Transmission and Scanning Electron Microscopy (TEM & SEM)

Core Concept

Electron microscopy (EM) uses a beam of electrons instead of light to image specimens. Because electrons have a far shorter wavelength than visible photons, EM achieves resolution thousands of times better than light microscopy — down to the sub-nanometer (atomic) scale. The two main forms differ in what they image: transmission electron microscopy (TEM) passes electrons through a thin specimen to reveal internal ultrastructure (organelles, membranes, macromolecules), while scanning electron microscopy (SEM) scans a focused electron beam across a specimen's surface and collects secondary electrons to produce a detailed three-dimensional surface image.

Key Components

• Electron source (gun): emits electrons, accelerated by high voltage (typically 60–300 kV for TEM, lower for SEM) to give them a short de Broglie wavelength (λ = h/p).
• Electromagnetic lenses: magnetic fields that focus and steer electrons (analogous to glass lenses for light).
• Vacuum column: electrons scatter in air, so the entire path is evacuated — a defining constraint.
• TEM: electrons transmit through an ultrathin section; image is formed by electrons that pass through (and are scattered by) the specimen; heavy-metal stains (osmium, uranyl, lead) increase contrast.
• SEM: a focused beam raster-scans the surface; detectors collect secondary electrons (knocked out of the sample's surface) and backscattered electrons; a conductive coating (gold/palladium or carbon) is applied.
• Detector/phosphor screen (TEM) and electron detectors (SEM): convert the electron signal into an image.

Mechanism / How It Works

1. Why electrons give high resolution. Resolution is limited by wavelength; a 100 keV electron has λ ≈ 0.0037 nm — far shorter than visible light (~500 nm) — so the diffraction limit is dramatically smaller. (Practical TEM resolution, ~0.1 nm, is limited by lens aberrations, not wavelength.)
2. TEM. A parallel electron beam illuminates an ultrathin section (typically 50–100 nm thick). Electrons that pass through are scattered differentially by dense (heavy-metal-stained) structures; denser regions scatter more electrons away and appear dark (electron-dense). The transmitted electrons are focused to form a 2-D projection of the section's internal structure. Contrast reveals membranes, organelles, and even individual macromolecules (with negative staining or cryo-EM).
3. SEM. A fine electron beam rasters point by point over the specimen surface. At each point, incident electrons eject secondary electrons from the near-surface atoms; the detector collects them, and their intensity maps to brightness. Because the yield depends on surface angle, the result is a topographic image with a strong 3-D appearance of the surface.
4. Preparation. Both require fixation (chemical or cryo), dehydration, and, for SEM, a conductive coating; TEM additionally requires embedding and ultrathin sectioning (or negative staining/cryo for macromolecules). Samples must be dry and vacuum-stable — meaning living cells cannot be imaged directly.

Comparison Table

Energy and Directionality

The "energy" in EM is the kinetic energy of accelerated electrons, set by the accelerating voltage (e.g., 100 keV). The electron wavelength follows the de Broglie relation λ = h/(2·m·e·V)^½, so higher voltage → shorter wavelength → higher potential resolution. No cellular ATP is involved; in fact, the electron beam's energy damages biological samples (radiation damage), which is a key limitation and a reason for cryo-EM (imaging at low temperature to slow damage).

Technique (How It Is Done)

• Fixation: chemical crosslinking (glutaraldehyde + osmium tetroxide) or rapid freezing (cryo-fixation) to preserve structure.
• TEM: dehydrate → embed in resin → cut ultrathin sections → stain with heavy metals → image in the vacuum column.
• SEM: dehydrate → critical-point dry (to avoid collapse) → coat with gold/palladium → raster-scan and collect secondary electrons.
• Cryo-EM (TEM variant): flash-freeze hydrated samples and image without stains, enabling near-native structures of proteins and complexes at atomic resolution.
• Limits: vacuum + dehydration preclude live cells; heavy preparation can introduce artifacts (shrinkage, damage); TEM sees only very thin sections.

How it works

1. Why electrons give high resolution. Resolution is limited by wavelength; a 100 keV electron has λ ≈ 0.0037 nm — far shorter than visible light (~500 nm) — so the diffraction limit is dramatically smaller. (Practical TEM resolution, ~0.1 nm, is limited by lens aberrations, not wavelength.)
2. TEM. A parallel electron beam illuminates an ultrathin section (typically 50–100 nm thick). Electrons that pass through are scattered differentially by dense (heavy-metal-stained) structures; denser regions scatter more electrons away and appear dark (electron-dense). The transmitted electrons are focused to form a 2-D projection of the section's internal structure. Contrast reveals membranes, organelles, and even individual macromolecules (with negative staining or cryo-EM).
3. SEM. A fine electron beam rasters point by point over the specimen surface. At each point, incident electrons eject secondary electrons from the near-surface atoms; the detector collects them, and their intensity maps to brightness. Because the yield depends on surface angle, the result is a topographic image with a strong 3-D appearance of the surface.
4. Preparation. Both require fixation (chemical or cryo), dehydration, and, for SEM, a conductive coating; TEM additionally requires embedding and ultrathin sectioning (or negative staining/cryo for macromolecules). Samples must be dry and vacuum-stable — meaning living cells cannot be imaged directly.

Comparison Table

Common confusions

• "TEM and SEM are interchangeable." — Wrong. TEM images internal structure through thin sections; SEM images surface topography. They answer different questions.
• "Higher accelerating voltage always gives sharper images of living cells." — Two errors: living cells cannot be in the vacuum, and higher voltage trades more radiation damage against shorter wavelength.
• "Electron microscopy can watch live cells." — No. The vacuum, dehydration, and beam damage make live-cell EM impossible (cryo-EM images frozen, not living, samples).
• "SEM gives atomic resolution." — No. SEM's resolution is ~1–10 nm; TEM (and cryo-EM) reach much finer, near-atomic resolution.
• "The dark spots in TEM are always where metal is." — Electron-dense regions (heavy-metal-stained structures) scatter more electrons and appear dark; it is the staining, not the biological material itself, that creates most contrast.

Quick review

• Electrons (short λ) → sub-nm resolution; vacuum required.
• TEM: transmits electrons through thin sections → internal ultrastructure (~0.1–0.5 nm).
• SEM: scans surface, collects secondary electrons → 3-D surface (~1–10 nm).
• Preparation: fix, dehydrate, embed/section + heavy-metal stain (TEM) or coat (SEM).
• Cryo-EM: frozen hydrated samples, near-atomic resolution.
• EM complements, not replaces, light microscopy.