Electron microscopy is an effective method for obtaining high-resolution images in several fields such as biomedical science, forensics, and technology. Electron microscopes can capture far higher resolution images than light microscopes, providing information that would otherwise be very difficult to attain.
The two most common types of electron microscopy are scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The way TEM and SEM function and the types of images they can capture vary. This article will provide you an overview of SEM and TEM, including their respective definitions, how they function, and how they compare to each other.
What is SEM?
SEM, which stands for Scanning Electron Microscopy or Scanning Electron Microscope, is a type of electron microscope that utilizes a fine beam of focused electrons to scan the surface of your sample. This microscope captures details about the interaction between the sample and the electrons, resulting in a magnified image. Using an SEM, you can magnify images up to 2 million times.
SEM images provide insight into the topography and elemental composition of a sample. Using an SEM, you’re able to capture black-and-white 3D image samples that are either thick or thin. The size of the electron microscope chamber dictates the resulting sample’s size.
How does SEM work?
An electron source (also known as an electron gun) generates a stream of high-energy electrons towards a sample to obtain a high-resolution image. Electromagnetic lenses aid the electron beam to focus. Once the focused stream reaches the sample, it scans the surface in a rectangular raster.
The interaction between the electron beam and the sample generates secondary electrons, backscattered electrons, and X-rays. Magnified images are the direct result of these captured interactions.
What is TEM?
TEM, which stands for Transmission Electron Microscopy or Transmission Electron Microscope, is a type of electron microscope that creates an image of a sample’s internal structure using a broad beam of electrons. This electron beam passes through the specimen, producing an image that details its morphology, composition, and crystal structure.
For the electrons to pass through, samples need to be extremely thin, usually less than 150 nm thick. After passing through the specimen, the electrons arrive at a detector below, where a 2-D image is formed.
TEMs have an incredible magnification capability of 10-50 million times. They’re able to provide details at the atomic level, which is the highest resolution of any electron microscope. Hence, TEMs are often used to investigate molecular and cellular structures.
How Does a TEM Work?
A beam of electrons is sent through an ultrathin sample by an electron source. As electrons penetrate through the sample, they also pass through the lenses below. Data is utilized to generate images displayed directly on a fluorescent screen or a computer screen using a charge-coupled device (CCD) camera.
SEM vs. TEM: Operational Differences
SEM and TEM differ in the manner in which users operate their systems.
SEMs usually use up to 30 kV of acceleration voltages, while TEM users can configure it between 60 and 300 kV.
TEM magnifications are also much higher than the capacity of SEMs. TEM users can magnify their samples more than 50 million times, while SEM users can only magnify up to 1–2 million times.
However, SEMs can achieve a greater maximum field of view (FOV) than TEMs. This feature means that TEM users can only process a tiny portion of their sample. Similarly, the depth of field of SEM systems is significantly greater than that of TEM systems.
The two systems also differ in the way they create and process images. Samples are placed at the bottom of the electron column in SEMs, as electron detectors collect back-scattered and secondary scattered electrons. Photomultipliers are used to transform this signal into a voltage signal and then amplified to produce the image on a computer screen.
On the other hand, you’ll find the specimen in the middle of the column in TEM microscopes. Transmitted electrons pass through the sample and a series of intermediate and projector lenses below it. The resulting image is displayed on a fluorescent screen or a PC screen via a charged-coupled device (CCD) camera.
SEM vs. TEM: a Side by Side Comparison
Given that they have operational differences, these high-resolution microscopes also have similarities, starting with their components. Each has an electron source/gun that emits an electron stream toward a sample in a vacuum, as well as lenses and electron apertures for controlling the electron beam and capturing images.
This table summarizes the differences between SEMs and TEMS:
|Type of Electrons||Scattered, scanning electrons||Transmitted electrons|
|Image Formation||Detectors capture and count electrons, and the image is shown on the PC screen.||Direct imaging on a fluorescent screen or via a charge-coupled device (CCD) camera onto a PC screen|
|Operational Requirement||Easy to use, requires little or no sample preparation||Strenuous sample preparation, equipment needs qualified users.|
|Surface image in 3D||2D projection image of the inner structure|
|Maximum Magnification||Up to 1-2 million times||Beyond 50 million times|
|Maximum Field of View||Large||Limited|
|Specimen Thickness||Thin and thick samples are okay||<150 nm in most cases|
|Optimal Spatial Resolution||-0.5 nm||<50 pm|
|High Tension||~1–30 kilovolts||~60-300 kV|
|Cost||Less Expensive||More Expensive|
|Sample Restriction||Less Restrictive||More Restrictive|
SEM vs. TEM: Summary of Advantages
Both SEMs and TEMs have unique advantages when compared to one another.
Compared to TEMs, SEMs cost less to procure, take less time to generate an image, require less time for specimen preparation, accept thicker samples that are much larger.
Compared to SEMs, TEMs generate higher-resolution images, provide atomic and crystallographic data, produce 2D images that are easier to interpret than 3D SEM images, and allow users to examine additional characteristics of a given sample.
Other Key Factors to Consider Before Purchasing an Electron Microscope
How many users will utilize the system? Did they undergo sufficient training? If not, how much time are they willing to spend to get to know the system? Desktop SEMs are simple enough to operate and need little or no sample preparation. Obtaining images can be as straightforward as pressing a couple of buttons.
Users with specific requirements can access the system’s more advanced features, provided they are willing to take the time to learn. In general, the operator training criteria for a desktop SEM are much lower, and the system itself is much more sturdy. Also, it doesn’t break as easily and has a much lower potential repair cost.
Is there a well-defined application routine? If there is, and the desktop SEM can provide the information you require, there’s little to no reason to spend more than you should. You should weigh concerns involving future requirements exceeding desktop capabilities against the certainty and timing of the said needs, as well as the availability of outside resources for more demanding applications.
Even if future requirements eventually exceed desktop capability, your initial investment in a desktop SEM can continue to pay off as you use your current system to supplement later floor model system versions. Either you utilize it in a screening capacity or continue performing routine analyses while the new floor model system is counted on to work on more demanding applications.
Finally, a desktop system also helps justify the procurement of a larger system, showing the value of SEM while enabling an experience-based assessment of the need for more advanced capacity from an external provider.
As previously mentioned, Desktop SEM systems only require minimal preparation for samples. Further, they also have lesser vacuum requirements and minimal evacuated volume. These allow the SEM to process images much quicker compared to regular floor model systems.
Also, consumers of the information typically run desktop SEMs. This setup eliminates the need for a dedicated operator to conduct any form of analysis, furnish a report, and communicate results.
Besides quicker responses, there’s significant intangible value in the analysis’s immediacy and each user’s ability to lead the investigation in real-time response to observations.
Finally, in some applications, such as inspection, longer delays have a measurable cost by jeopardizing more work-in-progress.
When choosing between SEM and TEM, the decision will ultimately depend on the type of analysis you require. From what we’re able to discuss, it’s apparent that there’s no better technique. While this is the case, the following are specific instances when one is the better choice over the other:
- Choose TEM if you want to get information from the inner structure.
- SEM works better when you require surface information.
- TEMs may be much more expensive and larger than SEMs and require more effort to acquire and interpret results, but they allow much more versatility and resolving power for their users.
Both TEM and SEM are valuable tools in the physical, biological, and chemical sciences. By having a firmer grasp on the differences between these two electron microscopes, you’ll be able to select the best type of microscope that’ll meet your needs.