What the SEM can't do
There are some things SEM can't do:
- SEM cannot take colour images. The colour is often added artificially in coloured SEM images. Note: Some SEMs can collect true colour images via a wavelength selective cathodolumenence (CL) detector.
- SEM cannot image through water.
Note: An ESEM using a wet Scanning Transmission Electron Microscope (STEM) detector can be used to image through thin water films.
- The SEM cannot reliably image charged molecules that are mobile in a matrix. For example, some species (e.g. Na+) are volatile under the electron beam because the negative electron beam exerts a force on charged material.
- SEM is not good for quantifying surface roughness at small scale. Atomic Force Microscope (Scanning Probe Microscopy) is more useful for this task.
- Measurements involving height (z-axis) cannot be taken directly in an SEM. This requires two images that have been tilted relative to one another to create a 3D image, and specialised processing software.
- Generally, SEMs are not used for experiments involving liquids, chemical reactions, and air-gas systems although some specialised machines and sample chambers do allow for these experiments.
- The resolution of the SEM is not high enough to image individual atoms (use a transmission electron microscope).
- Elemental analysis below micron scale.
Note: Analysis in the < 7kV range can provide elemental information on the sub-micron scale but is often problematical.
Where the SEM is not suitable, other techniques should be used. If in doubt consult the Technique Finder on the AMMRF web site.