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TEM vs SEM

Transmission Electron Microscope vs Scanning Electron Microscope

Electron microscopes are one of the most (if not the most) powerful imaging devices ever invented, and these are just about powerful enough to let us see the smallest details of any material right down to each atom.

There are two main types of electron microscopes, namely, transmission electron microscopes (TEM) and scanning electron microscopes (SEM). The question is, what is the difference between the two?

Here is a rundown of TEM vs SEM:

How it works

The key difference between TEMs and SEMs is the specific microscopy technique being used. While both devices use electron beams to image a specimen, TEMs transmit these electrons through the specimen, while SEMs use these electrons to scan the specimen.

Transmission electron microscope

STEM - Electron Gun

On a transmission electron microscope, the electrons are sourced from an electron gun designed with a tungsten filament cathode. It releases a high voltage beam of electrons that is accelerated by an anode at 40 to 100 keV, which is then focused by electromagnetic and electrostatic lenses.

This electron beam is then transmitted to the specimen, passing through it, and gets scattered. Afterwards, these electrons carrying information about the specimen’s structure are magnified by the special objective lenses of the microscope.

Finally, an image is generated through one of three ways- by projecting the electron image onto a coated fluorescent screen, by exposing it on a photographic plate to be photographed, or by guiding it through an optical lens system to a sensor of a digital camera with a fiber optic light guide.

Scanning electron microscope

Scanning electron microscope

On the other hand, scanning electron microscopes use 50 eV focused electron beams to probe the specimen by scanning it across a rectangular area of the specimen in a raster scan pattern. As the electrons come into contact with the specimen, it produces a variety of energy loss.

These forms of energy offer information about the specimen’s surface, composition, and topography, and include light, heat, X-ray, and most importantly, high energy backscattered electrons and low energy secondary electrons.

What happens next is that these electrons and other energy loss generate signals that are gathered by an electron detector, and the position and intensity of these signals are used to construct the image of the specimen- at least in most types of scanning electron microscopes.

Magnification

Another difference between TEM vs SEM is their magnification power. Transmission electron microscopes have a higher magnification of up to 50 million times, whereas scanning electron microscopes can typically magnify images around 500,000 times.

Transmission electron microscope

Generally, transmission electron microscopes have a magnification of at least 100,000 times, and it can go up to a stunning 50 million with the use of specialized high resolution transmission electron microscopy.

This is possible through the microscope’s use of multi-stage beam preparation optics, including high power electron optical lenses which are actually made from curved magnetic fields made of solenoid coils and ferromagnetic materials.

These lenses have a wide range of magnification power, and each one has the ability to focus and defocus the electron beam coming from the microscope’s electron gun, as well as collimate or magnify the beams transmitted from the specimen.

Scanning electron microscope

The thing about scanning electron microscopes is that these have a surprisingly wide range of magnification levels, from just as low as 10x, which is similar to that of a simple hand-held microscope, and up to a maximum of 500,000x, which is easily hundreds of times the magnification of even the best light microscope.

However, there are some scanning electron microscopes that can magnify an image for up to 3,000,000 times. Now, unlike conventional light microscopes, the magnification of a scanning electron microscope is not dependent on the use of objective lenses.

Rather, this is controlled by the raster size on the specimen compared to the display device’s raster. This can be manipulated by adjusting the voltage of the deflector plates and the current of the scanning coils.

Resolution

TEMs and SEMs also differ in terms of resolution or resolving power. Similar with their magnification levels, TEMs have a higher resolution of 50 picometers, while SEMs have a resolution of 1 nanometer.

Transmission electron microscope

Out of all the different types of electron microscopes, transmission electron microscopes have the highest resolution capacity, with a maximum resolution of up to 50 picometers or less than 0.5 angstrom.

A common issue with transmission electron microscopes, especially the older models, is the eventual spherical aberration. Although, the good thing is new generation hardware correctors have been developed to significantly reduce this problem.

This is more than enough to be able to see individual atoms and atomic columns, how these atoms are positioned within the material, and even how they interact with each other and affect any given property of that material.

Scanning electron microscope

Scanning electron microscope

Meanwhile, scanning electron microscopes have a relatively lower resolution at only a single nanometer. We say that it’s relatively low because even at such resolving power, scanning electron microscopes are still thousands of times better than most other types of microscopes.

As the microscope scans a few nanometers of the specimen’s surface, the secondary emitted electrons become highly localized, and it’s with this that image resolutions of 1 nanometer become possible.

Although, if deeper sections of the specimen are what needs to be imaged, the microscope then makes use of the backscattered electrons that are reflected from the specimen, and since these are scattered, the resulting image resolution is somewhat reduced.

What you can see

In terms of what kinds of specimens can be viewed under the microscope, and what details the microscope can present us with, both TEMs and SEMs can be used to view almost any material, although the difference is TEMs image the interior, while SEMs scan the surface.

Transmission electron microscope

Transmission electron microscopes are versatile imaging devices that can be used to image just about any type of material or specimen, including those of the organic and the inorganic variety, such as cells, tissue sections, and individual molecules.

There is just one requirement- the specimen needs to be sectioned thinly enough for the electrons to be successfully transmitted through the specimen without resulting in aberrations and image distortions, or an overall lack of contrast and resolution.

This is because transmission electron microscopes “probe” past the surface and into the interior of the specimen in order to show us intricate details regarding its structural makeup, which includes atomic columns and even individual atoms.

Scanning electron microscope

There is a wide variety of samples that can be viewed under a scanning electron microscope, such as biological samples including organisms, cells, and tissue sections, hard and dry materials like bone, wood, and metals, and even replicas of various specimens made from silicone and resin molds.

That said, many scanning electron microscopes are used to analyze semiconductor wafers. These microscopes typically require that the specimen be coated with a conductive material like gold for it to be electrically conductive, which makes for an accurate image.

Overall, scanning electron microscopes are high vacuum electron microscopes with a high depth of field, and have the ability to showcase the identity,  abundance, and distribution of different elements throughout the specimen, allowing for an accurate characteristic representation of the specimen’s three-dimensional surface structure.

Specimen preparation

Finally, there is also some difference in the sample preparation requirements of TEMs and SEMs, in such that TEMs need ultra-thin and high contrast specimen sections, whereas SEMs can do away with largely unprepared samples as long as they are electrically conductive.

Transmission electron microscope

One downside with transmission electron microscopes is the prerequisite for an extensive process of preparing the specimen in order for it to be stable within the microscope’s vacuum, and more importantly, for it to be readable by the electron beams.

As such, the specimen needs to be stabilized via dehydration, embedding, or chemical fixation before being sliced into ultra-thin sections of 100 nanometers or less, which is done by running the specimen through a focused ion beam.

This is especially important for biological specimens and other organic material. Moreover, many of these specimens may require additional sample preparation techniques such as staining, in order to increase their contrast, and in turn, their image resolution.

Scanning electron microscope

The good thing about scanning electron microscopes is that there is no need to extensively prepare the sample and slice it into electron thin sections. In fact, it can easily image four millimeter thick specimens with high resolution, as well as image bulk samples simultaneously.

This is primarily because the electron beam does not need to pass through the specimen, hence factors such as thickness, contrast, and transparency are irrelevant, especially since what’s being scanned is only the specimen’s surface.

Special types of scanning electron microscopes such as environmental scanning electron microscopes (ESEM) are equipped with low vacuum chambers and chamber gasses, and therefore can be used to image unfixed biological specimens without the need for dehydration and other sample preparation techniques.

Overview

Tem vs sem
Source

Transmission electron microscopes and scanning electron microscopes have a lot of differences in terms of how they operate, what their capabilities are, what you can see through them, and even the requirements for preparing the specimen.

To summarize this overview of TEM vs SEM, transmission electron microscopes are higher magnification and higher resolution imaging devices that operate by transmitting electrons through ultrathin sections of the specimen in order to show minute details like atomic columns.

On the other hand, scanning electron microscopes are somewhat lower resolution and magnification imaging tools, but still powerful enough to scan larger, electrically conductive materials to display information about its surface structure.

Read also: How much does an electron microscope cost?

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