Below the critical voltage, knock-on stops. Radiolysis does not care about the voltage.
The beam can move atoms (knock-on), break bonds (radiolysis) and heat the specimen. Before you call a change real, check each one. This lab gives you the numbers: the voltage below which knock-on stops, the test that tells radiolysis from knock-on, how many electrons per Å2 your settings deliver, and how hot a probe can make the spot under it.
Ed: one representative value per element. Real values depend on direction and on whether the atom sits at a surface or edge, where it is easier to remove.
Moving A or Ed switches the material to "your own". Knock-on happens only above the critical voltage. Below it, damage you see is something else.
A picture, not a model of one material: damage grows with each electron and heals at rate k2. Units are arbitrary. k2 = 0 is the knock-on-like case.
Critical dose: the dose at which the feature you care about is gone (for example, diffraction spots fade to 1/e). Use a value measured on your own material if you can.
Energy left per nm = average loss per inelastic event ÷ inelastic mean free path. About 0.26 eV/nm fits carbon at TEM voltages; it falls as the voltage rises. Heat sink: roughly the radius of the grid hole or the edge of the foil. Metals: 20 to 400 W/m·K, oxides 1 to 40, carbon films about 1.6, polymers 0.1 to 0.5.
| Check | What you do | Tab | What it tells you |
|---|---|---|---|
| 1 Knock-on | Compare your voltage with the critical voltage for the lightest atom that matters. | Knock-on | Below it, knock-on is impossible. Above it, lowering the voltage is the fix. |
| 2 Dose rate | Give the same total dose at two very different dose rates, on fresh areas. | Radiolysis | A different outcome means something heals or diffuses between hits: not knock-on alone. |
| 3 Dose series | Image one area again and again and track a feature (diffraction spots, a spectrum edge) against accumulated dose. | Dose budget | The dose at which it fades is your critical dose. Stay well below it. |
| 4 Your dose | Work out e/Å2 per second (TEM) or per frame (STEM) from current, beam size, pixel size and dwell. | Dose budget | How long, or how many frames, you have before the feature is gone. |
| 5 Heating | Estimate the temperature rise for your current and material. | Beam heating | Usually a few K or less in a thin foil. Large only for poor conductors, large currents or foils with poor contact. |
| 6 Control | Watch a region the beam has not seen. For in-situ tests, repeat with the beam blanked except for short snapshots. | If the change happens only where the beam was, the beam is part of it. |
The knock-on formula is exact for a free, resting nucleus. It leaves out atoms at surfaces and edges (easier to remove), thermal vibration (which lowers the threshold a little) and channelling. The displacement energies are single representative values. The radiolysis tab is a picture with arbitrary units, made to show the dose-rate test, not a rate model for any material; real radiolysis can also show a true dose-rate threshold. The dose tab assumes a uniform beam and, in STEM, a probe smaller than a pixel. The heating tab uses the thin-foil result of Egerton, Li and Malac with radiation neglected; it assumes a uniform foil connected to a heat sink at a fixed distance, and the value for energy left per nm is an estimate. Contamination and charging, two other common beam effects, are not covered.
On this site: Scattering in the TEM · FIB specimen prep: what the ion beam does · EELS thickness calculator · Quantitative HAADF · GPA strain mapping
Two tests. Lower the voltage below the critical voltage: knock-on stops, radiolysis often gets worse. Change the dose rate at the same total dose: radiolysis changes, knock-on does not.
Only for knock-on. At lower voltage each electron loses more energy in the sample, so radiolysis per electron goes up. For beam-sensitive organics and many oxides, a higher voltage can be better.
Cooling slows the movement of broken bonds and radicals, so less damage becomes permanent. It does little for knock-on, which is a direct collision.
Rarely, in a thin foil. The electrons pass through and leave only tens of eV each. A few K is typical. Poor conductors, large currents and foils that are cut off from their support are the exceptions. Estimates that use the full beam power are far too high for thin foils.
@misc{tripathy2026beamdamage,
author = {Tripathy, Manisha},
title = {Beam Damage in the TEM},
year = {2026},
howpublished = {\url{https://untetheredatom.com/tem/beam-damage}},
note = {Interactive web tool}
}