Transmission Electron Microscope
Introduction
Transmission Electron Microscopy (TEM) is one of the most important methods for studying materials, especially at the nanoscale, due to its exceptional ability to image nanoparticles. TEM analysis allows the study of fine structure, observation of nanoparticles, and examination of internal structures such as core-shell formations. A critical aspect of nanoparticle analysis is determining their size, and TEM is an effective method for this, providing both qualitative and quantitative information.
Definition and Principle
TEM is a type of imaging technology that relies on electrons transmitted through a studied sample. These transmitted electrons are the basis of modern TEM design. Today’s best TEMs can produce images with magnifications ranging from 2,000x to 50 millionx and resolutions up to 50 picometers. These microscopes are among the leading tools for studying the fine structures of various materials, especially nanomaterials.
Understanding the operation of TEMs requires knowledge of how transmitted electrons interact with a sample. The type of interacting electrons and how they are detected determine the imaging mode. TEM involves three categories of electrons: unscattered electrons, low-angle scattered electrons, and high-angle scattered electrons. Traditional electron detectors in TEM use a phosphorescent screen, which lights up upon electron impact, with brightness proportional to the number of electrons. A lack of electrons results in screen darkness.
Transmitted electrons are classified by their deviation angle from the initial beam direction. Less than 0.573° is considered unscattered; between 0.573° and 2.865° is low-angle scattering; anything greater is high-angle scattering. Each category has specific detectors: the bright-field (BF) detector for unscattered electrons, and the high-angle annular dark-field (HAADF) detector for high-angle electrons.
In TEMs, the electron gun is placed at the top, and the rest of the components align along the electron beam path—this setup is called the “electron column.” Modern TEMs feature diverse and complex components, enabling extremely high magnification and atomic-level resolution.
During operation, the entire column must be under vacuum, with vacuum levels depending on the electron gun type. The electron beam is typically accelerated between 80 and 300 kV. For samples composed of light or beam-sensitive elements, voltages under 100 kV are preferred, though this increases electron wavelength and may reduce image quality.
Condenser lenses narrow the electron beam diameter and help focus it on the sample. The condenser aperture removes unfocused parts of the beam. After interacting with and passing through the sample, the beam is refocused by the objective lens to form a primary image.
Next, the objective aperture filters parts of the transmitted electrons depending on the desired imaging mode. The final magnified image is formed by imaging lenses and sent to detectors, which transmit the data to a computer to generate the final image.
TEM Imaging Modes
When a mix of all three types of electrons reaches the screen, it results in a blurry image. The objective aperture helps separate them. In general, TEM imaging is categorized into two main types:
Bright-field (BF) Imaging: When the objective aperture blocks scattered electrons, a bright background appears. Regions in the sample that scatter electrons show up as dark spots due to blocking of unscattered electrons—these are often thicker or denser areas.
Dark-field (DF) Imaging: When the objective aperture blocks unscattered electrons and allows low-angle scattered ones to pass, the resulting image has a dark background. Scattering regions appear bright. This is known as dark-field imaging.
These modes help analyze the internal structure of the sample through electron interaction behavior.
Benefits of TEM
TEM is vital for R&D across many fields, providing insights that enhance materials and processes.
Ultra-high resolution imaging at the atomic level, enabling nanoscale structural analysis.
Ability to analyze chemical composition using integrated techniques like EDS (Energy Dispersive Spectroscopy).
Detailed information on crystal structures and spatial distribution of defects.
Real-time monitoring of changes during various processes, revealing kinetics and microscopic behavior.
Ideal for studying nanomaterials due to its high-resolution capabilities.
Can observe dynamic processes like chemical reactions or physical changes.
Helps detect and understand defects such as cracks and distortions and their impact on properties.
Enables analysis of interactions between different materials at the atomic level.
Sample Types
TEM analysis can be performed on all types of powder, solution, suspension, polymer, and biological samples.
Measurement Conditions
Consider the following when submitting samples:
Minimum sample weight: At least 5 mg per sample.
Dispersion solvent: If the dispersion medium is not water or ethanol, provide 5 mL of the used solvent.
Minimum visible particle size: About 5 nanometers.
Imaging types: Standard TEM is for imaging only. For diffraction patterns or other analyses, HR-TEM is needed.
Biological samples: Require special preparation using an ultramicrotome to obtain measurable slices.
Result Interpretation
Clear Image: Indicates the sample is thin enough for electrons to pass, revealing fine internal structure.
Contrast: Depends on atomic density—denser areas appear darker; less dense, lighter.
Nanodimension Analysis: TEM can accurately determine size and shape of nanoparticles, aiding in property evaluation.
Phase Identification: Images may reveal different phases; structural variations help in identifying them.
Selected Area Electron Diffraction (SAED): Provides crystallographic structure information, identifying crystal directions.
Chemical Composition: Using EDX, elements and their concentrations can be measured.
Structural Analysis: TEM can detect atomic arrangement, crystal defects, and impurities—key to understanding physical and chemical properties.
It is important for the analyst to be knowledgeable about how to interpret results and handle any variations or deviations that may arise. Valuable another information that can lead to advancements in research and development can be obtained by communicating with experts at the Photon Center.