What transmission electron microscopy is
Transmission electron microscopy (TEM) forms an image from electrons that pass through a very thin specimen. Instead of light and glass lenses, a TEM uses a focused electron beam and electromagnetic lenses. Because electrons have a far shorter wavelength than visible light, the instrument can resolve detail down to individual columns of atoms in crystalline materials.
The technique has a long history. Max Knoll and Ernst Ruska demonstrated the first TEM in 1931, their group exceeded the resolution of light microscopes in 1933, and the first commercial TEM appeared in 1939. Ruska later shared the 1986 Nobel Prize in Physics for this work.
TEM principle: how the image is formed
The working principle is easiest to follow along the column, from top to bottom.
- Electron source. A tungsten filament, a lanthanum hexaboride (LaB6) crystal or a field emission gun emits electrons, which are accelerated typically by 100 to 300 kV.
- Condenser lenses. They shape the beam and control how much of the specimen is illuminated and how intensely.
- Specimen. Electrons pass through the thin sample. Some go straight through, some are scattered elastically by atomic nuclei, and some lose energy in interactions with the sample's electrons.
- Objective lens and apertures. The objective lens forms the first image and diffraction pattern. Apertures select which electrons contribute to the final image, which is how contrast modes are chosen.
- Projector lenses. They magnify either the image or the diffraction pattern onto the detector.
- Detector. Older instruments used phosphor screens and film; modern ones use CCD or CMOS cameras and direct electron detectors that can count single electrons.
The whole column is kept under high vacuum, around 10-4 Pa or better, because air molecules would scatter the beam.
The key difference from an SEM: a TEM records electrons transmitted through the sample, giving internal structure, while a scanning electron microscope collects signals emitted from the surface. For the surface-imaging side, see our guide to how a scanning electron microscope works.
TEM imaging modes
One instrument can produce very different kinds of information depending on which electrons are used and what the lenses project.
| Mode | How it works | Typical use |
|---|---|---|
| Bright field | Objective aperture passes the direct beam; scattering regions appear dark | General morphology, cell ultrastructure, particle size |
| Dark field | Aperture selects scattered or diffracted electrons; those regions appear bright | Highlighting specific crystal orientations or phases |
| Electron diffraction (SAED) | Projector lenses image the diffraction pattern instead of the specimen | Crystal structure; spots for single crystals, rings for polycrystalline material |
| Diffraction contrast | Contrast from local changes in diffraction conditions | Dislocations, stacking faults and other lattice defects |
| HRTEM (phase contrast) | Interference of transmitted and scattered waves | Atomic lattice imaging, interfaces, nanoparticles |
| STEM | Convergent beam is rastered across the sample; annular detectors collect scattered electrons | Z-contrast imaging, mapping combined with spectroscopy |
Spectroscopy in the TEM
Electron energy loss spectroscopy (EELS) sorts transmitted electrons by the energy they lost, giving information on elements, bonding and electronic structure. Energy-dispersive X-ray spectroscopy is also widely fitted to TEM and STEM instruments for elemental mapping; the method is explained in our article on EDS/EDX in the electron microscope.
Resolution: how far TEM can go
Resolution in TEM is limited less by wavelength than by lens aberrations. For a 200 kV microscope without a corrector, the theoretical information cut-off is around 200 pm (2 Å). With spherical aberration correctors, the limit moves to the electron source and chromatic aberration, and recent corrector designs have reached below 0.5 Å, enough to image light atoms such as lithium in battery materials.
Interpreting high-resolution images is not straightforward. Phase-contrast images depend on focus, thickness and lens settings, so researchers often compare them with simulated images before drawing conclusions about atomic positions.
Sample preparation for TEM
The biggest practical constraint is thickness. A conventional TEM needs specimens that are electron transparent, typically under 100 nm, although the exact limit depends on accelerating voltage and the material.
- Biological tissue: fixation, resin embedding and ultramicrotomy into sections of roughly 70 to 350 nm, with heavy-metal stains such as osmium tetroxide and uranyl acetate for contrast.
- Particles and macromolecules: negative staining on a support film, or vitrification for cryo-EM.
- Metals and ceramics: mechanical polishing followed by ion milling, or chemical and electrolytic thinning for metals.
- Site-specific samples: focused ion beam (FIB) lift-out, which is precise but can implant gallium and damage the surface layer.
Cryo-EM and other advanced TEM methods
Cryogenic electron microscopy keeps samples frozen in vitreous ice, preserving biological molecules close to their native state. Jacques Dubochet, Joachim Frank and Richard Henderson shared the 2017 Nobel Prize in Chemistry "for developing cryo-electron microscopy for the high-resolution structure determination of biomolecules in solution". Single-particle analysis now routinely reaches about 3 to 4 Å for many proteins, with record structures far finer.
Electron tomography records a tilt series and reconstructs a 3D volume, though the sample cannot be tilted through a full 180°, leaving a "missing wedge" of information. In situ TEM uses special holders to heat, strain or bias samples during observation.
Uses and limitations of TEM
| Field | What TEM is used for |
|---|---|
| Materials science | Defects, grain boundaries, precipitates, thin films and interfaces |
| Nanotechnology | Size, shape and crystallinity of nanoparticles and nanowires |
| Semiconductors | Cross-sections of device layers and failure analysis |
| Cell biology | Organelles, membranes, viruses and cell ultrastructure |
| Structural biology | Protein and complex structures by cryo-EM |
The limitations are just as important when planning a project. Electron beams can damage specimens, especially biological and beam-sensitive materials. Preparation is slow and can itself change the structure. The field of view is small, so the region imaged may not represent the whole sample, and an image is a 2D projection through the thickness unless tomography is used.
Summary
Transmission electron microscopy works by passing a high-energy electron beam through an ultrathin sample and using electromagnetic lenses to form images or diffraction patterns. Bright field, dark field, diffraction, HRTEM and STEM modes, combined with EELS and EDS, make it one of the most versatile tools for studying structure from the cell to the atom. Its value depends heavily on good sample preparation and careful interpretation, which is why most projects combine TEM with complementary techniques such as SEM.