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Basic Principles and Applications of STEM (Scanning Transmission Electron Microscopy)
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- Universallab
- @universallab
Introduction
Throughout history, humanity has never ceased to explore the microscopic world. Electron microscopes, using electron beams with shorter wavelengths as light sources, have surpassed the diffraction limits of visible light. This breakthrough has enabled the observation of the nano-world, allowing researchers to study reactions at the scale of single atoms and molecules.
Scanning Transmission Electron Microscopy (STEM) is a development of Transmission Electron Microscopy (TEM). In recent years, with the introduction of aberration correctors, STEM has achieved sub-angstrom spatial resolution, making it possible to image individual atomic columns. Coupled with energy loss spectroscopy with sub-electron volt energy resolution, STEM facilitates characterization and analysis of material microstructures and fine chemical compositions at nano and atomic scales. It holds immense application potential in fields such as metallurgy, materials science, environmental science, and biology.
Development History of STEM
- In 1924, Louis de Broglie proposed the hypothesis of matter waves.
- In 1927, Davisson experimentally confirmed the wave-particle duality of electrons through electron diffraction, laying the theoretical foundation for electron microscopy.
- In 1938, Manfred von Ardenne, an employee of Siemens, built the first STEM instrument.
- In the 1970s, Albert Crewe and colleagues at the University of Chicago developed the cold field emission electron gun used in STEM, a precursor to modern STEM instruments.
- In 1973, Humphreys and others introduced the High Angle Annular Dark Field (HAADF) detector, which revealed that in high-angle detection mode, image contrast is proportional to the square of the atomic number (Z), leading to the term Z-contrast image.
- In 1988, Pennycook and colleagues at Oak Ridge National Laboratory first achieved high-resolution HAADF imaging of YBa2Cu3O7-x and ErBa2Cu3O7-x, marking the true capability of STEM for atomic resolution imaging.
- In 2003, Baston introduced aberration correctors into STEM instruments, focusing the imaging electron beam to a size of 0.078 nm, further enhancing the imaging quality of STEM.
STEM Working Principles and Structure
1. STEM Working Principles
The operating principles of STEM are illustrated in Figure 2. The electron beam generated by a field emission electron gun is converged into a nano-sized probe after passing through a complex condenser system. This highly focused electron probe scans the sample in a raster pattern under the control of scanning coils. Simultaneously, a high-angle annular dark field (HAADF) detector positioned beneath the sample collects scattered electrons of various angles generated by the interaction of the electron probe with the sample. These scattered electrons are then collected, converted into signals, and used to form images based on their scattering angles.
During this process, point-by-point scanning and imaging are performed. After continuously scanning an area of the sample, the final scanning transmission result is obtained.
In STEM operation, the point-by-point scanning and imaging mode correlates each scanned point on the sample with the corresponding pixel in the image. Unlike TEM, which uses a parallel beam of transmitted electrons for imaging, STEM employs a converged electron probe for scanning imaging on the sample. TEM is akin to taking a photograph, while STEM involves a meticulous drawing process.
In addition to imaging based on scattered signals collected by the HAADF detector, STEM utilizes a post-column electron energy-loss spectrometer (EELS) and X-ray energy dispersive spectrometer (EDS) for obtaining electron energy-loss spectra and micro-area elemental analysis results, thereby providing information on the chemical composition and electronic structure of the sample.
2. Instrument Structure of STEM
Scanning Transmission Electron Microscopy (STEM) combines the characteristics of scanning electron microscopy and transmission electron microscopy, integrating scanning imaging with transmission analysis. Its instrument structure can be seen as a synthesis of scanning electron microscopy and transmission electron microscopy, with the main difference being the addition of scanning accessories and the placement of electron signal detectors beneath the sample.
As shown in Figure 2, the STEM system mainly consists of the following components [3]: electron gun, electron optical system, sample chamber, annular detector, and imaging devices.
Electron Gun: Provides an electron beam with specific energy, current, velocity, and angle. STEM requires high-quality electron sources, often utilizing field emission electron guns capable of emitting electrons with higher speeds and energies.
Electron Optical System: Located below the electron gun, it consists of a series of electromagnetic lenses that converge the electron beam emitted by the electron gun into a sufficiently small spot at the atomic scale.
Sample Chamber: Houses the sample stage for sample placement, enabling selection of observation fields by moving the sample stage.
Annular Detector and Imaging Devices: Positioned beneath the sample chamber, the annular detector has a certain inner diameter to capture scattered electrons at a wide range of scattering angles. Imaging devices analyze and process the collected electron signals for image formation.
Vacuum System: Composed of multiple stages of vacuum pumps, maintaining a high-quality vacuum environment is crucial for electron microscopes to avoid image aberrations caused by collisions of high-energy electron beams with gas molecules and to prevent sample contamination.
In STEM, the integration of scanning capabilities with transmission electron microscopy allows for detailed imaging and analysis of samples at atomic scales, making it indispensable in various fields such as materials science, nanotechnology, and biology.
3. Sample Preparation Requirements for STEM
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Samples should generally be solid samples with a thickness less than 100 nm and dimensions of 5 μm × 5 μm.
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Samples should not be displaced under the electromagnetic fields of the electron microscope.
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Samples should maintain stability under high vacuum conditions.
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Samples should be free from moisture or other volatile substances. If samples contain moisture or other volatile substances, they should be dried beforehand.
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Samples intended for STEM analysis should be thin film samples with a thickness < 50 nm and uniform thickness.
Imaging Modes of STEM
The interaction between the incident electron beam and atoms in the sample results in elastic and inelastic scattering of electrons. These scattered electrons carry information about the sample. As shown in Figure 2, different signals collected by the annular detector positioned beneath the sample at different positions are utilized for different imaging modes.
STEM imaging includes bright field (Annular Bright Field, ABF), dark field (Annular Dark Field, ADF), and the highly acclaimed high angle annular dark field (HAADF). Due to the different scattering signal collection angles in various imaging modes, different images of the same location can be obtained simultaneously during experiments, reflecting different information about the material.
Annular Bright Field (ABF): In STEM, the axial bright field detector is positioned at the center of the transmitted electron beam cone, collecting electrons within a deviation angle θ1 ( θ1 < 10 mrads). It primarily includes transmitted electrons and partially scattered electrons. Bright field imaging can form phase contrast images, lattice images, etc., with higher resolution compared to dark field images. It is typically used to provide complementary images to ADF imaging results. The contrast in ABF images is proportional to the cube root of the atomic number Z, making it more sensitive to lighter elements.
Annular Dark Field (ADF): The annular dark field detector, located within an angular range θ2 (10 mrads < θ2 < 50 mrads), mainly detects electrons scattered by Bragg scattering. Under the same imaging conditions, ADF images exhibit less sensitivity to imaging aberrations compared to ABF images, resulting in better contrast.
High Angle Annular Dark Field (HAADF): In HAADF mode, using a detector that further expands the collection angle to θ3 ( θ3 > 50 mrads) in the annular dark field configuration, predominantly detects high-angle incoherent scattered electrons. These high-angle scattered electrons arise from the scattering of the incident electron beam by the 1s electrons in the atomic inner shells of the sample. This involves properties related to the atomic nucleus, including nuclear energy level structure and the chemical environment of the nucleus.
Applications of STEM
1.Observation of Lattice Structure and Atomic Distribution with HAADF-STEM Images
K. Kaneko et al. conducted in-situ transmission electron microscopy observations of the precipitation process in Al-Ge alloys, collecting STEM-HAADF images at different depths to study the atomic arrangement at precipitate interfaces. As shown in Figure 3a, they observed the presence of single and multiple twins in the cross-section of triangular plate-like Ge precipitates, and Figure 4c displays the existence of multiple twins in rod-like Ge precipitates. Utilizing 3D modeling simulations, they reconstructed the distribution of Ge precipitates in the alloy. During the study, the researchers identified the rapid coarsening phenomenon of rod-like Ge precipitates and speculated its correlation with the diffusion of solid-solution Ge. This discovery opens new perspectives for understanding the growth processes of precipitates in alloys and provides insights for effectively controlling the composition and heat treatment processes of alloys.
2.STEM-EDS Elemental Composition Analysis
Equipped with an EDS probe on the sample, which collects characteristic X-rays emitted due to the interaction between the sample and the electron beam, STEM-EDS analysis can be conducted. Based on the intensity and wavelength distribution of X-rays, elemental mapping and semi-quantitative compositional information of the sample can be obtained.
Scholars from Osaka Prefecture University utilized STEM technology to perform HAADF morphology imaging and EDS elemental line analysis of the LCO/Li2S-P2S5 interface. They observed mutual diffusion of Co, P, and S at the cathode/solid electrolyte interface. This interdiffusion behavior between elements leads to degradation at the interface of the LCO electrode and solid electrolyte, contributing to increased interfacial resistance in all-solid-state lithium batteries.
