADP is obtained by inserting a selected area aperture in the TEM column to restrict the diffraction signal to a specific region of interest in the sample. By analyzing the diffraction pattern, one can determine the crystal structure, orientation, and possible defects in the material. The presence of systematic absences, extra reflections, or splitting of spots in SADP provides insights into symmetry, twinning, and strain effects in the sample.

Figure 3 High-angle annular dark field image of NbTiCrZrB₂-SiC composite showing different phase contrast where the NbTiCrZrB₂¬ phase appears with bright contrast and the SiC phase appears with dark contrast. Correspondingly, the elemental maps shows the distribution of different elements present in different phases.

High-resolution TEM (HR-TEM) is a phase-contrast imaging technique that allows images to be captured with near-atomic resolution, allowing for the study of crystallinity, lattice planes, crystal phases, and defects. The high-resolution transmission electron microscopy (HRTEM) uses both the transmitted and the scattered beams to create an interference image. In the crystalline materials, specific diffraction angles are present per the Bragg’s law, thus the interference between the electron beams which are diffracted from different planes (which satisfy the Bragg’s condition), generates a periodic pattern as per the sample’s atomic structure.