What is a scanning electron microscope?
A scanning electron microscope is an instrument that uses electrons instead of light to examine the surface of a solid sample. Because electrons have a much shorter wavelength than visible light, an SEM can resolve details far smaller than an optical microscope can, and it also has a large depth of field, so rough surfaces appear sharp and three-dimensional.
The idea dates back to the 1930s. Max Knoll produced an early scanned electron-beam image in 1935, Manfred von Ardenne built a high-resolution scanning instrument in 1937, and the first commercial SEM, the Stereoscan from the Cambridge Scientific Instrument Company, reached the market in 1965.
How does a scanning electron microscope work?
Every SEM has the same basic building blocks: an electron source, a column of electromagnetic lenses, scan coils, a sample stage inside a vacuum chamber, and one or more detectors. The image is not captured all at once like a photograph. It is assembled point by point as the beam moves in a raster pattern.
- Electron gun. Electrons are emitted from a tungsten filament, a lanthanum hexaboride (LaB6) crystal or, in most modern instruments, a field emission source, and accelerated to energies typically between about 0.1 and 30 keV.
- Condenser and objective lenses. Electromagnetic lenses demagnify the beam to a spot that can be smaller than 10 nm on the sample surface.
- Scan coils. They sweep the beam line by line over a chosen area. A smaller scanned area means higher magnification.
- Beam–sample interaction. Electrons entering the sample scatter and lose energy within an interaction volume that can extend to around a micrometre below the surface.
- Detection. Detectors measure the signals leaving each point, and the software maps their intensity to pixel brightness.
Supporting infrastructure matters as much as the column. Facilities need stable power, vacuum pumps, cooling, a vibration-free floor and a room shielded from stray magnetic and electric fields.
Signals in scanning electron microscopy
The capability of an SEM depends largely on which detectors are fitted. The same beam generates several signals at once, and each carries different information.
| Signal | Origin | What it shows |
|---|---|---|
| Secondary electrons (SE) | Low-energy electrons (on the order of tens of eV) escaping from the top few nanometres | Surface topography and fine morphology |
| Backscattered electrons (BSE) | Beam electrons scattered back with high energy from deeper in the sample | Compositional contrast: heavier elements appear brighter |
| Characteristic X-rays (EDS) | Inner-shell ionisation of atoms | Elemental composition, spot analyses and element maps |
| Diffracted BSE (EBSD) | Backscattered electrons diffracted by crystal lattices | Crystal structure, grain orientation and texture |
| Cathodoluminescence (CL) | Visible light emitted by some materials | Trace-element zoning and defects |
Secondary vs backscattered electrons
Secondary electron images are the classic SEM pictures of insects, fracture surfaces and nanoparticles. Because SE come only from a very thin surface layer, they give the best spatial resolution. Backscattered electrons come from deeper in the interaction volume, so BSE images are less sharp, but their intensity depends on the mean atomic number of the material. This makes BSE ideal for quickly telling apart phases in alloys, ceramics or polished rock sections.
EDS: chemistry inside the microscope
Energy-dispersive X-ray spectroscopy (EDS) identifies elements from the energy of the X-rays they emit. It is fast and easy to use, but it has limits: very light elements such as H, He and Li cannot be detected, many detectors struggle with elements lighter than sodium, and sensitivity for trace elements is lower than with the wavelength-dispersive spectrometers used on electron microprobes.
Tip: choose the detector for the question. Use SE for shape, BSE for phase contrast, EDS for "what elements are here" and EBSD for "how are the crystals oriented". Combining BSE images with EDS maps answers most routine materials questions in one session.
Vacuum modes: high vacuum, low vacuum and ESEM
Electrons scatter in air, so a conventional SEM keeps the chamber under high vacuum, roughly 10-5 to 10-6 torr in many instruments. That works well for dry, stable, conductive samples, but not for wet or outgassing materials.
- High vacuum: best resolution and signal quality; samples must be dry and usually conductive or coated.
- Low vacuum / variable pressure: a small amount of gas in the chamber neutralises charge on insulating samples, so coating can often be skipped.
- Environmental SEM (ESEM): operates at pressures of a few torr and up to around 50 torr with controlled humidity, allowing hydrated or even wet samples to be observed closer to their natural state.
The trade-off is that gas molecules also scatter the beam, so resolution in low-vacuum and environmental modes is generally lower than in high vacuum.
Sample preparation for SEM
Many samples need little preparation, which is one of the reasons SEM is so widely used. A sample must be solid, fit on the stage and survive the chamber conditions.
- Mounting: samples are fixed to metal stubs with conductive tape or paint.
- Coating: insulators are sputter-coated with a few nanometres of gold, gold/palladium, platinum or similar metals for imaging, or carbon-coated when EDS analysis is planned.
- Biological samples: typically chemically fixed (for example with glutaraldehyde), dehydrated through a solvent series and dried by critical point drying with carbon dioxide to avoid collapse.
- Polished sections: for BSE and EDS of metals and rocks, a flat, polished surface gives the most reliable contrast and analysis.
Resolution and magnification of a scanning electron microscope
Resolution depends on the electron source, the beam energy, the detector and the sample itself. Depending on the instrument, it ranges from about 20 nm down to below 1 nm, with the best values achieved by field emission SEMs, and record secondary electron resolution has been reported at around 0.4 nm. Useful magnification runs from about 10× up to several hundred thousand times. In routine geological and analytical work, a spatial resolution of 50–100 nm and magnifications up to roughly 30,000× are common.
SEM vs TEM: key differences
Scanning and transmission electron microscopes are complementary. An SEM looks at surfaces of bulk samples, while a transmission electron microscope (TEM) sends the beam through an ultra-thin specimen to reveal internal structure, down to the atomic scale.
| Feature | SEM | TEM |
|---|---|---|
| Signal used | Electrons and X-rays emitted from the surface | Electrons transmitted through the specimen |
| Typical accelerating voltage | About 0.1–30 kV | Typically about 100–300 kV |
| Sample | Bulk, up to several centimetres | Electron-transparent, usually less than 100 nm thick |
| Information | Topography, composition, crystal orientation | Internal structure, crystallography, atomic arrangement |
| Preparation | Often minimal: mounting and coating | Demanding: ultramicrotomy, ion milling or FIB |
| Image | 3D-looking surface image | 2D projection through the sample |
Applications in materials and life science
In materials science, SEM is used for fracture analysis, coating thickness, grain structure, particle size and contamination checks. EDS and EBSD add chemistry and crystallography to the same session. In geology, BSE and CL imaging reveal mineral zoning and textures. In the life sciences, SEM documents cell surfaces, tissues, pollen, insects and biofilms, and environmental modes let researchers look at less heavily processed specimens.
These topics are recurring themes at microscopy meetings; for example, the materials science sessions of MCM 2019 covered many SEM-based studies. If you are planning to present your own work, see our guide to microscopy conferences in 2027.
Key takeaways
A scanning electron microscope combines a focused electron beam, raster scanning and several detectors to show both the shape and the composition of a surface. Choose high vacuum for top resolution, low vacuum or ESEM for insulating or hydrated samples, and pair imaging with EDS or EBSD when you need chemistry or crystal data. When you need internal structure at the atomic scale, TEM is the complementary technique.