What is the principle of sem and tem?

The Short Answer

Both the scanning electron microscope (SEM) and the transmission electron microscope (TEM) use a beam of electrons instead of light, focused by electromagnetic lenses inside a vacuum. Because electrons have a far shorter wavelength than visible light, both can resolve much finer detail than any light microscope.

• TEM principle: a broad electron beam passes through an extremely thin specimen. Parts of the specimen scatter electrons more or less strongly, and the electrons that come through are focused into a magnified image on a detector. It shows internal structure, like a very high-resolution shadow picture.
• SEM principle: a finely focused electron beam is scanned across the surface of the specimen point by point. At each point, the beam knocks out signals — mainly secondary and backscattered electrons — which a detector collects. The strength of the signal at each point builds up an image of the surface, which typically looks strikingly three-dimensional.

In short: TEM looks through a thin slice; SEM maps a surface.

Why Use Electrons Instead of Light?

A microscope’s ability to separate two close points (its resolving power) is limited by the wavelength of what it uses to “see”. Visible light has wavelengths of roughly 400 to 700 nanometres, which limits the best light microscopes to resolving detail of about 0.2 micrometres (200 nanometres).

Electrons accelerated through a high voltage behave as waves with wavelengths thousands of times shorter — a few picometres at the voltages used in TEM. In practice, lens imperfections rather than wavelength limit what electron microscopes can resolve, but they still reach detail on the scale of nanometres, and modern high-end TEMs can image individual columns of atoms.

That is why viruses, the internal membranes of cells and the arrangement of atoms in materials are electron-microscope territory.

What SEM and TEM Have in Common

Both instruments share the same basic building blocks:

• Electron source (gun). A heated filament (often tungsten or lanthanum hexaboride) or a field-emission tip produces electrons, which are accelerated by a high voltage.
• Vacuum. Air molecules would scatter the beam, so the column and sample chamber are kept under vacuum. This is one reason conventional samples must be dry.
• Electromagnetic lenses. Coils of wire create magnetic fields that bend and focus the electron beam, playing the role glass lenses play in a light microscope.
• Apertures. Small holes that limit the beam and improve contrast.
• Detectors. Because human eyes cannot see electrons, the image is formed on a fluorescent screen, a camera sensor or an electronic detector.

Electron images are fundamentally grayscale. They record the intensity of electrons, not colour. Coloured SEM or TEM images you see in books and on websites have been coloured afterwards, usually to make features easier to tell apart.

The Principle of TEM

1. Electrons are emitted and accelerated, typically at around 80 to 300 kilovolts.
2. Condenser lenses shape the beam and direct it evenly onto the specimen.
3. The specimen is extremely thin — for biological thin sections, usually well under 100 nanometres — so that electrons can pass through.
4. As electrons travel through, denser regions or regions with heavier atoms scatter more of them. Apertures remove strongly scattered electrons, so those regions appear darker.
5. Objective and projector lenses magnify the transmitted electrons into an image on a screen or camera.

TEM can also record diffraction patterns, which reveal how atoms are arranged in crystals, and with specialist detectors it can map chemical composition.

The Principle of SEM

1. Electrons are emitted and accelerated, usually at lower voltages than TEM — commonly up to about 30 kilovolts.
2. Lenses focus the beam to a very small spot on the specimen’s surface.
3. Scanning coils sweep that spot across the surface in a grid (raster), line by line.
4. At each point, the beam interacts with the sample and releases signals:
   • Secondary electrons, low-energy electrons from very near the surface, which are especially sensitive to surface shape. These give the classic, almost 3D-looking SEM image.
   • Backscattered electrons, beam electrons bounced back from the sample. Heavier elements bounce back more, so these show differences in composition.
   • X-rays, whose energies are characteristic of each element. With an energy-dispersive X-ray (EDS) detector, these can be used to identify and map elements.
5. A computer converts the detector signal at each point into the brightness of a pixel, building up the image.

SEM images have a large depth of field, which is why insect eyes, pollen grains and fractured metals look so sharp and sculptural. The magnification is set by how small an area the beam scans.

SEM vs TEM at a Glance

Sample Preparation

For SEM, conventional samples must be dry, stable in vacuum and electrically conductive at the surface. Biological specimens are usually chemically fixed, dehydrated and carefully dried (often by critical-point drying to avoid collapse), then coated with a very thin layer of metal such as gold or gold-palladium, or with carbon. Non-conductive materials like ceramics or plastics are coated in a similar way. Some instruments — low-vacuum or environmental SEMs — can image uncoated or slightly moist samples.

For TEM, the challenge is thinness. Biological tissue is typically fixed, embedded in resin and cut into ultrathin sections with a diamond knife on an ultramicrotome, then stained with heavy-metal salts to add contrast. Materials samples may be thinned by polishing, ion milling or a focused ion beam. Cryo-electron microscopy instead freezes samples rapidly in a thin layer of glass-like ice, preserving them closer to their natural state.

What Electron Microscopes Are Used For

• Biology and medicine: cell ultrastructure, viruses, kidney and muscle biopsies, protein structures (cryo-EM)
• Materials science: grain structure, defects, coatings, nanoparticles
• Semiconductors: inspecting and analysing microchip features and failures
• Geology and palaeontology: mineral textures, microfossils
• Forensics: gunshot residue, fibres, paint fragments
• Industry: quality control and failure analysis of fractured parts

Can an Amateur Use an Electron Microscope?

Rarely at home. Electron microscopes are expensive to buy and run: even compact benchtop SEMs typically cost tens of thousands of US dollars, full-size research SEMs considerably more, and advanced TEMs can run into the millions. They also need vacuum systems, stable power, careful maintenance and trained operators.

Some universities, museums and makerspaces offer outreach sessions or paid access, and a few dedicated hobbyists have restored decommissioned instruments. For most amateurs, though, light microscopy offers enormous scope at a tiny fraction of the cost; see light vs electron microscopes for a fuller comparison. To see where electron microscopes fit among other instrument types, read what are the four types of microscopes; for 3D techniques built on SEM and TEM, see what microscopes make 3D images.