Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have widespread applications in characterizing nanomaterials’ composition, interfacial strain and structure, morphology, phase quantification, and size distribution, all of which directly contribute to nanomaterials design.
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What is Scanning Electron Microscopy (SEM)?
SEM involves scanning and producing materials’ surface images from a convergent beam of electrons, which interact with the sample materials’ atoms (Zhou, Apkarian, Wang, & Joy, 2006). The technique generates several signals that can be converted into material composition and surface topology information.
The working principle of SEM includes the application of a high-energy electron beam (100-30000 eV), which is emitted from a thermal source.
SEM comprise lenses that converge the electron beam on the sample to < 10 nm spot; its lenses can penetrate up to 1 μm deep into the sample material and generate signals that are useful to form an image. An electron detector detects, and the indicator displays signals obtained from the scanned material. Image clarity can be manually controlled from the beam intensity and brightness. The angle of inclination of the beam and the materials’ topography has direct impacts on the SEM image results.
Transmission Electron Microscopy (TEM)
TEM is another electron microscopy technique that can provide images to a ~0.19 nm scale, all high resolution (Giannuzzi & Stevie, 1999).
It can image 100 nm thin samples through electron and sample interaction and is applicable in studying the chemistry and morphology of materials at the nanoscale.
TEM’s working principle comprises electron emission, which travels through a vacuum column of the microscope. A focused, thin beam is generated from an electromagnetic lens and passed into the sample and then the detector.
The sample’s crystal orientation and compositional density play a major role in TEM imaging. A 3D image of the sample can be produced by rotating the samples and taking multiple images per rotation. A sample’s chemical bonding, electronic properties, elemental composition, and valence can be measured using TEM.
Designing Nanomaterials Using SEM and TEM
Averaged particle size distribution of crystalline nanomaterials is determined from the Scherrer equation and X-ray diffraction’s (XRD’s) line-broadening fitting (Rabiei, et al., 2020).
Nanomaterials’ composition is analyzed from SEM and TEM by collecting the annular dark-field or elemental detector signals.
Phase identification of crystalline nanomaterials of > 5 nm large grain size is determined using selected area electron diffraction, an operational mode in TEM (Su, 2017).
A phase of a nanomaterial informs about the uniformity of its lattice sequence with its lattice parameters, and about the uniformity in its physical properties and composition.
Morphology of a wide variety of nanomaterials – nano-sheet, nanoplates (2D); nanowire, nanorod, nanotube (1D); and nanoparticles (0D), etc., can be characterized using SEM (Tiwari, Tiwari, & Kim, 2012).
The interfacial structure and strain of the nanomaterial correlate to the materials’ stability and catalytic activity, and it is evaluated at the atomic scale using TEM.
Current Applications and Challenges of SEM and TEM in Materials
SEM has widespread applications in materials fracture analysis, quality control, and research.
The field of high-temperature superconductors, mesoporous alloys, and nanomaterials all require SEM for sample analyses.
Similarly, TEM has diverse applications in academia, forensics, life sciences, medical biology, metallurgy, and nanotechnology.
A major challenge regarding SEM is obtaining consistent and reproducible data as the result is significantly dependent on the specific tool setting and the operator’s handling.
Moreover, SEM still lacks an automatically dedicated image recognition and algorithm-based pattern analysis program (Bolten et al, 2017).
The key challenges of TEM include the large size and cost of the equipment (Smith, 2015).
Sample preparation for TEM is laborious and analysis and operations demand specialized training.
Utility consumption of a TEM facility is intense: they require consistent voltage regulation, current maintenance in the electromagnetic coils, and a cooling water supply.
For both SEM and TEM, the samples require conductive coatings, are vacuum pressure-resistant, and are small. They are sensitive to electromagnetic fields and mechanical vibration and therefore, required to be placed away from such sensitive exposure.
Differences Between SEM and TEM
The major difference between SEM and TEM is their operability.
- Best suited to image nanomaterials’ surface morphology
- The sample’s surface is scanned by an electron beam
- Produces images from scattered electrons; it detects secondary electrons that are excited by the primary electrons and are emitted from the sample’s surface.
- Resolution is 2.00 – 0.20 nm
- Depth of field is higher than TEM
- Magnification (105 X) is at least 1 order lower than TEM
- Sample preparation for SEM is easier than TEM
- More suitable for studying nanomaterials’ internal properties
- Sample is penetrated by the electron beam
- Produces images from the transferred electrons by detecting the primary electrons that are transmitted from the sample
- Resolution is 10 – 0.5 nm
- Magnification is ~ 5*106 X
- Requires thin samples
References and Further Reading
Brodusch, N., Brahimi, S., De Melo, E., Song, J., Yue, S., Piché, N., & Gauvin, R. (2021). Scanning Electron Microscopy versus Transmission Electron Microscopy for Material Characterization: A Comparative Study on High-Strength Steels. Scanning. doi:10.1155/2021/5511618
Giannuzzi, L., & Stevie, F. (1999). A review of focused ion beam milling techniques for TEM specimen preparation. Micron. doi:10.1016/S0968-4328(99)00005-0
Rabiei, M., Palevicius, A., Monshi, A., Nasiri, S., Vilkauskas, A., & Janusas, G. (2020). Comparing Methods for Calculating Nano Crystal Size of Natural Hydroxyapatite Using X-Ray Diffraction. nanomaterials. doi:10.3390/nano10091627
Smith, D. (2015). Chapter 1:Characterization of Nanomaterials Using Transmission Electron Microscopy. In A. Kirkland, Nanoscience & Nanotechnology Series. doi:10.1039/9781782621867-00001
Su, D. (2017). Advanced electron microscopy characterization of nanomaterials for catalysis. Green Energy & Environment. doi:10.1016/j.gee.2017.02.001
Tiwari, J., Tiwari, R., & Kim, K. (2012). Zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices. Progress in Materials Science. doi:10.1016/j.pmatsci.2011.08.003
Zhou, W., Apkarian, R., Wang, Z., & Joy, D. (2006). Fundamentals of Scanning Electron Microscopy (SEM). Springer. doi:10.1007/978-0-387-39620-0_1