The two main types of electron microscopes used in research are scanning electron microscopes (SEMs) and transmission electron microscopes (TEMs).

While both scanning and transmission electron microscopes utilize a beam of electrons to image samples, they work in fundamentally different ways and are suited for imaging different types of samples.

SEM and TEM differ significantly in how they generate images, the types of information they provide, their capabilities and limitations, and their applications. This guide will compare SEM and TEM technology, resolution, sample requirements, imaging modes, and common uses.

The Difference in Working Principles of SEMs and TEMs

SEMs and TEMs both use a beam of electrons to image a sample. However, how the electrons interact with the sample differs between the two instruments.

Scanning Electron Microscope

In an SEM, the electron beam is focused to a fine point and rastered across the surface of the specimen. When the electron beam hits the sample, electrons and X-rays are ejected from the sample. Detectors collect these ejected particles and convert them into a signal that generates an image of the sample’s topography and composition.

SEM provides a 3D-like perspective of the sample’s surface. The excellent depth of field provided by SEM generates highly textured images that appear three-dimensional.

Transmission Electron Microscope

In TEM, the electron beam is transmitted through an ultra-thin sample. As electrons pass through the sample, some are scattered and disappear from the beam. The unscattered electrons hit a detector, generating a projection image of the sample.

Instead of scanning across the surface like SEM, TEM provides a 2D vertical projection through the entire thickness of the ultrathin sample. This allows visualization of subcellular details hundreds of nanometers below the surface.

Resolution

Resolution refers to the smallest detail that can be distinguished in an image.

• SEM resolution is about 1-20 nm.
• TEM resolution can reach below 0.5 nm.

The higher resolution of TEM allows visualization of finer details compared to SEM. For example, TEM can resolve small structures like membranes, vesicles, and macromolecular complexes that are below the resolution limits of SEM.

Sample Requirements

SEM and TEM also differ significantly in terms of sample preparation and requirements:

• SEM samples must be dry and stable in a vacuum. Samples are viewed from the surface, so only the top few microns need to be preserved.
• TEM requires samples to be chemically fixed and embedded into hard resin. Samples are cut into ultra-thin (50-100 nm) sections and mounted on grids. Only a thin sliver through the depth of the sample is viewed.

The stringent sample preparation for TEM preserves subcellular structures but provides a limited view. SEM has simpler sample requirements but only images of the surface.

Imaging Modes

SEM and TEM each offer multiple imaging modes:

SEM Imaging Modes

• Secondary electron imaging shows topography and morphology.
• Backscattered electron imaging provides compositional contrast.
• SEM can also analyze other signals like X-rays and cathodoluminescence.

TEM Imaging Modes

• Bright-field TEM shows direct beam transmission through the sample.
• Dark field TEM visualizes scattered electrons.
• Phase contrast highlights density differences within samples.
• Diffraction patterns provide crystallographic information.

TEM provides more diverse signals and contrast modes to probe sample details. SEM gives a more lifelike, 3D perspective of the sample surface.

Common Applications

The strengths and limitations of SEM and TEM lend each technique to specific applications:

SEM Applications

• Topographic imaging of surfaces at nanometer to micrometer scale
• Morphology of cells, tissues, nanoparticles, materials surfaces
• Qualitative and quantitative compositional analysis via energy dispersive X-ray spectroscopy (EDS)

TEM Applications

• Subcellular ultrastructure at molecular resolution
• Viruses, organelles, macromolecular complexes
• Quantitative analysis of internal features like vesicles, nuclear heterochromatin
• Crystallography of inorganic nanomaterials
• Chemical mapping via EDS

While SEM provides 3D topographic and compositional information on sample surfaces, TEM excels at high-resolution imaging of internal ultrastructure. Researchers often use both techniques to provide a comprehensive understanding of complex biological and material samples.

Comparative Table of SEM and TEM

FAQ

Can SEM see inside cells like TEM?

No, SEM cannot see below the surface of the sample. It images the surface topology rather than internal structures beneath the surface. TEM is required to see intracellular details at high resolution.

Can TEM analyze composition like SEM?

Yes, TEM can perform compositional analysis via techniques like energy-dispersive X-ray spectroscopy (EDS). However, SEM typically provides faster, more straightforward elemental analysis over larger surface areas.

Which has higher magnification – SEM or TEM?

TEM has much higher maximum magnifications, up to 10 million times, compared to SEM which maxes out around 300,000 times magnification. However, higher magnification does not directly translate to higher resolution. The maximum useful magnification is limited by the resolution of each instrument.

Can SEM see atoms like TEM?

No, SEM cannot resolve individual atoms. The resolution limit of even the most advanced SEMs is around 1 nm, which is much larger than a single atom. TEM can resolve individual atoms in some cases, especially when equipped with an aberration corrector.

Which is better for biological samples?

TEM is better for internal cell structure while SEM provides more information about surface morphology. Researchers often use both SEM and TEM to study biological samples, taking advantage of the complementary information provided by each technique.