SEM and TEM

Why it matters

Most materials properties come from microstructure: grain size, phases, porosity, cracks, interfaces and defects. Optical microscopes stop at roughly 200 nm resolution because of the wavelength of light. Electron microscopes go far smaller, and choosing the right one (and the right detector) decides whether you answer your question in an afternoon or waste a week on sample preparation.

Why electrons

An electron's wavelength depends on its accelerating voltage. At 200 kV it is about 2.5 pm, thousands of times shorter than visible light. In practice, resolution is limited by lens aberrations, not wavelength, but electron microscopes still resolve features from tens of nanometres (a typical SEM) down to individual atomic columns (an aberration-corrected TEM).

Electrons also interact strongly with matter, which gives several signals at once: secondary electronsLow-energy electrons from the top few nanometres of a sample, used in the SEM to image topography., backscattered electronsBeam electrons scattered back out of the sample; their signal scales with atomic number, so they show composition contrast between phases., characteristic X-rays and diffracted beams. Each detector answers a different question.

SEM: scanning electron microscopy

The SEM scans a focused beam (usually 1 to 30 kV) across the surface of a bulk sample and collects what comes back.

• Secondary electrons (SE) come from the top few nanometres. They show topography: fracture surfaces, particles, coatings, corrosion.
• Backscattered electrons (BSE) scale with atomic number. Heavier phases look brighter, so BSE shows composition contrast between phases on a polished section.
• EDSEnergy-dispersive X-ray spectroscopy, which reads characteristic X-rays to give elemental composition and maps; semi-quantitative without standards. (energy-dispersive X-ray spectroscopy) reads characteristic X-rays to give elemental composition and maps. It is semi-quantitative unless calibrated with standards, and light elements (B, C, N, O) are hard.
• EBSDElectron backscatter diffraction, an SEM technique that maps crystal orientation and grain boundaries on a well-polished surface. (electron backscatter diffraction) maps crystal orientation and grain boundaries on a well-polished surface.

Samples can be millimetres to centimetres across. Non-conductive samples (polymers, ceramics) charge up, so they are coated with a few nanometres of carbon or gold, or imaged at low voltage or in a low-vacuum mode.

TEM: transmission electron microscopy

The TEM sends a high-energy beam (often 80 to 300 kV) through a sample thin enough to transmit electrons, usually below about 100 nm.

• Bright-field and dark-field imaging show dislocations, precipitates, grain boundaries and stacking faults.
• Selected-area electron diffraction (SAEDSelected-area electron diffraction in the TEM, which gives the crystal structure and orientation of a small region.) gives the crystal structure and orientation of a region a few hundred nanometres across.
• High-resolution TEM and STEM (scanning TEM) image lattice fringes and atomic columns; STEM with EDS or EELS maps composition at the nanoscale.

The cost is sample preparation: electropolishing, ion milling or a focused ion beam (FIB) lift-out to cut a thin lamellaA thin slice cut from a specific spot, often by focused ion beam lift-out, so electrons can pass through it in the TEM. from a specific spot. Preparation can itself create artefacts (ion damage, amorphous layers), and a TEM sees a tiny volume, so you must check that it is representative.

Choosing a technique

A good rule: start with the cheapest technique that can answer the question. Light microscopy and SEM survey large areas; TEM confirms a specific nanoscale hypothesis. Combine imaging with bulk techniques such as XRD so a few images do not stand in for the whole sample.

Worked example

A steel bolt failed in service. You want to know whether it failed by fatigue and whether inclusions started the crack.

1. Photograph the fracture surface, then look at it in the SEM with SE imaging: beach marks and striations point to fatigue; dimples point to ductile overload.
2. Find the origin and use EDS on any particle there: manganese and sulfur together suggest a MnS inclusion.
3. Section and polish a sample near the origin and use BSE or EBSD to look at the microstructure.
4. Only if you need nanoscale detail (for example, precipitates that embrittled the steel) cut a FIB lamella for TEM.

Common mistakes

• Reading EDS numbers as exact. Without standards, light elements and small peaks are uncertain, and the X-ray signal comes from about a micrometre deep, not just the surface you see.
• Charging artefacts on uncoated insulators, which look like bright streaks and drift.
• Imaging one pretty spot and reporting it as typical. Image several regions at known magnifications and include scale bars.
• Forgetting beam damage: polymers and some oxides change under the beam.

Safety and training

Electron microscopes use high voltages, vacuum systems, and (for FIB and sample preparation) chemicals, cryogens and ion sources. You use them only after your facility's training and sign-off, under its procedures. This lesson builds the judgement you bring to that training.

Recap

• SEM images surfaces of bulk samples: SE for topography, BSE for composition contrast, EDS for elements, EBSD for orientation.
• TEM looks through thin samples for internal defects, diffraction and atomic-scale structure, at the cost of preparation.
• Pick the cheapest technique that answers the question, and check that images are representative.