SEM vs TEM at a glance
Both instruments use a beam of electrons instead of light, which is why they resolve details far smaller than any optical microscope. They differ in how the beam interacts with the sample, which signals are detected and what kind of specimen they can handle.
| Feature | Scanning electron microscope (SEM) | Transmission electron microscope (TEM) |
|---|---|---|
| Beam | Focused probe scanned point by point | Broad beam passing through the sample |
| Signals used | Secondary and backscattered electrons | Transmitted (and diffracted) electrons |
| Image | Surface topography with a 3D appearance | 2D projection of internal structure |
| Typical resolution | About 1–2 nm (field-emission instruments) | Below 0.1 nm with aberration correction |
| Magnification | Up to roughly 1–2 million times | Tens of millions of times |
| Accelerating voltage | About 1–30 kV | About 30–300 kV |
| Sample thickness | Bulk samples, limited by chamber size | Electron-transparent, usually under 100 nm |
| Field of view | Micrometres to several millimetres | Nanometres to several micrometres |
| Time to first image | Fast | Slower, mostly due to preparation |
How each microscope forms an image
Scanning electron microscope: building a picture pixel by pixel
In an SEM, electromagnetic lenses focus the beam into a fine probe that sweeps across the surface in a raster pattern. At every point, the beam knocks low-energy secondary electrons out of the top few nanometres of the specimen and scatters some primary electrons back as backscattered electrons. Detectors count these signals and the computer assigns a brightness to each pixel. Secondary electrons are sensitive to edges and slopes, which is why SEM images look strikingly three-dimensional. Backscattered electrons carry information about average atomic number, so heavier elements appear brighter. For a detailed walk-through of detectors and signals, see our guide on how a scanning electron microscope works.
Transmission electron microscope: looking through the sample
A TEM works more like a slide projector. A high-energy beam illuminates a thin region of the specimen, and electrons that pass through are focused by objective and projector lenses onto a camera. Areas that scatter more electrons, because they are denser, thicker or contain heavier atoms, appear darker. By switching lens settings the same instrument can record a diffraction pattern that reveals crystal structure, or a high-resolution phase-contrast image in which columns of atoms become visible. Our article on transmission electron microscopy covers bright-field, dark-field and diffraction modes in more depth.
SEM vs TEM resolution and magnification
Resolution, not magnification, is the number that matters. TEM wins clearly here because its electrons are accelerated to much higher energies, which shortens their wavelength, and because the image is formed from electrons that have travelled through a very thin slice rather than from a spreading interaction volume near the surface.
- SEM: modern field-emission instruments typically resolve features of about 1–2 nm. Resolution depends heavily on beam energy, working distance, detector and sample.
- TEM: conventional instruments resolve a fraction of a nanometre, and aberration-corrected TEMs go below 0.1 nm, enough to image individual atomic columns.
- Magnification: SEMs commonly cover from tens of times up to about a million times or more; TEMs reach tens of millions of times. Beyond the useful limit, higher magnification just enlarges blur.
Rule of thumb: if the feature you need is larger than a few nanometres and sits on the surface, an SEM will usually answer the question faster. If you need lattice planes, interfaces inside a material or the internal structure of a cell, you need a TEM.
SEM vs TEM images: what you actually see
An SEM micrograph of a pollen grain, an insect eye or a fractured metal looks like a greyscale photograph with depth, shadows and texture. The large depth of field keeps rough surfaces in focus from top to bottom, which is a major reason SEM images are so widely used in materials science, forensics and biology teaching.
A TEM micrograph looks flat. It shows the projected density of everything in the beam path: membranes and organelles inside a cell, dislocations and grain boundaries in a metal, or the shell and core of a nanoparticle. Both types of image are greyscale; the colours often seen in publications are added afterwards to highlight structures.
Sample preparation: the biggest practical difference
For many users, preparation decides the choice more than resolution does. An SEM accepts bulk specimens as long as they fit on the stage and tolerate vacuum. Conductive samples such as metals can often go in almost as they are; insulating or biological samples are usually dried and coated with a thin conductive layer, or imaged in a low-vacuum mode. Our step-by-step guide to SEM sample preparation explains fixation, drying and coating.
TEM specimens must be electron-transparent, which for most materials means thinner than about 100 nm. Typical routes are:
- Dispersing nanoparticles or powders on a support grid with a thin carbon film.
- Embedding biological tissue in resin and cutting ultrathin sections with an ultramicrotome.
- Staining sections with heavy-metal salts to increase contrast in soft matter.
- Thinning metals and ceramics by mechanical polishing followed by ion milling or electropolishing.
- Cutting a site-specific lamella with a focused ion beam (FIB) when a particular interface must be examined.
- Vitrifying proteins or cells in thin ice for cryo-TEM, avoiding chemical fixation altogether.
Each of these steps takes time and skill, and each can introduce artefacts, so interpreting TEM images always involves asking what the preparation may have changed.
Cost, size and ease of use
SEMs range from compact benchtop models to full floor-standing instruments, and they are generally less expensive to buy and run. Training is shorter, and many facilities let trained students operate them independently. TEMs are larger, often need a dedicated room with careful control of vibration, temperature and magnetic fields, and have a smaller pool of expert operators. Exact prices vary widely with configuration, detectors and service contracts, so quotes from suppliers are the only reliable guide.
| Question you are asking | Better choice | Why |
|---|---|---|
| What does the surface or fracture look like? | SEM | Topographic contrast and large depth of field |
| Which elements are present in this area? | Both (EDS) | SEM for quick mapping; TEM for nanoscale maps |
| What is the crystal structure or lattice spacing? | TEM | Electron diffraction and high-resolution imaging |
| What are the organelles inside this cell? | TEM | Thin sections reveal internal ultrastructure |
| How big are these particles across a large area? | SEM | Wide field of view and fast acquisition |
| Where exactly is a defect inside a device? | Both | SEM/FIB to locate and cut, TEM to analyse |
SEM vs TEM vs STEM
A scanning transmission electron microscope (STEM) combines ideas from both. Like a TEM, it uses a thin specimen and high-energy electrons; like an SEM, it scans a finely focused probe across the sample and builds the image point by point. Annular dark-field detectors in STEM produce contrast that rises strongly with atomic number, so heavy atoms stand out. Many modern TEMs can switch into STEM mode, and STEM is often paired with energy-dispersive X-ray spectroscopy or electron energy-loss spectroscopy for chemical mapping at the nanoscale.
Scanning vs transmission electron microscope for A-level biology
School syllabuses usually condense the comparison into a few points worth memorising:
- TEM has the higher resolution and magnification; SEM has lower resolution but shows surfaces in 3D.
- TEM images are 2D and show internal structures such as mitochondria, ribosomes and the nuclear envelope.
- Both require a vacuum, so living specimens cannot be observed.
- Both produce black-and-white images; colour is added artificially.
- TEM needs very thin sections; SEM can examine whole small specimens.
Conclusion
Choosing between a scanning vs transmission electron microscope starts with the question, not the instrument. Use an SEM when you need fast, intuitive images of surfaces, textures and particles on bulk samples. Use a TEM when the answer lies inside the material, at the scale of lattice planes, nanoparticle cores or cellular ultrastructure, and you can invest in thin-sample preparation. In many projects the two work best together: SEM to survey and locate, TEM to look deeper.