High-Resolution Transmission Electron Microscopy (HRTEM)
Introduction
High-Resolution Transmission Electron Microscopy (HRTEM) is an advanced technique used to image the fine structure of materials with atomic-level precision. This device enables comprehensive information acquisition about the microstructure of materials, allowing the study of internal structures, identification of defects, impurities, and crystal structure. HRTEM is used to determine crystal patterns, atomic arrangements, and material composition, as well as to analyze lattice defects such as vacancies and point defects. It also allows for the calculation of the volume fraction of defects, contributing to understanding the mechanical properties of materials.
Measurement Definition
HRTEM, known as High-Resolution Transmission Electron Microscopy, is a type of transmission electron microscope used to image and study very fine structural details (typically smaller than 4–5 nanometers) with high accuracy. HRTEM stands out due to its superior resolution compared to conventional TEM. It utilizes both transmitted and scattered electrons from the sample to generate images. The resulting image is phase-contrast based and can reach resolutions as small as the unit cell dimensions.
Using HRTEM, detailed analysis of structural defects, point defects, disorder, and various types of imperfections is possible. As with basic scanning electron microscopes, sample thickness in HRTEM must be very thin (ideally 2–3 microns) to allow a significant number of electrons to pass through.
One of the major challenges in HRTEM analysis is sample preparation. Different preparation methods are required based on sample types—such as powders, bulk materials, metals, or non-metals—and some of these will be explained later. As with other electron microscopes, various detectors such as EDS and EELS can also be attached to the HRTEM for further analysis.
Benefits of HRTEM Measurement
1. Determining the pore structure and morphology of materials.
2. Studying core-shell nanomaterials.
3. High-resolution microstructural analysis, including crystal structures, symmetry, orientation, and defects.
4. Capability for chemical analysis of sample components via emitted X-rays.
5. Analysis of fine crystals and crystallographic defects.
6. Identification of material composition using electron diffraction patterns.
Types of Analyzable Samples
1. Powder samples.
2. Bulk (solid) samples.
3. Colloidal samples.
Measurement Conditions and Sample Preparation
Powder Samples:
These can be delivered to the operator in two ways:
Dry samples: A specific amount of dry powder is added to alcohol or water, then dispersed using an ultrasonic device. For imaging, a small amount of the resulting colloidal solution is dropped onto a carbon-coated grid. Once dried, the sample is ready for imaging.
Colloidal samples: For samples that require specific colloidal conditions, they must be prepared in alcohol before delivery. After receiving the samples, the operator may use ultrasonic dispersion again upon request. All colloidal preparation conditions should be clearly provided to the operator, and coordination beforehand is recommended to avoid imaging issues.
Bulk Samples:
Preparation methods vary depending on material type, shape, and composition. Preparing bulk samples for TEM is generally complex and requires special techniques. It's highly recommended to consult with the operator before sending such samples.
Typical tools used for preparation include:
Ion Milling
Jet Polishing
Dimpling
Image Analysis
HRTEM Imaging: Bright Field and Dark Field Modes
Imaging in HRTEM can be performed in two modes: bright field and dark field, each with its specific applications in fine structure analysis.
1. Bright Field:
This is the conventional imaging mode. It utilizes electrons that pass through the sample with minimal path deviation, allowing accurate determination of particle shape and size. As atomic number, thickness, and density increase, particles appear darker. The image contrast depends on these physical parameters.
However, when the sample contains multiple phases or complex compositions, distinguishing between phases can be difficult due to thickness variations. Thus, darker regions may represent either a different phase or simply increased thickness, making interpretation uncertain.
2. Dark Field:
This imaging mode is particularly useful for overcoming the limitations of bright field imaging. It relies on electrons that have undergone significant path changes. In dark field images, variations in electron scattering due to atomic number or crystal structure differences allow better distinction of phases and features.