What is the Difference Between SEM and TEM

SEM and TEM are both types of electron microscopes. SEM stands for scanning electron microscope, while transmission electron microscope. There is a distinct difference between SEM and TEM based on their operating principles and applications.

What is the difference between SEM and TEM? SEM scans the surface of a specimen with a focused beam of electrons, whereas TEM transmits electrons through a thin specimen to create an image.

Key Areas Covered

1. What is SEM (Scanning Electron Microscope) 
      – Definition, Features 
2. What is TEM (Transmission Electron Microscope)
      – Definition, Features 
3. Similarities Between SEM and TEM
      – Outline of Common Features
4. Difference Between SEM and TEM 
      – Comparison of Key Differences
5. FAQ: SEM and TEM
      – Answers to Frequently Asked Questions

Key Terms

SEM, TEM, Transmission Electron Microscopy, Scanning Electron Microscopy

What is SEM

SEM (scanning electron microscope) is a powerful tool used in various fields of science. It is an instrument that identifies the morphology and composition of materials at very high resolutions. In order to provide a higher magnification and depth of field, a focused beam of electrons is used to generate an image. This narrow beam of electrons is emitted by an electron gun, which ejects electrons. These electrons are accelerated towards the specimen by an electric field. Interactions of the electrons with the atoms on the surface of the specimen generate various signals, including secondary electrons, backscattered electrons, and characteristic X-rays. These signals are detected and used in the creation of an image of the specimen with detailed information about the specimen’s composition and topography.

There are many advantages of using SEM over other types of microscopes. Researchers use its high magnification to examine particles existing at the nanoscale with greater detail, which are not detected by optical microscopes. SEM is capable of analyzing various materials such as biological samples, polymers, metals, and ceramics.

For advanced analytical techniques, SEM can use various accessories. Energy-dispersive X-ray spectroscopy (EDS) and wavelength-dispersive X-ray spectroscopy (WDS) are commonly used in conjunction with SEM to identify the elemental composition of a sample. SEM is useful in various fields such as forensics, geology, biology, nano technology, and even semiconductor manufacturing.

What is TEM

In TEM, electrons are transmitted through a thin specimen. As electrons pass through the specimen, they interact with its atoms, scattering or being absorbed depending on the material’s composition and thickness. These interactions produce various signals, including transmitted electrons that form the primary image and diffracted electrons used for structural analysis through techniques like electron diffraction.

One of the major features of TEM is its ability to visualize individual atoms within a sample. TEM can also be combined with other techniques to provide additional information. For instance, energy-dispersive X-ray spectroscopy (EDS) can be integrated into TEM setups to analyze the elemental composition of a sample, further enhancing its analytical capabilities.

Though TEM has its own advantages and properties, it is not without limitations. Some of these limitations include the complexity of sample preparation, as samples must be extremely thin for electrons to pass through, and the requirement of a high vacuum, limiting the selection of samples. However, TEM has proven to be an important instrument in various fields of science and industries, serving as a reliable analytical tool that provides accurate data on specimens.

Similarities Between SEM and TEM

1. These techniques offer high-resolution images.
2. Both SEM and TEM use similar sample preparation techniques.

Difference Between SEM and TEM

Definition 

• SEM (Scanning Electron Microscopy) is a microscopy technique that uses a focused beam of electrons to scan the surface of a sample. On the other hand, TEM (Transmission Electron Microscopy) is a microscopy technique that transmits a beam of electrons through an ultrathin sample.

Operation

• SEM scans the surface of a specimen with a focused beam of electrons to generate images, while TEM transmits electrons through a thin specimen to create an image.

Sample Preparation

• SEM typically requires coating the sample with a conductive material to prevent charging, while TEM involves thinning the sample to electron transparency using techniques like ultramicrotomy.

Resolution

• TEM generally offers higher resolution compared to SEM.

Depth of Field

• SEM has a larger depth of field, allowing for more of the sample to be in focus at once, whereas TEM has a much smaller depth of field due to its transmission nature.

Use

• SEM is more suited for studying surface morphology and topography of samples, while TEM is used for examining internal structures and ultrafine details of specimens.

Conclusion

SEM and TEM are two types of electron microscopes that obtain the details of smaller samples of objects. The main difference between SEM and TEM is that SEM creates an image by detecting reflected or knocked-off electrons, while TEM uses transmitted electrons (electrons that are passing through the sample) to create an image. While SEM is good at surface morphology analysis, TEM provides insight into internal structures and atomic-scale details.

FAQ: SEM and TEM

1. Why is SEM preferred over TEM?

Most prefer SEM over TEM because they cost less to procure, take less time to generate an image, require less time for specimen preparation, and accept thicker samples which are much larger.

2. Does SEM produce 3D images?

Scanning electron microscopy (SEM) is normally used to image the surface of cells, tissues, and whole multicellular organisms. Hence, SEM images of surfaces appear to be three-dimensional (3D).

3. Which has higher resolution, SEM or TEM?

Transmission electron microscopy (TEM) typically offers much higher resolution than scanning electron microscopy (SEM), often by a factor of 10 or more. Various factors, such as electron wavelength, spherical aberration coefficient of the objective lens, and other instrument parameters, contribute to resolution.

4. Can TEM produce 3D images?

While TEM can provide high-resolution images of the internal structure of a sample, it does not produce 3D images.

Reference:

1. “Scanning Electron Microscope.” Encyclopedia Britannica. 
2. “Transmission Electron Microscope.” Encyclopedia Britannica. 

Image Courtesy:

1. “SEM chamber1” (CC BY-SA 2.5) via Commons Wikimedia
2. “Scheme TEM en” By Gringer (talk) – Commons: Scheme TEM en.png (CC BY-SA 3.0) via Commons Wikimedia